The Effect of Drying/Re-Flooding on Trace Metal, As and Se Fluxes in a Treatment Wetland: Addressing Growing Environmental Concerns

SIMPLE SUMMARY: The potential exposure of wildlife to toxic levels of metals following re-flooding in metal-contaminated water impoundments and coastal areas subject to sea level rise is of primary concern. Treatment wetlands are similar systems which enhance biogeochemical processes to remove low levels of pollutants including metals from wastewaters. Wetlands convert many dissolved metals to insoluble precipitates which are unavailable for biological uptake. When wetlands are dried/re-flooded, metals can be released. In this work, we present mass flux data for 11 metals, As and Se following drying/re-flooding in a constructed wetland used to treat oil refinery effluent. Following re-flooding, Co, Cr, Mg, Mn, Ni, S and Sr were continuously released to outflow, Ba, Cu, Fe, Mo and Zn showed zero net flux and As and Se were removed from inflow. We propose a mechanistic hypothesis consistent with the different flux patterns for metals which form sulfide precipitates. Our results suggest that following re-flooding, less-soluble sulfide metals may be immobilized prior to more-soluble metals in coastal systems and indicate that ponding strategies should be used to minimize metal pollution downstream. Research is urgently needed in these systems to improve metal removal efficiency, determine best management practices and for wildlife risk assessment. ABSTRACT: The retention of heavy metals in water treatment wetlands is well documented, but little understood. Fluxes to and from sediments for moderate concentrations of dissolved metals are particularly unknown. Treatment wetlands are dried out seasonally or occasionally for maintenance. The extent to which heavy metals may be released by drying/re-flooding is of particular concern because of the potential for toxic levels of metals to be mobilized. A 36 ha treatment wetland receiving treated oil refinery effluent in California was dried for 6 months, then re-flooded to an average depth of >10 cm. The concentrations of 11 metals, As and Se in inflow, outflow, and porewaters were measured weekly for 4 months. Mass flux rates showed that the wetland acted as a sink for As and Se, six metals (Co, Cr, Mg, Mn, Ni, and Sr) and S were overall sources and five showed zero net flux (Ba, Cu, Fe, Mo, and Zn). Porewater results indicate that oxidation of the sediments caused the source metals to be released. Removal for As > Cu, Fe, Mo, Zn > Co, Mn, Ni was consistent with the thermodynamically-predicted ‘sulfide ladder’, suggesting that available sulfide was insufficient to re-sequester the entire pool of mobile chalcophile elements. Our results suggest that less-soluble sulfide metals may be immobilized prior to more-soluble metals following drying/re-flooding in coastal systems with multiple metal contaminants. Ponding for up to several weeks, depending on the metals of concern, will facilitate metal re-immobilization within sediments before waters are released and minimize impacts downstream. Research on how to speed-up the conversion of soluble metals to their insoluble sulfides or other immobilized forms is urgently needed.