Natural enrichment of arsenic in Loch Lomond sediments

Geochimrca et Cosmachrmica .dcfa Vol. 50. pp. 2059-2067 B Pxgamoo Journals Ltd. 1986. Printed in U.S.A.

Natural enrichment of arsenic in Loch Lomond sediments J. G. FARMERand M. A. LOVELL Department of Forensic Medicine and Science, University of Glasgow, Glasgow G I2 RQQ, Scotland, U.K. (Received January 20, 1986; accepted in revised_brm June 10. 1986)

Attract-~oncent~tions of arsenic in surface or near-surface sediment in 12 of I3 cores from the three major basins of Loch Lomond are substantially enhanced, up to 675 mg_/kg compared with background values of 15-50 m&kg. Sedimentary As profiles are attributed to post-depositional enrichment processes since there are no recent significant sources of environmental As contamination, anthropogenic or natural. in the Loch Lomond area. The enrichments can he interpreted in terms of postdepositional diagenetic remobilization processes in sediment reducing zones, followed by upward m&ation of As through interstitial water and oxidation/adsorption/precipitation reactions in near-surface oxidizing layers. Confirmation of the diagenetic/adsorption hypothesis is provided directly by the analysis of As species ( I .5-8 1 @g/l)in pore water and indirectly via comparison with sedimentary phosphorus. INTRODUCTION THE CONCENTRATIONOF arsenic in soils is typically 0.1-40 m&kg but can be greatly eIevated in mineralized areas (WOOL.SON,1983). Levels of arsenic in freshwater lake sediments might be expected to reflect the naturally occurring arsenic content of the soils of the corresponding catchment areas except where there has clearly been significant input from nearby industrial (e.g. smelting, mining) or agricultural (e.g. pesticideapplication) activities. Thus, enriched surficial sediment concentrations of arsenic of up to 2 17 mg/kg for Lake Washington, 307 mg,lkg for Browns Lake, Wisconsin, 650 mg/kg for Kelly Lake, Canada, and 3,500 mg/kg for lakes in the No~west Territories, Canada, have been attributed to the effects of the Tacoma copper smelter, Washington (CRRCELIUS,19751, additions of arsenical herbicides (KOBAYASHIand LEE, 19781, atmospheric deposition from smelters near Sudbury, Ontario (NRIAGU, 1983) and mine tailings (WAGEMANN et al., 1978), respectively. In 1979, however, FARMER and CROSS (1979) reported a highly elevated arsenic concentration of 474 mg/kg, in association with an enhanced iron content, in the O-I cm section of a sediment core from the southern basin of Loch Lomond, Scotland. In the absence of any known significant local source of arsenic contamination, this CQ.25-fold enhancement (relative to a “baseline” arsenic level at depth in the sediment column of 18 + 5 mg/kg) and the vertical profile of arsenic were tentatively attributed to the combined effects of upward migration, following diagenetic remobilization under reducing conditions, and subsequent readsorption on oxides and hydroxides of iron in aerobic layers. More recently, similar postdepositional enrichment mechanisms have been invoked to account for enhanced levels of arsenic in surficial layers of sediment from Lake Biwa, Japan (TARAMATSUei al., 1985), from Lake Ohakuri in an active geothermal, high-arsenic, area of New Zealand (AGGETT and O’BRIEN, 1985) and, to a lesser extent, from Lake

Washington, U.S.A. (PETERSON and CARPENTER, 1986). Following the eariier work carried out in this laboratory, we decided to investigate the dist~bution of arsenic in Loch Lomond sediments in a more comprehensive fashion via the analysis of arsenic, iron and manganese in the sediments and pore waters of a representative set of cores from the three main basins of the loch. The results for cores collected between November 198 1 and November 1982 are presented and discussed below. MATERIALS AND METHODS ~nv~ronmental setting of Loch Lomond Loch Lomond, 30 km to the nor&h-west of Glasgow, is an important drinking water reservoir, centre for recreational pursuits and the largest freshwater body (area 7 I km’, length 36.4 km; maxm. width 8 km) in Great Britain (Fig. 1). It is a glacial loch consisting of two distinct major basins, a “lower” and an “upper”, separated by a smaller, central basin of intermediate character. The lower basin, lying over sedimentary rocks of the Carboniferous and Devonian, is wide, relatively shallow (maxm. depth 3 I m) and biologically more productive than the narrow, deeper (maxm. depth 200 m) oligotrophic basins to the north, excavated in metamorphosed Dahadian mica-schists, schistose grits and slates. The River Faiioch, at the northern tip, drains a catchment area of metamorphic rocks overlain by peat, poor in available nutrients. The main inflow to the lower loch is the Endrick Water, draining productive fa~iand on beds of boulder clay and glacial and marine deposits. Outflow is via the River Leven. Even during periods of thermal stratification, more pronounced in the intermediate and upper region, where bottom temperatures are low ail year, dissolved oxygen levels remain high (>72W saturation) throughout the loch (MAULOOD and BONEY, 1980). Bottom sediments range from brown muds, with a thin nearsurface rust-red layer, in the south, to black, micaceous oozes, overlain by thin dark-brown unconsolidated material, in the north. Porosities are high, 80-98% and 90-99% over the top 20 cm in the south and north, respectively. Surliciai organic carbon content is greater in the north (- 10%) than in the south ( - 5%)and decreases more slowly with depth as a consequence of the increased acidity and lower temperature of northern bottom water as well as of the greater proportion of peaty ai~~hthonous material in the north compared with the more autochthonous planktonic debris in the south (SLACK,

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J. G. Farmer and M. A. Lovell were filtered through a 0.45 Mm Gelman cellulose biter i&+6) to remove particulates and stored at 4°C in polypropylen
Endrick

u

‘\ R.Leven

FIG. 1.Loch Lomond, showing main inflows, from the north and south-east, and outflow, to the south. and sediment core collection sites (1-6, southern basin: 7-9, central basin: IO13, northern basin). November 198 I-July 1982. Core ( 1). collected 15/12/8 1, water depth 2 I m; (2), 3/ I2/8 I,20 m; (3). 4/3/82, 31 m; (4), 3/12/81. 24 m; (5). 15/12/81, 19 m; (6), 19/l l/81, 18 m; (7), 18/2/82, 37 m: (8), 18/2/82. 55 m: (9), 4/3/82,60 m: (IO). 19/5/82,60 m: C11). 9/7/82, 140 m; (12). 2/6/82, 175 m: (13). 2/6/82. 55 m.

1954; LOVELL, 1985). Industrial activity and resident human population in the immediate vicinity of Loch Lomond are very low, especially when compared with the massive industrial conurbation of Clydeside. centred on Glasgow, to the south. Sample

collection

All sediment cores, loch water and biological samples were collected from the Glasgow University Field Station catamaran, Fiona. based at Rowardennan on the east shore of Loch Lomond (LoVELL. 1985). Thirteen sediment cores were collected between November I98 1 and July 1982 from the lower (cores l-6; length 1 I - 19 cm), middle (cores 7-9; length 7-18 cm) and upper (cores 10-13; length 23-44 cm) basins of Loch Lomond (Fig. I), using the Craib coring technique (i.d. 5.7 cm; CRAB, 1965) to ensure retention of intact, undisturbed, topmost layers of sediment (BAXTER et al., 1981a). After removal of overlying water, each core was extruded stepwise and sliced with a perspex blade into l-cm thick sections which. after storage in polypropylene jars, were weighed both wet and postdrying at 30°C. Each section was gently ground with a mortar and pestle prior to analysis. An Eckman grab was also used to sample surface sediment at various locations in the lower loch. A further core was collected between sites 1 and 5 in November 1982 for the extraction of pore water using a method adapted from DAVISON et al. (1982) and basedon the syringing of sediment from a predrilled, taped-up, plastic Craib core tube, f&ration throu& a 0.45 pm Millipore filter unit (MHWP 037 AO) into a second syringe under inert conditions and injection into nitrogen-filled self-sealing vials, prior to analysis. Loch water was collected at various depths at the coring sites using a 6-litre plastic Van Dom sampler. Sub-samples

methods

Sub-samples (0. I g) of loch sediment sections. stream seuiments and atmospheric particulate material were dissolved using an aqua regia/hydrofluoric acid/boric acid procedurc~ which enabled determination of total arsenic by graphite furnace atomic absorption spectrometry (PE-306fHGA-74/A%I) (LOVELLand FARMER,1983a) and major elements, siiicon. aluminium, manganese and iron, by flame (NZ0/C2H2) a-a.‘(FARMERand GIBSON, 1981). These methods werr validated by accurate analysis of standard reference materials for soil and sediment and, in the caSe of arsenic. LKIadditional corroboration by instrumental neutron activation analyst> (FARMERand CROSS, 1979) and the hydride generation a.a.3 method of PAHLAVANPOUR et al. (1984). Additional aliquots of loch sediment were extracted at 20°C using (a) 0.1 M hydroxylammonium chloridef0.0 1 M nitnc acid (0.5 hr) to release manganese and associated adsorbed elements from authigenic manganese oxide fractions (CHAO. 1972) with minimal attack on secondary amorphous oxide5 and hydroxides of iron, which were solubilized using (b) 1 M hydroxylammonium chloridef25oioacetic acid (4 hr) (CHESTER and HUGHES,1967: GIB.%ON and FARMER.1986). Manganese. iron, lead and zinc were measured directly III the leachates by flame (airfCzH2) a.a.s. The residues, collected on Whatman No. 4 1 ashless filter paper, were totally dissolved and analysed for arsenic, enabling calculation of the arsenic extracted in (a~ and (b) by subtraction from the total arsenic concentration. Readily oxidizable organic carbon was detcrmmed in sc lected loch sediment samples by the method of WALKS (I 947), as modified by LURING and RANTALA( l977). whdr total phosphorus and “inorganic” phosphorus were drtermined, post- and pre-ignition and concentrated hydrochloric acid digestion, spectrophotometrically using the phosphomolybdate method, correcting for any arsenic interferencl(JOHN, 1970; JOHNSONand PILSON. 1972). Loch water and pore waters were analysed for arsemc h) graphite furnace a.a.s., using nickel nitrate matrix moditication, and for dissolved iron and manganese by flame a.a.s Selected pore waters were also analyzed using an ion-exchange chromatography/hydride generation a.a.s. method for the speciation ofarsenic (GRABINSKI,198I; LOVFLL and FARMER. 1983b). Nitricfsulphuricfperchlonc acid (62.5:22.5: 15 i digests oi plankton were analysed by hydride generation a.a.s. for amnlc (LOVELL.1985). RESULTS Total arsenic, manganese, iron and iron/alumtmum ratlob in each of the 13 cores are plotted in Figs. 1. -1. 5 and 6. respectively. The results of the selective chemical extraction procedures are summarized for manganese, iron and arsenic in Fig. 8. Lead, zinc and arsenic profiles for cores I, 5 and I ! are shown in Fig. 3, phosphorus and arsenic profiles for cores 3,5 and 10 in Fig. 10. Pore water manganese, iron and arsenic data and associated sectional sediment results for the additional core are plotted in Fig. 9.

206 I

As in Loch Lomond sediments Arsenic (mgjkg)

FIG. 2. Profiles of total arsenic concentration @g/kg dry wt.) with depth, over collected core lengths, up to 20 cm, in 13 sediment cores from Loch Lomond: l-6 (southern basin), 7-9 (central basin), 10-i 3 (northern basin).

Filtered loch water at all sites and depths was found to contain less than 0.2 &I arsenic. Atmospheric arsenic was not detected above 0.1 n&m’ over 24-hour periods. The average arsenic con~n~tions of ph~oplan~on and zoopiankton in the loch were 3.8 rt 1.3 mglkg and 2.9 of:0.6 m&kg, respectively. Arsenic levels in stream sediments were less than 20 mg/kg except for the 62 m&kg recorded for one stream near the River Falloch at the northern tip. Overall analytical reproducibility (+ 1(I, RSD) ranged typically from ~2-590 for major element determinations to +510% for trace element measurements.

of elevated arsenic further distinguished sediment arsenic profiles in the northern from those of the southern basin. There does not appear to have been any recent source of environmental arsenic pollution which could have produced the observed substantial enrichment of arsenic in the top few cm of Loch Lomond sediment. Small local iron smelters had closed down by the mid18th century. Further afield, industrial Clydeside to the south has certainly cont~buted (via atmosphe~c transport and deposition of particulates) to the ap proximately ten-fold increase in the levels of lead and zinc in Loch Lomond sediments since the late- 18th century (FARMER et al., 1980). The atmospheric arsenic level of GO. 1 r&m3 at Loch Lomond, however, is in line with typical rural values of ~2 ng/m3 and considerably below the representative urban and “smelter-environment” values of 20 and 200 ng/m’. respectively, quoted by Woo~so~ ( 1983). Filtered Loch Lomond waters are also low in arsenic (~0.2 &l) when compared with the 8 100pg,fl reported for New Zealand thermal waters (AGGETT and ASPELL, 1979) and levels of 190-1260 @g/l found in streams and lakes near mining operations (WILSON and HAWKINS, 1978; BROOKS et al., 1982). There are no records of sodium arsenite being used in Loch Lomond to control the growth of rooted plants, as has been practised elsewhere (RUPPERT et a/., 1974; KOBAYASHI and LEE, 1978). and no indication that any arsenical herbicides which, unbeknown to us, may have been

used on the farmlands of the Endrick basin, at the south-east comer of the loch, have significantly influenced water or sediment concentrations. Arsenic concentrations in plankton are typical of uncontaminated freshwater en~ronments (FOWLER, 1983).

Conccntmtion

(w/kg)

DEXXJSSION

Arsenic Sediment core sections significantly elevated in arsenic were found at all sites except core 13 (Fig. 2). ~~irnurn sectional con~ntmtions (mgjkg) of 526 (core l), 373(2), 424(3), 480(4), 675(5), 17716) (southem basin), 288(7), 306(8), 355(9) (central basin) and 204( lo), 200( 11) and 174( 12) (northern basin) contrast with typical “background” values (to 20 or 30 cm depth) in the range 15-50 mdkg, comparable with the baseline figure of 18 Itr 5 mglkg quoted by FARMER and CROSS (1979) for the 76-cm core collected

near

site 3. These arsenic peaks were located in 1-cm sections between 1 and 3 cm in the southern basin, 2 and 5 cm in the central and 6 and 9 cm in the northern (Fig. 2). Above the peaks in most cores were sections which had arsenic concentrations (~50 mglkg) similar to those found at depth. A greater thickness of the zone

/

.:

20~

Pb

2n

AS

FIG. 3. Profiles of lead and zinc (me/kg dry wt.; I M hydroxylammonium chloride/25% acetic acid extractable) and of total arsenic (mg/kg dry wt.) with depth in Loch Lomond sediment cores 1, 5 and 11.

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I

12

13

FIG. 4. Profiles of total manganese concentration (5%dry wt.) with depth, over collected core lengths, up to 20 cm, in 13 sediment cores from Loch Lomond: I-6 (southern basin), 7-9 (central basin), IO-13 (northern basin). Stream sediment data confirm the apparent absence of localized inputs of arsenic to the loch from anthropogenic sources in the catchment area. The concen-

(Fig. 4). Manganese peaks usually occurred in the top 2 cm but could be as deep as 3-4 cm in the north (core 10). The thickness of the zone of highly elevated manganese tended to be greater outwith the southern bastn (e.g. cores 9 and 10) but was normally -r:5 cm. The high manganese concentration at 9- 10 cm III core i ..1 may reflect a combination of two profiles. possibly artributable to recent slumping of sediment. as suggested by a discontinuity in the porosit). organic carbon and silicon contents for sections between 7 and Y cm. As with manganese, there are sediment sections eievated in iron within general zones of enhancement (Fig. 5). The latter, however. are less well-defined. especially in cores from the north, because ot‘thc cornparatively low level of enhancement in total iron. r& \a ative to average core concentrations of O.i-9’: much of this “background” iron is present HI the con stituent minerals of the non-organic fraction ot‘ the sediment, trends in iron profiles can be more readil! identified if each sectional iron level is “corrected” b> normalizing to the corresponding sectional concentration of aluminium, a major element which increases downward in the sediment as organic carbon decreases. Profiles of Fe/Al are shown in Fig. 6. In the southern and central basins, the zones of enhanced iron genera& occur within the top 5 cm. In the north. the peak sections are generally found further down the cores III elongated enhanced zones although the picture is less clear in the dual profile of core 12 The iron pattern

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(8,

trations are similar to background (at depth) values for Loch Lomond sediments (FARMERand CROSS.1979: MACKENZIE et al., 1983), although it is worth noting that arsenic levels in Scottish soils derived from quartzmica-schists (which outcrop in the northern catchment area of Loch Lomond) are double the overall average for Scottish soils (WEST, 1979). As was observed in the original Loch Lomond sediment core LLRPM 1 (FARMERet al.. 1980). the onset of significant increase in levels of the heavy metal contaminants lead and zinc occurs much lower down the cores and the upwards rise in concentration is much more gradual, zones of enhancement occupying at least 14 cm (core 1) to more than 30 cm (core 11) of the sediment column, in marked contrast to the relatively narrow and well-defined sections of arsenic enrichment (Fig. 3). This contrast strongly suggests that the influences affecting the levels and distribution of arsenic in the Loch Lomond sediment column are different from those controlling lead and zinc and confirms that external arsenic contamination is not a significant factor. Manganese

and irotz

All sediment profiles of manganese followed the same upwards trend towards greatly elevated levels, of up to 9.1%, in the upper few cm of the sediment column

FIG.5. Profiles of total iron concentration (‘5,dry wt.) wnt depth, over collected core. lengths, up to 20 cm. in I3 sedimenr cores from Loch Lomond: l-6 (sorrthem basin), 7-9 (central basin), IO- 13 (northern basin)

As in Loch Lomond sediments Fe/AI

FIG. 6. Profilesof iron/aluminium (Fe/Al) with depth, over collected core lengths, up to 20 cm, in 13 sediment cores from Loch Lomond: l-6 (southern basin), 7-9 (central basin), lo13 (northern basin). for core 13 is more akin to that of southern and central cores. The depth profiles of manganese and iron, characterized by surface or near-surface enhancements overlying fairly uniform concentrations, are typical of postdepositional diagenetic remobilization. After burial of surface sediment, manganese and iron are solubilized under reducing conditions at depth in the sediment column and migrate upward through the pore water

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as divalent ions to be reoxidised and fixed in aerobic layers of sediment. As a consequence of thermodynamic and kinetic factors, the resultant manganese peak in oxic surface layers of sediment is displaced upwards relative to that of iron (BERNER, 1980: FARMERand LOVELL, 1984). Secondary enrichment processes in the catchment. leading to large enrichments prior to the suspended load entering the loch, can be ruled out as a plausible isolated explanation in view of the findings that (i) many peaks of manganese (and of iron) in the loch cores are sub-surface and (ii) total manganese in stream sediments from the catchment area was typically less than 0.15% and not more than 0.4%, values considerably below the enriched levels of near-surface sediment. It is significant, however, that the selective chemical extractant (a) released about 90% of the manganese from the sediment sections highest in manganese in each core, indicative of the secondary (authigenic) nature of the bulk of the elevated manganese in upper sections. While only an average 3.6% of total iron in the maximum concentration sections of the 13 cores was removed by (a), a corresponding 36% was solubilized by (b), still reflecting, relative to manganese, the comparatively modest extent of the secondary nature of the less dramatically enhanced iron. Relationships between arsenic, manganese and iron in sediment Figure 7 is a diagrammatic representation of the sectional peak locations and zones of enhancement for these three elements. The criteria for inclusion in the zones have been chosen somewhat arbitrarily but do correspond to significant enhancements in the case of manganese (> 1.1%) and of arsenic (>90 mg/kg) and represent an attempt to delineate the zones for the moderately enhanced iron in terms of Fe/Al > 1.O in

Element abc

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FIG. 7. Sectional peak locations (m) and zones of enhancement (B) for manganese (a), iron/aluminium (Fe/Al) (b) and arsenic (c), to a depth of 17 cm, in 13 sediment cores from Loch Lomond: l-6 (southern basin), 7-9 (central basin), IO-13 (northern basin). Criteria for inclusion in zones o’enhancement: manganese (a) >l.l%; Fe/Al (b) al.0 (cores l-10). ~1.2 (cores 1l-13); arsenic (c) >90 mg/kg.

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concentrations solubilized.

of arsenic,

of which 55 - :’ :‘. was alert

Ihagenetic/adsorption contra/ c$ ur.seui~ in Loch Lomond sediments

abcde As

concentrotlon

band

FIG. 8. Mean amounts of manganese, iron and arsenic (expressed as percentages of the total) released successively from Loch Lomond sediment sections by 0.1 M hydroxylammonium chloride/O.01 M nitric acid (Cl) and 1 M hydroxylammonium chloride/25% acetic acid (a) for several total arsenic concentration bands (a: 90- 119 mg/kg; b: 120- 149 mg/kg; c: 150-199 mg/kg; d: 200-399 mg/kg: e: 400 me/kg; f: peak arsenic sections in cores I- 12). From untreated. original dried sediment, the second chemical extractant removes (0) + (8). The proportion remaining (a) is not released under the extraction conditions employed.

In oxic surface sediments. arsenic is strongi? assaciated with iron. presumably in the foml of arsenatr adsorbed on ferric oxides and hydroxides or of precq?itated FeAsO, (KANAMORI. 1965: FERGLWS ami G~vls,1972: CRECELIUS. 1975;PRICE. lWh). Under reducing conditions accompanying the mlcrobioloacal decomposition of organic matter, lower in the sediment column, ferric oxides and hydroxides are soiubilired releasing adsorbed arsenate, which would undergo :-;: duction to arsenite and ASS: depending on pIZ and ih< activity of HS-. Upward migration of arscrm m I-!‘. duced form As(III) or as As(.V). reoxldatlon to +,s( \. i and readsorption on Fe(II1) oxides and hydroxides (11 precipitation as FeAs04 w’ould yield diagenetica!i\ produced zones of enhancement for arsenic similar TV: those of manganese and iron. The upward displacement of the manganese l~ai\ relative to the arsenic and iron peaks in 12 cores (Figs 2,4-6) reflects the precipitation of the authigenic oxides, of upwards-migrating manganese at higher Eh i: i’. more oxic conditions) than for the oxides and hydrox. ides of iron, with which arsenic. predominantly in the form of arsenate (As(V)), is associated. The northwards increase, both in the length of the displacement between

cores l-10 and > 1.2 in cores 1l- 13. The main features are as follows: (i) iron and arsenic are closely associated, most notably where the maximum sectional Fe/Al ratio coincides with the arsenic peak in five of the six southern cores and in two of the three central cores. (ii) the manganese peak is displaced upwards relative to the arsenic peak, the displacement between manganese and arsenic (and. therefore. iron) increasing in a northerly direction. (iii) the thickness of the zone ofenhancement tends to increase on going from the southern to central to northern basins. The association between Iron and arsenic in (i) is supported by the results of the selective chemical extraction procedures (Fig. 8). Of the 58 sediment sections of arsenic content > 90 mg/kg for which chemical leaching data are available, the first solution, for the selective dissolution of oxides and hydroxides of manganese, extracted an average 12% of total arsenic while the second, for secondary amorphous iron compounds. removed a further 29%. Similarly. for the sectional arsenic peaks in cores l-12. the corresponding figures were 11% and 34%. 1.c 45% of total arsenic was solubilized by 1 M hydroxylammonium chloride/25% acetic acid. In cores 2. 3, 7. 8 and 11. unusually high iron levels of 3.3-5.S%, some 36--540/F of total iron. were released by this solution from sections of peak

I,

I Sediment

“,.i<,

Wii,,.‘

FIG. 9. Profiles of sedimentary and pore waler manganebe. iron and arsenic in a Loch Lomond sediment core collected between sites 1 and 5 on 26/I 1/X2

As in LochLomond sediments Concentration 29004cyo 0-M

(w/kg -

200 1

1 400 I

600 t

f x

sorption processes, for which supporting evidence, both direct and indirect, is given below. Pore water data

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lo-15-B

P FIG. 10. Profiles of total phosphorus and of total arsenic (mgfkg dry wt.) with depth in Loch Lomond sediment cores 3, 5 and 10.

the manganese and the iron-arsenic peaks and in the thickness of the zones of enhan~ment for each element (Fig. 7), is consistent with a more gradual transition from surface oxic to deeper anoxic conditions in the northern basin sediments, as suggested by the geographical trends in the relative rates of decline of organic carbon with depth in the sediments. In southern Loch Lomond sediments, organic carbon declines markedly from -5% at the surface to -2.5% at 15 cm; in the north it is as high as 10% and fairly constant over the top 20-40 cm, reflecting some inhibition of organic matter decomposition, attributable to a significant allochthonous component less susceptible to biodegradation and to the increased acidity and lower temperatures of the northern basin. The resultant redoxcline should thus be less pronounced in the north, leading to the observed greater separation between manganese and arsenic-iron peaks and to the thicker (and, for arsenic and iron, less well-defined) zones of enhancement. The lower rate of com~~ion, as reflected in the porosity profiles, in the northern basin will also contribute to the observed ‘smearing’ and dilution (in concentration) of the arsenic-iron peaks in the north. This is in spite of comparable sedimentation rates (2030 mg/cm2/yr) for the northern and southern basins as determined by *‘*Pb dating (BAXTER et al., 198 Ib; LOVELL, 1985). Biological mixing does not appear to be of serious consequence in Loch Lomond sediments (FARMER et al., 1980; BAXTER et al., 198 lb). In any case, the effects of any potential mixing have clearly been insufficient to homogenize surface layers and obscure the well-marked zones of manganese, iron and arsenic enhancement, attributable to diagneticjad-

Pore water concentrations for the core collected between sites 1 and 5 in the southern basin were enhanced by factors of l7- 1300 (Mn), 6-2000 (Fe) and 20-l 100 (As) relative to levels in the overlying water column. Althou~ the total amount of arsenic in the pore water to a depth of 15 cm was only about 0.2% of total arsenic in this segment of the sediment column, the trends in pore water profiles (Fig. 9) strongly support the concept of diagenetic/adsorption control of arsenic. The pore water trends are consistent with upward diffusion of the three elements from a reduction zone, followed by a sudden drop in concentration due to oxidation/precipitation in the oxidation zone. Again, the close correspondence between iron and arsenic and the differences between manganese and iron-arsenic are apparent in the detailed structure of the concentration changes in the oxidation zone. Description of the pore water profiles solely in terms of oxidation/reduction reactions is oversimplified (ANDREAE, 1979; LEMMOet al., 1983). For example, dissolved manganese may well be controlled, especially lower down the core, by chemical equilibrium with precipitated MnC03 (BERNER, 1981; JAQUET et al., 1982). ~nfo~unately, no info~ation is av~lable on the alkalinity or on the levels of nitrate, sulphate, phosphate and their reduced products in pore water from this core. In addition to thermodynamic control, kinetic factors can be of importance, e.g. in the precipitationfadso~tion reactions of arsenic species with oxides of iron, the oxidized form, arsenate, is adsorbed more rapidly than the reduced form, arsenite (PIERCE and MOORE, 1982). This, despite the extreme sensitivity of the As(III)/As(V) ratio to slight changes in pE (PETERSONand CARPENTER, 1986), may help to explain the observed consistent distribution between oxidation states of pore water arsenic with depth: As(II1) averaged 65 + 9% and As(V) 35 & 9% of the sum of the two inorganic species over five pore water samples from 3-4 cm to 14-l 5 cm (NOVELL,1985). Thus, although chemical oxidation of As(III), perhaps catalysed by manganese (OSCARSONet al., 198 la,b; HUANG et al., 1982). may well be taking place towards the surface, the relative proportion of the product As(V) does not increase, because of its greater aRnity for amorphous oxides and hydroxides of iron. Biological intervention e.g. in the methylation of dissolved inorganic arsenic species, could also produce deviations from thermodynamically predicted equilibria (HOLM et al., 1979) but no evidence was found for the presence of methylated species monomethyiarsonic acid and dimethylarsinic acid in speciated pore waters &WELL, 1985). Phosphorus In the three Loch Lomond cores analysed for phosphorus, there was a strong resemblance between the

J. G. Farmer and M. A. Love11

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profiles of arsenic and its Group Vb neigbbour and chemical analogue, phosphorus, peak sediment concentrations coinciding in all three cases (Fig. 10). Of considerable relevance to this finding is the work by CARIGNANand FLETT ( 198 1) on the post-depositional mobility of phosphorus in sediments from Lake Memphremagog on the Quebec-Vermont border, North America. They showed that the upward movement of phosphorus suggested by solid phase and interstitial phosphorus trends, including sub-surface maxima at sediment depths of 2-5 cm and - 10 cm respectively. was controlled by a ~Iution-lotion-p~pi~tion cycle of manganese or iron, with phosphorus co-precipitating or co-dissolving with manganese or iron or both. In the three Loch Lomond sediment cores 3, 5 and 10, the apparent degree of enhancement of phosphorus is clearly less than that of arsenic. This is probably due, as in the case of iron, to a relatively greater proportion of the phosphorus residing in the primary mineral fraction, i.e. inert + apatite-bound phosphorus. The chemical p~tr~tment used here in the analysis of phosphorus does not ~~inguish between apatitebound inorganic phosphorus and non-apatite-bound (i.e. largely iron hydroxide-bound) phosphorus but does provide a crude estimate of organic phosphorus, which ranges, as a percentage of total phosphorus, from 1530% in core 3,9-27% in core 5 and 9-20% in core 10. Elevated levels of arsenic and phosphorus have also been measured in “crusts” and “coatings” collected from lower Loch Lomond. In shallow water (11 m) to the west of site 4, a stiff 4-cm thick crust overlying a 0.25 km2 barren area of glacial clay was found to consist of a 2-cm thick upper manganese-rich ( 11%)layer and a 2-cm thick lower orange iron-rich (22%) layer which contained 1% phosphors and 980 mg/kg arsenic. Xray diffraction of this layer identified iron phosphate hydroxides, but no mineral species of iron or manganese could be so identified in this material or in upper layers of Loch Lomond sediment, where it is presumed that amorphous hydrous oxides of manganese and iron predominate. CONCLUSIONS This work has shown that arsenic in the lacustrine sediment column is subject to control by diagenetic processes and adsorption m~hanisms. Consequently. it is possible that some of the published literature on arsenic in sediments may have erroneously interpreted vertical arsenic profiles by concentrating solely on temporal variations in local pollution inputs and in-

* See NAPS document no. 04417 for 14 pages of supplementary material. Order from NAPS % Microfiche Pubhcations, P.O. Box 35 13, Grand Central Station. New York, N.Y. 10163. Remit in advance in U.S. funds only $7.75 for photocopies or $4.00 for microfiche. Gutside the U.S. and Canada, add postage of $4.50 for the first 20 pages and S 1.OOfor each of IO pages of material thereafter, S I .50 for microfiche postage.

5uences. Prediction of the fate of arsemc in treshwater systems in the light of changing environmental conditions (e.g. eutrophication, acid precipitation, etc.) will therefore need to take into account the effects of such modifications on the processes which lead to arsenic enrichment and on the nature and stability of the a~thigenic mineral species and complexes of enhanced arsenic content. ‘4c~owl~~e~e~s-We thank P. R. 0. Barnen oftheScott& Marine Biological Association, Oban, for loan of a Cmib corer. R. S. Tippett and R. McMath of tbe Glasgow University Field Station, Loch Lomond, for considerable assistance with the sampling programme, A. B. Mackenzie, SURRC, East Kilbride, for *lePb analysis, the Geology Department, IJniversity of Glasgow, for X-ray diffraction facilities and the Natural Environment Research Council for post-graduate funding 01 M.A.L. Tables containing sectional results of arsenic, manganese. iron, porosity, orgauic carbon, phosphorus, silicon and aluminium for each sediment core and of arsenic. manganese and iron in the overlying loch water and pore water for the additional core have been deposited with the Nationat Auxiliary Publication Service.* E~jt~ria~han~iing: G. Michard REFERENCES AGGEIT J. and ASPELLA. C. (iY79) Release of arsenic from

geothermal sources in the Waikato catchment. In Trace Substancesin EnvironmentalHealthkxi. D. D. HEMPHILL). Vol. 13, pp. 84-92. University of Missouri Press. AGGET J. and O’BRIENG. A. (1985) Detailed model for the mobility of arsenic in lacusttine sediments based on measurements in Lake Ohakuri. Environ.Sci’.Technol.19.23 6-238. ANDREAEM. 0. (I 979) Arsenic speciation in seawater and interstitiat waters: The inIluence of ~oio~~~h~i~ interactions on the chemistry of a trace etement. Limnol. Oceania. 24,440-452. BAXTEPM. S., FARMERJ. CL, MCKINLEY 1.G., SWAN Il. S. and JACKW. ( 198I a) Evidenceof the unsuitahiity of gmvity

coring for collecting sediment in poIIution and sedimentation rate studies. Environ. Sci. Technol. 15, 843-846. BAXTERM. S., CRA~PORD R. W., SWAND. S. and FARMER J. G. (1981b) z’ePb dating of a Loch Lomond sediment core by conventional and particle track methods and some geochemical observations. Earth Planer.Sci. Letf 53,434444. BERNERR. A. (1980) Ear/y Dlagenesis. Princeton University Press. 241 p. BERNERR. A. (198 1) Authigenic mineral formation resulting from organic matter decomposition in modem sediments. Fffrrsc~r. Miner. 59, 1IT- 135. BROOKSR. R., FERGUSSONJ. E., HOLZBECHERJ., RVAN D. E., ZHANGH. F., DALEJ. M. and FREEDMANB. (1982) PolIution by arsenic in a gold-mining district in Nova Scotia. Environ. PoNut.,Ser. B 4, 109- 117. CARIGNANR. and FLET~ R. J. ( 198 I) Post depositional mobility of phosphorus in lake sediments. Limnol. Oceanogr. 26,36 l-366. CHAOT. T. ( 1972) Selective dissolution of manganese oxides from soils and sediments with acidified hydroxylamine hy drochloride. Soil Sci. Sot. Amer. Proc. 36, 764-768. CHESTERR. and Huo~~s M. J. (1967) A chemical technique for the separation of ferromanganese minerals, carbonate minerals and adsorbed trace elements from pelagic scdiments. Chem. Geol. 3, 199-2 1.2. CRAB J. S. (1965) A sampler for taking short undisturbed marine cores. J Cons. Perm. Int. Explor. Mer 361.34-39.

As in Loch Lomond sediments CRECELIUSE. A. (1975) The geochemical cycle of arsenic in

Lake W~~n~on and its relation to other elements. Limnoi. Oceanogr. 20,44 i-450. DAVISONW., WOOF C. and TURNERD. R. (1982) Handling and measurement techniques for anoxic interstitial waters. Nature 294, 582-583. FARMERJ. G. and CROSSJ. D. (1979) The determination of arsenic in Loch Lomond sediment by instrumental neutron activation analysis. Radiochem. Radioanal. Lett. 39,429440. FARMERJ. G. and GIBSONM. J. (1981) Direct determination of cadmium, chromium, copper and lead in siliceous standard reference materials from a fluoboric acid matrix by graphite furnace atomic absorption spectrometry. Af. Speftrosc. 2, 176-178. FARMERJ. G. and LOYELLM. A. (1984) Massive diagenetic enhancement of manganese in Loch Lomond sediments. Environ. Technol. Lett. 5, 257-262. FARMERJ. G., SWAND. S. and BAXTERM. S. ( 1980) Records and sources of metal pollutants in a dated Loch Lomond sediment core. Sci. Total Environ. 16, 13 I- 147. FERGUSONJ. F. and GAG%J. (1972) A review of the arsenic cycle in natural waters. Water Res. 6, 1259-l 274. FOWLERB. A. (1983) Arsenic metabolism and toxicity to freshwater and marine species. In Biological and Environmental Eficts ofArsenic (ed. B. A. FOWLER),Chap. 4, pp. 155- 170. Elsevier. GIBSONM. J. and FARMERJ. G. (I 986) Multi-step sequential chemical extraction of heavy metals from urban soils. Environ. Po~i~f., Ser. I3 11, 117-135. GRABINSKIA. A. (198 1) Determination of a~ni~Il~), arsenic(V), monome~y~nate and ~methy~nate by ionexchange chromatography with flameless atomic absorption spectrometric detection. Anal. Chem. 53,966-968. HOLM T. R., ANDERSONM. A., IVERSOND. G. and STANFORTHR. S. (1979) Heterogeneous reactions of arsenic in aquatic systems. In Chemical Modeling in Aqueous Systems, ACS Symp. Series 93 (ed. E. A. JENNE),Chap. 3 1, pp. 7 ll736. American Chemical Society, HUANGP. M., OXAR~ON D. W., Ll~w W. K. and HAMMER U. T. ( 1982) Dynamics and mechanisms of arsenite oxidation by freshwater lake sediments. Hydrobiol. 93, 3 15322. JAQUETJ.-M., NEMBRIMG., GARCIAJ. and VERNETJ.-P. (1982) The manganese cycle in Lac IKman, Switzerland: the role of Me~ll~enium. Hydrobiol. 91, 323-340. JOT M. K. (1970) Calorimetric destination of phosphorus in soil and plant materials with ascorbic acid. Soil Sci. 109, 2 14-220. JOHNSOND. L. and P~LSONM. (1972) Spectrophotometric determination of arsenite, arsenate and phosphate in natural waters. Anal. Chim. Acta 58,289-299. KANAMORIS. (1965) Geochemical study of arsenic in natural waters. III. The significance of ferric hydroxide precipitate in stratification and sedimentation of arsenic in lake waters. J. Earth Sci., Nagoya Univ., 13, 46-57. KOBAYASHIS. and LEEG. F. (1978) Accumulation of arsenic in sediments of lakes treated with sodium arsenite. Environ. Sci. Technol. 12, 119% 1200. LEMMON. V., FAUSTS. D., BELTONT. and TUCKERR. ( 1983) Assessment of the chemical and biological signi~~an~ of arsenical ~om~unds in a heavily contaminated watershed. Part 1. The fate and speciation of arsenical compounds in aquatic environments-a literature review. J. Environ. Sci. Health A18,335-387. LORINGD. H. and RANTALA R. T. T. (1977) Geochemical analyses of marine sediments and suspended particulate matter. Fisheries and Marine Service Technical Report No. 700. Environment Canada. 58 p. LOVELLM. A. (1985) Arsenic cycling in the freshwater sedi-

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me&s of Loch Lomond and some analytiicat speciation studies of arsenic metabolism, Ph.D. dissertation, University of Glasgow. LOVELLM. A. and FARMERJ. G. (1983a) The determination of arsenic in soil and sediment digests by graphite furnace atomic absorption spectrometry. Inrern. J. Environ. Anal. Chem. 14, 181-192. LOVELLM. A. and FARMERJ. G. (1983b) The determination of arsenic species in water and human urine by ion-exchange chromatography-hydride generation atomic absorption spectrometry (abstr.) Anal. Proc. 20, 274. MACKENZIEA. B., Scorr R. D., MCKINLEY1. G. and WEST J. M. (1983) A studv of lona term f IO’-104 v) elemental migration iu saturated clays and sediments: ‘Rep. Fluid Processes Unit Inst. Geol. Sci. FLPU 83-6. Institute of Geological Sciences, N.E.R.C. 48 p. MAULOODK. and BONEYA. D. f 1980) A seasonal and ecological study of the ph~oplan~on of Loch Lomond. Hydrobioi. 71, 239-259. NRIAGUJ. 0. (1983) Arsenic enrichment in lakes near the smelters at Sudbury, Ontario. Geochim. Cosmochim. Acta 47, 1523-1526. OSCAR~OND. W., HUANG P. M. and LIAW W. K. ( 198 la) Role of manganese in the oxidation of arsenite by freshwater lake sediments. Clays Clay Minerals 29, 2 19-225. OSCARSOND. W., HUANG P. M., DEFOSSEC. and HERBILLON A. (1981 b) Oxidative power of Mn(IV) and Fe(lII) oxides with respect to As(III) in terrestrial and aquatic environments. Nature 291, SO-5 1. PAHLAVAN~URB., WEATLEYM. and THOMPSONM. (1984) The determination of arsenic, an~mony and bismuth in soils especially in those con~ning much organic matter. In Environmental Contamination. DD. . . 8 16-820. CEP. PETERSONM. L. and CARPENTERR. (1986) Arsenic distributions in porewaters and sediments of Puget Sound, Lake Washington, the Washington coast and Saanich Inlet, B.C. Geochim. Cosmochim. Acta SO. 353-369. PIERCEM. L. and MOOREC. B. (1982) Adsorption of arsenite and arsenate on amorphous iron hydroxide. Water Res. 16, 1247-1253. PRICE N. B. (1976) Chemical diagenesis in sediments. In Chemical Oceanography (eds. J. P. RILEYand R. CHESTER), Vol. 6, 2nd edn., Chap. 1, pp. l-58. Academic Press, RUPPERT D. F., HOPKE K., CLUTE P., METZGERW. and CROWLEY D. ( 1974) Arsenic ~n~ntmtions and disttibution in Chau~uq~ Lake sediments. f. Radioanal. Chem. 23, 159-169. SLACKH. D. (1954) The bottom deposits of Loch Lomond. Proc. ROY. Sot. Edin. F&5.213-238. TAKAMAT& T., KAWASHIM‘A M. and KOYAMAM. (1985) The role of Mn2+-rich hydrous manganese oxide in the accumulation of arsenic in lake sediments. Water Res. 19, 1029-1032. WAGEMANNR., SNOWN. B., ROSEBERGD. M. and LUTZA. ( 1978) Arsenic in sediments, water and aquatic biota from lakes in the vicinity of Yellowknife, Northwest Territories. Canada. Arch. Environ. Contam. Toxicol. 7, 169-19 I. WALKEYA. (1947) A critical examination of a rapid method for determining organic carbon in soil. Soil Sci. 63, 25 l263. WEST T. S. ( 1979) Bio~gnifi~n~ and analysis of trace elements in agricultural soils. The first Tom Miller Memorial Lecture. The North of Scotland College of Agriculture. 33 p. WILSON I?.H. and HAWKINSD. B. (1978) Arsenic content of streams. stream sediments and groundwater of the Fairbanks area, Alaska. Environ. Geol. 2, 195-202. WOOLSONE. A. ( 1983) Emission, cycling and effects of arsenic in soil ecosystems. In Biological and Environmental Eficts of Arsenic (ed. B. A. FOWLER),Chap. 2, pp. 5 1- 139.