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Journal of Environmental Quality Abstract - Heavy Metals in the Environment

Nitrate Controls Methyl Mercury Production in a Streambed Bioreactor


This article in JEQ

  1. Vol. 40 No. 5, p. 1586-1592
    Received: Mar 1, 2011

    * Corresponding author(s): wroberts@sciborg.uwaterloo.ca
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  1. Rita Shiha,
  2. William D. Robertson *a,
  3. Sherry L. Schiffa and
  4. David L. Rudolpha
  1. a Dep. of Earth and Environmental Sciences, Univ. of Waterloo, 200 University Ave. W., Waterloo, ON N2L 3G1, Canada. Assigned to Associate Editor Dan Kaplan


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.

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