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 Wabigoon-English 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 in Clay Lake routinely exceeded 3 µg/g (ppm). Field studies in 1978–1981 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.