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Sediment-borne Contaminants in Rivers Discharging into the Humber Estuary, UK
PII:
Marine Pollution Bulletin Vol. 37, Nos. 3±7, pp. 316±329, 1998 Ó 1999 NERC. Published by Elsevier Science Ltd. All rights reserved Printed in Great Britain 0025-326X/99 $ ± see front matter S0025-326X(99)00055-7
Sediment-borne Contaminants in Rivers Discharging into the Humber Estuary, UK J.G. REES*, J. RIDGWAY, R.W.O.B. KNOX, G. WIGGANS and N. BREWARD British Geological Survey, Keyworth, Nottingham NG12 5GG, UK As part of a project to characterise and quantify the volume of sediments which comprise the Holocene (10,000 years to present) ®ll of the Humber Estuary, a study was undertaken to determine the geochemistry and heavy mineralogy of bed sediments in the river systems that discharge into the estuary. A total of 19 sediment samples in the Trent and Ouse river systems were taken for analysis. Contamination was evaluated by comparison of the sample geochemistry with that of the appropriate catchment (using existing data), and by evaluation of the proportion of anthropogenic heavy minerals (including natural minerals which have been mobilised by mining) in the sediment sample. Heavy metals fall into two groups with dierent patterns of distribution. Pb±Zn concentrations are greatest in catchments and rivers draining the Pennine ore®elds. Levels of these metals remain high between source areas and the Humber Estuary suggesting that large quantities are trapped in sediments stored within the ¯uvial systems. By way of contrast other heavy metals, associated with manufacturing industry, such as Cu and Co, have high concentrations near source cities, but decrease rapidly in amount down the river systems because of dilution by other sediments. The diering behaviour of mining and industrial related contaminants is generally re¯ected by the heavy minerals. Concentrations of gangue minerals, such as barytes and ¯uorite, are generally highest in rivers draining mining areas; ÔfurnaceÕ materials, such as slags are highest in industrialised rivers such as the Aire and the Don. The anthropogenic origin of all the contaminants is illustrated by comparison of the catchment and river sediment geochemistry and heavy mineralogy with that of early Holocene ¯uvially-derived sediments cored in boreholes drilled in the present Humber Estuary. The ®ndings of the study corroborate those of others focused on water chemistry and ¯oodplain sediments (Neal et al., 1996, 1997; Macklin et al., 1997). However, the comparison of pre-existing data on catchment geochemistry with a limited number of river samples, as demonstrated here, provides a fast and cost eective tool for the determination of contamination characteris*Corresponding author.
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tics in river systems. Ó 1999 NERC. Published Elsevier Science Ltd. All rights reserved
by
Background In recent years levels of pollutants in the North Sea have caused increasing concern to nations that border it, leading to the formulation of policies which will reduce the volume of pollutants entering the sea (Brown, 1993). Such concerns have also highlighted the need for further research into the transport of contaminants into the sea from onshore sources. The Land±Ocean Interaction study (LOIS), funded by the UK Natural Environment Research Council (NERC) is aimed at quantifying the exchange, transformation, and storage of materials at the land±ocean boundary, and determining how these parameters vary in time and space (NERC, 1994; Wilkinson et al., 1997). Part of the LOIS programme, the Land±Ocean Evolution Perspective (LOEPS), is identifying the long-term changes in ¯ux into the coastal zone, particularly in response to post-glacial sea-level rise. The central component of this focuses on the evolution of the Humber Estuary on EnglandÕs North Sea coast. One of the aims of the LOEPS research is to characterise and quantify the volume of sediments which comprise the Holocene (10,000 years to present) ®ll of the Humber Estuary. This is being done by examining the bulk sediment geochemistry, heavy mineralogy and clay mineralogy of cored sequences through the ®ll, and determining the relative in¯uences of ¯uvial and marine systems. This paper describes a pilot project undertaken to: (1) determine the geochemistry and concentration of contaminants in sediments of the major rivers draining into the Humber; (2) compare these with catchment geochemistry using existing datasets; (3) examine the heavy mineral content of the river sediments; and (4) compare these geochemical signatures and heavy mineral data with those from older (pre-anthropogenic) Holocene ¯uvial sediments of the Humber Estuary ®ll. Other contamination studies within LOIS have focused
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on mapping of heavy metals within these river systems by long-period monitoring of dissolved and acid-available particulate phases in waters (Neal et al., 1996; 1997), and studies of the historical variation of heavy metal supply in selected river ¯oodplains (Macklin et al., 1997). The work described here attempts to ®nd a cost-effective approach to the characterisation of contaminants entering the Humber by comparing new data on the geochemistry of nineteen major river-bed sediment and borehole samples with existing data on the geochemistry of their catchment basin sediments. Heavy mineral data are used to con®rm the geochemical ®ndings. A signi®cant factor in the rationale behind the study was that stream sediment based geochemical datasets, covering almost the whole of the UK land area are available through the ongoing British Geological Survey (BGS) regional geochemistry surveys, currently covering all of northern Britain, and the work of the Applied Geochemistry Research Group of Imperial College, London covering England and Wales (Wolfson Geochemical
Atlas data, Webb et al., 1978) and Northern Ireland (Webb et al., 1973). Any methodology developed could thus have widespread application.
Setting of the River Systems The Humber Estuary receives freshwater runo from two major river systems, the Trent and the Yorkshire Ouse (Fig. 1). The catchments of these have a combined area of 24 000 km2 , or approximately one ®fth of the area of England. The catchments of the Trent system, and that of the Yorkshire Derwent in the Ouse system, receive between 750 and 800 mm of rain per year. The Pennine catchments of the Ouse system, which are generally of higher elevation, receive between 850 and 1150 mm per year. Agricultural land-use within them is very varied, ranging from the upland pasture on some margins of the system watersheds to intensively-farmed lowlands in the lower parts of the river basins.
Fig. 1 The main rivers of the Trent and Ouse systems, major cities, and sites of river samples.
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Some of the major tributaries feeding the Trent (Fig. 2) are dominantly rural, and have no notable urban centres on them; these include the Dove, Idle and Torne. Others are widely urbanised and contain many of EnglandÕs principal industrial conurbations which, together, account for almost half of national metallurgical, coal and power production (Arnett and Justice, 1991; Jarvie et al., 1997). Some catchments are associated with cities which have lesser concentrations of heavy industry, such as the Soar, upon which Leicester is sited. All of the other tributaries host cities associated with manufacturing industry, including a large part of the West Midlands conurbation, incorporating Birmingham, on the River Tame, and Derby on the Derbyshire Derwent. A number of large cities associated with manufacturing industry occur on the Trent itself, including Stoke-onTrent, Burton-upon-Trent, and Nottingham. Several of the rivers within the Ouse system are not associated with urban centres, but with areas that have a long history of Pennine Pb±Zn mining (Dunham and Wilson, 1985). These include the Ouse (above York), into which the Swale and Ure drain, the Nidd and the Wharfe. The Aire, by way of contrast is less associated with metalliferous mining, but drains cities in the Aire Valley, including Leeds, and also the mill towns on its tributary, the Calder. The southernmost of the major rivers of the Ouse system, the Don, likewise is associated with cities which have a long history of heavy manufacturing in-
dustry (particularly iron and steel), including Sheeld and Doncaster.
Methodology River Sampling To ascertain the likely character of modern ¯uvial sediments discharging into the Humber Estuary a reconnaissance survey was undertaken of the geochemistry and mineralogy of the Trent and Ouse river systems, including important tributaries. The river sampling programme was undertaken at nineteen sites during two consecutive days in early February 1997. The samples at Newark and Acaster occur just upstream of the tidal limits of the Trent (at Cromwell Lock) and Ouse (at Naburn) respectively. The only samples taken in the tidal reaches of the systems were those from Gainsborough on the Trent, and Cawood, on the Ouse. The samples from the tributaries of the Trent and Ouse were taken from a point near their con¯uence with these rivers, or in the case of samples on the Torne, Idle, Yorkshire Derwent, Don and Aire just above their tidal limits. At each site muddy sands or sandy muds were sampled from the river bed within 2 m of the bank in water depths of less than 0.3 m using a plastic scoop. Some winnowing of ®ne-grained material was inevitable, but only the surface of the sample was aected and this was minimised by covering the sample by hand during transport through the water column. Samples for bulk sediment geochemistry and for heavy mineral analysis were stored separately. Visual and textural analysis indicates that all samples comprise a mixture of ®ne-sand, very ®ne-sand, silt and clay. In all cases, the sediment sampled is considered to represent a mixture of bedload material and suspended matter deposited under waning ¯ood ¯ow conditions, probably over several cycles. Such ÔactiveÕ drainage sediments are generally accepted as being representative of the upstream catchment area (see Ottesen and Theobald, 1994). However, it is recognised that there is a possibility of local heterogeneity in the river bed and the question of sampling reproducibility must be addressed in future work.
Sample Preparation and Analysis
Fig. 2 Catchments of the principal tributaries of the Rivers Trent and Ouse. The catchment of the river Sow, as shown here, includes the headwaters of the Trent. Likewise, that of the Swale includes the headwaters of the Ouse, as well as the river Ure. The sites of boreholes discussed in the text are shown.
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Geochemical samples were dried for 12 h at 60°C before being reduced to powder in an agate mill and pressed into pellets of c. 12 g mass for determination of a range of major oxides and trace elements using highprecision X-ray ¯uorescence (XRF) facilities at the British Geological Survey. Calibrations were veri®ed by analysis of Reference Materials and a stable silica glass disc, containing all the analytes of interest, was analysed regularly as a quality control standard with the data plotted on QC charts to monitor performance. The samples were analysed as part of a larger programme involving sediments from the LOEPS project and cor-
Volume 37/Numbers 3±7/March±July 1998 TABLE 1 River sediment sample sites and catchment data. Catchment identi®ed by main river Sow Tame Trent (1) Dove Derwent Soar Trent (2) Trent (3) Idle Torne Trent (4) Swale Nidd Ouse (1) Wharfe Aire Don Derwent Ouse (2)
River sample site location Gt. Haywood Alrewas Burton Hatton Spondon Kegworth Wilford Newark Bawtry Rossington Gainsborough Newton Cattal Acaster Boston Spa Beal Doncaster Kexby Cawood
River sample site grid reference 39962 41874 42588 42140 44155 44942 45693 47920 46562 46288 48145 45108 44472 47049 44327 45340 45665 47049 45744
32242 31400 32367 32947 33392 32720 33808 35386 39270 39959 38910 46000 45400 45118 44574 42560 40371 45118 43790
relation coecients for pairs of replicate analyses of core samples (resampled, prepared and analysed, all at a later date) are shown in Table 1, demonstrating the high precision of the general methodology. Samples for heavy mineral analysis were ultrasonically cleaned and wet sieved. Heavy minerals were separated from the 63±125 lm grain size fraction by gravity-settling in bromoform (S.G. 2.90). The heavy mineral fractions were mounted in Canada Balsam for optical study. The analyses were carried out by conventional optical microscopy; this involved a count of 200 non-opaque detrital grains per sample (where grain recovery permitted) to determine the overall composition of the suite. While the data obtained are thus not representative of the sand fraction as a whole, they provide a sound and time-ecient basis for comparison in a reconnaissance study of this kind.
System *tidal
Catchment area (km2 )
Number of catchment samples
Trent Trent Trent Trent Trent Trent Trent Trent Trent Trent Trent* Ouse Ouse Ouse Ouse Ouse Ouse Ouse Ouse*
871 1510 3114 1079 1203 1353 7361 8197 818 123 9542 2691 510 3064 740 1960 1299 1710 4662
356 279 838 363 254 357 1910 2097 90 12 2345 1029 184 1202 247 394 201 511 1729
alytical and quality control procedures are described in the Atlas.
Comparison of River and Catchment Geochemical Datasets Drainage basin geology (Fig. 3) clearly will in¯uence the geochemistry of the river samples and it is thus necessary to examine the background geochemical signature of the catchments which contribute sediments to the sample station. The only geochemical dataset which covers catchments for the entire Ouse and Trent systems is that from the Wolfson Geochemical Atlas (Webb et al., 1978), based on stream sediments, for which sampling started in 1969. The mean sampling density was about 1 per 2.5 km2 . At each site two 100 g samples of active stream-bed sediment were taken at least 20 m upstream of any road, and sieved at 200 lm. No samples were taken in urban areas. Most elements were analysed spectrographically, though four, including As and Zn, were determined by atomic absorption spectrophotometric (AAS) and colorimetric methods. Sampling, an-
Fig. 3 Geology of the Trent and Ouse systems. Superimposed on this are the segments of the catchments of the rivers Trent and Ouse systems discussed in the text. Catchment Ouse 2 comprises the area marked, as well as that of catchment Ouse 1. Likewise Trent 2 includes Trent 1, Trent 3 includes Trent 1 and 2, and Trent 4 includes Trent 1, 2 and 3. Pleistocene deposits in the region are principally lodgement tills of local origin, although some will contain sediment transported from the North Sea area during one or more of the Pleistocene glaciations. As the provenance and boundaries of dierent Pleistocene formations are unclear, and because they will (largely) re¯ect bedrock character, they are not shown here.
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The stream sediments from the Wolfson survey can be expected to re¯ect natural background variations in geochemistry, but also might show variations related to mining activity in upland areas and to agricultural practices.
Data Compatibility The BGS XRF data and the Wolfson spectrographic data are both based on analytical methods which yield ÔtotalÕ concentrations and are thus directly comparable. The Wolfson data for As and Zn are based on techniques which provide ÔpartialÕ concentrations, although in most cases these will be close to ÔtotalÕ and comparison can be made with appropriate caution. Grain size also can in¯uence element concentrations, ®ne-grained sediments normally having higher concentrations of trace elements than coarse-grained ones. The BGS LOEPS geochemical dataset is based on analysis of the <2 mm sediment fraction. This size fraction was used because it allows direct comparison with oshore geochemical data, for which ®ne-grained sample material is often dicult to acquire, and enables sucient material for analysis to be obtained from cores, particularly where coarser grained sediment is present. Appleton (1995) compared the Wolfson and BGS stream sediment data and attributed variations in heavy metal concentrations between the two to the use of dierent size fractions and sieving techniques, the BGS data being based on slightly ®ner-grained material. However, within this study, the unsieved river sediments and the Wolfson sieved sample material are very similar in grain size, both being composed of sediment of ®ne sand grade or smaller. Correlation coecients for both the river sample and Wolfson datasets show no strong relationships between trace metals and Al, a common proxy for grain size. In addition, scavenging by Fe±Mn oxyhydroxides and complexing by organic matter also could aect trace element concentrations. Wholesale normalisation to a grain size proxy could, therefore, have an unpredictable eect on some elements and no attempt has been made, to compensate for grain-size, although there is scope for signi®cant variation within the size fractions used. The list of determinands in the Wolfson and BGS LOEPS datasets also is dierent and this study is restricted to those elements which are common to both. Although, it has not been possible to reanalyse any of the Wolfson sample material for direct comparison with the BGS XRF data, the similarity of many of the geochemical signatures for catchments and river samples shown in Figs. 4±8 suggests that the two datasets are compatible.
Interpretational Approach Given that the Wolfson and BGS datasets are based on slightly dierent sediment size fractions, have been 320
derived using dierent analytical techniques and involve dierences in the range of determinands, a robust method of interpretation is required. Furthermore, in comparing catchment geochemistry (based on many samples) with major river sample and borehole geochemistry (based on single samples), calculation of average values and, in some instances, values weighted according to catchment size is necessary. Most standard statistical techniques, such as principal components analysis, are not appropriate in these circumstances. Instead, average catchment and river sample geochemistry are compared by means of ÔspidergramsÕ in which each element value has been normalised to the upper crustal average given by Wedepohl (1995) and plotted on a logarithmic scale. This allows elements with widely diering concentrations to be viewed on a single diagram. Oxide values in percentage are converted to ppm before normalisation. Dierences in concentration levels due to grain size and to analytical techniques may still be present and the diagrams shown (Figs. 4±7) and discussed below must be viewed with this in mind. The advantages of the spidergram approach are that patterns of variation can be compared even though absolute concentrations are dierent and large dierences can be viewed at the same time as close similarities.
Trent Tributary Catchments The Trent tributary catchments can be split into four groups, re¯ecting topographic, geological and anthropogenic in¯uences. The catchments of the rivers Torne and Idle are both subdued topographically and dominantly rural, though the fringes of Doncaster impinge on that of the Torne, and small manufacturing towns occur in the Idle catchment. Both catchments contain mainly PermoTriassic rocks. The Torne is the smallest catchment considered (Table 1) and the systematic dierences for most elements between the catchment and the river sample geochemistry are the largest in this study (Fig. 4). The reasons for this are not clear, but may be related to systematic grain-size dierences as indicated by the relatively high Al and Fe±Mn concentrations in the river sample from Rossington (Table 2), which could be related to the clay mineral content and Fe±Mn oxyhydroxide coatings on clay minerals respectively. Such factors are considered to exert a strong in¯uence on trace element concentrations (e.g. Watters, 1983). Notwithstanding this, Zn (Table 2, Fig. 4) still appears abnormally high in the river sediment and perhaps represents contamination from lowland agricultural practices. The Idle catchment is also relatively small, but the dierences between average catchment and river sample geochemistry are less systematic than in the Torne and the noticeable river sample highs in Co, Cu and Zn could be due to contamination. The Derbyshire Derwent and the Dove and their tributaries drain the higher ground associated with
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Fig. 4 Geochemistry of the tributaries of the Trent system. The mean spidergram of the catchment samples is shown by the catchment name; the river sample by the sample site name (see Table 1).
Carboniferous rocks of the Peak District at the southern end of the Pennines, though they pass over Triassic rocks towards their con¯uence with the Trent (Fig. 3). Mining, mainly for Pb±Zn, has been extensive in the past, particularly in the catchment of the Derwent (Mostaghel, 1983), which also has had a greater association with industry, particularly engineering in Derby. Catchment and river sediment geochemistry are very similar here (Fig. 4), although the river is particularly low in Mn and As, for which there is no obvious reason. Both graphs display a sharp change in slope due to an increase in Zn and Pb values that most probably represents upland, mining related contamination. The Dove catchment and the river sample from Hatton show remarkably similar geochemistry suggesting, as might be
expected, that there is little industrial contamination in this basin. The catchments of the rivers Sow (which, as shown in Fig. 2, also includes the headwaters of the River Trent), and Tame, both are associated with Triassic and Carboniferous rocks. The mineral resources of the latter have been a focus for industry over the last two centuries, and have led to the growth of important manufacturing cities such as Stoke-on-Trent and Birmingham. Both the Great Haywood and Alrewas samples show evidence of contamination stemming from industrial activity there being little or no mining in these catchments. Industrial contamination is particularly prominent in the enhanced Cr, Ni, Cu and Zn values at Alrewas. 321
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Fig. 5 Geochemistry of segments of the catchments of the river Trent (Fig. 3) discussed in the text. The mean spidergram of the catchment samples is shown by the catchment name; the river sample by the sample site name (see Table 1).
The catchment of the River Soar constitutes a largely rural area of thickly drift-covered Triassic and Jurassic rocks. Although the city of Leicester occurs in the catchment it is generally not associated with heavy industry, or mining. River and catchment geochemistry are closely matched in this drainage basin and there is little sign of contamination.
Trent Catchments To determine how the geochemical signature changes downstream, and the likely input to the Humber Estuary, the Trent basin has also been divided into four catchments of successively increasing size (Fig. 3), such that basin 2 includes basin 1 and 3 includes basins 1 and 2 and so on. Only at Burton is there any sign of signi®cant industrial contamination (Fig. 5) and this clearly can be related to the input from the Tame valley (Fig. 4). The other three catchments show very similar patterns in the geochemical signatures of both river sediment and average catchment with little sign of industrial contamination, except perhaps for As and possibly Fe and Cr at Gainsborough. Relatively high concentrations of Pb and Zn most probably re¯ect a continuing in¯uence of mining related contamination. The signatures of Trent 2 and river sediment from Wilford show the biggest dierences, although the general pattern of variation is similar, with none of the major change of slope dierences shown by the Trent 1 and Burton patterns. The river sediment from Wilford contains generally lower concentrations of the elements studied than the Trent 2 catchment, including Al, a common proxy for grain size. It is likely that the observed concentration dierences are due to the Wilford sediment being coarser grained than that of the 322
catchment as a whole. Increased levels of Ca and Sr in the tidal section of the Trent at Gainsborough could be related to shelly material in the sediment or possibly to the presence of carbonate cements from the dissolution and reprecipitation of Cretaceous chalk fragments.
Ouse Tributary Catchments The catchments of the Swale (here taken to include the headwaters of the River Ouse, as well as the Ure), Nidd and Wharfe extend onto the Pennines on their western ¯anks. The high ground is formed of Carboniferous rocks which have a long association with mining, particularly of Pb±Zn. The lower parts of catchments occur on drift-covered Triassic and Jurassic strata. In general, the average catchment and river sediment geochemical signatures are very similar in pattern for the Swale and Nidd (Fig. 6). Probable mining related contamination is seen in the enhanced Ba and Pb levels in the Newton (Swale) river sample and the similarly enhanced Ba, Cu, Zn and Pb in the Cattal (Nidd) sample. Relatively high concentrations of Zn and Pb in the catchment signatures could stem directly from upland mining contamination or from naturally enhanced background levels in a mineralised area. The Wharfe is rather dierent with the catchment signature displaying higher concentrations than the river sample for most elements. Both signatures show relatively high Ba levels (see Table 2) and the catchment in particular is high in Zn and Pb. A notable feature of the Boston Spa river sample is the low level of As and the relationship between river and catchment signatures for the Wharfe is very similar to that described above for the Derbyshire Derwent.
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Fig. 6 Geochemistry of the tributaries of the Ouse system, and segments of the catchments of the river Ouse (Fig. 3) discussed in the text. The mean spidergram of the catchment samples is shown by the catchment name; the river sample by the sample site name (see Table 1).
The catchments of the Aire, including the Calder, and the Don drain Carboniferous rocks of the Pennines. However, the amount of Pb±Zn mining within them is considerably smaller than those to the north. The Aire and Don catchments have had the highest concentration of heavy industry in the Trent and Ouse systems, associated with mineral resources in the Carboniferous rocks. The cities of Leeds, Sheeld and Doncaster, and many other manufacturing towns occur within these basins. The Aire at Beal shows the eects of industrial contamination in high levels of Cr, Cu, Zn and Pb relative to the catchment geochemistry. In the case of the Don, catchment and river (Doncaster) signatures are very similar, but levels of Cr, Co, Ni, Cu, Zn and Pb are
high in both (Fig. 6 and Table 2), re¯ecting the industrialised nature of much of the catchment. The Yorkshire Derwent drains a rural catchment underlain by Jurassic rocks. The geochemical signatures of river (Kexby) and catchment are very similar and show no signs of contamination.
Ouse Catchments The Ouse catchment has been divided into two in a similar manner as for the Trent. The Ouse 1 catchment (Fig. 6) includes the Nidd and Swale and the relationship between the river sediment sampled Acaster, and the average catchment geochemistry is very similar to 323
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Fig. 7 Comparative geochemistry of the ¯uvial sediments of the Trent and Ouse river systems and early Holocene ¯uvially-derived sediments. The locations of boreholes referred to are shown in Fig. 2, and depths of sediment bodies in Table 4.
Fig. 8 Comparative geochemistry of the Derbyshire Derwent (DDERWENT) and Dove catchments showing the higher concentrations of Pb and Zn related to mining in the Derwent basin.
those shown in the tributaries with enhanced levels of Ba and Pb in the river sample. Further downstream, at Cawood, below the con¯uence with the Wharfe, the river sediment shows evidence of industrial contamination in relatively high levels of As, Cr and Cu. There may also be enrichment in Fe, Co and Ni, but for these metals, in comparison with the catchment, the dierences in absolute concentrations and the departure from the general trend are smaller. Cock Beck, draining north-eastern Leeds, enters the Wharfe below Boston Spa and this may account for the elevated metal levels. The Ouse at Cawood is tidal, and the high Ca and Sr values here may perhaps have the same explanation as was advanced for the Trent at Gainsborough, being caused by the presence of shelly material or the formation of carbonate cement. The tidal nature of the sites at Cawood and Gainsborough provides an alternative explanation for some of the observed metal enrichments. Middleton and Grant (1993) have shown a spatial uniformity in heavy metal concentrations in sediments of the Trent±Ouse±Humber system which extends landwards beyond the normal limit of seawater intrusion. A signi®cant proportion of the bed sediment of the Humber is considered to be clay originating from erosion of Pleistocene tills on the 324
Holderness coastline (Al-Bakri, 1986). Within the estuary the titanium dioxide industries between Immingham and Grimsby discharge large amounts of Fe, which facilitates the development of Fe coatings on clay particles and the consequent scavenging of heavy metals from the dissolved phase. The contaminated sediments are then redistributed by tidal pumping and may penetrate upestuary to the tidal limit. While this could account for the increased metal concentrations in the tidal river sediments (As, Fe, Cr, Co, Ni and Cu at Cawood; As and possibly Fe and Cr at Gainsborough), the depletion of Zn and Pb (in comparison with the catchment) at both sites and the selectively large enrichments in Cr and Cu at Cawood suggest that it is not the full explanation.
Anthropogenic Heavy Minerals in River Samples Throughout the river samples taken, the non-opaque heavy mineral assemblages include a wide variety of minerals derived from bed-rock sandstones, including anatase, apatite, garnet, monazite, rutile, staurolite, tourmaline and zircon, accompanied in varying proportion by relatively unstable minerals derived from the overlying Pleistocene tills and sands, including amphi-
Volume 37/Numbers 3±7/March±July 1998 TABLE 2 Geochemical data from samples and catchments. r river sample, c mean for catchment (DDERWENT Derbyshire Derwent; YDERWENT Yorkshire Derwent).
ROSSINGTON TORNE BAWTRY IDLE SPONDON DDERWENT HATTON DOVE HAYWOOD SOW ALREWAS TAME KEGWORTH SOAR BURTON TRENT 1 WILFORD TRENT 2 NEWARK TRENT 3 GAINSBOROUGH TRENT 4 NEWTON SWALE CATTAL NIDD BOSTON WHARFE ACASTER OUSE 1 CAWOOD OUSE 2 BEAL AIRE DONCASTER DON KEXBY YDERWENT
Al2 O3
CaO
Mn
Ba
As
Sr
K2 O
Fe2 O3
Cr
Co
Ni
Cu
Zn
Pb
r c r c r c r c r c r c r c r c r c r c r
10.40 3.33 9.30 4.55 5.10 5.64 6.20 4.90 7.60 4.38 7.50 4.53 5.80 4.59 6.80 4.48 3.00 4.75 6.10 4.78 8.50
4.75 4.28 2.30 4.10 1.86 2.29 1.45 1.67 1.55 1.07 1.40 1.65 2.77 2.43 0.98 1.32 0.76 1.77 1.53 1.90 6.01
2090 477 1440 608 490 1557 1300 1095 2750 598 3080 1265 570 580 1700 941 400 991 870 943 1030
1267 240 794 555 1449 1311 805 1041 611 328 693 350 402 251 580 334 342 607 583 576 470
18 7 10 8 1 15 12 15 8 11 19 9 7 13 15 10 6 12 10 12 25
205 30 106 93 79 109 62 50 73 43 98 34 81 43 71 36 45 51 66 55 157
2.26 0.98 2.27 1.29 0.90 0.86 1.40 1.08 1.48 1.12 1.77 1.26 1.71 1.20 1.46 1.22 0.94 1.15 1.36 1.18 1.83
6.12 2.16 4.06 2.32 3.12 3.48 3.73 3.23 3.67 2.53 5.00 3.28 3.01 3.92 4.01 2.91 1.83 3.25 3.17 3.27 4.45
85 33 78 44 37 53 47 53 58 42 295 49 54 59 171 44 22 50 36 51 85
29 6 18 7 14 17 17 16 30 11 26 17 9 12 23 14 5 15 12 14 15
42 10 24 15 24 38 27 45 22 24 198 32 16 25 88 28 19 32 20 32 28
79 26 54 27 19 29 67 38 59 26 442 45 24 24 295 34 16 32 19 31 39
1264 215 438 171 526 472 182 221 457 149 1165 226 88 114 698 193 131 220 152 211 154
93 62 89 72 329 412 79 108 384 67 218 79 46 45 217 74 53 121 200 114 58
c r c r c r c r c r c r c r c r c
4.72 11.10 4.31 9.60 4.92 3.70 5.11 10.80 4.42 9.60 4.56 9.80 5.92 6.90 7.09 5.50 4.59
2.06 4.74 2.03 1.99 1.36 2.61 4.09 3.32 2.00 6.46 2.26 1.68 1.94 1.73 1.17 1.58 2.86
932 1690 978 3020 1940 590 1999 1670 950 1380 1200 1450 1626 2510 2755 710 533
556 1979 605 3496 672 1943 1153 2471 554 643 723 914 360 481 322 287 184
12 8 9 5 7 1 14 7 8 34 9 17 16 12 17 4 8
59 134 62 114 55 102 130 124 60 160 72 99 47 69 54 56 46
1.20 1.52 0.56 1.21 0.70 0.57 0.64 1.44 0.62 1.85 0.66 1.34 0.78 1.07 1.03 1.05 0.68
3.24 5.04 2.69 4.39 3.27 2.51 3.22 4.92 2.68 5.67 2.79 6.23 3.86 6.97 4.45 2.81 2.97
51 65 37 50 39 25 36 63 37 105 37 229 50 181 103 50 74
14 20 11 22 13 8 14 20 10 19 11 23 17 26 27 14 11
31 29 19 26 27 10 22 32 19 34 21 41 34 73 60 15 19
30 25 15 37 13 8 18 29 15 68 16 115 30 86 49 10 13
204 516 390 568 162 148 677 525 350 251 362 427 183 352 201 75 87
108 417 172 484 153 116 293 477 153 113 167 166 81 167 145 22 28
bole, epidote, pyroxene and titanite. However, this study focused on the occurrence of minerals whose occurrence can be related to the activities of man within the various river catchment areas. These activities fall into two groups, those associated with lead±zinc mining in the Pennines and those associated with smelting in industrial centres. The principal mining-related minerals are the gangue minerals barytes and ¯uorite. Small amounts of the ore mineral sphalerite have also been recorded. The relative proportions of barytes and ¯uorite vary considerably. This may re¯ect diering mineral proportions within the various mines but could also relate to the eects of hydraulic separation of minerals of dierent density. The ore-bodies which were the source of these minerals are all hosted in Dinantian strata (Carboniferous Limestone), and Table 3 demonstrates that there is a broad relationship between the occurrence of barytes/ ¯uorite and the proportion of Dinantian strata in the catchment. Since the supply of ore-related minerals will be primarily controlled by the intensity and location of
mining activity, a more precise correlation cannot be expected. The greatest apparent anomalies are found in the Nidd and Trent 2 samples, with 47% and 56% (as a percentage of total heavy minerals) barytes and ¯uorite relating to respective catchment areas with only 2% and 7% Dinantian strata. The minor amounts of barytes and ¯uorite that occur in rivers with no Dinantian component in their catchments may be derived from Pleistocene tills or from Pleistocene sands related to catchments that diered slightly from those of the present day. Alternatively, they may be related in some way to local industrial activity. The presence of furnace-derived components is most clearly indicated by the occurrence of sharply angular fragments of glass (fragmented slag) and glassy spheres (¯y ash). These are accompanied by a variety of minerals not encountered in uncontaminated sedimentary assemblages. Positive identi®cation of these minerals is dicult without recourse to methods, such as electron microprobe analysis, that were beyond the scope of this study. The slag mineral melilite could, however, be 325
Marine Pollution Bulletin TABLE 3 Proportion of presumed anthropogenic materials in heavy mineral assemblages.a % Mine-generated minerals
River Barytes
Fluorite
Sphalerite
Total
% Dinantian
% Furnace
Trent
Sow Tame Trent (1) Dove Derwent Soar Trent (2) Trent (3) Idle Torne Trent (4)
1 3 3 27 18 3 21 1 R 2 1
2 1 1 3 59 1 35 6 R R 3
0 0 0 1 R 0 0 0 0 0 0
3 4 4 30 77 4 56 7 1 2 4
0 0 0 20 23 0 7 6 0 0 5
11 1 2 0 5 2 1 0 20 0 2
Ouse
Swale Nidd Ouse (1) Wharfe Aire Don Derwent Ouse (2)
13 18 8 51 24 7 0 51
17 29 18 26 4 3 0 35
0 0 0 0 0 0 0 0
30 47 26 77 28 10 0 86
20 2 17 36 11 0 0 18
0 0 6 1 14 46 0 2
a %Mine % minerals derived from mining activity in the Pennine ore®elds: Ba barytes, Fl ¯uorite, Sp sphalerite. % Dinantian % Dinantian rocks in the catchment area; %Slag % minerals and glass derived from furnace slag.
identi®ed on the basis of its distinctive optical properties. All minerals that lie outside the normal range of sedimentary occurrence have therefore been grouped, along with glass particles, into a single ÔfurnaceÕ category. Concentrations of furnace components are most pronounced in the Sow, Idle, Aire and Don samples, re¯ecting proximity to the industrial centres such as Stoke-on-Trent, Leeds, Sheeld and Doncaster.
Sediment-borne Contaminants Entering the Humber The sediment entering the Humber Estuary from the Trent system is composed of material from the Torne, Idle and Trent 4 catchments. Similarly the contribution from the Ouse system comprises the Don, Aire and Ouse 2 catchment sediments. An attempt has been made to model the geochemical signatures of the sediment inputs by summing the individual river components in proportion to the catchment basin size. This has been done for both average catchment geochemistry based on the Wolfson dataset as well as for the river samples. The results are shown in Fig. 7.
R rare.
Also shown on Fig. 7 are the signatures of sediment bodies towards the base of boreholes HMB 16, 18, 19 and 20, drilled through the Holocene sequence of the Humber Estuary as part of the LOIS project (Fig. 2). Microfaunal assemblages of these bodies suggest deposition in freshwater or brackish environments (Mitlehner, personal communication) and they are considered to represent early Holocene discharge of the Trent (HMB 16, 18 and 20) and Ouse (HMB 19) systems. On the basis of radiocarbon dating of peats in boreholes HMB 18 and 19 within the LOIS project, the sediments were deposited between 7450 75 and 5075 55 years BP (Innes, personal communication). They may, therefore, provide pre-anthropogenic standards of ¯uvial sediments discharging into the estuary. Table 4 shows the grid references of these boreholes, depths and chemistry of the ¯uviatile sediments which are sandy silts. In the Trent system the model geochemical signatures for catchment and major river samples are very similar. However, when compared with the early Holocene geochemistry, both catchment and river signatures show signi®cant enhancements of Cu, Zn and Pb, although
TABLE 4 Base Holocene sedimentary units showing depths in metres, oxide chemistry in weight percent, trace elements in ppm and National Grid References. HMB No. 16 18 19 20
326
Depth
Al2 03
CaO
MnO
Ba
As
Sr
K2O Fe2 03
Cr
Co
Ni
Cu
Zn
Pb
9.8±13.55 8.65±10.85 10.2±12.9 10±13.95
12.90 10.80 11.22 11.88
3.53 3.32 2.24 2.98
0.13 0.20 0.09 0.17
612 625 394 487
15 14 5 10
123 113 72 104
2.25 1.99 2.15 2.08
84 73 58 72
19 17 17 17
36 29 29 30
17 14 18 12
141 127 52 100
22 18 16 20
6.41 5.90 5.12 4.97
Grid Reference 4859349232 4828840744 4686742463 4822942315
Volume 37/Numbers 3±7/March±July 1998
the levels and patterns for most elements are very similar, adding support to the view that the borehole sediments were indeed derived from the Trent system. In the Ouse more clear-cut dierences can be seen. The river sample data are notably higher in Cr and Cu than the Wolfson catchment data and both are enriched in Zn and Pb in comparison with the borehole sediments. As in the Trent, the general patterns for the river sample and Wolfson catchment data are similar, but the pattern for the borehole data shows notable dierences. It is probable that although ¯uvial in origin, the early Holocene sediment near the base of HMB 19 does not represent discharge from the Ouse system, in its present day form, but from a more limited early Holocene catchment con®guration (Rees et al., 1998).
Discussion The marked similarity, in many cases, between the geochemical signatures of average catchments derived from the Wolfson dataset and single major river samples suggests that the latter are representative of upstream catchment areas (e.g. the Soar catchment and Kegworth river sample, Fig. 4; the Yorkshire Derwent and Kexby river sample, Fig. 6), except where modi®ed by local industrial contamination (e.g. the Tame catchment and Alrewas river sample, Fig. 4; the Aire catchment and Beal river sample, Fig. 6). Comparison of catchment and river sample chemistry thus provides a simple means for the estimation of the contribution of industrial sources to sediment contamination. Likewise, mining contamination may be recognised in disparities between signatures from catchments with similar geology, but dierent mining histories. The Derbyshire Derwent and the Dove have similar catchment geology and geochemical signatures, except that the more heavily mined Derwent basin contains signi®cantly higher concentrations of Zn and Pb (Fig. 8). In catchments draining the Pennine ore®eld areas (Derbyshire Derwent, Dove, Wharfe, Nidd and Swale) catchment and river samples have comparable, and relatively high, levels of Pb and Zn suggesting a widespread source of metals, most probably related to mining in upland areas which was at its most active in the 18th and 19th Centuries. However, land-clearance in mineralised regions could lead to enhanced metal levels from natural sources. Metal levels remain relatively high for considerable distances downstream (e.g. at Newark on the Trent and Cawood on the Ouse, Figs. 5 and 6) and enrichment can still be seen in the model signatures for the whole catchments, whether based on Wolfson or major river sediment data (Fig. 7). The implication is that large quantities of Pb and Zn are stored in the Trent and Ouse systems and are being continuously released into the rivers. Macklin et al. (1997), Neal et al. (1996) and Neal et al. (1997) express similar views based on studies of ¯oodplain sediments and water chemistry respectively.
By way of contrast, industrial contamination in the Tame and Sow (Fig. 4) appears to undergo rapid dilution downstream, being readily apparent at Burton but not at Wilford, near Nottingham (Figs. 1 and 5). In the model signatures for the Trent catchments (Fig. 7) only Cu of the industry related metals shows any sign of enhancement in either river sediment or Wolfson patterns. This suggests that the Trent river system does not contribute signi®cant quantities of sediment-borne industrial heavy metal contaminants to the Humber Estuary. The Ouse at Cawood, Aire at Beal and Don at Doncaster all carry industrial metal contaminants (Fig. 6) and must contribute contaminated sediment to the Humber, as indicated by the model river sediment signature for Ouse rivers (Fig. 7). However, it is likely that this model signature overestimates the level of contamination, because although dilution by sediment entering the Ouse from the Yorkshire Derwent is taken partially into account, the in¯uence of the (probably uncontaminated) sediment contribution to the rivers downstream of the sampling sites at Beal, Cawood, Doncaster and Kexby is not. The elevated levels of industrial heavy metals found in sur®cial sediments of the estuary (Jones, 1979, Middleton and Grant, 1990, Millward and Glegg, 1997) and reaching the North Sea (Millward et al., 1996) may re¯ect deposition of riverine dissolved contaminants, which are adsorbed onto precipitates within either the rivers or estuary (Neal et al., 1997), or inputs from local sources along the banks of the estuary, such as the titanium dioxide processing industry between Immingham and Grimsby. There is evidence from river sediments that the Ouse system contributes As, Cr and Cu and the Trent Cu contamination to the Humber (Fig. 7), and that both rivers input mining related Pb and Zn contamination. Examination of boreholes drilled through the estuary ®ll (eg. HMB 4, HI1; Fig. 2) as part of the LOIS project and cores of saltmarsh sediment (Ciavola and Covelli, 1995), indicates that although Pb±Zn contamination started much earlier than that of industrial As, Cr and Cu, it too has increased during this century and this increase must, in part, be attributable to industrial and other anthropogenic development. Distributions of mining and industry (furnace) related heavy minerals mirror those of mining and industry related metals. Mining-related heavy minerals such as barytes and ¯uorite maintain a strong presence in sediments well downstream of mining areas, whereas the proportion of industrial heavy minerals, mainly of ÔfurnaceÕ materials, is highest in those catchments with high levels of industrial development and decreases rapidly downstream. However, there is commonly a poor correlation between recorded mineral percentages and element levels because the heavy mineral analysis technique used is not quantitative. For instance, high proportions of barytes in the heavy mineral fractions of the Wharfe at Boston Spa and the Ouse at Cawood 327
Marine Pollution Bulletin
(Ouse 2 in Table 3; 51% in both cases) are not re¯ected in similar Ba levels. The Ouse contains Ba at approximately upper crustal average levels, while the Wharfe shows an enrichment of approximately 3 times the upper crustal average of Wedepohl (1995) (1943 ppm compared with 668 ppm). Similarly, the high proportion of ÔfurnaceÕ material in the Don (46%), as compared with the Aire (14%) is not matched by the dierences in heavy metal levels, the Aire generally containing the higher concentrations. In the heavy mineral suites of the early Holocene sediments in the boreholes, mining related (non-opaque) heavy minerals (barytes, ¯uorite and sphalerite) are either absent or present in very small quantities (<0.5%). This supports the view that the widespread mobilisation of metals from Carboniferous rocks into the drainage system is a result of anthropogenic activity, either through mining or land-clearance, and not simply the result of natural erosion of a mineralised area.
Conclusions The active drainage sediment approach to contamination within the Ouse±Trent±Humber system con®rms the general ®ndings of other workers using water chemistry (e.g. Neal et al., 1997) and ¯oodplain sediment chemistry (e.g. Macklin et al., 1997). Heavy metals fall into two main groups with dierent patterns of distribution. Pb±Zn concentrations are greatest in catchments and rivers draining the Pennine ore®elds and remain high between source areas and the Humber Estuary, suggesting that large quantities of mining related waste are trapped in sediments stored within the ¯uvial systems. In contrast, heavy metals associated with manufacturing industry, such as Cr, Co, Ni and Cu, have high levels near source cities, but decrease rapidly in amount down the river systems because of dilution by other sediments. The diering behaviour of mining and industrial related contaminants is generally re¯ected by the heavy minerals. Gangue minerals, such as barytes and ¯uorite, are generally highest in rivers draining mining areas and persist for relatively long distances downstream; ÔfurnaceÕ materials, such as slags, are highest in industrialised rivers such as the Aire and the Don. Both the Trent and Ouse systems contribute mining related contamination to the Humber, but the latter carries the highest proportion of industrial contaminants. Comparison of catchment basin geochemistry at different scales, using existing stream sediment data, with the geochemistry of sediments from major rivers, has the potential to provide a rapid and cost eective method of distinguishing between contamination and natural, geology-related, background variation and between contamination from mining, industrial and other sources. This approach could be applied throughout the UK 328
because of the widespread availability of stream sediment geochemical data. Contamination in the Ouse±Trent±Humber system is illustrated by comparison of the catchment and river sediment geochemistry and heavy mineralogy with that of early Holocene ¯uvial sediments cored in boreholes drilled in the present Humber Estuary. Similar methodology could be applied to other estuaries. The research described in this paper was undertaken as part of the Natural Environment Research Council-funded Land±Ocean Interaction Study (LOIS). We wish to thank Professor Iain Thornton and Drs Mike Ramsey and Barry Coles for the supply of the Wolfson dataset. Margaret Slater digitised the catchments. The paper bene®ted from reviews by three unnamed referees, Dr C. D. R. Evans and Dr B. G. Rawlins. This paper is published with permission of the Director, British Geological Survey (NERC). LOIS Publication No 629. Al-Bakri, D. (1986) Provenance of the sediments in the Humber Estuary and the adjacent coasts, Eastern England. Marine Geology 72, 171±186. Appleton, J. D. (1995) Potentially harmful elements from natural sources and mining areas: characteristics, extent and relevance to planning and development in Great Britain. British Geological Survey Technical Report WP/95/03. Arnett, R. R. and Justice, M. (1991) Environmental Pollution: (5) Freshwater Quality in Humberside. Working Paper 12, School of Geography and Earth Resources, University of Hull. Brown, A (editor). (1993) The UK Environment 1993. Department of the Environment. HMSO, London. Ciavola, P. and Covelli, S. (1995) Coastal-estuary ¯ux of heavy metals: the use of estuarine salt marshes as a recorder of pollution in the Humber Estuary, UK. In LITTORAL `94. Proceedings of the Second International Symposium of the European Coastal Zone Association for Science and Technology, Lisbon. (Lisbon: Instituto de Hidraulica e recursos hidricos), pp. 231±245. Dunham, K. C. and Wilson, A. A. (1985) Geology of the Northern Pennine Ore®eld. Volume 2, Stainmore to Craven. Economic memoir covering the areas of the one-inch and 1:50 000 geological sheets 40, 41 and 50, and parts of 31, 32, 60 and 61, New Series. London: HMSO. Jarvie, H. P. Neal, C. and Robson, A. J. (1997) The geography of the Humber catchment. The Science of the Total Environment 87±100. Jones, L. H. (1979) Heavy metals in the Humber Estuary and its organisms. In The Humber Estuary. Natural Environment Research Council Publication, Series C20, 13±15. Macklin, M. G. Hudson-Edwards. and Dawson, E. J. (1997) The signi®cance of pollution from historic metal mining in the Pennine ore®elds on river sediment contaminant ¯uxes to the North Sea. The Science of the Total Environment 194±195, 391±398. Middleton, R. and Grant, A. (1990) Heavy metals in the Humber estuary: Scrobiculariaclay as a pre-Industrial datum. Proceedings of the Yorkshire Geological Society 48, 75±80. Middleton, R. and Grant, A. (1993) Trace metals in sediments from the Humber Estuary: A statistical analysis of spatial uniformity. Netherlands Journal of Aquatic Ecology 27, 111±120. Millward, G. E. Allen, J. I. Morris, A. W. and Turner, A. (1996) Distributions and ¯uxes of non-detrital particulate Fe, Mn, Cu Zn in the Humber coastal zone, UK. Continental Shelf Research 16, 967±993. Millward, G. E. and Glegg, G. A. (1997) Fluxes and Retention of Trace Metals in the Humber Estuary. Estuarine, Coastal and Shelf Science 44, 97±105. Mostaghel, M. A. (1983) Evolution of the South Pennine Ore®eld: I: Regional distribution of major non-metallic minerals. Bulletin of the Peak District Mines Historical Society 8, 369±372. Natural Environment Research Council. (1994) Land±Ocean Interaction Study (LOIS) Implementation Plan. Natural Environment Research Council, Swindon. Neal, C. Smith, C. J. Jeery, A. Jarvie, H. P. and Robson, A. J. (1996) Trace element concentrations in the major rivers entering the Humber estuary, NE England. Journal of Hydrology 182, 37± 64.
Volume 37/Numbers 3±7/March±July 1998 Neal, C. Robson, A. J. Jerey, H. A. Harrow, M. L. Neal, M. Smith, C. J. and Jarvie, H. P. (1997) Trace element inter-relationships for the Humber rivers: inferences for hydrological and chemical controls. The Science of the Total Environment 194±195, 321±344. Ottesen, R. T. and Theobald, P. K. (1994) Stream sediments in mineral exploration. In Drainage Geochemistry, eds. M. Hale and J. A. Plant, pp. 147±184. Handbook of Exploration Geochemistry, Vol. 6, ed. G. J. S. Govett. Elsevier, Amsterdam. Rees, J. G. Ridgway, J. Newsham, R. Knox, R. W. OÕB. Outhwaite, K. E. and Balson P. S. (1998) Post-glacial sediment storage in the Humber Estuary: an overview for estuarine management. British Geological Survey Technical Report WB/98/35C. Watters, R. A. (1983) Geochemical exploration for uranium and other heavy metals in tropical and subtropical environments using heavy
mineral concentrates. Journal of Geochemical Exploration 19, 103± 124. Webb, J. S. Nichol, I. Foster, R. Lowenstein, P. L. and Howarth, R. J. (1973) Provisional geochemical atlas of Northern Ireland. Applied Geochemistry Research Group, London, Technical Communication 60, p. 86. Webb, J. S. Thornton, I. Howarth, R. J. Thompson, M. and Lowenstein, P. (1978) The Wolfson Geochemical Atlas of England and Wales. Clarendon Press, Oxford. Wedepohl, K. H. (1995) The composition of the continental crust. Geochimica et Cosmochimica Acta 59, 1217±1232. Wilkinson, W. B. Leeks, G. J. L. Morris, A. and Walling, D. E. (1997) Rivers and coastal research in the Land±Ocean Interaction Study. The Science of the Total Environment 194±195, 5±14.
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Marine Pollution Bulletin Vol. 37, Nos. 3±7, pp. 316±329, 1998 Ó 1999 NERC. Published by Elsevier Science Ltd. All rights reserved Printed in Great Britain 0025-326X/99 $ ± see front matter S0025-326X(99)00055-7
Sediment-borne Contaminants in Rivers Discharging into the Humber Estuary, UK J.G. REES*, J. RIDGWAY, R.W.O.B. KNOX, G. WIGGANS and N. BREWARD British Geological Survey, Keyworth, Nottingham NG12 5GG, UK As part of a project to characterise and quantify the volume of sediments which comprise the Holocene (10,000 years to present) ®ll of the Humber Estuary, a study was undertaken to determine the geochemistry and heavy mineralogy of bed sediments in the river systems that discharge into the estuary. A total of 19 sediment samples in the Trent and Ouse river systems were taken for analysis. Contamination was evaluated by comparison of the sample geochemistry with that of the appropriate catchment (using existing data), and by evaluation of the proportion of anthropogenic heavy minerals (including natural minerals which have been mobilised by mining) in the sediment sample. Heavy metals fall into two groups with dierent patterns of distribution. Pb±Zn concentrations are greatest in catchments and rivers draining the Pennine ore®elds. Levels of these metals remain high between source areas and the Humber Estuary suggesting that large quantities are trapped in sediments stored within the ¯uvial systems. By way of contrast other heavy metals, associated with manufacturing industry, such as Cu and Co, have high concentrations near source cities, but decrease rapidly in amount down the river systems because of dilution by other sediments. The diering behaviour of mining and industrial related contaminants is generally re¯ected by the heavy minerals. Concentrations of gangue minerals, such as barytes and ¯uorite, are generally highest in rivers draining mining areas; ÔfurnaceÕ materials, such as slags are highest in industrialised rivers such as the Aire and the Don. The anthropogenic origin of all the contaminants is illustrated by comparison of the catchment and river sediment geochemistry and heavy mineralogy with that of early Holocene ¯uvially-derived sediments cored in boreholes drilled in the present Humber Estuary. The ®ndings of the study corroborate those of others focused on water chemistry and ¯oodplain sediments (Neal et al., 1996, 1997; Macklin et al., 1997). However, the comparison of pre-existing data on catchment geochemistry with a limited number of river samples, as demonstrated here, provides a fast and cost eective tool for the determination of contamination characteris*Corresponding author.
316
tics in river systems. Ó 1999 NERC. Published Elsevier Science Ltd. All rights reserved
by
Background In recent years levels of pollutants in the North Sea have caused increasing concern to nations that border it, leading to the formulation of policies which will reduce the volume of pollutants entering the sea (Brown, 1993). Such concerns have also highlighted the need for further research into the transport of contaminants into the sea from onshore sources. The Land±Ocean Interaction study (LOIS), funded by the UK Natural Environment Research Council (NERC) is aimed at quantifying the exchange, transformation, and storage of materials at the land±ocean boundary, and determining how these parameters vary in time and space (NERC, 1994; Wilkinson et al., 1997). Part of the LOIS programme, the Land±Ocean Evolution Perspective (LOEPS), is identifying the long-term changes in ¯ux into the coastal zone, particularly in response to post-glacial sea-level rise. The central component of this focuses on the evolution of the Humber Estuary on EnglandÕs North Sea coast. One of the aims of the LOEPS research is to characterise and quantify the volume of sediments which comprise the Holocene (10,000 years to present) ®ll of the Humber Estuary. This is being done by examining the bulk sediment geochemistry, heavy mineralogy and clay mineralogy of cored sequences through the ®ll, and determining the relative in¯uences of ¯uvial and marine systems. This paper describes a pilot project undertaken to: (1) determine the geochemistry and concentration of contaminants in sediments of the major rivers draining into the Humber; (2) compare these with catchment geochemistry using existing datasets; (3) examine the heavy mineral content of the river sediments; and (4) compare these geochemical signatures and heavy mineral data with those from older (pre-anthropogenic) Holocene ¯uvial sediments of the Humber Estuary ®ll. Other contamination studies within LOIS have focused
Volume 37/Numbers 3±7/March±July 1998
on mapping of heavy metals within these river systems by long-period monitoring of dissolved and acid-available particulate phases in waters (Neal et al., 1996; 1997), and studies of the historical variation of heavy metal supply in selected river ¯oodplains (Macklin et al., 1997). The work described here attempts to ®nd a cost-effective approach to the characterisation of contaminants entering the Humber by comparing new data on the geochemistry of nineteen major river-bed sediment and borehole samples with existing data on the geochemistry of their catchment basin sediments. Heavy mineral data are used to con®rm the geochemical ®ndings. A signi®cant factor in the rationale behind the study was that stream sediment based geochemical datasets, covering almost the whole of the UK land area are available through the ongoing British Geological Survey (BGS) regional geochemistry surveys, currently covering all of northern Britain, and the work of the Applied Geochemistry Research Group of Imperial College, London covering England and Wales (Wolfson Geochemical
Atlas data, Webb et al., 1978) and Northern Ireland (Webb et al., 1973). Any methodology developed could thus have widespread application.
Setting of the River Systems The Humber Estuary receives freshwater runo from two major river systems, the Trent and the Yorkshire Ouse (Fig. 1). The catchments of these have a combined area of 24 000 km2 , or approximately one ®fth of the area of England. The catchments of the Trent system, and that of the Yorkshire Derwent in the Ouse system, receive between 750 and 800 mm of rain per year. The Pennine catchments of the Ouse system, which are generally of higher elevation, receive between 850 and 1150 mm per year. Agricultural land-use within them is very varied, ranging from the upland pasture on some margins of the system watersheds to intensively-farmed lowlands in the lower parts of the river basins.
Fig. 1 The main rivers of the Trent and Ouse systems, major cities, and sites of river samples.
317
Marine Pollution Bulletin
Some of the major tributaries feeding the Trent (Fig. 2) are dominantly rural, and have no notable urban centres on them; these include the Dove, Idle and Torne. Others are widely urbanised and contain many of EnglandÕs principal industrial conurbations which, together, account for almost half of national metallurgical, coal and power production (Arnett and Justice, 1991; Jarvie et al., 1997). Some catchments are associated with cities which have lesser concentrations of heavy industry, such as the Soar, upon which Leicester is sited. All of the other tributaries host cities associated with manufacturing industry, including a large part of the West Midlands conurbation, incorporating Birmingham, on the River Tame, and Derby on the Derbyshire Derwent. A number of large cities associated with manufacturing industry occur on the Trent itself, including Stoke-onTrent, Burton-upon-Trent, and Nottingham. Several of the rivers within the Ouse system are not associated with urban centres, but with areas that have a long history of Pennine Pb±Zn mining (Dunham and Wilson, 1985). These include the Ouse (above York), into which the Swale and Ure drain, the Nidd and the Wharfe. The Aire, by way of contrast is less associated with metalliferous mining, but drains cities in the Aire Valley, including Leeds, and also the mill towns on its tributary, the Calder. The southernmost of the major rivers of the Ouse system, the Don, likewise is associated with cities which have a long history of heavy manufacturing in-
dustry (particularly iron and steel), including Sheeld and Doncaster.
Methodology River Sampling To ascertain the likely character of modern ¯uvial sediments discharging into the Humber Estuary a reconnaissance survey was undertaken of the geochemistry and mineralogy of the Trent and Ouse river systems, including important tributaries. The river sampling programme was undertaken at nineteen sites during two consecutive days in early February 1997. The samples at Newark and Acaster occur just upstream of the tidal limits of the Trent (at Cromwell Lock) and Ouse (at Naburn) respectively. The only samples taken in the tidal reaches of the systems were those from Gainsborough on the Trent, and Cawood, on the Ouse. The samples from the tributaries of the Trent and Ouse were taken from a point near their con¯uence with these rivers, or in the case of samples on the Torne, Idle, Yorkshire Derwent, Don and Aire just above their tidal limits. At each site muddy sands or sandy muds were sampled from the river bed within 2 m of the bank in water depths of less than 0.3 m using a plastic scoop. Some winnowing of ®ne-grained material was inevitable, but only the surface of the sample was aected and this was minimised by covering the sample by hand during transport through the water column. Samples for bulk sediment geochemistry and for heavy mineral analysis were stored separately. Visual and textural analysis indicates that all samples comprise a mixture of ®ne-sand, very ®ne-sand, silt and clay. In all cases, the sediment sampled is considered to represent a mixture of bedload material and suspended matter deposited under waning ¯ood ¯ow conditions, probably over several cycles. Such ÔactiveÕ drainage sediments are generally accepted as being representative of the upstream catchment area (see Ottesen and Theobald, 1994). However, it is recognised that there is a possibility of local heterogeneity in the river bed and the question of sampling reproducibility must be addressed in future work.
Sample Preparation and Analysis
Fig. 2 Catchments of the principal tributaries of the Rivers Trent and Ouse. The catchment of the river Sow, as shown here, includes the headwaters of the Trent. Likewise, that of the Swale includes the headwaters of the Ouse, as well as the river Ure. The sites of boreholes discussed in the text are shown.
318
Geochemical samples were dried for 12 h at 60°C before being reduced to powder in an agate mill and pressed into pellets of c. 12 g mass for determination of a range of major oxides and trace elements using highprecision X-ray ¯uorescence (XRF) facilities at the British Geological Survey. Calibrations were veri®ed by analysis of Reference Materials and a stable silica glass disc, containing all the analytes of interest, was analysed regularly as a quality control standard with the data plotted on QC charts to monitor performance. The samples were analysed as part of a larger programme involving sediments from the LOEPS project and cor-
Volume 37/Numbers 3±7/March±July 1998 TABLE 1 River sediment sample sites and catchment data. Catchment identi®ed by main river Sow Tame Trent (1) Dove Derwent Soar Trent (2) Trent (3) Idle Torne Trent (4) Swale Nidd Ouse (1) Wharfe Aire Don Derwent Ouse (2)
River sample site location Gt. Haywood Alrewas Burton Hatton Spondon Kegworth Wilford Newark Bawtry Rossington Gainsborough Newton Cattal Acaster Boston Spa Beal Doncaster Kexby Cawood
River sample site grid reference 39962 41874 42588 42140 44155 44942 45693 47920 46562 46288 48145 45108 44472 47049 44327 45340 45665 47049 45744
32242 31400 32367 32947 33392 32720 33808 35386 39270 39959 38910 46000 45400 45118 44574 42560 40371 45118 43790
relation coecients for pairs of replicate analyses of core samples (resampled, prepared and analysed, all at a later date) are shown in Table 1, demonstrating the high precision of the general methodology. Samples for heavy mineral analysis were ultrasonically cleaned and wet sieved. Heavy minerals were separated from the 63±125 lm grain size fraction by gravity-settling in bromoform (S.G. 2.90). The heavy mineral fractions were mounted in Canada Balsam for optical study. The analyses were carried out by conventional optical microscopy; this involved a count of 200 non-opaque detrital grains per sample (where grain recovery permitted) to determine the overall composition of the suite. While the data obtained are thus not representative of the sand fraction as a whole, they provide a sound and time-ecient basis for comparison in a reconnaissance study of this kind.
System *tidal
Catchment area (km2 )
Number of catchment samples
Trent Trent Trent Trent Trent Trent Trent Trent Trent Trent Trent* Ouse Ouse Ouse Ouse Ouse Ouse Ouse Ouse*
871 1510 3114 1079 1203 1353 7361 8197 818 123 9542 2691 510 3064 740 1960 1299 1710 4662
356 279 838 363 254 357 1910 2097 90 12 2345 1029 184 1202 247 394 201 511 1729
alytical and quality control procedures are described in the Atlas.
Comparison of River and Catchment Geochemical Datasets Drainage basin geology (Fig. 3) clearly will in¯uence the geochemistry of the river samples and it is thus necessary to examine the background geochemical signature of the catchments which contribute sediments to the sample station. The only geochemical dataset which covers catchments for the entire Ouse and Trent systems is that from the Wolfson Geochemical Atlas (Webb et al., 1978), based on stream sediments, for which sampling started in 1969. The mean sampling density was about 1 per 2.5 km2 . At each site two 100 g samples of active stream-bed sediment were taken at least 20 m upstream of any road, and sieved at 200 lm. No samples were taken in urban areas. Most elements were analysed spectrographically, though four, including As and Zn, were determined by atomic absorption spectrophotometric (AAS) and colorimetric methods. Sampling, an-
Fig. 3 Geology of the Trent and Ouse systems. Superimposed on this are the segments of the catchments of the rivers Trent and Ouse systems discussed in the text. Catchment Ouse 2 comprises the area marked, as well as that of catchment Ouse 1. Likewise Trent 2 includes Trent 1, Trent 3 includes Trent 1 and 2, and Trent 4 includes Trent 1, 2 and 3. Pleistocene deposits in the region are principally lodgement tills of local origin, although some will contain sediment transported from the North Sea area during one or more of the Pleistocene glaciations. As the provenance and boundaries of dierent Pleistocene formations are unclear, and because they will (largely) re¯ect bedrock character, they are not shown here.
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The stream sediments from the Wolfson survey can be expected to re¯ect natural background variations in geochemistry, but also might show variations related to mining activity in upland areas and to agricultural practices.
Data Compatibility The BGS XRF data and the Wolfson spectrographic data are both based on analytical methods which yield ÔtotalÕ concentrations and are thus directly comparable. The Wolfson data for As and Zn are based on techniques which provide ÔpartialÕ concentrations, although in most cases these will be close to ÔtotalÕ and comparison can be made with appropriate caution. Grain size also can in¯uence element concentrations, ®ne-grained sediments normally having higher concentrations of trace elements than coarse-grained ones. The BGS LOEPS geochemical dataset is based on analysis of the <2 mm sediment fraction. This size fraction was used because it allows direct comparison with oshore geochemical data, for which ®ne-grained sample material is often dicult to acquire, and enables sucient material for analysis to be obtained from cores, particularly where coarser grained sediment is present. Appleton (1995) compared the Wolfson and BGS stream sediment data and attributed variations in heavy metal concentrations between the two to the use of dierent size fractions and sieving techniques, the BGS data being based on slightly ®ner-grained material. However, within this study, the unsieved river sediments and the Wolfson sieved sample material are very similar in grain size, both being composed of sediment of ®ne sand grade or smaller. Correlation coecients for both the river sample and Wolfson datasets show no strong relationships between trace metals and Al, a common proxy for grain size. In addition, scavenging by Fe±Mn oxyhydroxides and complexing by organic matter also could aect trace element concentrations. Wholesale normalisation to a grain size proxy could, therefore, have an unpredictable eect on some elements and no attempt has been made, to compensate for grain-size, although there is scope for signi®cant variation within the size fractions used. The list of determinands in the Wolfson and BGS LOEPS datasets also is dierent and this study is restricted to those elements which are common to both. Although, it has not been possible to reanalyse any of the Wolfson sample material for direct comparison with the BGS XRF data, the similarity of many of the geochemical signatures for catchments and river samples shown in Figs. 4±8 suggests that the two datasets are compatible.
Interpretational Approach Given that the Wolfson and BGS datasets are based on slightly dierent sediment size fractions, have been 320
derived using dierent analytical techniques and involve dierences in the range of determinands, a robust method of interpretation is required. Furthermore, in comparing catchment geochemistry (based on many samples) with major river sample and borehole geochemistry (based on single samples), calculation of average values and, in some instances, values weighted according to catchment size is necessary. Most standard statistical techniques, such as principal components analysis, are not appropriate in these circumstances. Instead, average catchment and river sample geochemistry are compared by means of ÔspidergramsÕ in which each element value has been normalised to the upper crustal average given by Wedepohl (1995) and plotted on a logarithmic scale. This allows elements with widely diering concentrations to be viewed on a single diagram. Oxide values in percentage are converted to ppm before normalisation. Dierences in concentration levels due to grain size and to analytical techniques may still be present and the diagrams shown (Figs. 4±7) and discussed below must be viewed with this in mind. The advantages of the spidergram approach are that patterns of variation can be compared even though absolute concentrations are dierent and large dierences can be viewed at the same time as close similarities.
Trent Tributary Catchments The Trent tributary catchments can be split into four groups, re¯ecting topographic, geological and anthropogenic in¯uences. The catchments of the rivers Torne and Idle are both subdued topographically and dominantly rural, though the fringes of Doncaster impinge on that of the Torne, and small manufacturing towns occur in the Idle catchment. Both catchments contain mainly PermoTriassic rocks. The Torne is the smallest catchment considered (Table 1) and the systematic dierences for most elements between the catchment and the river sample geochemistry are the largest in this study (Fig. 4). The reasons for this are not clear, but may be related to systematic grain-size dierences as indicated by the relatively high Al and Fe±Mn concentrations in the river sample from Rossington (Table 2), which could be related to the clay mineral content and Fe±Mn oxyhydroxide coatings on clay minerals respectively. Such factors are considered to exert a strong in¯uence on trace element concentrations (e.g. Watters, 1983). Notwithstanding this, Zn (Table 2, Fig. 4) still appears abnormally high in the river sediment and perhaps represents contamination from lowland agricultural practices. The Idle catchment is also relatively small, but the dierences between average catchment and river sample geochemistry are less systematic than in the Torne and the noticeable river sample highs in Co, Cu and Zn could be due to contamination. The Derbyshire Derwent and the Dove and their tributaries drain the higher ground associated with
Volume 37/Numbers 3±7/March±July 1998
Fig. 4 Geochemistry of the tributaries of the Trent system. The mean spidergram of the catchment samples is shown by the catchment name; the river sample by the sample site name (see Table 1).
Carboniferous rocks of the Peak District at the southern end of the Pennines, though they pass over Triassic rocks towards their con¯uence with the Trent (Fig. 3). Mining, mainly for Pb±Zn, has been extensive in the past, particularly in the catchment of the Derwent (Mostaghel, 1983), which also has had a greater association with industry, particularly engineering in Derby. Catchment and river sediment geochemistry are very similar here (Fig. 4), although the river is particularly low in Mn and As, for which there is no obvious reason. Both graphs display a sharp change in slope due to an increase in Zn and Pb values that most probably represents upland, mining related contamination. The Dove catchment and the river sample from Hatton show remarkably similar geochemistry suggesting, as might be
expected, that there is little industrial contamination in this basin. The catchments of the rivers Sow (which, as shown in Fig. 2, also includes the headwaters of the River Trent), and Tame, both are associated with Triassic and Carboniferous rocks. The mineral resources of the latter have been a focus for industry over the last two centuries, and have led to the growth of important manufacturing cities such as Stoke-on-Trent and Birmingham. Both the Great Haywood and Alrewas samples show evidence of contamination stemming from industrial activity there being little or no mining in these catchments. Industrial contamination is particularly prominent in the enhanced Cr, Ni, Cu and Zn values at Alrewas. 321
Marine Pollution Bulletin
Fig. 5 Geochemistry of segments of the catchments of the river Trent (Fig. 3) discussed in the text. The mean spidergram of the catchment samples is shown by the catchment name; the river sample by the sample site name (see Table 1).
The catchment of the River Soar constitutes a largely rural area of thickly drift-covered Triassic and Jurassic rocks. Although the city of Leicester occurs in the catchment it is generally not associated with heavy industry, or mining. River and catchment geochemistry are closely matched in this drainage basin and there is little sign of contamination.
Trent Catchments To determine how the geochemical signature changes downstream, and the likely input to the Humber Estuary, the Trent basin has also been divided into four catchments of successively increasing size (Fig. 3), such that basin 2 includes basin 1 and 3 includes basins 1 and 2 and so on. Only at Burton is there any sign of signi®cant industrial contamination (Fig. 5) and this clearly can be related to the input from the Tame valley (Fig. 4). The other three catchments show very similar patterns in the geochemical signatures of both river sediment and average catchment with little sign of industrial contamination, except perhaps for As and possibly Fe and Cr at Gainsborough. Relatively high concentrations of Pb and Zn most probably re¯ect a continuing in¯uence of mining related contamination. The signatures of Trent 2 and river sediment from Wilford show the biggest dierences, although the general pattern of variation is similar, with none of the major change of slope dierences shown by the Trent 1 and Burton patterns. The river sediment from Wilford contains generally lower concentrations of the elements studied than the Trent 2 catchment, including Al, a common proxy for grain size. It is likely that the observed concentration dierences are due to the Wilford sediment being coarser grained than that of the 322
catchment as a whole. Increased levels of Ca and Sr in the tidal section of the Trent at Gainsborough could be related to shelly material in the sediment or possibly to the presence of carbonate cements from the dissolution and reprecipitation of Cretaceous chalk fragments.
Ouse Tributary Catchments The catchments of the Swale (here taken to include the headwaters of the River Ouse, as well as the Ure), Nidd and Wharfe extend onto the Pennines on their western ¯anks. The high ground is formed of Carboniferous rocks which have a long association with mining, particularly of Pb±Zn. The lower parts of catchments occur on drift-covered Triassic and Jurassic strata. In general, the average catchment and river sediment geochemical signatures are very similar in pattern for the Swale and Nidd (Fig. 6). Probable mining related contamination is seen in the enhanced Ba and Pb levels in the Newton (Swale) river sample and the similarly enhanced Ba, Cu, Zn and Pb in the Cattal (Nidd) sample. Relatively high concentrations of Zn and Pb in the catchment signatures could stem directly from upland mining contamination or from naturally enhanced background levels in a mineralised area. The Wharfe is rather dierent with the catchment signature displaying higher concentrations than the river sample for most elements. Both signatures show relatively high Ba levels (see Table 2) and the catchment in particular is high in Zn and Pb. A notable feature of the Boston Spa river sample is the low level of As and the relationship between river and catchment signatures for the Wharfe is very similar to that described above for the Derbyshire Derwent.
Volume 37/Numbers 3±7/March±July 1998
Fig. 6 Geochemistry of the tributaries of the Ouse system, and segments of the catchments of the river Ouse (Fig. 3) discussed in the text. The mean spidergram of the catchment samples is shown by the catchment name; the river sample by the sample site name (see Table 1).
The catchments of the Aire, including the Calder, and the Don drain Carboniferous rocks of the Pennines. However, the amount of Pb±Zn mining within them is considerably smaller than those to the north. The Aire and Don catchments have had the highest concentration of heavy industry in the Trent and Ouse systems, associated with mineral resources in the Carboniferous rocks. The cities of Leeds, Sheeld and Doncaster, and many other manufacturing towns occur within these basins. The Aire at Beal shows the eects of industrial contamination in high levels of Cr, Cu, Zn and Pb relative to the catchment geochemistry. In the case of the Don, catchment and river (Doncaster) signatures are very similar, but levels of Cr, Co, Ni, Cu, Zn and Pb are
high in both (Fig. 6 and Table 2), re¯ecting the industrialised nature of much of the catchment. The Yorkshire Derwent drains a rural catchment underlain by Jurassic rocks. The geochemical signatures of river (Kexby) and catchment are very similar and show no signs of contamination.
Ouse Catchments The Ouse catchment has been divided into two in a similar manner as for the Trent. The Ouse 1 catchment (Fig. 6) includes the Nidd and Swale and the relationship between the river sediment sampled Acaster, and the average catchment geochemistry is very similar to 323
Marine Pollution Bulletin
Fig. 7 Comparative geochemistry of the ¯uvial sediments of the Trent and Ouse river systems and early Holocene ¯uvially-derived sediments. The locations of boreholes referred to are shown in Fig. 2, and depths of sediment bodies in Table 4.
Fig. 8 Comparative geochemistry of the Derbyshire Derwent (DDERWENT) and Dove catchments showing the higher concentrations of Pb and Zn related to mining in the Derwent basin.
those shown in the tributaries with enhanced levels of Ba and Pb in the river sample. Further downstream, at Cawood, below the con¯uence with the Wharfe, the river sediment shows evidence of industrial contamination in relatively high levels of As, Cr and Cu. There may also be enrichment in Fe, Co and Ni, but for these metals, in comparison with the catchment, the dierences in absolute concentrations and the departure from the general trend are smaller. Cock Beck, draining north-eastern Leeds, enters the Wharfe below Boston Spa and this may account for the elevated metal levels. The Ouse at Cawood is tidal, and the high Ca and Sr values here may perhaps have the same explanation as was advanced for the Trent at Gainsborough, being caused by the presence of shelly material or the formation of carbonate cement. The tidal nature of the sites at Cawood and Gainsborough provides an alternative explanation for some of the observed metal enrichments. Middleton and Grant (1993) have shown a spatial uniformity in heavy metal concentrations in sediments of the Trent±Ouse±Humber system which extends landwards beyond the normal limit of seawater intrusion. A signi®cant proportion of the bed sediment of the Humber is considered to be clay originating from erosion of Pleistocene tills on the 324
Holderness coastline (Al-Bakri, 1986). Within the estuary the titanium dioxide industries between Immingham and Grimsby discharge large amounts of Fe, which facilitates the development of Fe coatings on clay particles and the consequent scavenging of heavy metals from the dissolved phase. The contaminated sediments are then redistributed by tidal pumping and may penetrate upestuary to the tidal limit. While this could account for the increased metal concentrations in the tidal river sediments (As, Fe, Cr, Co, Ni and Cu at Cawood; As and possibly Fe and Cr at Gainsborough), the depletion of Zn and Pb (in comparison with the catchment) at both sites and the selectively large enrichments in Cr and Cu at Cawood suggest that it is not the full explanation.
Anthropogenic Heavy Minerals in River Samples Throughout the river samples taken, the non-opaque heavy mineral assemblages include a wide variety of minerals derived from bed-rock sandstones, including anatase, apatite, garnet, monazite, rutile, staurolite, tourmaline and zircon, accompanied in varying proportion by relatively unstable minerals derived from the overlying Pleistocene tills and sands, including amphi-
Volume 37/Numbers 3±7/March±July 1998 TABLE 2 Geochemical data from samples and catchments. r river sample, c mean for catchment (DDERWENT Derbyshire Derwent; YDERWENT Yorkshire Derwent).
ROSSINGTON TORNE BAWTRY IDLE SPONDON DDERWENT HATTON DOVE HAYWOOD SOW ALREWAS TAME KEGWORTH SOAR BURTON TRENT 1 WILFORD TRENT 2 NEWARK TRENT 3 GAINSBOROUGH TRENT 4 NEWTON SWALE CATTAL NIDD BOSTON WHARFE ACASTER OUSE 1 CAWOOD OUSE 2 BEAL AIRE DONCASTER DON KEXBY YDERWENT
Al2 O3
CaO
Mn
Ba
As
Sr
K2 O
Fe2 O3
Cr
Co
Ni
Cu
Zn
Pb
r c r c r c r c r c r c r c r c r c r c r
10.40 3.33 9.30 4.55 5.10 5.64 6.20 4.90 7.60 4.38 7.50 4.53 5.80 4.59 6.80 4.48 3.00 4.75 6.10 4.78 8.50
4.75 4.28 2.30 4.10 1.86 2.29 1.45 1.67 1.55 1.07 1.40 1.65 2.77 2.43 0.98 1.32 0.76 1.77 1.53 1.90 6.01
2090 477 1440 608 490 1557 1300 1095 2750 598 3080 1265 570 580 1700 941 400 991 870 943 1030
1267 240 794 555 1449 1311 805 1041 611 328 693 350 402 251 580 334 342 607 583 576 470
18 7 10 8 1 15 12 15 8 11 19 9 7 13 15 10 6 12 10 12 25
205 30 106 93 79 109 62 50 73 43 98 34 81 43 71 36 45 51 66 55 157
2.26 0.98 2.27 1.29 0.90 0.86 1.40 1.08 1.48 1.12 1.77 1.26 1.71 1.20 1.46 1.22 0.94 1.15 1.36 1.18 1.83
6.12 2.16 4.06 2.32 3.12 3.48 3.73 3.23 3.67 2.53 5.00 3.28 3.01 3.92 4.01 2.91 1.83 3.25 3.17 3.27 4.45
85 33 78 44 37 53 47 53 58 42 295 49 54 59 171 44 22 50 36 51 85
29 6 18 7 14 17 17 16 30 11 26 17 9 12 23 14 5 15 12 14 15
42 10 24 15 24 38 27 45 22 24 198 32 16 25 88 28 19 32 20 32 28
79 26 54 27 19 29 67 38 59 26 442 45 24 24 295 34 16 32 19 31 39
1264 215 438 171 526 472 182 221 457 149 1165 226 88 114 698 193 131 220 152 211 154
93 62 89 72 329 412 79 108 384 67 218 79 46 45 217 74 53 121 200 114 58
c r c r c r c r c r c r c r c r c
4.72 11.10 4.31 9.60 4.92 3.70 5.11 10.80 4.42 9.60 4.56 9.80 5.92 6.90 7.09 5.50 4.59
2.06 4.74 2.03 1.99 1.36 2.61 4.09 3.32 2.00 6.46 2.26 1.68 1.94 1.73 1.17 1.58 2.86
932 1690 978 3020 1940 590 1999 1670 950 1380 1200 1450 1626 2510 2755 710 533
556 1979 605 3496 672 1943 1153 2471 554 643 723 914 360 481 322 287 184
12 8 9 5 7 1 14 7 8 34 9 17 16 12 17 4 8
59 134 62 114 55 102 130 124 60 160 72 99 47 69 54 56 46
1.20 1.52 0.56 1.21 0.70 0.57 0.64 1.44 0.62 1.85 0.66 1.34 0.78 1.07 1.03 1.05 0.68
3.24 5.04 2.69 4.39 3.27 2.51 3.22 4.92 2.68 5.67 2.79 6.23 3.86 6.97 4.45 2.81 2.97
51 65 37 50 39 25 36 63 37 105 37 229 50 181 103 50 74
14 20 11 22 13 8 14 20 10 19 11 23 17 26 27 14 11
31 29 19 26 27 10 22 32 19 34 21 41 34 73 60 15 19
30 25 15 37 13 8 18 29 15 68 16 115 30 86 49 10 13
204 516 390 568 162 148 677 525 350 251 362 427 183 352 201 75 87
108 417 172 484 153 116 293 477 153 113 167 166 81 167 145 22 28
bole, epidote, pyroxene and titanite. However, this study focused on the occurrence of minerals whose occurrence can be related to the activities of man within the various river catchment areas. These activities fall into two groups, those associated with lead±zinc mining in the Pennines and those associated with smelting in industrial centres. The principal mining-related minerals are the gangue minerals barytes and ¯uorite. Small amounts of the ore mineral sphalerite have also been recorded. The relative proportions of barytes and ¯uorite vary considerably. This may re¯ect diering mineral proportions within the various mines but could also relate to the eects of hydraulic separation of minerals of dierent density. The ore-bodies which were the source of these minerals are all hosted in Dinantian strata (Carboniferous Limestone), and Table 3 demonstrates that there is a broad relationship between the occurrence of barytes/ ¯uorite and the proportion of Dinantian strata in the catchment. Since the supply of ore-related minerals will be primarily controlled by the intensity and location of
mining activity, a more precise correlation cannot be expected. The greatest apparent anomalies are found in the Nidd and Trent 2 samples, with 47% and 56% (as a percentage of total heavy minerals) barytes and ¯uorite relating to respective catchment areas with only 2% and 7% Dinantian strata. The minor amounts of barytes and ¯uorite that occur in rivers with no Dinantian component in their catchments may be derived from Pleistocene tills or from Pleistocene sands related to catchments that diered slightly from those of the present day. Alternatively, they may be related in some way to local industrial activity. The presence of furnace-derived components is most clearly indicated by the occurrence of sharply angular fragments of glass (fragmented slag) and glassy spheres (¯y ash). These are accompanied by a variety of minerals not encountered in uncontaminated sedimentary assemblages. Positive identi®cation of these minerals is dicult without recourse to methods, such as electron microprobe analysis, that were beyond the scope of this study. The slag mineral melilite could, however, be 325
Marine Pollution Bulletin TABLE 3 Proportion of presumed anthropogenic materials in heavy mineral assemblages.a % Mine-generated minerals
River Barytes
Fluorite
Sphalerite
Total
% Dinantian
% Furnace
Trent
Sow Tame Trent (1) Dove Derwent Soar Trent (2) Trent (3) Idle Torne Trent (4)
1 3 3 27 18 3 21 1 R 2 1
2 1 1 3 59 1 35 6 R R 3
0 0 0 1 R 0 0 0 0 0 0
3 4 4 30 77 4 56 7 1 2 4
0 0 0 20 23 0 7 6 0 0 5
11 1 2 0 5 2 1 0 20 0 2
Ouse
Swale Nidd Ouse (1) Wharfe Aire Don Derwent Ouse (2)
13 18 8 51 24 7 0 51
17 29 18 26 4 3 0 35
0 0 0 0 0 0 0 0
30 47 26 77 28 10 0 86
20 2 17 36 11 0 0 18
0 0 6 1 14 46 0 2
a %Mine % minerals derived from mining activity in the Pennine ore®elds: Ba barytes, Fl ¯uorite, Sp sphalerite. % Dinantian % Dinantian rocks in the catchment area; %Slag % minerals and glass derived from furnace slag.
identi®ed on the basis of its distinctive optical properties. All minerals that lie outside the normal range of sedimentary occurrence have therefore been grouped, along with glass particles, into a single ÔfurnaceÕ category. Concentrations of furnace components are most pronounced in the Sow, Idle, Aire and Don samples, re¯ecting proximity to the industrial centres such as Stoke-on-Trent, Leeds, Sheeld and Doncaster.
Sediment-borne Contaminants Entering the Humber The sediment entering the Humber Estuary from the Trent system is composed of material from the Torne, Idle and Trent 4 catchments. Similarly the contribution from the Ouse system comprises the Don, Aire and Ouse 2 catchment sediments. An attempt has been made to model the geochemical signatures of the sediment inputs by summing the individual river components in proportion to the catchment basin size. This has been done for both average catchment geochemistry based on the Wolfson dataset as well as for the river samples. The results are shown in Fig. 7.
R rare.
Also shown on Fig. 7 are the signatures of sediment bodies towards the base of boreholes HMB 16, 18, 19 and 20, drilled through the Holocene sequence of the Humber Estuary as part of the LOIS project (Fig. 2). Microfaunal assemblages of these bodies suggest deposition in freshwater or brackish environments (Mitlehner, personal communication) and they are considered to represent early Holocene discharge of the Trent (HMB 16, 18 and 20) and Ouse (HMB 19) systems. On the basis of radiocarbon dating of peats in boreholes HMB 18 and 19 within the LOIS project, the sediments were deposited between 7450 75 and 5075 55 years BP (Innes, personal communication). They may, therefore, provide pre-anthropogenic standards of ¯uvial sediments discharging into the estuary. Table 4 shows the grid references of these boreholes, depths and chemistry of the ¯uviatile sediments which are sandy silts. In the Trent system the model geochemical signatures for catchment and major river samples are very similar. However, when compared with the early Holocene geochemistry, both catchment and river signatures show signi®cant enhancements of Cu, Zn and Pb, although
TABLE 4 Base Holocene sedimentary units showing depths in metres, oxide chemistry in weight percent, trace elements in ppm and National Grid References. HMB No. 16 18 19 20
326
Depth
Al2 03
CaO
MnO
Ba
As
Sr
K2O Fe2 03
Cr
Co
Ni
Cu
Zn
Pb
9.8±13.55 8.65±10.85 10.2±12.9 10±13.95
12.90 10.80 11.22 11.88
3.53 3.32 2.24 2.98
0.13 0.20 0.09 0.17
612 625 394 487
15 14 5 10
123 113 72 104
2.25 1.99 2.15 2.08
84 73 58 72
19 17 17 17
36 29 29 30
17 14 18 12
141 127 52 100
22 18 16 20
6.41 5.90 5.12 4.97
Grid Reference 4859349232 4828840744 4686742463 4822942315
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the levels and patterns for most elements are very similar, adding support to the view that the borehole sediments were indeed derived from the Trent system. In the Ouse more clear-cut dierences can be seen. The river sample data are notably higher in Cr and Cu than the Wolfson catchment data and both are enriched in Zn and Pb in comparison with the borehole sediments. As in the Trent, the general patterns for the river sample and Wolfson catchment data are similar, but the pattern for the borehole data shows notable dierences. It is probable that although ¯uvial in origin, the early Holocene sediment near the base of HMB 19 does not represent discharge from the Ouse system, in its present day form, but from a more limited early Holocene catchment con®guration (Rees et al., 1998).
Discussion The marked similarity, in many cases, between the geochemical signatures of average catchments derived from the Wolfson dataset and single major river samples suggests that the latter are representative of upstream catchment areas (e.g. the Soar catchment and Kegworth river sample, Fig. 4; the Yorkshire Derwent and Kexby river sample, Fig. 6), except where modi®ed by local industrial contamination (e.g. the Tame catchment and Alrewas river sample, Fig. 4; the Aire catchment and Beal river sample, Fig. 6). Comparison of catchment and river sample chemistry thus provides a simple means for the estimation of the contribution of industrial sources to sediment contamination. Likewise, mining contamination may be recognised in disparities between signatures from catchments with similar geology, but dierent mining histories. The Derbyshire Derwent and the Dove have similar catchment geology and geochemical signatures, except that the more heavily mined Derwent basin contains signi®cantly higher concentrations of Zn and Pb (Fig. 8). In catchments draining the Pennine ore®eld areas (Derbyshire Derwent, Dove, Wharfe, Nidd and Swale) catchment and river samples have comparable, and relatively high, levels of Pb and Zn suggesting a widespread source of metals, most probably related to mining in upland areas which was at its most active in the 18th and 19th Centuries. However, land-clearance in mineralised regions could lead to enhanced metal levels from natural sources. Metal levels remain relatively high for considerable distances downstream (e.g. at Newark on the Trent and Cawood on the Ouse, Figs. 5 and 6) and enrichment can still be seen in the model signatures for the whole catchments, whether based on Wolfson or major river sediment data (Fig. 7). The implication is that large quantities of Pb and Zn are stored in the Trent and Ouse systems and are being continuously released into the rivers. Macklin et al. (1997), Neal et al. (1996) and Neal et al. (1997) express similar views based on studies of ¯oodplain sediments and water chemistry respectively.
By way of contrast, industrial contamination in the Tame and Sow (Fig. 4) appears to undergo rapid dilution downstream, being readily apparent at Burton but not at Wilford, near Nottingham (Figs. 1 and 5). In the model signatures for the Trent catchments (Fig. 7) only Cu of the industry related metals shows any sign of enhancement in either river sediment or Wolfson patterns. This suggests that the Trent river system does not contribute signi®cant quantities of sediment-borne industrial heavy metal contaminants to the Humber Estuary. The Ouse at Cawood, Aire at Beal and Don at Doncaster all carry industrial metal contaminants (Fig. 6) and must contribute contaminated sediment to the Humber, as indicated by the model river sediment signature for Ouse rivers (Fig. 7). However, it is likely that this model signature overestimates the level of contamination, because although dilution by sediment entering the Ouse from the Yorkshire Derwent is taken partially into account, the in¯uence of the (probably uncontaminated) sediment contribution to the rivers downstream of the sampling sites at Beal, Cawood, Doncaster and Kexby is not. The elevated levels of industrial heavy metals found in sur®cial sediments of the estuary (Jones, 1979, Middleton and Grant, 1990, Millward and Glegg, 1997) and reaching the North Sea (Millward et al., 1996) may re¯ect deposition of riverine dissolved contaminants, which are adsorbed onto precipitates within either the rivers or estuary (Neal et al., 1997), or inputs from local sources along the banks of the estuary, such as the titanium dioxide processing industry between Immingham and Grimsby. There is evidence from river sediments that the Ouse system contributes As, Cr and Cu and the Trent Cu contamination to the Humber (Fig. 7), and that both rivers input mining related Pb and Zn contamination. Examination of boreholes drilled through the estuary ®ll (eg. HMB 4, HI1; Fig. 2) as part of the LOIS project and cores of saltmarsh sediment (Ciavola and Covelli, 1995), indicates that although Pb±Zn contamination started much earlier than that of industrial As, Cr and Cu, it too has increased during this century and this increase must, in part, be attributable to industrial and other anthropogenic development. Distributions of mining and industry (furnace) related heavy minerals mirror those of mining and industry related metals. Mining-related heavy minerals such as barytes and ¯uorite maintain a strong presence in sediments well downstream of mining areas, whereas the proportion of industrial heavy minerals, mainly of ÔfurnaceÕ materials, is highest in those catchments with high levels of industrial development and decreases rapidly downstream. However, there is commonly a poor correlation between recorded mineral percentages and element levels because the heavy mineral analysis technique used is not quantitative. For instance, high proportions of barytes in the heavy mineral fractions of the Wharfe at Boston Spa and the Ouse at Cawood 327
Marine Pollution Bulletin
(Ouse 2 in Table 3; 51% in both cases) are not re¯ected in similar Ba levels. The Ouse contains Ba at approximately upper crustal average levels, while the Wharfe shows an enrichment of approximately 3 times the upper crustal average of Wedepohl (1995) (1943 ppm compared with 668 ppm). Similarly, the high proportion of ÔfurnaceÕ material in the Don (46%), as compared with the Aire (14%) is not matched by the dierences in heavy metal levels, the Aire generally containing the higher concentrations. In the heavy mineral suites of the early Holocene sediments in the boreholes, mining related (non-opaque) heavy minerals (barytes, ¯uorite and sphalerite) are either absent or present in very small quantities (<0.5%). This supports the view that the widespread mobilisation of metals from Carboniferous rocks into the drainage system is a result of anthropogenic activity, either through mining or land-clearance, and not simply the result of natural erosion of a mineralised area.
Conclusions The active drainage sediment approach to contamination within the Ouse±Trent±Humber system con®rms the general ®ndings of other workers using water chemistry (e.g. Neal et al., 1997) and ¯oodplain sediment chemistry (e.g. Macklin et al., 1997). Heavy metals fall into two main groups with dierent patterns of distribution. Pb±Zn concentrations are greatest in catchments and rivers draining the Pennine ore®elds and remain high between source areas and the Humber Estuary, suggesting that large quantities of mining related waste are trapped in sediments stored within the ¯uvial systems. In contrast, heavy metals associated with manufacturing industry, such as Cr, Co, Ni and Cu, have high levels near source cities, but decrease rapidly in amount down the river systems because of dilution by other sediments. The diering behaviour of mining and industrial related contaminants is generally re¯ected by the heavy minerals. Gangue minerals, such as barytes and ¯uorite, are generally highest in rivers draining mining areas and persist for relatively long distances downstream; ÔfurnaceÕ materials, such as slags, are highest in industrialised rivers such as the Aire and the Don. Both the Trent and Ouse systems contribute mining related contamination to the Humber, but the latter carries the highest proportion of industrial contaminants. Comparison of catchment basin geochemistry at different scales, using existing stream sediment data, with the geochemistry of sediments from major rivers, has the potential to provide a rapid and cost eective method of distinguishing between contamination and natural, geology-related, background variation and between contamination from mining, industrial and other sources. This approach could be applied throughout the UK 328
because of the widespread availability of stream sediment geochemical data. Contamination in the Ouse±Trent±Humber system is illustrated by comparison of the catchment and river sediment geochemistry and heavy mineralogy with that of early Holocene ¯uvial sediments cored in boreholes drilled in the present Humber Estuary. Similar methodology could be applied to other estuaries. The research described in this paper was undertaken as part of the Natural Environment Research Council-funded Land±Ocean Interaction Study (LOIS). We wish to thank Professor Iain Thornton and Drs Mike Ramsey and Barry Coles for the supply of the Wolfson dataset. Margaret Slater digitised the catchments. The paper bene®ted from reviews by three unnamed referees, Dr C. D. R. Evans and Dr B. G. Rawlins. This paper is published with permission of the Director, British Geological Survey (NERC). LOIS Publication No 629. Al-Bakri, D. (1986) Provenance of the sediments in the Humber Estuary and the adjacent coasts, Eastern England. Marine Geology 72, 171±186. Appleton, J. D. (1995) Potentially harmful elements from natural sources and mining areas: characteristics, extent and relevance to planning and development in Great Britain. British Geological Survey Technical Report WP/95/03. Arnett, R. R. and Justice, M. (1991) Environmental Pollution: (5) Freshwater Quality in Humberside. Working Paper 12, School of Geography and Earth Resources, University of Hull. Brown, A (editor). (1993) The UK Environment 1993. Department of the Environment. HMSO, London. Ciavola, P. and Covelli, S. (1995) Coastal-estuary ¯ux of heavy metals: the use of estuarine salt marshes as a recorder of pollution in the Humber Estuary, UK. In LITTORAL `94. Proceedings of the Second International Symposium of the European Coastal Zone Association for Science and Technology, Lisbon. (Lisbon: Instituto de Hidraulica e recursos hidricos), pp. 231±245. Dunham, K. C. and Wilson, A. A. (1985) Geology of the Northern Pennine Ore®eld. Volume 2, Stainmore to Craven. Economic memoir covering the areas of the one-inch and 1:50 000 geological sheets 40, 41 and 50, and parts of 31, 32, 60 and 61, New Series. London: HMSO. Jarvie, H. P. Neal, C. and Robson, A. J. (1997) The geography of the Humber catchment. The Science of the Total Environment 87±100. Jones, L. H. (1979) Heavy metals in the Humber Estuary and its organisms. In The Humber Estuary. Natural Environment Research Council Publication, Series C20, 13±15. Macklin, M. G. Hudson-Edwards. and Dawson, E. J. (1997) The signi®cance of pollution from historic metal mining in the Pennine ore®elds on river sediment contaminant ¯uxes to the North Sea. The Science of the Total Environment 194±195, 391±398. Middleton, R. and Grant, A. (1990) Heavy metals in the Humber estuary: Scrobiculariaclay as a pre-Industrial datum. Proceedings of the Yorkshire Geological Society 48, 75±80. Middleton, R. and Grant, A. (1993) Trace metals in sediments from the Humber Estuary: A statistical analysis of spatial uniformity. Netherlands Journal of Aquatic Ecology 27, 111±120. Millward, G. E. Allen, J. I. Morris, A. W. and Turner, A. (1996) Distributions and ¯uxes of non-detrital particulate Fe, Mn, Cu Zn in the Humber coastal zone, UK. Continental Shelf Research 16, 967±993. Millward, G. E. and Glegg, G. A. (1997) Fluxes and Retention of Trace Metals in the Humber Estuary. Estuarine, Coastal and Shelf Science 44, 97±105. Mostaghel, M. A. (1983) Evolution of the South Pennine Ore®eld: I: Regional distribution of major non-metallic minerals. Bulletin of the Peak District Mines Historical Society 8, 369±372. Natural Environment Research Council. (1994) Land±Ocean Interaction Study (LOIS) Implementation Plan. Natural Environment Research Council, Swindon. Neal, C. Smith, C. J. Jeery, A. Jarvie, H. P. and Robson, A. J. (1996) Trace element concentrations in the major rivers entering the Humber estuary, NE England. Journal of Hydrology 182, 37± 64.
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