Geochemical mapping of stream water for environmental studies and mineral exploration in the UK

Journal of Geochemical Exploration, 49 ( 1993 ) 63-88

63

Elsevier Science Publishers B.V., Amsterdam

Geochemical mapping of stream water for environmental studies and mineral exploration in the UK P.R. Simpson a, W.M. Edmundsb, N. Breward a, J.M.Cook c, D. Flighta, G.E.M. H a l l d a n d T . R . L i s t e r~ aGeochemistry Group, British GeologicalSurvey, Nottingham, NG12 5GG, UK bHydrogeology Group, British GeologicalSurvey, Wallingford, OXI O8BB, UK CAnalytical Geochemistry Group, British GeologicalSurvey, Nottingham, NG I 2 5GG, UK dGeological Survey of Canada, 601 Booth Street, Ottawa, Ontario, K1A OE8, Canada (Received 2 November 1992; accepted after revision 19 July 1993 )

ABSTRACT The stream water hydrogeochemical database, prepared by the British Geological Survey's Geochemical Survey Programme, in conjunction with databases for stream sediments, soil samples and mineral concentrates, has recently been enhanced for a range of economic and environmental objectives. The density of systematic stream water sampling and analysis has been increased to one sample per km 2 and a broader spectrum of determinands introduced. An orientation suite of hydrogeochemical maps has been produced from water samples collected at 1279 sample sites in North Wales to test the methodology. Preliminary results indicate that bedrock geology and mineralisation are the most important variables which influence the surface water chemistry. The primary control by geological parameters is variously modified by secondary influences which include geomorphological factors (especially altitude) atmospheric (climatic and coastal effects), and anthropogenic (agriculture, urban and industrial developments). Regional hydrogeochemical stream water maps have a wide range of economic and environmentalapplications, especially when interpreted in conjunctionwith geological data. Results obtained so far indicate that interpretation is likely to be further enhanced in the future by intercomparison with regional stream sediment, hydrogeological, geomorphological, pedological, agricultural, landuse, climatic and remotely sensed datasets in a GIS environment. Hydrogeochemical sampling and analysis represents a cost-effective addition to the Regional Geochemical Survey of the UK.

INTRODUCTION

A pilot hydrogeochemical survey of North Wales has been prepared to test the robustness and applications of a new series of hydrogeochemical maps for stream water prepared as part of the British Geological Survey's (BGS) Geochemical Survey Programme (GSP). The current hydrogeochemistry programme based on low density sampling and determination of U, F - , pH, conductivity and alkalinity of stream water has recently been enhanced by higher 0375-6742/93/$06.00 © 1993 Elsevier Science Publishers B.V. All fights reserved.

64

P.R. SIMPSON ET A L

density sampling and the introduction of a broader spectrum of determinands. These include anions and a wide selection of minor and trace elements. The hydrogeochemical datasets are prepared in conjunction with databases for the geochemistry of stream sediments, soil samples and mineral concentrates described more fully elsewhere (British Geological Survey, 1991). Preliminary results are presented here for an orientation suite of hydrogeochemical maps for water collected from surface drainage systems during the summer of 1989 under ideal baseflow conditions at 1279 sample sites in North Wales. These include U determined by laser-induced fluorescence; As by hydride-generation quartz-tube AAS; F - by ion-selective electrode, chloride and nitrate by automated colorimetry, alkalinity, pH, conductivity and a suite consisting mainly of cations (A1, Ba, Cd, Co, Cu, Fe, Mg, Mn, Na, Ni, P, Pb, SO 2-, Si, Zn) by ICP-AES. The region is notable for a wide variety of land use including heavy industry, urban, agriculture, industry, former base metal mining and upland wilderness, with topographic relief (Plate 1 ) extending from sea level to the Snowdon Massif ( 1085 m above O.D. ) and geological formations which range in age from Precambrian to Permo-Triassic. OUTLINE GEOLOGYOF THE STUDYAREAIN NORTH WALES The oldest rocks in the area are those of the late-Precambrian Mona complex, exposed on the Isle of Anglesey and on the Lleyn Peninsula (Fig. 1 ). They comprise a suite of igneous rocks ranging from serpentinised ultrabasics and gabbros to felsites, rhyolites and granites (notably the Coedana granite) and metasediments (Plate 2). These rocks are metamorphosed from greenschist to blueschist facies. On the mainland, the Arfonian Formation, a thick sequence of acidic ashflow tufts of Late Precambrian age are overlain by a thick Cambrian succession of turbiditic sandstone, siltstone and mudstones in the northern part of Snowdonia and in the Harlech Dome. The Ordovician comprises mainly siltstones and mudstones with impersistent sandstones with thick sequences of bimodal acid/basic volcanic rocks. The Silurian strata are dominated by turbiditic mudstones and siltstones. Devonian strata are restricted to a suite of calcareous sandstones on Anglesey overlain by Carboniferous limestones (Visean), with some sandstones at the base of the succession. On the mainland Carboniferous limestone crops out along the coast, east of Llandudno, and bounds the Upper Carboniferous and Permo-Triassic fault-trough of the Vale of Clwyd. On Anglesey, Upper Carboniferous Coal Measures occupy a narrow syncline beneath Malltraeth Marsh, while in the east, Upper Carboniferous (coal measures) occur in the Flint Coalfield and a fault-bound block in the Vale of Clwyd. Permo-Triassic

65

GEOCHEMICALMAPPINGOF STREAMWATERIN THE UK Liverpool Bay

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Fig. 1. Mining fields of North Wales (after Foster-Smith, 1979).

dune-bedded sandstones, which were probably marginal to an ephemeral hypersaline lake (playa) (Smith and Taylor, 1992 ) overlie the coal measures in the Vale of Clwyd, and to the east and south of the Flint Coalfield. Glacial and post-glacial deposits such as boulder clay, solifluction head and blown sand are widespread in the area, especially on the lower ground. M I N E R A L DEPOSITS

Metalliferous mineralisation is well developed in a series of discrete orefields, some of which have been worked since Roman times. Lead-Zn ores were worked mainly in the Llanrwst and Llansannan areas and also beyond the eastern margin of the hydrogeochemical study area at Halkyn mountain (Fig. 1 ). Copper and Pb ores were worked on a small scale around Beddgelert in the Snowdon Massif and Cu-Pb-Zn ores on a large scale at the volcanogenic massive sulphide (VMS) deposit at Parys Mountain near Amlwch, Anglesey. Further south, the mainly Pb-Zn-Cu orefield between Barmouth and Dolgellau, beyond the southern margin of the study area (Fig. 1 ), has

66

P.R. SIMPSON ET AL.

also yielded considerable quantities of Au from the Dolgellau Au belt, notably at Clogau Gold Mines. SAMPLE COLLECTION

Field sampling is undertaken annually by the British Geological Survey for the Geochemical Survey Programme over approximately the same eleven week period ranging from mid June to the end of September. The GSP systematically covers areas of approximately 5000 km 2 each season at relatively high density (approximately 1 sample site/km2). Field sampling for the stream water samples was undertaken as part of the GSP field campaign in 1989. Time series studies (Bricker, 1992) have demonstrated that if water sampiing is undertaken at the same time of year each year this will tend to minimise the impact on the data of seasonal effects due to surges in the baseflow conditions. This has also been demonstrated in studies of seasonal variations of river water chemistry in former metalliferous mining areas of Wales (Pearce, 1993). Since the GSP conducts sampling during the same period each year this practical expedient is expected to help reduce the effects of seasonal and climatic variation on the hydrogeochemical data so that data collected in following years can be merged to make synoptic maps. Regional hydrogeochemical data would also benefit from comparison with the results from time series studies at local flumes; these facilities were not available in the study area however. The site data are recorded at each site on a separate field card. Information on the drainage conditions describing actual stream flow characteristics at time of sampling, and weather conditions during the previous 12, 24, and 48 hours and the period between 2 and 7 days prior to sampling, are all recorded on the field card. The cards are stored permanently and are available for reference should interpretation of the datasets require it. Sampling is not normally suspended during storm conditions and the data present here are uncorrected for any hydrographic or climatic effects. In order to optimise the conditions of sampling and storage for all of the parameters to be determined, six separate water samples were collected at each site. Care was taken to minimise contamination when handling the sample bottles an d all sample bottles and filtration equipment were rinsed thoroughly with several aliquots of the sample before collection. Three 30 ml polystyrene Sterilin ® vials were filled directly from the stream after rinsing. The vial to be used for pH and conductivity determinations was filled and sealed underwater to minimise degassing. The remaining vials contained samples for fluoride and U determinations respectively. An additional unfiltered sample was collected in a 250 ml polyethylene bottle for the measurement of alkalinity. A plastic syringe and a filter holder containing a 0.45/tm membrane filter

GEOCHEMICAL MAPPING OF STREAM WATER IN THE U K

67

Plate 1. Landsat TM image of North Wales (© BGS/NERC) illustrating topography and landuse patterns. The image is an infrared false-colour composite of Landsat Thematic Mapper scene 204-023 taken from a digital mosaic of Wales. It forms part of the database of enhanced imagery covering the UK mainland processed by the BGS Remote Sensing Group, 1992.

Plate 2. North Wales Geology summarised from British Geological Survey 1 : 250 000 scale solid geology maps for Anglesey (1982), Liverpool Bay ( 1978 ), Cardigan Bay (1982), and MidWales and Marches (1990).

68

P.R. SIMPSON ET AL.

were used to collect two 30 ml samples filtered into Sterilin ® vials, one for the determination of chloride and nitrate, and the other for analysis by ICPAES and for hydride generation quartz-tube AAS. At the end of each day's sampling, the second sample was acidified to 1% v/v with high purity nitric acid. An acidified deionised water blank in a 30 ml Sterilin ® vial was included with every 100 samples as a check on contamination from the containers and the acid. ANALYTICAL METHODOLOGY

At BGS fieM base Measurements of pH and conductivity were made on return to the the field base, usually within a few hours of collection and certainly on the same day. Acidity (pH) was determined using a glass combination electrode connected to a high performance pH meter (Radiometer Model PHM 80 ). Specific electrical conductance was measured using a standard 1 cm path cell in conjunction with an ElL conductivity bridge; results were automatically temperature compensated and are standardised and quoted with reference to 25 ° C. Total alkalinity (bicarbonate, HCO3- ) was determined the day after collection by titrating 100 ml aliquots of samples against sulphuric acid to an end point of pH ~ 4.5 using bromocresol green as indicator.

At BGS fieM laboratory The remaining water samples were assembled, documented, and returned to the field laboratory on a weekly basis where the following procedures were undertaken. Uranium was determined using a laser-induced fluorescence technique. Several drops of saturated ammonium phosphate were added to the sample to complex the various dissolved U species present into a single high-luminescent form, before analysis on a Scintrex UA3 instrument. This technique is sensitive, with a practical limit of determination of around 0.05 ppb, with relative freedom from interferences and fast sample throughput if concentrated nitric acid is also added to maintain pH below 4. Fluoride determinations (with a 0.02 ppm limit of detection) were carried out using a fluoride ion selective electrode connected to a sensitive, high-impedance meter, following the addition of a total ionic strength buffer/decomplexing agent to the samples and standards.

A t BGS analytical geochemistry laboratories All remaining samples were batched up in the field laboratory and returned to BGS Analytical Geochemistry laboratories where automated colorimetry

GEOCHEMICAL MAPPING OF STREAMWATERIN THE UK

69

was used to determine chloride and nitrate in the filtered unacidified water samples (Cook and Miles, 1980). Inductively coupled plasma atomic emission spectrometry (ICP-AES) was employed for the measurement of all the cations and SO4-S (Miles and Cook, 1982).

At GSC geochemical methods development section Following abstraction of sufficient sample for the analysis by ICP-AES, the remainder was resealed and sent to the Geological Survey of Canada for determination of As using a hydride generation system coupled to a quartz-tube flowcell attachment on an atomic absorption spectrometer (HGQTAAS). TABLE 1 Analytical detection limits in this study, by method

A utomated colorimetry Chloride Nitrate

2 ppm 0.1 ppm

Hydride generation quartz tube AAS As

0.1 ppb

Ion-selective electrode F-

0.02 ppm

Alkalinity HCO3-

1 ppm

Conductivity specific electrical conductivity

10/tS/cm

Fluorometry by Scintrex UA3 Laser Fluorometer U

0.05 ppb

ICP-AES Element

Wavelength ( n m )

Order

Detection limit (ppb)

Cd Ba Mn Fe P

226.50 493.41 257.61 259.94 178.29 180.73 279.08 251.61 589.59 213.86 308.21 317.93 324.75 220.35 228.62 231.60

2 1 2 2 3 3 2 2 1 3 2 2 2 3 3 3

5 2 5 3 30 100 20 20 10 4 20 l0 4 30 4 10

SO~Mg Si Na Zn A1 Ca Cu Pb Co Ni

Plate 3. pH (acidity), conductivity (specific electrical conductivity), bicarbonate (alkalinity), chloride.

Plate 4. Fluoride, sulphate, nitrate, aluminium.

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?late 5. Arsenic, calcium, barium, magnesium.

Plate 6. Lead, cadmium, zinc, nickel.

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72

P.R. SIMPSONET AL.

PREPARATION OF REGIONAL GEOCHEMICAL MAPS

The verified analytical data for every sample site were loaded into the BGS ORACLE database for permanent storage and interrogation. The data files were used to prepare gridded images from the site data, using the Interactive Surface Modelling (ISM) package running on the VAX mainframe computer. These images were transfered via an Ethernet link to a Macintosh II workstation equipped with Image image processing software on which further processing was undertaken to generate the percentile-binned colour images presented in this study. The U, As, F-, alkalinity (bicarbonate), conductivity, chloride, nitrate and ICP-AES detection limits (Table l ) were calculated on the basis of 5o; analytical data falling below this limit of detection have been suppressed in the images presented (Plates 3-8 ). The radius of influence around grid nodes was set to l km and blank areas in Plates 3-8 indicate regions for which sample and or analytical data are presently unavailable. INTERPRETATION OF THE RESULTS

Physicochemical parameters and anions Acidity (pH) Acidic stream waters ( p H < 5.0) are present over large parts of Central Snowdonia (Plate 3). High relief, base-poor bedrock (Precambrian to Ordovician sediments and volcanics), thin soils with poor buffering capacity and high, naturally acidic rainfall with high sulphur deposition (Bull, 1991 ), combine to give low-pH streams. Very low pH concentrations are present in the general vicinity of the former mining areas of Llanrwst and Parys Mountain where oxidation of sulphide minerals generates highly acidic waters ( < 3.5 ). In contrast, high pH values occur over the Lower Carboniferous limestone on Anglesey, around the Vale of Clwyd and also over the more basic and calcareous members of the Precambrian Mona Complex on Anglesey and in the Lleyn Peninsula. In the more intensively farmed areas around the coast and on the lower ground inland, agricultural liming of pasture may also be responsible for more localised high pH values in the stream waters.

Specific electrical conductivity (gSIcm) The specific electrical conductivity (SEC) of a stream water is a useful indication of the total dissolved load of ions (Plate 3 ). The map demonstrates that the areas of highest conductivity are underlain by the Lower Carboniferous limestone in Anglesey, the Vale of Clwyd, and the calcareous Mona Complex rocks of the western Lleyn Peninsula; bicarbonate is the major anion in these waters. The high values of conductivity recorded over western Anglesey

GEOCHEMICAL MAPPING OF STREAM WATER IN THE UK

73

and the Menai Straits area are probably related to high chloride concentrations of marine origin. Around the mining centres of Llanrwst and Parys Mountain, sulphate derived from the oxidation of the primary sulphide minerals is present in high concentrations (see below), which is reflected locally by high conductivity. Low conductivity stream waters occur mostly over Snowdonia and the Denbigh Moors, reflecting the combination of high rainfall and resistant, base-poor bedrock in these areas.

Alkalinity (bicarbonate, HCO~ ) Bicarbonate concentration is a reliable measure of the extent of acid neutralisation that has taken place in natural water (Edmunds and Kinniburgh, 1986). The bicarbonate anion is often the dominant anion in natural stream waters and has a fundamental influence on the solubility of a number of cations, especially U (as the uranyl cation, UO 2+ ) with which it forms stable complexes. High concentrations of dissolved bicarbonate are present over the calcareous, carbonate-rich lithologies of the region such as the Carboniferous limestone, the Precambrian in the west of the Lleyn Peninsula and the Upper Carboniferous and Permo-Triassic sediments in the Vale of Clwyd (Plate 3 ). Relatively high values are also present over a wide variety of lithologies over low ground in Anglesey and Lleyn, suggesting that the lower rainfall and more intensive agriculture have an important influence. Much lower concentrations are present over the high ground of Snowdonia, corresponding to the carbonate-poor Precambrian-Ordovician sediments and volcanics and high rainfall and sulphur deposition; this correlates well with low conductivity and low pH values in this area. On Anglesey, the highly acid waters around the Parys Mountain mining area are marked by a small low-bicarbonate area.

Chloride (CI-) The principal influences on the chloride distribution pattern are attributed to a combination of marine aerosols, rainfall, lithology and land use. The influence of marine spray is particularly well-developed on the western coasts of Anglesey and the Lleyn Peninsula, which face the prevailing south-westerly winds (Plate 3 ). Moderate values are present over most of Anglesey, eastern Lleyn and the Denbigh Moors, and high concentrations are present over the Vale of Clwyd where high C1- inputs from bedrock correlate with high fluoride, sulphate and Na attributed to weathering of the Upper Carboniferous and Permo-Triassic sandstones which yield a halite signature. Very low chloride concentrations are present over the Snowdonia area, probably due to high rainfall.

Fluoride (F-) Fluoride shows some similarities to chloride in its distribution pattern, with higher concentrations around the western coasts of Anglesey and the Lleyn

Plate 7. Manganese, iron, cobalt, copper.

Plate 8. Silica, sodium, phosphorus, uranium.

g-

en

GEOCHEMICAL MAPPING OF STREAM WATER IN THE UK

75

Peninsula (Plate 4). A postulated buried granite (Cornwell and Smith, 1993 ) which is a potential source of F - in mineralising systems, underlies the area in northwest Anglesey where stream water is anomalously enriched in F - . An aerosol contribution from an aluminium smelter near Holyhead on Holy Island (Fig. 1 ) however seems to be a more likely source since further evidence of granite-related mineralisation is lacking. Concentrations are high over the Vale of Clwyd where they are also associated with high chloride, sulphate and Na concentrations related to evaporite bearing Upper Carboniferous and Permo-Triassic sandstones. Fluoride concentrations are low over the highrelief, high rainfall area of Snowdonia but, unlike the chloride distribution, fluoride concentrations are high in the vicinity of the Parys Mountain sulphide deposit on Anglesey. Sulphate (SOe4 - ) The distribution of sulphate in stream water is also very similar to that of chloride. A well-defined coastal belt with high concentrations in Anglesey and around the Menai Straits is attributed to marine influences (Plate 4). There is also a strong correlation with Upper Carboniferous and Permo-Triassic sandstones in the Vale of Clwyd which yield a halite signature. The centres of mineralisation at Parys Mountain and Llanrwst show very high sulphate concentrations attributable to weathering of the sulphide ore bodies. Values of up to 940 ppm are present dissolved in waters draining the Parys Mountain area, where SO42- is the dominant anion in solution. As with chloride and fluoride, very low concentrations are present over the high ground in central Snowdonia, a function of the high rainfall, resistant rocks and thin, leached podsol soils. Nitrate (NO5-) Bedrock lithology is unlikely to contribute directly to the nitrate concentration in stream water. Thus land use and rainfall evidently control the principal nitrate inputs to streams in the area. Higher concentrations occur mostly over the lower ground in Anglesey and the Lleyn Peninsula where agriculture is more intensive; over the upland moors of Snowdonia the higher rainfall and hill farming regime results in lower nitrate concentrations (Plates 1 and 4). Relatively high concentrations over the Denbigh Moors reflect the presence of deeper soils and more intensive agriculture. The results are generally indicative of the extent of nitrate pollution arising from intensive agriculture and may be indicative of surface waters in catchments which have some potential to contaminate acquifers in recharge zones. The data are likely to be useful in regional baseline monitoring and assessing the extent of any further degradation of the surface water environment in the future.

P.R.SIMPSONETAL.

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Metals Aluminium The distribution of high concentrations of dissolved AI is closely related to areas of low pH and, in particular, the centres of mineralisation such as Parys Mountain ( < 3.5 ), Llanrwst and Beddgelert where acid mine drainage derived from the oxidation of sulphides contributes to the low pH of the water (Plate 4). At low pH ( < 5.5 ) A1 is mobilised and maintained in solution. Areas of low pH and high A1 concentrations, unrelated to mineralisation, are also present over large parts of the region, notably in central Snowdonia where base-poor bedrock, poorly-buffered soils and high rainfall contribute to soil acidification and acid runoff. In contrast, A1 concentrations are usually very low over calcareous terrain. Arsenic High dissolved As concentrations correlate well with centres of mineralisation at Llanrwst, the lower Conway valley, Beddgelert and Porthmadog and also on the southwestern aureole of the VMS deposit at Parys Mountain, though not centred on the deposit (Plate 5 ). Elsewhere, the As distribution has only a relatively weak correlation with pH but is more strongly correlated with the distribution of dissolved Fe, especially in the marshy areas of Angle-

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2

GEOCHEMICAL MAPPING OF STREAM WATER IN THE UK

77

sey (Malltraeth, Llyn Alaw) and the Vale of Clwyd. This suggests that reducing conditions (i.e. low Eh) and possibly the formation of colloidal complexes may be locally important for the dissolution and retention of both of these elements in solution. Both areas partly coincide with outcrops of Upper Carboniferous rocks and, in the latter case, also with Permo-Triassic rocks, which have similar sandstone lithologies in the Vale of Clwyd. Lithological control of As distribution by these formations is therefore also likely. Correlation with the incidence of Au observed in heavy mineral concentrates is recognised at several discrete locations on the Lleyn peninsula and at a new Au centre in the vicinity of Beddgelert where Au has not hitherto been recorded (Fig. 2). These observations indicate that the potential use of dissolved As may have applications as a Au pathfinder but requires further evaluation beyond the scope of the present study. Calcium Dissolved Ca shows a very simple relationship to the geology, being high over the Carboniferous limestones of SE and Central Anglesey, and both Upper Carboniferous and Permo-Triassic sanstones in the Vale of Clwyd (Plate 5 ). High concentrations on Holy Island on the west of Anglesey are related to Ca-rich serpentinised metagabbros and later dolerite dykes in the Precambrian Mona complex. Basic rocks and meta-limestones within the Gwna group of the Mona complex which are exposed at the western point of the Lleyn Peninsula show high to intermediate Ca concentrations in stream waters. Intermediate Ca concentrations are present over large areas of the more basic rocks of the Mona complex, the Ordovician shales on Anglesey, and the Silurian sediments of the Denbigh Moors. Low concentrations are present over the Ca-poor igneous and sedimentary rocks of Snowdonia which range from Precambrian to Ordovician in age. The distribution pattern delineates the areas which are most base-deficient and with least buffering capacity, and therefore the most susceptible to acidification. Barium Dissolved Ba shows a similar distribution to that of Ca, probably due to the limited Ba 2+ substitution for Ca 2÷ in calcite and carbonate-rich lithologies. High concentrations are therefore associated with the Carboniferous limestone in Anglesey and with both Carboniferous and Permo-Triassic sandstones in the Vale of Clwyd area, and also with carbonate-rich facies of the Mona Complex on the Lleyn Peninsula (Plate 5 ). High concentrations which are uncorrelated with carbonate-bearing lithologies, occur in association with rocks of the Mona Complex in south Anglesey, where high Ba concentrations noted in panned concentrates by BGS surveys are probably indicative of barite mineralisation. High concentrations are also present in the Llanrwst mining field, where they correlate well with high Pb, Zn and Cd concentrations.

78

P.R. SIMPSON ET AL.

Accessory barite is known in this area but was mined only as a by-product. It should be noted that the Halkyn mountain mining field, to the east of the Vale of Clwyd, produced barite in addition to Pb and Zn (Lewis, 1967; Dunham et al., 1979, see especially pp. 288-289).

Magnesium Dissolved Mg generally shows a very close correlation with Ca and Ba, with high concentrations reflecting carbonate-rich lithologies and the more basic parts of the Mona Complex, especially the ophicalcites in central Anglesey and Holy Island (Greenly, 1919 ) and the Mona Complex rocks on the northwestern tip of the Lleyn peninsula (Plate 5 ). The Lower Carboniferous rocks of eastern Anglesey, which outcrop between Llangefni and the east coast are an exception, since the high Ca and Ba values are not matched by high Mg concentrations, probably due to the local absence of dolomitic alteration. Relatively high Mg concentrations are present over the Ordovician rocks on Anglesey. The Mg content varies from low concentrations (Ordovician), to intermediate concentrations (Silurian) in the Denbigh Moors, and to high concentrations (Upper Carboniferous and Permo-Triassic sandstones) in the Vale of Clwyd. Areas of low Ca content, together with low concentrations for Mg (and Ba), delineate the base-deficient area of central Snowdonia, which is also characterised by thin soils and high rainfall, resulting in low acid neutralising capacity and a high susceptibility to acidification (Hornung et al., 1990 ). The map also indicates surrounding areas with higher Mg concentrations over lower ground and base-rich lithologies which are more likely to have wellbuffered soils and groundwaters. Lead The mean abundance of dissolved Pb in near neutral pH natural waters in this dataset is very low and the detection limit for Pb by ICP-AES is relatively high (30 ppb at 5a level) which is close to the 90th percentile of the dataset. Variation in the analytical results below this level has therefore been suppressed (Plate 6). Some of the anomalous concentrations recorded can be accounted for by geological factors, especially when coincident anomalies of Cu, Zn, Cd, AI and low pH are also considered. The most notable of these are the mining areas of Parys Mountain, Llanrwst and the Llansannan orefield between Llanrwst and Denbigh. At Parys Mountain and Llanrwst the pH is very low; however in the Llansannan orefield pH is higher (between 5.6 and 7). Cadmium The mean abundance of dissolved Cd in this dataset is also relatively low compared with the ICP-AES detection limit (5 ppb at 5a level) hence only

GEOCHEMICAL MAPPING OF STREAM WATER IN THE UK

79

anomalous concentrations, above the 95th percentile, are shown (Plate 6). The dissolved Cd distribution shows a very similar pattern to that for Pb, with marked anomalies over the mining centres of Parys Mountain, Llanrwst and Llansannan. However, Cd in water is potentially toxic and the WHO guideline concentration for drinking water is 5 #g 1-1. These data, although only locally anomalous, may therefore be relevant to studies of the drinking water quality and epidemiology in Pb-Zn mining areas of this region.

Zinc Dissolved Zn has a much greater mean abundance in this dataset than Cd, but a closely comparable analytical detection limit by ICP-AES (5 ppb at the 5t~ level). This limit coincides closely with the 50th percentile of the Zn data. Zinc also has a similar mean abundance to that of Pb which has a much higher detection limit (30 ppb at the 5a level). Zinc therefore provides a more varied and interesting distribution pattern (Plate 6) than either Pb or Cd. Anomalous concentrations of dissolved Zn are closely comparable with both Pb and Cd as could be expected from the geochemical similarity of these two metals under most conditions. However there are some notable differences; Zn additionally shows a marked anomaly over Bodafon Mountain, to the southeast of Parys Mountain on Anglesey and also to the south of Conway. In southern Anglesey the anomalous Zn, Pb and Cd concentrations are slightly displaced relative to each other. Additional coincident Z n - P b - C d anomalies occur in western Snowdonia which may be indicative of mineralisation comparable to that in the Beddgelert area. Zinc tends to be lower over all lithologies younger than Silurian, with generally low concentrations on the Carboniferous of Anglesey and in the Vale of Clwyd. These low regional concentrations are generally interspersed, however, with scattered high concentrations which are probably attributable to mineralisation. Indeed, the distribution of dissolved Zn may prove to be one of the most reliable and easily determined pathfinders to base metal mineralisation.

Nickel The mean concentration of dissolved Ni in this dataset is usually well below the ICP-AES detection limit (10 ppb at the 5a level), thus only anomalous concentrations above the 90th percentile are presented (Plate 6 ). In addition to high concentrations over the base metal mining areas of Parys Mountain, Llanrwst and Llansannan, there is also a correlation between anomalous Ni concentrations and the more basic units of the Mona complex in central and western Anglesey and at the western end of the Lleyn Peninsula, and associated with Ordovician rocks in the north-central belt of Anglesey.

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Manganese The concentration of dissolved Mn is particularly high over parts of Anglesey, notably around Parys Mountain, the north-central Ordovician belt, the west coast and the area around and north of Malltraeth Marsh (Plate 7). In contrast, very low concentrations are present over the calcareous Devonian sandstones and the Carboniferous limestone: the transition between very high and low concentrations closely follows the geological boundary with the older rocks. Low Mn concentrations are also present over the northern Mona complex outcrop between Amlwch and Cemaes, but rise again west of Cemaes. The nature of the patterns on Anglesey suggests that surface drainage conditions may provide an important control on Mn content of waters in some areas, especially in the poorly-drained areas of Malltraeth and the low ground around Llyn Alaw. The high organic content in these marshy areas results in a relatively low Eh environment with high concentrations of dissolved Mn, Fe and As. The Mn is retained in solution as Mn 2+ although the pH is above 7 in most of the high-Mn areas. On the mainland, high Mn concentrations correlate fairly well with areas of low pH and occur in several discrete centres from the Lleyn peninsula to the Vale of Clwyd, and include the mining areas of Llanrwst, Beddgelert and the lower Conway valley. This suggests that high Mn concentrations are probably associated with, and indicative of, mineralisation. However this signature is not reliable where secondary environmental factors predominate. Iron There are many similarities in the Fe distribution pattern to those shown by Mn, especially the very high concentrations over Parys Mountain where the pyritic orebody gives rise to high dissolved Fe and Mn concentrations and low pH (Plate 7 ). Both Fe and Mn also show anomalies in the Beddgelert and Llanrwst mining areas where the pH is low. Poorly drained areas with reducing conditions (low Eh), which are enriched in dissolved Fe, Mn and As, possibly as colloidal complexes, occur at Malltraeth and Llyn Alaw on Anglesey and over the low-lying land to the north of Porthmadog, near Beddgelert. The more basic units of the Mona complex and the locally Fe-rich horizons in the Ordovician also make local contributions to the dissolved Fe concentrations. In the Vale of Clwyd the Fe and Mn anomalies are sharply defined, suggesting a more limited dispersion in the slightly alkaline conditions. The areas of Devonian calcareous sandstone and Carboniferous limestone show very low dissolved Fe (and Mn) concentrations. Cobalt The mean abundance of dissolved Co in this data set falls well below the detection limit by ICP-AES (4 ppb at the 5a level) thus only anomalous concentrations above the 90th percentile of the dataset are presented and only

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important anomalies are detected (Plate 7). There is a less pronounced association with basic rocks than is shown by Ni, but anomalous concentrations can be observed in association with mineralisation in the Parys Mountain, Llanrwst and Llansannan mining areas, and also in association with areas of low pH in central Snowdonia.

Copper The distribution of high dissolved Cu concentrations over the area indicates areas of known mineralisation and present and historical mining activity, the most significant anomaly being Parys Mountain, the site of the large Cu deposit (Plate 7 ). High Cu concentrations also reflect the presence of the disused Beddgelert Cu mines and the mining fields of Llanrwst and Llansannan. High Cu, Pb, Zn and As concentrations on Carboniferous limestones along the western margin of the Vale of Clwyd are indicative of mineralisation locally. As with several other metals, there is a good correlation of high Cu concentrations with low pH, since metals are thereby stabilised in solution and are less prone t~ sorption or precipitation, although interpretation is complicated by acid mine drainage in some areas.

Silica The map of dissolved silica is complex and bears no systematic relationship to lithology. Controls on dissolved silica vary between the mainland and Anglesey, probably as a result of differences in the secondary environment and geology (Plate 8 ). For example, on the mainland there is a general positive correlation between areas of low pH, low silica and high A1; and high pH with high silica and low A1. Hence high silica concentrations are observed on the Lleyn peninsula, on the northwestern slopes of Snowdonia and in the Vale of Clywd and low silica concentrations are notable throughout Snowdonia, Betws-Y-Coed and Llanrwst. The solution chemistry of silica is complex; it is probably present in solution as silicic acid and various polymeric forms, but in the presence of A1 it can react to give protocrystalline "clay" minerals which eventually flocculate and precipitate out. This is one of the important routes for the removal of A1 from solution (Birchall, 1991 ) and thus the relative distribution on the mainland could be accounted for. On Anglesey, however, the more complex geology, especially the presence of Carboniferous limestone, generates conditions of high pH and low silica and low A1 locally, whereas the large VMS Cu deposit at Parys Mountain is associated with low pH, high silica and high A1. In the lower-lying areas a relationship probably exists between higher silica and increasing drift thickness, so the higher dissolved silica concentrations in these areas therefore may be attributable to longer residence time of the groundwater which is in contact with siliceous and permeable rocks and soils.

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Sodium The principal influences on the dissolved Na distribution pattern (Plate 8 ) are similar to those for chloride, fluoride, and to a lesser extent sulphate, discussed above, and are attributed to a combination of marine aerosols, rainfall, evaporite-bearing lithologies and land use. The influence of marine spray is particularly notable on the western and southwestern seaboard of Anglesey and the Lleyn Peninsula, which face the prevailing south-westerly winds. Moderate Na concentrations are present over most of Anglesey and western Lleyn which are particularly exposed to oceanic weather systems, and also over the Denbigh Moors. High concentrations are present over the Vale of Clwyd area which precisely match the chloride distribution, where inputs are attributed to weathering of the Upper Carboniferous and Permo-Triassic sandstones which yield a halite signature. Very low Na concentrations are present over the Snowdonia area, probably due to the high rainfall pattern.

Phosphorus The principal influences on the dissolved P distribution pattern (Plate 8 ) are comparable to those for chloride. High concentrations which are attributable to a marine origin are noted on western and central Anglesey and the Lleyn Peninsula. High to moderate P concentrations are present over the Denbigh Moors. High P concentrations are present over the Vale of Clwyd which are comparable with the chloride distribution. Coincident inputs of chloride, fluoride, sulphate and P in stream waters of the Vale of Clwyd are attributed to weathering of the Upper Carboniferous and Permo-Triassic sandstones which yield a halite signature. Very low P concentrations are present over the Snowdonia area, probably due to the high rainfall pattern.

Uranium The distribution of dissolved U in stream waters is influenced by a combination of factors including geology, hydrochemical conditions, relief and land use. Uranium has a high solubility over a wide pH range due to the formation of stable, soluble complexes formed between uranyl ions (UO 2+ ) and the most abundant anions (bicarbonate and sulphate). The principal contrasts in the distribution pattern lie between the relatively high concentrations over the Mona Complex on western Anglesey and the Lleyn Peninsula and the very low concentrations present over the Precambrian-Ordovician sediments and volcanics of Central Snowdonia (Plate 8). Higher concentrations are also present over the Silurian of the Denbigh Moors and the Upper Carboniferous and Permo-Triassic sandstones of the Vale of Clwyd which yield a halite signature. There is general correlation between high U concentrations and enhanced conductivity, bicarbonate and fluoride concentrations in the stream waters of the region. High U concentrations coincide with the Cu deposit at Parys Mountain where U concentrations of 12 ppb coincide with very low pH

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values ( < 3.5 ) derived by sulphide oxidation, suggesting intense leaching of the bedrock and release of U into surface waters as a sulphate complex. DISCUSSION

These results demonstrate that bedrock geology and the presence of mine workings and mineralised areas are evidently amongst the most important variables which influence the surface water chemistry in the North Wales orientation study area. These primary geological parameters are variously modified by secondary influences which include geomorphological (especially altitude), atmospheric (climatic and coastal effects) and anthropogenic (agriculture, industry and urban developments). These factors, when taken together, require a systematic multielement and regional approach to surface water hydrogeochemicalmapping for their evaluation and understanding. The data collected also provide baseline information on the regional geochemistry of drainage systems for multi-user applications ranging from environmental studies to mineral exploration. The synoptic survey approach adopted in this study, which has resulted in the production of maps which can readily be interpreted in relation to geology, and which do not appear to be significantly affected by variations in stream flow, is also generally supported by the results of time series experiments elsewhere. The Shenandoah Valley in Northern Virginia USA is a good example where over the last decade the chemistry of stream water samples is closely comparable when collected at similar times of the year, year on year over a ten year period. The effect of storm events is transient and the base load characteristics are re-established within a relatively short period (Bricker and Rice, 1989; Bricker, 1992 ). This has also been demonstrated in studies of seasonal variations of river water chemistry in former metalliferous mining areas of Wales (Pearce, 1993). Ca and Mg show seasonal patterns of high summer/low winter concentrations, whereas metals such as Zn, Pb, Cu, Cd, A1, Mn, and Ba tend to show low summer/high winter variations. Stream water hydrogeochemical data also form an important component of the national water-quality assessment programme in the USA (Leahy et al., 1990) where they provide information concerning the regional occurrence and concentration of dissolved trace elements and of industrial and agricultural contaminants, including the relative magnitude of various local and regional contributions to surface water contamination. In addition, hydrogeochemical data can provide a primary source of data for policy questions regarding key substances in water in need of regulation and for which further research is required regarding toxicity, human exposure and drinking water treatment. Consideration must also be given to additional aspects when interpreting the results of regional stream water surveys. For example water geochemistry

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involves the study of a geochemically dynamic system. The interpretation of regional hydrogeochemical maps therefore involves not only consideration of single elements or parameters but also their interactions with others which contribute to the hydrogeological system. Several observations of general significance which link more than one parameter can therefore be made on the geochemical maps. For example the solubility of many elements in this dataset is controlled by the stream water pH, notably that of the transition metals, which show higher concentrations around the low-pH areas such as Parys Mountain and Llanrwst, especially when the metal concentration is locally enhanced due to mineralisation. Acidity also exerts a major control on speciation and therefore influences the toxicity and bioavailability of several potentially toxic trace metals, which is important for water quality studies and agricultural and epidemiological outcomes. The base deficiency and inherently weak buffering capacity of the soils in the high ground of Snowdonia, which is due to a variety of factors including Ca-poor bedrock and high rainfall, is indicated in the datasets by low pH, bicarbonate, conductivity and low dissolved Ca and Mg concentrations coupled with high dissolved A1. The Critical Loads for acidity of soils are exceeded in this area, as indicated on the maps showing exceedences of acidity, and the soils are therefore vulnerable in this area to acidic precipitation (Department of the Environment, 1991 ). The combination locally of high dissolved A1 with low silica and low pH in stream waters should be evaluated further since there may be implications for human and animal health if used for drinking. There is a well defined signature in water samples from base metal mineralisation in the region. There is also new evidence of precious metal anomalies from the dissolved As data, some of which are unrelated to current or historical mining activity. These relationships are well developed in the Lleyn peninsula where coincidence is noted between Au in heavy mineral concentrates and As in water; and in Anglesey and western Snowdonia, where high levels of Ba in water correspond to the presence of barite in heavy mineral concentrates. Lithology has a demonstrable effect on the distribution patterns of many dissolved elements. This is true even for relatively immobile elements, such as Ni, which are closely related to the presence of basic rocks within the Mona Complex on Anglesey. This suggests that hydrogeochemical data can usefully be integrated with data from geological maps to describe the relative distribution and mobility of chemical elements from bedrock which are dispersed as weathering products in stream water, stream sediment and heavy mineral concentrates. CONCLUSIONS

The preliminary results obtained indicate that bedrock geology and mineralisation are the most important variables influencing the surface water

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chemistry and they are modified to varying degrees by atmospheric (climatic and coastal effects), anthropogenic (agriculture and urban developments) and geomorphological influences. Examples of some of the possible applications of the maps are outlined below. Synoptic hydrogeochemical datasets which are produced to a rigorous baseline standard can be used for long range monitoring to assess the impacts of changes in industrial and agricultural activities and the extent of atmospheric pollution burdens on surface water quality. For example the gradual regional degradation of surface waters which results from the application of fertilisers in agricultural areas and from urbanisation and industrialisation can be identified, and the effect of remedial measures assessed. The multielement datasets can also be used as the basis for comparison with generally accepted trigger values to determine where the geochemical and physicochemical parameters of surface water lie outside the range of values considered as acceptable for irrigation and livestock watering. This is presently the case in certain regions in the area under investigation. Some of the stream water also lies outside the range of acceptable levels for human consumption of drinking water. This may be an important consideration in country areas especially where stream water is used for drinking. The baseline data are useful for sensitivity analysis of surface waters to acidification where they contribute to the Critical Load study which is designed to provide information used in developing transnational legislation for controlling emissions of SO2 and NOx. Regional contamination of surface waters arising from the operation of major industrial plant can be detected in the datasets. This is an aspect which will come increasingly under the scrutiny of both Her Majesty's Inspectorate of Pollution (HMIP) and Local Authorities under the UK Environment Act 1990. For example the data can be used for assessment of the impact of acid mine drainage and wastes especially toxic heavy metals on the surface water chemistry (Runnells et al., 1992) and estimating inputs from these sites by surface drainage to the coastal zone (Boult, 1993; Parkman, 1993 ), and the contribution of fluoride aerosol emissions to surface waters from major industrial plants. The data provide multielement input to epidemiological studies concerning the biosphere including plant, animal and human health. For example, studies of dissolved A1 and A1/Si ratios could be applied to human epidemiology studies of degenerative diseases such as Alzheimers syndrome and determination of naturally occurring levels of fluoride in surface water can be studied in relation to incidence of and requirements for the control of dental caries in children. There is also a major potential input to agricultural and land use planning since the water chemistry is indicative of the availability or absence of soluble metals in the soil profile which are essential for various types of agricultural

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production. For example the presence of base-deficient soils, which are unsuitable for use in intensive agriculture and are also likely to be sensitive to certain types of afforestation, is indicated here by low dissolved Ca, Mg, P and bicarbonate and high pH. The data also provide input to planning and site selection for a monitoring grid of time-series studies suitable for the detection of pollution events in the drainage system. The applications to mineral exploration include the use of dissolved As as a pathfinder for Au mineralisation, which could also be used in conjunction with multielementgeochemical data for Au and Au pathfinders in stream sediments, combined with inspection of concentrates for visible Au. Fluoride is an excellent pathfinder for hydrothermal mineralisation centres and pH, and dissolved metals, especially Ba, Cd, Pb, Zn, Co, Ni, Cu, P and U may be pathfinders for a wide variety of mineral deposit types. Regional hydrogeochemical stream water maps therefore have a range of economic and environmental applications and especially so when they are interpreted in conjunction with geological data, and in the future with regional stream sediment, hydrogeological, geomorphological, pedological, agricultural, landuse, climatic and remotely sensed datasets in a GIS environment. More comprehensive results and a further assessment will therefore be undertaken when the results of the stream waters reported here can be compared with those from the stream sediment samples from identical sampling sites. The results obtained so far generally indicate that high density water sampling, followed by multielement analysis, and carried out in conjunction with drainage sample surveys is a practical, valuable and cost-effective addition to the Regional Geochemical Survey database. ACKNOWLEDGMENTS

The research by G.E.M. Hall for this project and the analyses of dissolved As prepared by the Geochemical Methods Development Section of the Geological Survey of Canada, Ottawa represent a contribution to the work plan previously agreed under the Memorandum of Understanding between the Geological Survey of Canada and the British Geological Survey. D. Appleton, J. Baldock, F. Fordyce, M.J. Gallagher, D. Grey, H. Haslam, M. Howells, D. Kinniburgh, all of the British Geological Survey, and two anonymous referees are thanked for critically reading the manuscript and suggesting improvements to the text. Published by permission of the Director, British Geological Survey (NERC). REFERENCES Birchall, J.D., 1991. Trace elements in the environment and effects on human health (Plenary lecture). In: Programme and Abstracts, Industrial and Environmental Mineralogy, Winter

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Conference, National Museum of Wales, University of Wales College of Cardiff. Mineralogical Society, 12 pp. Boult, S., 1993. Metal transport in a stream polluted by acid mine drainage - - The Afon Goch, Anglesey, UK. In: Abstracts, Society for Environmental Geochemistry and Health, SEGH European Conference 1993, Aberystwyth. Bricker, O.P., 1992. Geochemistry of Small Drainage Basins: Massanutten Headwater Streams - - Mill Run and Shelter Run, V.M. Goldschmidt Conference; Field Trip May 8 1992. U.S. Geological Survey, Reston, VA. Bricker, O.P. and Rice, K.C., 1989. Acidic deposition to streams, a geology based method predicts their sensitivity. Environ. Sci. Technol., 23 (4): 379-385. British Geological Survey, 1978, 1982, 1990. 1 : 250 000 Scale Solid Geology Maps. 1978: Liverpool Bay, Sheet 53 ° N-04 ° W; 1982: Anglesey, Sheet 53 ° N-06 ° W; 1982: Cardigan Bay, Sheet 52 ° N-06 ° W; 1990: Mid-Wales and Marches, Sheet 52 ° N-04 ° W. British Geological Survey, Keyworth, Nottingham. British Geological Survey, 1991. Regional geochemistry of the East Grampians area. British Geological Survey, Keyworth, Nottingham, 95 pp. British Geological Survey, 1992. Landsat Thematic Mapper, digital mosaic of Wales. Remote Sensing Group, British Geological Survey, Keyworth, Nottingham. Bull, K., 1991. Critical Loads Maps for the United Kingdom. Institute of Terrestrial Ecology, Monks Wood and Members of the Critical Loads Advisory Group to the Department of the Environment. NERC News, (July): 30-33. Cook, J.M. and Miles, D.L., 1980. Methods for the chemical analysis of groundwater. Rep. Inst. Geol. Sci., 80/5, HMSO, London, 55 pp. Cornwell, J.D. and Smith, I.F., 1993. A possible concealed granite beneath part of Anglesey, North Wales, J. Geol. Soc. London, 150: 83-87. Department of the Environment, 1991. "Acid Rain" Critical and Target Loads Maps for the United Kingdom. Air Quality Division, Department of the Environment, 6 pp. Dunham, K., Beer, K.E, Ellis, R.A., Gallagher, M.J., Nutt, M.J.C. and Webb, B.C. 1979. United Kingdom. In: S.H.U. Bowie, A. Kvalheim and H.W. Haslam (Editors), Mineral Deposits of Europe, 1. Inst. Mining and Metallurgy and Mineralogical Society, London, pp. 263-317. Edmunds, W.M. and Kinniburgh, D.G., 1986. The susceptibility o f U K groundwaters to acidic deposition. J. Geol. Soc., 143: 707-720. Foster-Smith, J.R., 1979. The Mines of Cardiganshire, British Mining No. 12. Northern Mine Research Society, Sheffield, 99 pp. Greenly, E., 1919. The Geology ofAnglesey, Vol. 1. HMSO, London, 388 pp. Hornung, M., Le Grice, S., Brown, N. and Norris, D., 1990. The role of geology and soils in controlling surface water acidity in Wales. In: R.W. Edwards, A.S. Gee and J.H. Stoner (Editors), Acid Waters in Wales, Kluwer Academic Publ., Dordrecht, 337 pp. Leahy, P.P., Rosenshein, J.S. and Knopman, D.S., 1990. Implementation plan for the national water-quality assessment program. U.S. Geological Survey, Open-File Report 90-174, Reston, VA, 10 pp. Lewis, W.J., 1967. Lead Mining in Wales. University of Wales Press, Cardiff, 415 pp. Miles, D.L. and Cook, J.M., 1982. The determination of sulphate in natural waters by inductively-coupled plasma emission spectrometry, Anal. Chim. Acta, 141 : 207-212. Parkman, R.H., 1993. Metal fixation and mobilisation in the vsediments of the Afon Goch Estuary - - Dulas Bay. In: Abstracts, Society for Environmental Geochemistry and Health, SEGH European Conference 1993, Aberystwyth. Pearce, F.M., 1993. Seasonal variations of river water chemistry in former metalliferous mining areas of Wales. In: Abstracts, Society for Environmental Geochemistry and Health, SEGH European Conference 1993, Aberystwyth.

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Runnells, D.D., Shepherd, T.A., Angino, E.E., 1992. Metals in water, determining natural background concentrations in mineralised areas. Environ. Sci. Technol., 26(12 ): 2316- 2323. Smith, D.B. and Taylor, J.C.M., 1992. Permian. In: J.C.W. Cope, J.K. Ingham and P.F. Rawson (Editors), Atlas of Palaeogeography and Lithofacies. Geol.Soc., London, pp. 87-96.