Methylmercury Production in Denitrifying Woodchip Bioreactors

Abstract

Several operational woodchip bioreactors were installed at the outlets of agricultural drainage systems located in east central Illinois. The potential for monomethylmercury (MMHg) production and export in these bioreactors was investigated from summer 2008 to summer 2010. The basic approach was to compare the chemistry of simultaneously-collected bioreactor inlet and outlet water samples in order to assess the extent of nitrate depletion, consumption of sulfate, and production of MMHg, plus other low-charge mercury species (LCHg). In making such a comparison, we implicitly assume that the reactor is near steady state, which is a reasonable approximation given hydraulic residence times on the order of hours. All mercury (Hg) speciation measurements were made using a first-generation mercury thiourea complex ion chromatography system for Hg speciation analysis, which reliably separates MMHg and HgII (mercuric mercury), but combines MMHg and a newly-discovered, unidentified Hg species of low charge (LCHg). Due to this analytical artifact, the results reported here constitute an upper bound on true Hg methylation. In no season was MMHg ever detected in inlet samples at concentrations at much above the detection limit of ~0.1 ng/L. However, levels of MMHg+LCHg over 2 ng/L were observed in the outlets during warm seasons when nitrate had become depleted within the bioreactor. Sulfate depletion was also observed in most samples with elevated [MMHg+LCHg]. The combination of sulfate depletion and MMHg production is consistent with nitrate inhibition of iron and sulfate reduction and with MMHg concentrations observed in other highly anaerobic environments, e.g., lake hypolimnia and wetland porewaters. The maximum [MMHg+LCHg] observed in any given bioreactor followed an inverse function of the bioreactor loading density, i.e., the ratio of the area drained to the area of the bioreactor pit. The function has a form similar to that observed for bioreactor denitrification efficacy and suggests that optimal bioreactor designs that permit substantial denitrification while minimizing Hg methylation are feasible. Finally, extremely high MMHg+LCHg levels were observed when stagnant water conditions occurred within the bioreactors. Thus, it is recommended that bioreactors not be built with bottom depresssional areas where stagnant water can reside, in order to avoid developing anoxic conditions where methylation occurs. For the same reasons, bioreactors should not be used simultaneously with controlled drainage (water table management) if restricting the drainage results in keeping the bioreactors flooded for long periods of time.Illinois Sustainable Technology Center (Grant No. HWR09215)Ope