Nitrate controls methyl mercury production in a streambed bioreactor

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 (NO?) in an agricultural drainage ditch in southern Ontario (Avon site). The reactor provides 30 to 100% removal of NO?-N concentrations of 0.6 to 4.4 mg L(-1), but sulfate (SO?(2-)) reducing conditions develop when NO? 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 NO?(-) reduction (or denitrification) ( = 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 SO?(2-) reducing conditions were present ( = 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 SO?(2-) reducing conditions develop; however, maintaining NO?-N concentrations > 0.5 mg L suppresses the production of MeHg.