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Map of the English-Wabigoon River system showing the location of the four sampled lakes (Clay, Ball, Separation and Tetu).

Map of the English-Wabigoon River system showing the location of the four sampled lakes (Clay, Ball, Separation and Tetu).

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The English-Wabigoon River system in Northwestern Ontario, Canada, was one of the most heavily mercury-contaminated waterways in the world due to historical discharges in the 1960s from a chlor-alkali plant. This study examines long-term (1970-2010) monitoring data to assess temporal trends in mercury contamination in Walleye, Northern Pike and Lak...

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Context 1
... a variety of fish species from lakes within the river system for mercury content. From the 1970s to the late 1980s, fish were sampled on an annual basis, and then either every two, three or five years until the most recent sampling event in 2010. Samples were most consistently taken from four representative lakes: Clay, Ball, Separation and Tetu (Fig. 1). Clay Lake, the closest to the historical point-source of mercury, is approximately 87 km downstream of Dryden, followed by Ball Lake ($50 km down- stream from Clay), then Separation ($50 km from Ball) and Tetu ($65 km from Separation). Walleye, Northern Pike and Lake Whitefish were sampled from 1970 to 2010 in Clay Lake, and from 1974 ...
Context 2
... In general, initial concentrations in the early 1970s for Walleye were higher than Northern Pike (6-14%) and Lake Whitefish (63-94%) in all four lakes. The raw data for each size class showed either immediate declines or slight initial increases in mercury concentrations, followed by overall declines to 2010, with annual or biannual variability (Fig. S1 ...

Citations

... Another program assessed Hg concentrations in umbilical cord blood collected at the local hospital [3,4]. Biomarker Hg concentrations from Grassy Narrows First Nation followed a similar pattern to Hg levels in local fish from the contaminated river system, with extremely high concentrations in the early 1970's, a sharp decline until 1977 and a less pronounced decline until 1987, after which, mean concentrations remained relatively stable [5,6]. ...
... In this community, fish, especially walleye, is the source of Hg exposure [4,5]. Table 2 contains the responses to the GN-CHA questions on past and current fish consumption frequency. ...
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Background The watershed in Asubpeeschoseewagong Netum Anishinabek (Grassy Narrows First Nation) territory has been contaminated by mercury (Hg) since 1962, resulting in very high Hg concentrations in fish, central to the community’s culture, traditions, economy and diet. Biomarkers of Hg exposure (umbilical cord blood and hair/blood samples), monitored between 1970 and 1997, decreased over time. A recent Grassy Narrows Community Health Assessment (GN-CHA) survey included current symptoms of nervous system dysfunction. The present study aimed to cluster self-reported symptoms and examine their associations with past Hg exposure. Methods The GN-CHA included 391 adults. Symptom clustering used a two-step segmentation approach. Umbilical cord Hg and/or yearly measurements of equivalent hair Hg were available for 242 participants. Structural Equation Models (SEM) displayed the associations between Hg exposure and clusters, with Hg exposure modelled as a latent variable or in separate variables (prenatal, childhood and having had hair Hg ≥ 5 μg/g at least once over the sampling period). Longitudinal Mixed Effects Models (LMEM) served to examine past hair Hg with respect to clusters. Results A total of 37 symptoms bonded into 6 clusters, representing Extrapyramidal impairment, Sensory impairment, Cranial nerve disturbances, Gross motor impairment, Neuro-cognitive deficits and Affect/Mood disorders. Median Hg concentrations were 5 μg/L (1–78.5) and 1.1 μg/g (0.2–16) for umbilical cord and childhood hair, respectively. More than one-third (36.6%) had hair Hg ≥ 5 μg/g at least once. In SEM, latent Hg was directly associated with Extrapyramidal and Sensory impairment, Cranial nerve disturbances and Affect/Mood disorders. Direct associations were observed for prenatal exposure with Affect/Mood disorders, for childhood exposure with Extrapyramidal impairment and Cranial nerve disturbances, and for hair Hg ≥ 5 μg/g with Extrapyramidal and Sensory impairment. For all clusters, a further association between past Hg exposure and symptom clusters was mediated by diagnosed nervous system disorders. LMEM showed higher past hair Hg among those with higher scores for all clusters, except Affect/Mood disorders. Conclusion Our findings provide evidence that in this First Nation community, past Hg exposure from fish consumption was associated with later-life clusters of coexisting symptoms of nervous system dysfunction.
... Between 1962 and 1969 the Dryden Chemical Company located in Dryden, northwestern Ontario, discharged approximately 10 tonnes of mercury from a chlor-alkali plant into the Wabigoon River ( Fig. 1; Armstrong and Hamilton 1973). In 1974, mercury contamination of this river and lake system was found in fish as far as Tetu Lake, 250 km downstream of Dryden (Neff et al. 2012). During 1972 in the east basin of Clay Lake, which is the first lake (86 km) downstream of Dryden, surface sediment and water mercury concentrations were >6000 ng/g and 120 ng/L, respectively (Armstrong and Hamilton 1973). ...
... More recent studies of sediment and fish mercury concentrations in Clay Lake (Lockhart et al. 2000;Kinghorn et al. 2007;Neff et al. 2012) found that recovery was initially rapid for both sediments and fish. However, since the early 1980s, sediments have declined much more slowly and walleye mercury concentrations in muscle have plateaued at about 2 μg/g, wet weight, 4-and 10-fold higher than commercial and subsistence consumption guidelines, respectively. ...
... Some natural recovery in the sediments has occurred during the intervening decades, with the rate being rapid at first and then slowing to a very low rate. Fish also recovered rapidly at first but have not decreased since the mid 1980s (Fig. 8b, Neff et al. 2012). The divergence of fish recovery from sediment recovery is likely linked to the continuing inputs of MeHg to Clay Lake from upstream. ...
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Between 1962 and 1969, 10 tonnes of mercury were discharged from a chlor-alkali plant in Dryden, Ontario, to the English–Wabigoon River. Present-day fish mercury concentrations are amongst the highest recorded in Canada. In 2017, the Grassy Narrows Science Team found no evidence of ongoing discharges from the plant site to the river water, even though large quantities of mercury remain at the site. Instead, our data suggest that ongoing erosion of high mercury particles by the river, as it meanders through contaminated floodplains, is responsible for present-day transport of mercury to Clay Lake and to Ball Lake, located 154 km downstream. In Clay Lake, surface sediment total mercury concentrations and inflow water concentrations are still about 15 times above background (86 km downstream), and in Ball Lake mercury concentrations in sediments appeared to be still increasing. The remobilization of legacy inorganic mercury from riverbank erosion between Dryden and Clay Lake stimulates methyl mercury production there, in Clay Lake, and in Ball Lake. The large quantities of methyl mercury produced between Dryden and Clay Lake are mostly dissolved in water and are swept downstream, elevating concentrations in water and biota throughout the system. Several options for remediating the ongoing contamination are discussed.
... • Blood mercury in Penobscot River marsh birds (3 -8 μg/g ww, GM) were 6x to 10x greater than at Maine reference sites Estuarine wetlands provide critical foraging and nesting habitat for breeding marsh birds (Correll et al., 2016;Craig and Beal, 1992;Shriver et al., 2004), yet the geochemistry of wetland sediments can enhance mercury methylation and so increase avian exposure to mercury (Hg; Hall et al., 2008;Shriver et al., 2006). Anthropogenic releases of Hg have tripled with industrial development (Lorey and Driscoll, 1999), increasing aerial deposition across entire continents (Driscoll et al., 2007) and creating localized hotspots near industrial dischargers (Eggleston, 2009;Neff et al., 2012). When inorganic Hg accumulates in wetland habitat, bacteria in anoxic marsh sediments transform inorganic mercury (Hg +2 ) into methyl Hg (Gilmour et al., this issue;Merritt and Amirbahman, 2009), which biomagnifies in the food web (Wiener et al., 2003;Eagles-Smith et al., 2016). ...
Article
Mercury (Hg) concentrations in the blood and feathers of five species of migratory marsh birds, Nelson's sparrow (Ammodramus nelson subvirgatus), song sparrow (Melospiiza melodia), swamp sparrow (Melospiza geogiana), red-winged blackbird (Agelaius phoeniceus), and Virginia rail (Rallus limicola), breeding in marshes along the lower Penobscot River, Maine, far exceeded reference concentrations, exceeded concentrations associated with reproductive health, and are the highest Hg concentrations reported to date for several species. Blood Hg concentrations in adult Nelson's sparrows were greatest in 2007, at 6.6 μg/g ww (geometric mean) and in 2012, at 6.5 μg/g ww and greatest in red-winged blackbirds in 2012, 8.0 μg/g ww. Mercury in blood increased with residence time on the contaminated marshes at an estimated rate of 0.04 to 0.07 μg/g ww per day. Feather mercury concentrations in specific primary, secondary and tail feathers (P1, S2, R6) were strongly associated with exposure location at the time of feather formation. Geometric mean Hg concentrations in primary feathers (P1) reached 39.6 μg/g fw in 2010 in Nelson's sparrows. The paper documents the dynamic nature of Hg concentrations in avian blood and feathers, an important consideration in contaminant study design, and the increased risk to marsh birds posed by Hg deposition from upstream sources.
... Localized discharges from mercury-cell chlor-alkali plants are among the greatest point sources of Hg to aquatic systems (Bloom et al., 1999;Fimreite et al., 1971). While this production method has been largely discontinued, past discharges continue to pose a threat to aquatic systems long after plant operations have ceased (Munthe et al., 2007;Neff et al., 2012). ...
Article
Mercury (Hg) concentrations in aquatic biota, including fish and shellfish, were measured over the period 2006 – 2012 in the lower Penobscot River and upper estuary (Maine, USA). The Penobscot is a system contaminated with Hg by a chlor-alkali plant that operated from 1967 to 2000, discharging 6 – 12 tons of mercury into the river. Mercury levels in aquatic biota were highest at sites downstream of the chlor-alkali plant and spatial trends were similar to those of sediments. Mean total Hg concentrations in fish muscle (adjusted for size or age) in the most affected areas were 521 (480, 566; 95% CI) ng/g ww in American eels, 321 (261,395) in mummichog, 121 (104, 140) in rainbow smelt, 155 (142,169) in tomcod, 55.2 (42.7,71.4) in winter flounder, and 328 (259,413) in American lobster tail and 522 (488,557) ng/g dw in blue mussel. Levels exceeded the 50 ng/g ww considered protective for piscivorous predators and were of concern for human health, with American eels and American lobster exceeding Maine’s mercury action level of 200 ng/g ww. Calculations of trophic position (using nitrogen isotopes) suggested that the spatial patterns observed in total Hg concentrations were not due to changes in feeding habits of the species. Fish feeding in benthic food webs, as defined by stomach content and stable carbon isotope analyses, showed no change in Hg concentrations over time. In contrast, declining trends in Hg were found in two species dependent on pelagic food webs. The absence of declines in Hg concentrations in the benthically-based food webs, despite the fact that most Hg was discharged into the system more than 40 years ago, is consistent with the long recovery predicted from dated sediment cores and from similar studies elsewhere.
... Data pertaining to lakes downwind and downslope from major anthropogenic American Journal of Climate Change Hg release locations (e.g. [34]) are not part of this analysis. ...
... Skinless boneless dorsal fillets were taken and stored at − 20°C. Total mercury analysis using MOECC protocols were performed, including acid digestion and cold vapour flameless atomic absorption spectroscopy as described by Bhavsar et al. (2010) and Neff et al. (2012). We considered top predator fish because they often exhibit higher Hg levels than lower trophic level fish due to biomagnification (Kamman et al., 2005). ...
... For example, northwestern Ontario has shown more extensive decreases in fish Hg level historically compared to other regions. This is in response to the highly polluted conditions of lakes found in this watershed, including Wabigoon Lake, Clay Lake, Tetu Lake, Ball Lake, and Separation Lake (Neff et al., 2012), prior to the implementation of stricter policies on industrial emissions. From the late 1800s to early 1900s, small local gold mines near Dryden and Kenora may have contributed to elevated levels of Hg found in this area until 1920, in which mercury amalgamation was phased-out (Bruce, 1925;Pirrone et al., 1998). ...
Article
Mercury (Hg) levels in Ontario top predator fishes have been increasing in recent decades. These increases may be a result of many additive factors, including global climate change. Only recently has research been conducted on how climate change may impact Hg levels in freshwater fishes at large-scales. We examined the relationship between Hg trends and (1) local weather, (2) large-scale climate drivers, and (3) anthropogenic Hg emissions, in native cool water (walleye and northern pike) and warm water (smallmouth bass and largemouth bass) predatory fishes in Ontario, Canada, for historical (1970-1992) and recent (1993-2014) time periods. For each fish species studied, > 25% of Ontario's secondary watersheds shifted from historically declining to recently increasing fish Hg trends, and ≥ 50% of watersheds experienced increasing trends between 1993 and 2014. Recent fish Hg increased at up to 0.20µg/g/decade; which were significant (p < 0.05) for walleye, northern pike and smallmouth bass. Multiple linear regressions revealed a complex interplay of local weather, large-scale climate drivers, and anthropogenic Hg emissions influencing fish Hg levels. Recent Hg levels for walleye and largemouth bass increased with changes in global climate drivers, while higher precipitation influenced smallmouth bass Hg levels the most. Walleye Hg levels increased during the positive phases of global climate drivers, reflecting the local influence of local temperatures and precipitation indirectly. Differentiating the effects of climate-related parameters and emissions is increasingly crucial to assess how changing multiple environmental stressors may impact health of wildlife and humans consuming fish.
... The mercury discharges from the Dryden facility were particularly impactful on the First Nations reserves of Grassy Narrows and Wabaseemong situated on the English-Wabigoon river system where approximately 10 metric tonnes of inorganic mercury was discharged into the river system in the 1960's, prompting consumption and sport-fishing bans on locally caught fish due to elevated levels of MeHg of up to 20 mg/Kg wet weight ( Kinghorn et al., 2007). Concentrations in top predatory species were within the ranges reported in species sampled from well-known, highly contaminated water systems such as Minamata Bay in Japan, ( Neff et al., 2012), and biomonitoring data from community members reflecting elevated blood MeHg levels of up to 323 μg/L ( Wheatley et al., 1997). Since the 1970's, mercury concentrations in regional fish have declined in the areas where this legacy point-source pollution occurred ( Kinghorn et al., 2007;Neff et al., 2012;Weis, 2004), as have concentrations in biomonitoring data ( Paradis, 1996, 1995). ...
... Concentrations in top predatory species were within the ranges reported in species sampled from well-known, highly contaminated water systems such as Minamata Bay in Japan, ( Neff et al., 2012), and biomonitoring data from community members reflecting elevated blood MeHg levels of up to 323 μg/L ( Wheatley et al., 1997). Since the 1970's, mercury concentrations in regional fish have declined in the areas where this legacy point-source pollution occurred ( Kinghorn et al., 2007;Neff et al., 2012;Weis, 2004), as have concentrations in biomonitoring data ( Paradis, 1996, 1995). Historic mercury biomonitoring data in some First Nations populations has been collected since the 1970's, and on aggregate has shown a decline in exposures ( Wheatley and Paradis, 1995). ...
Article
Methyl Mercury (MeHg) exposure is a global environmental health concern. Indigenous peoples around the world are susceptible to MeHg exposure from often higher fish consumption compared to general populations. The objective of this study was to estimate dietary exposure to methylmercury (MeHg) among First Nations living on-reserve in the province of Ontario, Canada. A total diet study was constructed based on a 24-h recall from the First Nations Food, Nutrition, and Environment Study (FNFNES), and measured contaminant concentrations from Health Canada for market foods, and FNFNES for traditional foods. A probabilistic assessment of annual and seasonal traditional food consumptions was conducted for 1429 adult participants. Results were compared to exposures in the general Canadian population and reference values from Health Canada for adults and women of childbearing age (ages 19–50). Results indicated traditional foods to be the primary contributor to the dietary total MeHg intake (72%). The average dietary total MeHg exposure in the First Nations population in Ontario (0.039 μg/kg/d) was 1.6 times higher than the general Canadian population; however, the majority (97.8%) of the population was below the reference values. Mercury concentrations in participants’ hair samples (n = 744) ranged from 0.03 to 13.54 µg/g, with an average of 0.64 µg/g (geometric average of 0.27 µg/g). Less than 1% of the population had a hair mercury value above the 6 µg/g level, and 1.3% of women of child bearing age had values greater than 2 µg/g. Fish species contributing to the MeHg intake included pickerel-walleye, pike, perch and trout. Only 7.9% of the population met the recommended fish consumption rate of two, 3.5 oz servings per week from the American Heart Association. Therefore, consumption of lower trophic level fish can be promoted to provide the maximum nutritional benefit with minimal risk of MeHg exposure.
... Another example of Minamata disease, and the cat dancing disease connected with anthropogenic mercury pollution coming from the local chlor-alkali plant, was found in the English-Wabigoon River system in Northwestern Ontario (Canada) in 1970. [61,62]. The brains of two cats from a village next to the English River, which had been fed fish from the river, had Hg levels at 16.4 and 6.9 mg Hg/kg. ...
... Strom [68] suggests that as the point-source discharges have now been eliminated or greatly reduced, the decreased level of THg in the river otters observed by him was likely the result of regional or geological factors. However, as shown in other papers on North American river otter, mink, bats, and other wildlife and fish from the Swedish lakes, a few decades after the cessation of Hg pollution for industrial plants and the use of organic Hg compounds in agriculture, the residues still affect vertebrates and invertebrates that prey on animals that live constantly in water or periodically, such as amphibians and some insects [61,86,87]. ...
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Mercury (Hg), lead (Pb) and cadmium (Cd) are involved in mammalian brain damage. However, little is known about Pb and Cd brain levels in wildlife that reflect the geochemical background. The aims of the study include the estimation of Hg, Pb and Cd concentrations, and the determination of relationships between these elements in the brains of 94 mesocarnivores. Road-killed or hunted animals were obtained from north-western Poland near the Polish-German border. The investigation covered the native Eurasian otter Lutra lutra, badger Meles meles, pine marten Martes martes, beech marten M. foina, European polecat Mustela putorius, red fox Vulpes vulpes, and alien species: feral and ranch American mink Neovison vison, raccoon Procyon lotor and raccoon dog Nyctereutes procyonoides. Depending on the diet and environmental pollution, the carnivore brains accumulated toxic metals in varying amounts. The highest median Hg levels (in mg/kg dry weight, dw) were found in the piscivorous Eurasian otter and feral mink (2.44 and 3.96), Pb in the omnivorous raccoon (0.47), while Cd in minks (~0.06). We indicated that Pb-based ammunition is a significant source of the element in scavengers from hunting area, and we also found a significant correlation between Pb and Cd levels in the fox brain. Finally, this study is the first to suggest background levels for brain Pb and Cd in mesocarnivores (<0.50 and <0.04 mg/kg dw, respectively).
... heterodon), brassy minnow (Hybognathus hakinsoni), and yellow perch (Perca flavescens). Muscle [Hg] in pike and walleye of a given size within a given geographic location is relatively robust to seasonal variation and short-term annual cycles (approximately 1-2 % change/year Rasmussen et al. 2007;Neff et al. 2012); therefore, samples were pooled from summer collections in 2013 and 2014 for this analysis. Only prey species collected in summer 2013 were considered for analysis in an attempt to minimize seasonal influences over the whole body [Hg] in these fish (Korthals and Winfrey 1987;Greenfield et al. 2013). ...
Article
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Mercury (Hg) contamination can pose risks to human and animal health as well as commercial fisheries. Reservoir construction in riverine systems produces flooded conditions amenable to Hg(II)-methylating bacteria, which can transform this relatively benign environmental contaminant into the bioaccumulative, environmentally relevant, and neurotoxic methyl-Hg (MeHg). Hg concentrations ([Hg]) in fishes from reservoirs can take decades to decrease to pre-dam levels, but less is known about Hg exported downstream and its dynamics within downstream fish populations. We examined and compared the multidecadal rates of biotic [Hg] decrease and contemporary factors affecting [Hg] in fish collected from a hydroelectric reservoir (Tobin Lake) and a related downstream fishery (Cumberland Lake) along the Saskatchewan River, Canada. Rates of [Hg] decrease were considered in four species—northern pike (Esox lucius), sauger (Sander canadensis), goldeye (Hiodon alosoides), and walleye (S. vitreus)—all of which showed a significant decrease over time (p < 0.001) and are now lower than Health Canada consumption guidelines (0.5 μg/g). Rates of decrease ranged from 0.5 to 3.9 %/year and were similar between sites in the cases of northern pike and sauger. Contemporary factors affecting [Hg] in walleye collected downstream include fish length (p < 0.001), fish age (p < 0.001), and trophic magnification through the food web (p < 0.001), and relationships between [Hg] and trophic level in predatory and prey fish are now similar to those found in non-Hg-inundated systems at a similar latitude. Together, these results suggest connected contamination between the two sites and delineate the timeline during which [Hg] in a variety of fish species decreased to nontoxic levels in both locations.
... In 1962 a chlor-alkali plant was built at Dryden. Liquid Hg(0) was initially employed in the chlor-alkali process, with the result that the river system became severely polluted with Hg discharged from the plant (Armstrong and Hamilton, 1973;Jackson, 1980;Woychuk, 1980a, 1981;Jackson et al., 1980a,b;1982;Rudd et al., 1983;Parks and Hamilton, 1987;Parks, 1988;Parks et al., 1989;Neff et al., 2012). From 1962 to 1970 the Hg pollution proceeded unchecked; but restrictions were imposed in 1970, and in 1975 the pollution virtually ceased because the chlor-alkali process employing Hg(0) for Cl and NaOH manufacture was discontinued (Jackson et al., , 1982Jackson and Woychuk, 1980a;Parks and Hamilton, 1987;Neff et al., 2012), although small quantities of Hg (amounting to~1% of the uncontrolled discharges) continued to leak into the river Parks et al., 1980). ...
... Liquid Hg(0) was initially employed in the chlor-alkali process, with the result that the river system became severely polluted with Hg discharged from the plant (Armstrong and Hamilton, 1973;Jackson, 1980;Woychuk, 1980a, 1981;Jackson et al., 1980a,b;1982;Rudd et al., 1983;Parks and Hamilton, 1987;Parks, 1988;Parks et al., 1989;Neff et al., 2012). From 1962 to 1970 the Hg pollution proceeded unchecked; but restrictions were imposed in 1970, and in 1975 the pollution virtually ceased because the chlor-alkali process employing Hg(0) for Cl and NaOH manufacture was discontinued (Jackson et al., , 1982Jackson and Woychuk, 1980a;Parks and Hamilton, 1987;Neff et al., 2012), although small quantities of Hg (amounting to~1% of the uncontrolled discharges) continued to leak into the river Parks et al., 1980). By that time, however, major ecological damage had been done: The river system had been polluted with~10 ± 1 tonnes of Hg, most of which had accumulated in fine-grained sediments, and this had led to a huge upsurge in microbial CH 3 Hg þ production (Jackson, 1980;Jackson and Woychuk, 1980a;Jackson et al., 1982;Rudd et al., 1983;Parks and Hamilton, 1987;Neff et al., 2012) (and Cl À ions from the chlor-alkali plant may have exacerbated the problem by solubilising inorganic Hg(II), thereby increasing its availability for methylation (Jackson et al., 1982;Jackson, 1998a,b)). ...
... From 1962 to 1970 the Hg pollution proceeded unchecked; but restrictions were imposed in 1970, and in 1975 the pollution virtually ceased because the chlor-alkali process employing Hg(0) for Cl and NaOH manufacture was discontinued (Jackson et al., , 1982Jackson and Woychuk, 1980a;Parks and Hamilton, 1987;Neff et al., 2012), although small quantities of Hg (amounting to~1% of the uncontrolled discharges) continued to leak into the river Parks et al., 1980). By that time, however, major ecological damage had been done: The river system had been polluted with~10 ± 1 tonnes of Hg, most of which had accumulated in fine-grained sediments, and this had led to a huge upsurge in microbial CH 3 Hg þ production (Jackson, 1980;Jackson and Woychuk, 1980a;Jackson et al., 1982;Rudd et al., 1983;Parks and Hamilton, 1987;Neff et al., 2012) (and Cl À ions from the chlor-alkali plant may have exacerbated the problem by solubilising inorganic Hg(II), thereby increasing its availability for methylation (Jackson et al., 1982;Jackson, 1998a,b)). As a result, fish and other organisms in the system were heavily contaminated with CH 3 Hg þ , and the skeletal muscle tissues of fish came to have Hg concentrations far in excess of the legal limit for edible fish marketed in Canada (0.5 ppm), rendering them unfit for human consumption and posing a threat to fish-eating wildlife (Armstrong and Hamilton, 1973;Annett et al., 1975;Bishop and Neary, 1976;Parks and Hamilton, 1987;Parks, 1988;Kinghorn et al., 2007;Neff et al., 2012). ...
Article
The Wabigoon River (Ontario, Canada) was affected by dams starting in 1898 and was polluted with pulp and paper mill wastes starting in 1913 and mercury from a chlor-alkali plant from 1962 to 1975. A dated sediment core from a riverine lake was analysed to investigate resultant changes in the biogeochemistry of mercury as revealed by variations in mercury isotope ratios and sediment chemistry. A total mercury maximum formed by the mercury pollution coincided with minimums in the δ-values of the 198Hg/202Hg, 199Hg/202Hg, 200Hg/202Hg, and 201Hg/202Hg ratios, and the δ-values decreased in the order δ201Hg > δ200Hg > δ199Hg > δ198Hg. Thus, mass-dependent fractionation caused depletion in lighter isotopes, implying evaporation of Hg(0) and pollution of the atmosphere as well as the river-lake system. Concurrently, mass-independent fractionation caused 199Hg enrichment, possibly reflecting an independently documented upsurge in methylmercury production, and 201Hg depletion, suggesting removal of methylmercury with anomalously high 201Hg/199Hg ratios by aquatic organisms and accumulation of 201Hg-depleted inorganic Hg(II) in sediments. The δ201Hg/δ199Hg ratio rose abruptly when mercury pollution began, reflecting the resultant increase in methylmercury production, and remained high but gradually declined as the pollution abated, paralleling trends shown by methylmercury in aquatic organisms. The δ201Hg/δ199Hg ratio of pre-1962 background mercury increased ca. 1898 and ca. 1913–1929, suggesting accelerated methylmercury production due to stimulation of microbial activities by the damming of the river and the input of pulp and paper mill wastes, respectively. Other variations were linked to economic and technological factors that affected pulp and paper manufacture.