Accelerating recovery of the mercury-contaminated Wabigoon/English River system

Abstract

Mercury levels (essentially methyl mercury -MeHg) in sportfish in a 250 km section of the WabigoonEnglish River system remain seriously elevated as a result if the discharge of approximately 10 tonnes of inorganic Hg from a chlor-alkali plant at Dryden, Ontario, Canada which occurred primarily between 1962 and 1970. The discharges resulted in elevated mercury concentrations in water, sediments and biota. For example, Hg in adult Northern Pike inClay Lake routinely exceeded 3/~g/g (ppm). Field studies in 1978-81 suggest that partitioning of inorganic and MeHg between surface sediment, water and suspended particles occurs within days. MeHg levels in water were partitioned with total (essentially inorganic) Hg. Temperature affects both Hg and MeHg levels in water; concentrations fluctuated seasonally by an order of magnitude at some sites. Hg in contaminated surface sediments is almost certainly the primary source of the mercury now entering the water and biota in this contaminated watercourse. Mercury levels in biota decline less dramatically with distance downstream of Dryden than mercury concentrations in sediments. Natural erosion, resuspension and sedimentation processes have helped to reduce the amount of mercury in the active layer at the sediment/water interface and the most effective means of accelerating the recovery of the system will probably involve measures to accelerate these natural processes. Enclosure experiments, regional surveys and geochemical studies all provide evidence that the biological uptake of upstream anthropogenic Hg loadings at any given site would likely be reduced dramatically by the continuous addition of very modest quantities of pristine clay sediment. The quantities contemplated, when resuspended, would result in suspended solids concentrations on the order of 15-25 ppm, a value higher than for most shield waters but well within the range of many other productive watercourses in North America. The ability to mitigate local sources and ameliorate the adverse biological effects of anthropogenic loadings from upstream sources by resuspension of clean clay sediments permits 'targeting' of sites for restoration and opens a wide array of ameliorative options. The authors believe that some of these options would be more effective and less costly than other restoration procedures commonly considered such as dredging and on land disposal of contaminated sediment.

Effect of Loading Rate on the Fate of Mercury in Littoral Mesocosms

“…Other loss mechanisms included partitioning to particles and sediments and, to a lesser extent, to periphyton. Spike Hg partitioned to main environmental compartments within days to weeks, which is also in agreement with previous studies ( , ).…”

Mercury concentration in fish from Piracicaba River (Minas Gerais, Brazil)

“…In aquatic ecosystems, pH values between 4.7 and 6.0 have favorable physic-chemical characteristics to the Hg methylate. It increases the Hg diffusion of the sediment in the water column and from this to the biota, where it finishes by accumulating high concentration in the organisms on top of the food chain, such as the fish (Phillips et al 1987;Rada et al 1993;Akagi et al 1979;Parks and Hamilton 1987). In the present study, the water pH at the sampling sites was practically constant as it slightly revealed acid to neutral, which coincided with the one obtained from the studies made by Oberdá in 1996(Oberdá 1996.…”

Mercury in the Northern Crayfish, Orconectes virilis (Hagen), in New England, USA

“…Mercury concentrations exhibited high variability in the crayfish Orconectes virilis across three New England states, and half of the sites (14 of 28 or 50%) had representative animals with mercury levels at or above expected background concentrations ( £ 0.1 lg/g; Parks and Hamilton, 1987). This background concentration represents mercury levels detected in crayfish from isolated water bodies located in pristine habitats of Canada with geology and vegetation similar to our study areas.…”

“…Methylmercury halflives within abdominal muscle are very long, leading to bioaccumulation within individuals, and MeHg may account for >75% of the total mercury found in crayfish (Headon et al, 1996). In northeastern North America, background mercury concentrations in crayfish tail muscle are expected to be £ 0.1 lg/g based on surveys of natural lakes in remote Canadian locations removed from any potential point-source contamination (Parks and Hamilton, 1987).…”