Mercury in lake sediments of the Precambrian Shield near Huntsville ...

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Mercury in lake sediments of the Precambrian Shield near Huntsville, Ontario, Canada P. E. Rasmussen 7 D. J. Villard 7 H. D. Gardner 7 J. A. C. Fortescue 7 S. L. Schiff W.W. Shilts

Abstract Long sediment cores ( 1 1 m) were collected from eight Precambrian Shield lakes in southern Ontario, Canada and analyzed for mercury (Hg), loss-on-ignition (LOI), and a suite of 36 other elements. Results indicated at least 100-fold variation in sediment Hg concentrations between lakes in close proximity (from 450 ppb), comparable to the variation reported for lakes across the whole of Canada. Strong areal correlations between Hg concentrations and LOI (r 2p0.77), between Hg and other trace element concentrations (Pb, Zn, Cd, Sb, As, Br), and similarities in the vertical concentration profiles of Hg and LOI, all point to the importance of organic matter in the release, transport and redistribution of metals in watershed systems. The spatial pattern of Hg concentrations in deep, precolonial sediments ( 1 20 cm) was found to mirror the pattern of Hg concentrations in modern surface sediments, an observation that was confirmed in a follow-up survey (r 2p0.85; np25 lakes), indicating that natural processes govern the unequal distribution of Hg among these lakes. Between-lake differences in surface sediment Hg concentrations normalized to organic carbon (Hg/C) were also reflected by Hg concentrations in smallmouth bass normalized to 35 cm length (R 2p0.63;

Received: 31 October 1996 7 Accepted: 27 May 1997 P. E. Rasmussen (Y) Geological Survey of Canada, 499–601 Booth Street, Ottawa K1A 0E8, Canada D. J. Villard 7 J. A. C. Fortescue Ontario Ministry of Northern Development and Mines, Sudbury, Ontario P3E 6B5, Canada H. D. Gardner Ontario Ministry of Natural Resources, Kemptville, Ontario K0G 1J0, Canada S. L. Schiff Department of Earth Sciences, University of Waterloo, Waterloo, Ontario N2L 3G1, Canada W.W. Shilts Illinois State Geological Survey, 615 East Peabody Drive, Champaign, Illinois 61802, U.S.A.

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Environmental Geology 33 (2/3) February 1998 7 Q Springer-Verlag

np15 lakes). The latter relationship suggests that smallmouth bass and lake sediment indicators provide mutually supportive information regarding Hg loading to the lacustrine environment from geological sources in the watershed system. Key words Mercury 7 Fish 7 Natural sources 7 Sediment profiles

Introduction Long sediment cores ( 1 1 m) were collected from eight lakes near Huntsville, Ontario, Canada (Fig. 1) to investigate the distribution of Hg and related elements in precolonial and modern lake sediments. This area is characterized by anomalously high Hg concentrations in fish in some lakes (Fig. 2), an anomaly confirmed by surveys repeated over two decades (B.P. Neary, Ontario Ministry of Environment and Energy, OMEE , pers. comm.). In many lakes, fish Hg concentrations exceed consumption guidelines of 0.5 ppm (OMEE/OMNR 1990), and in some lakes (Vernon and Fairy) fish Hg concentrations can be as high as in lakes affected by direct point-source discharges. In contrast, neighboring lakes such as Peninsula have low fish Hg concentrations (Fig. 2). Previous investigations have determined that there are no local industrial sources that would explain the anomalous Hg concentrations in the fish of the Huntsville area lakes (OMEE, unpub. data; Reguly 1991). The chief objective of the present study is to document the distribution pattern of Hg in modern sediments and in deep, precolonial sediments (deposited since the last deglaciation) to gain insight into processes influencing the distribution of Hg between lakes. The study was also designed to investigate possible correlations between Hg and other elements that have a known geological association with Hg. The application of these data to the quantification of Hg loading rates is the subject of a separate study (Bonham-Carter and others 1996) and is not within the scope of this paper. McMurtry and others (1989) reported that Hg concentrations in smallmouth bass in 90 lakes in Ontario are inversely correlated with variables that reflect water hard-

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Fig. 1 Location of study area; locations of long sediment core sampling sites for preliminary survey (details of individual vertical profiles illustrated in Rasmussen 1991); close-up of linear structural features controlling the configuration of streams and lakes upstream from the town of Huntsville

ness (especially Ca concentration in water) and are positively correlated with variables that reflect acidity. However, Rasmussen (1993a) found that the distribution of anomalous Hg concentrations in smallmouth bass in the Huntsville lakes is not explained by Ca concentration, pH or alkalinity, based on water chemistry survey data for 47 lakes in this region (Hornbrook and others 1984; Hornbrook and Friske 1989). Thus, the Huntsville area lakes do not appear to be representative of the lakes studied by McMurtry and others (1989). This raises the question of possible geological controls, particularly since there are numerous areas where Hg concentrations are elevated in glacial deposits ( 1 200 ppb in the ~2-mm fraction) in this region and in other parts of Ontario (Kettles 1988; Kettles and Shilts 1994; Coker and others 1995). Conversely, others have concluded that long-range atmospheric transport is the chief source of the Hg in this general area (Mierle 1990; Mierle and Ingram 1991), a conclusion that is based on the assumption that natural con-

tributions of Hg from weathering of glacial deposits or bedrock sources are negligible. Natural causes for elevated concentrations of Hg in fish have been documented in other areas, for example geothermal zones (Weissberg and Zobel 1973; Dunnette 1988) and watersheds influenced by volcanogenic sulfide mineralization (Rannie and Punter 1987) and Proterozoic black shales (Loukola-Ruskeeniemi 1990; Shilts and Coker 1995). Thus, geological processes and pathways need to be considered as potential sources of Hg for the aquatic food chain as well as anthropogenic sources. Detailed biogeochemical surveys have indicated that there are numerous localized Hg anomalies in the Huntsville watershed system, characterized by concentration ratios ranging from 2 to 12, in vegetation, soils, and stream sediments (Rasmussen 1993a, 1993b). Some of the anomalous Hg contents are associated with linear structural features that control the configuration of the Round-Buck-FoxVernon chain of lakes (Fig. 1), and others are associated


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Fig. 2 Spatial distribution of Hg concentrations in smallmouth bass (Micropterus dolomieui) in Huntsville area lakes, from the OMEE/OMNR Sport Fish Contaminants Program (Neary, OMEE, pers. comm.). The Hg concentrations are reported for dorsal tissue (wet weight), normalized for 35 cm, based on concentration-length regressions of a minimum of 20 samples per lake (Neary and others 1980 for sampling procedure)

with elevated Hg concentrations in the underlying glacial till (Rasmussen 1993a). These terrestrial studies suggest that both fault-related Hg dispersion halos and weathering of glacial deposits are potentially significant sources of Hg in this system (Rasmussen 1993a, 1993b).

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Sampling and analysis Sample location and collection Huntsville (long. 79712b, lat. 45720b) is located about 250 km north of Toronto in the Grenville Structural Province near the southern margin of Ontario’s Precambrian Shield region (Fig. 1). Direction of water flow in the Huntsville system is from Round Lake to Fairy Lake, from Peninsula Lake to Fairy Lake, and from Fairy Lake south to Mary Lake. The largest proportion of the popu-

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lation (11 000) resides in the town of Huntsville, between Vernon and Fairy Lakes, with resort development along the shore of Peninsula Lake, and fewer residences and cottages distributed around the upstream lakes. Sample locations were selected using government bathymetric maps and fishing maps (OMEE and Ontario Ministry of Natural Resources, OMNR, respectively), subbottom acoustic profiles (GSC), and in some places an echosounder. In the preliminary survey, 22 long sediment cores were collected from undisturbed basins of eight lakes to obtain vertical concentration profiles extending from the sediment surface to the glaciolacustrine clay (3–5 m below the sediment-water interface). Core sample locations within the eight study lakes of the preliminary survey are indicated on Fig. 1. Each of the two transects A–Ab and B–Bb (Fig. 1) consists of a series of six core samples spaced 200 m apart in Fox Lake and 70 m apart in Lake Vernon. The western chain of lakes is aligned along prominent linear structural features trending in a NW–SE direction (Fig. 1), visible on satellite images of the area (Rasmussen and Gardner 1992). The sampling transects intersect this structural trend at approximately 907. The lake sediment cores were collected in clean 5 cm-diameter acrylic sleeves inside stainless steel core barrels. Coring was performed by Metaprobe Inc. (Guelph, Ontario) during February and March 1991, using a mobile “vibracore” coring system which operates from the ice surface and is capable of penetrating 5 m of lake sediment in water depths of 20 m or less. The concept of obtaining long sediment cores in order to verify short core work was first tested using traditional winter coring methods (Fortescue 1986) and later improved using the vibracore system (Fortescue 1988). The device operates with a hydraulically driven high-frequency vibratory head designed to minimize sample compaction and yield an undisturbed sediment core, with the sediment-water interface preserved intact. Sample contamination is avoided because (1) the sediment sample contacts only the clean acrylic inner sleeve, and (2) no drilling lubricants are required. In the follow-up survey (February 1993), the study area was expanded to include an additional 17 lakes located between longitudes 79700b–80740b and latitudes 45700b–46745b (Fig. 2). Organic sediment cores were collected in duplicate from profundal basins of each lake, with each core reaching a depth of at least 1 m below the sediment-water interface. In this second survey, cores were collected manually using a modified piston core sampler and clean plastic core barrels, with the assistance and guidance of Dr. J. McAndrews, Department of Botany, Royal Ontario Museum (ROM), Toronto. Sample preparation For the preliminary survey, the top interval of each core (0–20 cm) was sectioned into 20 samples of 1-cm thickness. The next interval (20–100 cm) was sectioned into 8 samples of 10 cm thickness. The remaining interval (100 cm to the bottom) was sectioned into samples of 25 cm thickness. The interval from 40 to 100 cm was sectioned

at 10 cm intervals, and the remainder was sectioned at 25 cm . Qualitative descriptions of each sample interval are recorded in Rasmussen (1991). The follow-up survey cores were sampled at 10 cm intervals from the sediment-water interface to a depth of 1 m (11 samples per core). Samples submitted for Hg determination were treated differently from samples submitted for determination of the other 36 elements, as described below. Counting from the sediment-water interface, every other sample was sent to Bondar Clegg Ltd. (Ottawa) for determination of Hg and loss-on-ignition (LOI), while the remainder were sent to Chemex Ltd. (Mississauga) for multielement analysis and LOI measurement. In order to provide enough material for multielement analysis, it was necessary to combine all samples from the top 20 cm of some cores. In several cores collected in the preliminary survey, some disruption of sediment layering occurred in the top few centimeters due to freezing. As the sediment froze from the outside, inside particles from the waterlogged strata at the top of the cores tended to migrate upwards. This displacement caused a thin, cone-shaped spike of sediment particles (10–15 cm in height) to develop above the sediment-water interface in a few cores. These sediment particles were not included in the analysis. Such disruption of sediment layering would be expected to affect the resolution of the geochemical profiles in the more porous material near the sediment-water interface. No cores were allowed to freeze in the second survey. A comparison of geochemical profiles from cores collected in the same location in the first survey (frozen) and in the second survey (unfrozen) indicated that freezing had no observable effect at the macroscopic scale of observation in this study. Chronology Analysis of ragweed pollen (Ambrosia) and carbon-14 dating provided a chronological framework for data interpretation, permitting a definition of the surface samples (collected from 0–15 cm interval) as “modern” or “postcolonial” and the deep samples (collected from depths greater than 20 cm below the sediment-water interface) as “precolonial”. Pollen analysis of cores from Axe Lake and Fawn Lake was performed at the Department of Botany, Royal Ontario Museum, and carbon-14 dating was performed on 12 deep core samples from six lakes by the Brock University Earth Sciences Department Radiocarbon Laboratory. Sedimentation rates based on C14 dating averaged 17 year/cm (SD year/cm) for the six lakes, and varied from 28 year/cm (Peninsula) to 12 year/ cm (Fairy), estimated on the assumptions that sedimentation rate is constant and that zero centimeter depth equals zero time. Watershed and lake areas Watershed boundaries and lake boundaries were digitized on topographic maps of two scales (1 : 50 000 and 1 : 250 000), and areas were calculated using Geographic Information Systems software (ARC/INFO). A compari-


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son with area data provided by government fishing maps (OMNR) indicated that error was within 4%. Watershed to lake area ratios were calculated using only the surface area of the lake water (islands were excluded) for the denominator and the remaining area of the drainage basin, including islands, for the numerator. Thus, where lakes occur in chains, the numerator includes all water and land surfaces within the remaining area of the drainage basin.

insula Lake (near or below the detection limit of 5 ppb) to maximum values in Axe Lake (477 ppb at 8 cm below the sediment-water interface). It is noteworthy that the lake sediment with the lowest Hg concentrations (Peninsula Lake) is located within the most populated zone of the study area, while the lake sediment with the highest Hg concentrations (Axe Lake) is found in an isolated watershed with only a single cottage near the shore. This suggests that factors other than proximity to human settlement influence the Hg distribution in these lakes. The 100-fold variation in sediment Hg concentrations among lakes in the Huntsville area (Fig. 3) is comparable to the amount of variation found across the entire country. The National Geochemical Reconnaissance (NGR) database maintained by the Geological Survey of Canada contains Hg concentration data for 69,884 lake sediment sites as of 1995, for which the median is 60 ppb and the 5th and 95th percentiles are 20 and 175 ppb, respectively (Friske and Coker 1995). In fact, Hg concentrations as high as 320 ppb (at 40 cm) were observed in deep sediment samples from Axe Lake, exceeding the 99th percentile of the NGR database which is 250 ppb (P.W.P. Friske, Geological Survey of Canada, pers. comm.). Note that surface sediment concentrations in this study cannot be directly compared with the NGR database as the NGR samples were generally collected at depths greater than 30 cm below the sediment-water interface (Friske and Coker 1995).

Mercury determination Samples were oven-dried on aluminum foil drying boats, a technique previously demonstrated not to significantly affect Hg concentrations (Rasmussen and others 1991). The dried samples were double-bagged using Zip-Loc plastic bags, and shipped for analysis. The analytical method consisted of concentrated HNO3–HCl digestion, SnCl2 reduction and cold vapour AAS determination at a detection limit of 5 ppb. Loss on ignition (LOI) was determined at 500 7C for the remainder of each sample. A bulk sediment standard was prepared by homogenizing a large sample thoroughly for 4 h in a bucket of distilled water, and then dividing it into 15 replicates which were subjected to the same handling procedures as for regular sample preparation. The mean Hg concentration was 39 ppb (SD 4.9 ppb; RSD 12.5%). In addition, 24 samples were analyzed in duplicate for Hg. The Hg contents of the duplicates ranged from below the detection limit to 142 ppb, and RSD averaged 3.7% and ranged from 0 to 12%. The follow-up samples were analyzed by ACME laAreal correlation between shallow and deep boratories (Vancouver, Canada) for total Hg using the sediments same analytical method and for organic carbon using a Results of the preliminary survey showed a very strong LECO analyzer. Accuracy of Hg determination was within areal correlation (R 2p0.85; np8 lakes) between Hg con13% based on nine blind replicates of provisional standard LKSD-2 and six blind replicates of provisional standard LKSD-3 (Lynch 1990). Multielement determination The samples were dried pulverized to approximately -150 mesh using a ceramic ZrO2 pulverizer, and digested using a perchloric-nitric-hydrofluoric acid mixture by Chemex Ltd. (Mississauga). Atomic absorption spectrophotometry (AAS) was used for the determination of Ag and Pb. Inductively coupled plasma-atomic emission spectrometry (ICP-AES) was used for the determination of Al, Ba, Be, Bi, Ca, Cd, Co, Cr, Cu, Fe, K, Mg, Mn, Mo, Na, Ni, P, Sr, Ti, V and Zn. Neutron activation analysis (NAA) was used for the determination of Au, As, Br, Hf, La, Lu, Sb, Sc, Ta, Th, U, and W. The raw data, detailed quality assessment data, and error assessment for the individual elements have been reported elsewhere (Rasmussen 1991).

Results Areal distribution of Hg in sediments In the Huntsville area, Hg concentrations vary widely from lake to lake, ranging from minimum values in Pen174


Fig. 3 Relationship between Hg concentrations in deep and surface sediments in the preliminary survey. “Surface” refers to modern Ambrosia horizon 0–15 cm; “deep” refers to greater than 20 cm ( 1 240 years B.P.). Sedimentation rates are based on C-14 dates. Average concentrations were calculated for each lake using all data for the respective deep and surface intervals from all cores. Typical variation between sampling sites within a lake is 26% RSD for surface sediments and 21% RSD for deep sediments

Cases and solutions

centrations in surface and deep sediments (Fig. 3), indicating that the between-lake variation in the Hg content of surface sediments matches the between-lake variation in deep sediments. Similar surface-to-deep sediment correlations were observed for LOI (R 2p0.89) and Br concentrations (R 2p0.86), suggesting that the correlation in Fig. 3 is related to the strong affinity between Hg and sediment organic matter. Such correlations were not observed for surface and deep concentrations of other trace metals, probably due to differences in their behaviour during postdepositional diagenesis compared to Hg, in particular their weaker affinity for organic matter. As this surface-to-deep sediment relationship has significant bearing on the interpretation of the history of these lakes, a follow-up survey was conducted to determine if the relationship holds for a larger dataset. Results confirmed that an areal correlation (R 2p0.85; np25) exists between total Hg concentrations in modern and precolonial sediments of unperturbed lakes (Fig. 4). An anomalously high surface-to-deep concentration ratio (Fig. 4) was observed in the site located at the west end of Fairy Lake, which is the receiving basin for the canal waters flowing east through the town of Huntsville (Fig. 1). As this is the only sampling site located in the receiving waters of a settlement or other potential point source, this anomaly appears to reflect human perturbation of the natural spatial variation, perhaps related to the construction and dredging of the Huntsville canal. The sediment samples collected at this site were also anomalously enriched in Ag, but not in other elements that show strong areal correlations with Hg in unperturbed sediments (e.g., Pb, As, Sb, Cd, total C; discussed later).

Fig. 4 Confirmation of relationship between Hg concentrations in deep and surface sediments in the follow-up survey. “Surface” refers to 0–15 cm modern Ambrosia horizon; “deep” refers to 70–100 cm interval ( 1 840 years B.P.). Each data point represents average Hg concentrations calculated using all data for the respective depth intervals for each lake (min. two cores per lake). See Fig. 1 for location of the perturbed canal outflow site; this outlier was confirmed in repeat sampling (three cores total). Cores from two distal basins in Fairy Lake fall within the trend of unperturbed lakes

Relationship between Hg, organic matter, and watershed morphology One important influence on sediment Hg concentrations is the variation in organic content between lakes, as indicated by the strong areal correlation (R 2p0.77) between LOI and Hg in deep sediments (Fig. 5). Thus, the difference in Hg concentrations between Axe Lake and Peninsula Lake sediments may be partly attributed to differences in the organic content of the sediment. The higher organic content of Axe Lake sediment reflects the fact that the lake drains a large expanse of low, wet land. Results for the larger dataset (np25 lakes) of the follow-up survey indicate that the correlation between Hg concentration and organic C (by LECO analysis) remains moderately strong in precolonial sediments (R 2p0.51) and in surface sediments (R 2p0.54). The NGR database also shows a positive relationship between Hg and LOI, at LOI contents of less than 50% (Garrett 1996). These observed relationships between Hg and organic matter are consistent with results of many previous studies that show dissolved and particulate organic matter to be an important control on the release and transport of Hg and other metals from geological sources in the watershed to the lake bottom (Jonasson 1976; Coker and others 1979; Rasmussen 1994). As watershed morphology is considered to be an important factor in atmospheric source models (Swain and others 1992), relationships between sediment geochemistry and watershed morphology were also investigated. According to Swain and others (1992) the ratio of watershed area to lake area is critical as it explains most of the variation in Hg accumulation in lakes of their study area (upper Midwest USA). They report that their model also accounts for the effect of dissolved organic matter on Hg transport, based on observations of Engstrom (1987) that the humic content of a lake is related to the relative size

Fig. 5 Mercury versus loss on ignition (LOI) in deep lake sediments (20 cm to bottom of core; 1 240 years B.P.) from preliminary survey. Average concentrations were calculated for each core using all data from the deep interval; average variation between sampling sites within a lake was 26% RSD for Hg and 7% RSD for LOI


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Fig. 6 Relationship between Hg, organic matter, and watershed morphology in the Huntsville area. Watershed-to-lake area ratios show no relationship with Hg concentrations or the organic carbon content (LECO analysis) of modern and precolonial sediments in the study area. Note that the x-axis is a logarithmic scale

of the watershed. For the Huntsville area, watershed-tolake area ratios are plotted against Hg concentrations and the organic carbon content of modern and precolonial sediments (Fig. 6). The results indicate that, in this study area, relative watershed size does not explain the between-lake variations in Hg concentrations or organic matter content of lake sediment. Differences in sedimentation rates (Fig. 3) also do not account for the variation in Hg concentrations between lakes.

Fig. 7 Relationship between Hg to C ratio in surface sediments (Ambrosia layer) and Hg concentration of smallmouth bass in Muskoka-Haliburton lakes (follow-up survey). Lake locations shown in Figure 2. Scatter plot includes all lakes for which organic carbon data (LECO analysis only), sediment Hg data (unperturbed sites in Fig. 4), and fish Hg data (as in Fig. 2) were available

decrease in Hg concentrations in the upper 10 cm of the sediment column, followed by a more gradual decrease in Hg concentrations all the way to the bottom of the cores. This same trend–a steep concentration gradient just below the sediment-water interface followed by a more gradual decrease down core–is also present for LOI (Figs. 8 and 9). Vertical changes in sediment lithology and geochemistry, in particular the relative proportions of mineral and organic matter, are thus important influences on the shape of the Hg concentration profile. Lowest concentrations of Hg and LOI are observed in clayey samples collected from the bottom of cores where the coring device penetrated into the glaciolacustrine sediment. The large variability of deep sediment Hg concentrations (100–200 cm) in Lake Vernon (Fig. 9) is related to abrupt sediment composition changes in one core (Rasmussen 1991). Cores from some other lakes also showed a subsurface maximum in Hg content (Axe, Fawn, Buck, and Haller) but accompanying lithological changes were not observed in these other cores. Possible reasons for these localized subsurface maxima need to be investigated further in order to understand factors governing Hg distribution in addition to its affinity for organic matter.

Comparison of Hg distribution in fish and sediments Normalizing sediment Hg concentrations with respect to organic matter content is a method that has been used to predict Hg bioavailability, even in the absence of suitable indicator species (Bryan and Langston 1992). Using the core data collected in the follow-up survey, and Hg concentration data for dorsal tissue of smallmouth bass (35cm length) obtained from OMEE/OMNR Sportfish Contaminants Monitoring Programme (as in Fig. 2), it was possible to test this method for a set of 15 lakes. The resulting scatter plot (Fig. 7) shows a simple relationship between the Hg burden in fish tissue and Hg/C ratios in modern sediment of unperturbed lakes (R 2p0.63, np15). Note that the elevated fish Hg concentrations of Fairy Areal correlations between Hg and other elements Lake are reflected by the sediment concentrations obThe coefficients of determination (R 2) for linear correlaserved in unperturbed basins of Fairy Lake, and not by tions between LOI, Hg and the other elements are disthe Hg anomaly in the canal outflow site (characterized played as a scatter plot in Fig. 10. From the resulting patby a Hg/C ratio of about 1.5!10–3; not plotted in Fig. 7). tern, the degree to which organic matter content influences the spatial distribution of Hg and the other elements may be inferred. Elements that have an affinity for Vertical concentration profiles In general, total Hg concentrations decrease down profile organics (As, Sb, Pb, Ni, Cr, Cu, Cd, and Zn) correlate positively with Hg, as indicated by the positive axis. Eleas shown by the multiple core data from Fox Lake and Lake Vernon (Figs. 8 and 9). There tends to be an abrupt ments that are associated with the inorganic fraction (Al, 176


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Fig. 8 Representative vertical profiles of Hg and LOI in Fox Lake sediment, based on total of six cores collected along transect A-Ab (Fig. 1). Data points represent the mean concentration for each depth interval; error bars represent the 95% confidence intervals for the mean estimates. Sedimentation rate of 17.2 year/cm estimated from C-14 dating of 12 sediment samples from six surrounding lakes

K, Ba, Sr, Na, Mn, and Ti) correlate negatively with Hg, as indicated by the negative axis. This relationship is linear when element correlations are plotted with respect to Al to represent the mineral fraction, and with respect to LOI to represent the organic fraction (Fig. 11). The positions of Br and K in Fig. 11 suggest that these two elements could equally be used to represent the organic and mineral fractions, respectively. Inferences from Fig. 11 regarding possible partitioning of the trace elements between the organic and mineral fractions of the sediment are under further investigation using sequential chemical extraction techniques. The elements As and Sb and some other metals (Cr, Ni,

Fig. 9 Representative vertical profiles of Hg and LOI in Lake Vernon sediment, based on total of six cores collected along transect B-Bb (Fig. 1). Data points represent the mean concentration for each depth interval; error bars represent the 95% confidence intervals for the mean estimates. Sedimentation rate of 16.7 year/cm estimated from C-14 dating of samples collected from depths of 100–110 cm and 440–455 cm at centre of Lake Vernon transect B-Bb

Cu, and Pb) correlate more strongly with Hg than with LOI (Fig. 10), suggesting that an additional factor other than organic matter influences the distribution of these elements. The significance of these relationships in terms of source and transport mechanisms is discussed below.


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Discussion

Fig. 10 Coefficients of determination (R 2): summary graph showing relationships between Hg, LOI and other elements in Huntsville area lake sediments. Positive correlations are indicated by the positive axis, and inverse correlations are indicated by the negative axis. Average concentrations were calculated for each of 22 cores using all data from the 0–20 cm interval in the preliminary survey (Fig. 1). Brackets () indicate median below detection limit (d.l.): 9 samples were above d.l. for As (1 ppm) and 11 samples were above d.l. for Sb (0.2 ppm)

Fig. 11 Coefficients of determination (R 2): summary graph showing relationships between Al, LOI and other elements in Huntsville area lake sediments. Positive correlations are indicated by the positive axis, and inverse correlations are indicated by the negative axis. Average concentrations were calculated for each of 22 cores using all data from the 0–20 cm interval in the preliminary survey (Fig. 1). Brackets () indicate median below detection limit (d.l.): 9 samples were above d.l. for As (1 ppm) and 11 samples were above d.l. for Sb (0.2 ppm)

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The strong correlation between Hg concentrations in surface and deep sediments (Figs. 3 and 4) indicates that the spatial distribution pattern of Hg was virtually the same in precolonial time as it is now. In other words, factors governing Hg distribution in the present, causing one lake to display high Hg concentrations (Axe Lake) and another lake to display low Hg concentrations (Peninsula Lake), also governed Hg distribution in the past. The outlier in Fig. 4, which represents an exception to this trend, is interpreted to represent human perturbation of the surface sediments and a possible local anthropogenic source. Apart from this site, possible effects of human activity (erosion, water level fluctuations, atmospheric Hg loading) are not reflected by any observable changes in Hg distribution between the study lakes. This similarity of recent and precolonial patterns suggests instead that it is natural processes that control the unequal distribution of Hg between the lakes in both sediment and fish indicators. The simple relationship established between fish and sediment, based on the partitioning of Hg in sediments (Fig. 7), is an expected result according to the observations of Bryan and Langston (1992). Sediments and smallmouth bass are both considered to be useful measures of Hg loading to waterbodies (Bloomfield and others 1980; Bryan and Langston 1992). Bryan and Langston (1992) indicate that the Hg content of sediments is an effective measure of the loading of Hg due to the strong adsorption of Hg onto particulate matter, especially organic matter, and that measurements of Hg in sediments show much greater distinctions in levels of contamination between waterbodies than do measurements of Hg in water. Smallmouth bass are also useful for such comparisons for two reasons (Bloomfield and others 1980). First, the species is widely distributed as a result of its wide range of habitat. Second, smallmouth bass are top level predators, and as such are likely to contain appreciable amounts of Hg due to biomagnification of Hg along the food chain (Bloomfield and others 1980). The technique of normalizing soil or sediment Hg concentrations with respect to organic matter content has different applications in different disciplines. In geochemical exploration studies Hg/C ratios have been used to correct for the strong affinity between Hg and organic matter in determining source strength or proximity to a buried source, while in toxicology studies Hg/C ratios have been used for predicting Hg bioavailability. The correlation between Hg in fish and Hg/C ratios in sediment in the Huntsville area (Fig. 7) shows that both indicators provide mutually supportive information for comparisons of Hg loading to the various lakes. This observation further supports the conclusion that the geological anomalies discovered by the terrestrial biogeochemical surveys upstream from the town of Huntsville (Fig. 1) are a significant and biologically available source of Hg in this system (Rasmussen 1993a).

Cases and solutions

Figures 3 and 4 indicate that Hg concentrations in surface sediments are generally higher than Hg concentrations in deep sediments by a factor of 2–5. This is also apparent in the shape and magnitude of the vertical concentration profiles (Figs. 8 and 9), which are similar to Hg profiles reported elsewhere in the Precambrian Shield of North America and Scandinavia (Ouellet and Jones 1983; Bjorklund and others 1984; Evans 1986; Johnson 1987; Lindqvist and others 1991; Swain and others 1992; Lucotte and others 1995). This commonly observed twoto fivefold enrichment in Hg concentrations in sediments of remote lakes (located hundreds or thousands of kilometers from industrial sources) is widely interpreted as evidence of a two- to fivefold increase in atmospheric loading at the continental or global scale since the onset of industrial activity. However, the relative influence of postdepositional diagenetic processes is unknown or assumed to be negligible in these studies. In general, redox cycling tends to increase the metal content of the upper layers of sediment, producing metal concentration profiles that show high concentrations near the sediment surface and lower concentrations at depth in the sediments (Shaw and others 1990; Farmer 1991; Matty and Long 1995). Recent research has determined that these diagenetic processes similarly influence Hg concentration profiles (Matty and Long 1995). Further research is needed to determine the extent to which postdepositional diagenesis and remobilization affect previous interpretations of sediment Hg profiles as historic records of global change (Rasmussen 1994). The theory that surface enrichment of Hg in lake sediments reflects an historic increase in atmospheric Hg loading does not satisfactorily explain the similarity between the vertical LOI and Hg profiles in the Huntsville lakes (Figs. 8 and 9). Likewise, the atmospheric theory does not explain the correlation between surface and deep sediment concentrations (Figs. 3 and 4) given the wide variation in watershed retentions and watershed area to lake area ratios in the survey area (Fig. 6). Another explanation for the steeper near-surface concentration gradient might be increased erosion of terrestrial organic material since colonization, an explanation which does address the similarity between the Hg and LOI profiles (Figs. 8 and 9). However, the consistency of the vertical increase in sediment Hg concentrations across all the study lakes (Figs. 3 and 4) is not addressed by this explanation, due to the fact that the intensity of activities that enhance soil erosion (construction activity, deforestation) varies widely amongst the watersheds. Similarities between vertical Hg and LOI profiles (Figs. 8 and 9), particularly in precolonial sediments, suggest that the gradual decrease in Hg content down core may be related to compositional changes in organic matter during decay. This model is supported by experimental evidence indicating that the breakdown of organic material during diagenesis is accompanied by a reduction in the integrity of the organo-Hg association with depth (Lindberg and Harriss 1974). As sedimentation progresses and surface sediments become buried under anoxic conditions, reac-

tions causing release of Hg species to pore water will be favored, while sediment compaction will continuously force the Hg-enriched pore water up into the porous, newly deposited surface sediment. The steeper Hg concentration gradient near the top of the cores (Figs. 8 and 9) may be related to the REDOX boundary near the sediment-water interface, as demonstrated by Matty and Long (1995) and as is the case for other metals (Shaw and others 1990; Farmer 1991; Friske 1995). In many cores from this study, a red to reddishbrown color was observed in sediments in the top 10–15 cm which contrasted with the dark brown color of the underlying sediment, suggesting a change in the oxidation state of iron in the surface horizon. In the case of Hg, REDOX-controlled reactions would include the microbial breakdown of organic matter, the abiotic reduction of Hg 2c to Hg 0 by fulvic acids, and adsorption of inorganic Hg species and organo-Hg complexes by oxides and hydroxides in near-surface sediments. The cores collected in this study provide important information at sediment depths below 30 cm, which is the limit of many Hg concentration profile studies. The gradual decrease of LOI and Hg concentrations with increasing depth in precolonial sediments (Figs. 8 and 9) suggests that diagenetic processes do come into play. The observations that the Hg distribution pattern is identical in surface and deep sediments and that the two- to fivefold surface enrichment ratio occurs across all the study lakes (Figs. 3 and 4) are also consistent with a diagenetic model. The strong spatial correlations between Hg, Pb, As, Sb, and other metals observed in the Huntsville lake sediments (Figs. 10 and 11) suggest that these elements have a common source and transportation mechanism. Although it has been suggested that long-range atmospheric transport is the chief source for the Hg, the elemental associations would suggest a local rather than distant source, as it is unlikely that the elemental associations in raw material (coal and ore) would be maintained during long-range atmospheric transport. The physical and chemical behavior of each element during combustion and atmospheric transport is very different: for example, while most Hg and some As is thought to be emitted from stacks in vapor form (Galloway and others 1982), Sb belongs to the class of elements that condenses on particulate matter following combustion (Fernandez and others 1992). Previous investigations have found no local industrial source of Hg releases (Reguly 1991; Ontario Ministry of Environment and Energy, unpub. data). Moreover, the distribution of elevated Hg, Pb, As, and Sb concentrations in the lake sediments is inconsistent with (1) the location of roadways, (2) the location of the most heavily populated areas, and (3) prevailing wind directions. Therefore, the elemental associations in sediments of the Huntsville area may reflect enhanced metal concentrations in the surrounding till and bedrock, and it is also possible that Hg and the related elements are migrating from deep bedrock sources to the surface environment


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along faults and fractures. Bedrock outcrops are unfortunately scarce in the study area, which is characterized by thick sequences of glacial drift. Samples collected from 37 bedrock exposures were strongly weathered and generally contained Hg concentrations below the 5 ppb detection limit (Rasmussen 1993a), with the exception of one outcrop of pelitic gneiss (8–23 ppb). This carbonaceous pelitic gneiss unit (mapped by Nadeau 1990) trends northwest to southeast in the study area, adjacent to and underlying the Hg-enriched chain of lakes. This unit may be considered a potential bedrock source for the Hg in this area, as elevated levels of Hg in sediments and biota are reported to be associated with Proterozoic black shale occurrences (Cameron and Jonasson 1972; Loukola-Ruskeeniemi 1990; Friske and Coker 1995, Shilts and Coker 1995). Other possible bedrock sources include mafic and ultramafic sequences which locally contain base metal sulfides (Marmont and Johnston 1987), or metacarbonate-hosted mineralization at depth. The latter suggestion, that a metacarbonate sequence may exist at depth, is based on the presence of marble in an outcrop near Huntsville (A. Davidson, Geological Survey of Canada, pers. comm.), and on the similarity of elemental associations between this site and marble-hosted polymetallic sulfide deposits elsewhere in the Grenville Province, where elevated Hg concentrations occur with elevated concentrations of Sb, As, Cu, Cd, Pb, and Zn (Carter and others 1980). Examples include a number of deposits: the stratiform Cu-Sb-Ag-Hg deposit at Clyde Forks and the stratabound Cu-Sb-Au-Ag deposits located in Lavant and Darling Townships (Carter and others 1980), and the carbonate-hosted Zn deposit at the Balmat mine, New York (Bloomfield and others 1980). Unweathered rock samples obtained by diamond drilling are required to further understand the contribution of deep bedrock sources in the Huntsville area. In addition, the use of stable Pb isotopic signatures holds promise as a method to investigate potential bedrock sources of Hg, due to the strong spatial association between Pb and Hg in the sediments of this area (R 2p0.82; np22).

ments, and thus localized enrichment of Hg and related elements in organic-rich environmental media is not in itself evidence of an anthropogenic influence. The consistency of the surface-to-deep concentration ratios, among lakes which display more than 100-fold variation in Hg concentration, suggests that postdepositional diagenesis has an overriding influence on the shape of the vertical concentration profiles. Thus, although it is possible that an atmospheric component may be superimposed on the landscape, the vertical lake sediment profiles provide no clear evidence regarding the potential significance of atmospheric deposition. An outlier on a scatter plot of modern versus precolonial Hg concentrations identifies human perturbation and a possible local anthropogenic source of Hg in or near Huntsville, but in this case the pathway is more likely to be aqueous than atmospheric. Differences in interpretation of the vertical concentration profiles do not affect the main conclusion, which is that smallmouth bass and lake sediment indicators provide mutually supportive information regarding Hg loading to the lacustrine environment from geological sources in the watershed system. This conclusion is supported by the observation that the surface-to-deep Hg concentration ratio is about the same for all the lakes, and by the lack of correlation between Hg concentrations and sedimentation rates. Research into geological sources of Hg has particular relevance to the environmental assessment of lands proposed for hydroelectric reservoir creation. It has been demonstrated that the immersion of soil and vegetation by flooding releases Hg into the aquatic ecosystem and creates conditions which favour methylation processes (Hecky and others 1987a, 1987b; Morrison and Thérien 1991). The flooding of landscapes enriched in Hg due to local geological sources can lead to particularly high concentrations of Hg in fish. In British Columbia, for example, elevated concentrations of Hg in the fish of Williston Reservoir have been attributed to the adjacent mercuriferous Pinchi fault zone (Watson 1992). In Manitoba, elevated Hg concentrations in fish of the Churchill River Diversion have been attributed to the mobilization of Hg from volcanogenic sulfides (Canada Manitoba Agreement Conclusions and implications 1987). Based on their study of the Churchill River DiverNatural geological and geochemical processes are an im- sion, Rannie and Punter (1987) recommended the use of portant influence on the distribution of Hg in the modsoil and vegetation surveys to evaluate geological sources ern lacustrine environment. In the Huntsville area, beof Hg for future reservoir development proposals. They drock and till geochemistry and permeability and beobserved that a gap exists in the understanding of geodrock structure vary widely from watershed to watershed. logical pathways by which Hg moves from bedrock to the In addition to the influence of organic transport and reaquatic ecosystem, a comment which echoed earlier condistribution, the variability of these geological factors re- cerns expressed by Azzaria and Habashi (1976) regarding sults in large variations in Hg concentrations from lake the lack of research on natural inputs of Hg to lakes. Azto lake. The extent to which anthropogenic processes zaria and Habashi (1976) identified a particular need for have modified this environment is difficult to assess, but information in northwestern Quebec, where the relative they are by no means the only processes influencing Hg importance of natural and anthropogenic sources of Hg concentrations in lake sediments and biota. The strong in lakes and reservoirs remains a subject of debate to this correlations between sediment organic matter and metals day (Girard and Dumont 1995). observed in this study result from the dominant role of Further research is needed to elucidate the natural bioorganic matter in the biogeochemical cycling of these ele- geochemical cycling of Hg and other metals of concern, 180


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to determine the pathways through which these metals enter the food chain from geological sources, and to develop improved diagnostic tools for distinguishing natural and anthropogenic sources of Hg and other metals in the environment.

diment geochemistry applied to mineral exploration. In: Hood P J (ed). Geophysics and Geochemistry in the Search for Metallic Ores. Economic Geology Report 31, Geological Survey of Canada, Ottawa, Canada, pp 435–478 Coker WB, Kettles IM, Shilts WW (1995) Comparison of mercury concentrations in modern lake sediments and glacial drift in the Canadian Shield in the region of Ottawa/Kingston Acknowledgements We thank Rick Keevil, Chris Marmont and to Georgian Bay, Ontario, Canada. Water Air Soil Pollut, Jenny Reed (preliminary survey) and J. McAndrews, Z. Zu and 80 : 1025–1029 Helena Karam (follow-up survey) for their valuable assistance Dunnette DA (1988) Assessment of health risk from lithogenic in sample collection and preparation. We also thank Bernie mercury (abstract). In: Book of Abstracts, 196th ACS National Neary (Ontario Ministry of Environment and Energy) for proMtg., Los Angeles, USA. PB: American Chemical Society viding the smallmouth bass data, Lindsay Milton for his assistWashington, D.C. ance in calculating watershed-to-lake area ratios, Susan Davis Engstrom DR (1987) Influence of vegetation and hydrology on for drafting, and Melody Myers for proofreading. Palynological the humus budgets of Labrador lakes. Can J Fish Aquat Sci analysis was performed by J. McAndrews, Department of Bota44 : 1306–1314 ny, Royal Ontario Museum, and carbon-14 dating was perform- Evans RD (1986) Sources of mercury contamination in the seed by H. Melville, Brock University Radiocarbon Laboratory. diments of small headwater lakes in south-central Ontario. The preliminary survey was conducted as part of P.R.’s PhD Arch Environ Contam Toxic 15 : 505–512 thesis which was co-supervised by Jerome Nriagu and Sherry Farmer JG (1991) The perturbation of historical pollution reSchiff, and funded by the Ontario Ministry of Environment and cords in aquatic sediments. Environ. Geochem. Health, Energy, a NSERC Strategic Grant (Pam Welbourn, P.I.), the On13 : 76–83 tario Ministry of Northern Development and Mines, and two Fernandez MA, Martinez L, Segarra M, Garcia JC, FerOntario Graduate Scholarships. The follow-up survey was jointran E (1992) Behaviour of heavy metals in the combustion ly funded by Natural Resources Canada and Ontario Hydro gases of urban waste incinerators. Environ Sci Technol (IPP 860033). Sincere thanks go to Peter Friske, Bob Garrett, 26 : 1040-1047 and Gwendy Hall for their thoughtful review of an earlier draft, Fortescue JAC (1986) Geochemical stratigraphy of organic and to Stephen Cook and another anonymous reviewer for their lake sediments from selected lakes north and east of Lake Suvaluable comments on the final manuscript. perior. Ontario Geological Survey Map 80757 (Geochemical Series), Compiled 1985, Ontario Geological Survey, Toronto, Canada Fortescue JAC (1988) The use of geochemistry of long lake sediment cores for verification of regional geochemical results; Goudreau Lake area, District of Algoma. In: Summary of Field Work and Other Activities 1987, Miscellaneous Paper Azzaria LM, Habashi F (1976) Mercury pollution - an exami137, Ontario Geological Survey, Toronto, Canada, pp 482-488 nation of some basic issues. CIM Bulletin 69 : 101–107 Friske PWB (1995) Effects of limnological variation on element Bjorklund I, Borg H, Johansson K (1984) Mercury in Swedistribution in lake sediments from Tatin Lake, central Britdish lakes - its regional distribution and causes. Ambio ish Columbia - implications for the use of lake sediment data 13 : 118–121 in exploration and environmental studies. In: Current ReBloomfield JA, Quinn SO, Scrudato RJ, Long D, Richards search 1995-E. Geological Survey of Canada, Ottawa, Canada, A, Ryan F (1980) Atmospheric and watershed inputs of merpp 59–67 cury to Cranberry Lake, St. Lawrence County, New York. EnFriske PWB, Coker WB (1995) The importance of geological viron Sci Res 17 : 175–210 controls on the natural distribution of mercury in lake and Bonham-Carter GF, Rasmussen PE, Rencz AN, Sangster stream sediments across Canada. Water Air Soil Pollut DF (1996) Mass-balance model for mercury applied to Hunts80 : 1047–1051 ville lakes: a GIS study. In: Proceedings of 1995 Canadian Mercury Network Workshop, September 29–30, York Univer- Galloway JN, Thornton JD, Norton SA, Volchok HL, Mclean RAN (1982) Trace metals in atmospheric deposition: sity, Toronto, Canada, EMAN Occasional Paper Series, Report a review and assessment. 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The study and monitoring ter, Air Soil Pollut., 80 : 13–19 of mercury in the Churchill River Diversion. Environment Hecky RE, Bodaly RA, Ramsey DJ, Strange NE (1987a) EnCanada Inland Waters Directorate. Winnipeg, Manitoba, Canhancement of mercury bioaccumulation in fish by flooded ada terrestrial materials in experimental ecosystems. In: CanaCarter TR, Colvine AC, Meyn HD (1980) Geology of base da-Manitoba Agreement on the Study and Monitoring of metal, precious metal, iron, and molybdenum deposits in the Mercury in the Churchill River Diversion. Environment CanaPembroke-Renfrew Area. Ontario Geological Survey Mineral da Inland Waters Directorate, Winnipeg, Manitoba, Canada. Deposits Circular 20, Ontario Ministry of Natural Resources, Technical Appendix to the Summary Report Toronto, Canada Hecky RE, Bodaly RA, Strange NE, Ramsey DJ, Anema C, Coker WB, Hornbrook EHW, Cameron EM (1979) Lake seFudge, RJP (1987b) Mercury bioaccumulation in yellow

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