Nitrate Controls Methyl Mercury Production in a Streambed Bioreactor
- Rita Shiha,
- William D. Robertson *a,
- Sherry L. Schiffa and
- David L. Rudolpha
Organic carbon bioreactors provide low-cost, passive treatment of a variety of environmental contaminants but can have undesirable side effects in some cases. This study examines the production of methyl mercury (MeHg) in a streambed bioreactor consisting of 40 m3 of wood chips and designed to treat nitrate (NO3−) in an agricultural drainage ditch in southern Ontario (Avon site). The reactor provides 30 to 100% removal of NO3−–N concentrations of 0.6 to 4.4 mg L−1, but sulfate (SO42−) reducing conditions develop when NO3− removal is complete. Sulfate reducing conditions are known to stimulation the production of MeHg in natural wetlands. Over one seasonal cycle, effluent MeHg ranged from 0.01 to 0.76 ng L−1 and total Hg ranged from 1.3 to 3.4 ng L−1. During all sampling events when reducing conditions were only sufficient to promote NO3− reduction (or denitrification) (n = 5, late fall 2009, winter 2010), MeHg concentrations decreased in the reactor and it was a net sink for MeHg (mean flux of −5.1 μg m−2 yr−1). During all sampling events when SO42− reducing conditions were present (n = 6, early fall 2009, spring 2010), MeHg concentrations increased in the reactor and it was a strong source of MeHg to the stream (mean flux of 15.2 μg m−2 yr−1). Total Hg was consistently removed in the reactor (10 of 11 sampling events) and was correlated to the total suspended sediment load (r2 = 0.69), which was removed in the reactor by physical filtration. This study shows that organic carbon bioreactors can be a strong source of MeHg production when SO42− reducing conditions develop; however, maintaining NO3−–N concentrations > 0.5 mg L−1 suppresses the production of MeHg.Please view the pdf by using the Full Text (PDF) link under 'View' to the left.
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