Abstract
Simple SummaryThe 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.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.
Highlights
Increasing heavy metal pollution in soils and surface waters is a global concern [1,2].Low-lying coastal areas, river flood plains, water retention basins and natural wetlands are susceptible to metal accumulation from sources such as industrial activity, mining, acid rain, agricultural runoff and overbank flooding [2,3]
Removal for As > Cu, Fe, Mo, Zn > Co, Mn, Ni is consistent with the thermodynamicallypredicted sulfide ladder, suggesting that the extent to which these elements were released to or removed from surface waters was at least partially regulated by a limited supply of sulfide in porewaters
Relative differences between mass removal rates for As > Cu, Fe, Mo, Zn > Co, Mn, Ni followed the increasing solubility products for the respective metal sulfides, the thermodynamically-predicted sulfide ladder, suggesting that mass exchange between pore- and surface waters for these elements was at least partially regulated by a limited supply of sulfide in porewaters
Summary
Increasing heavy metal pollution in soils and surface waters is a global concern [1,2].Low-lying coastal areas, river flood plains, water retention basins and natural wetlands are susceptible to metal accumulation from sources such as industrial activity, mining, acid rain, agricultural runoff and overbank flooding [2,3]. Increasing heavy metal pollution in soils and surface waters is a global concern [1,2]. Drying and flooding cycles can mobilize metals from these systems [4,5], resulting in exposing wildlife to potentially toxic levels of metals as well as polluting discharges to the environment. Increasing our understanding of the processes which control metal mobility in these systems and which metals are more prone to mobilization are important areas of research today, as many of these systems are subject to sea level rise and intensifying wet/dry cycles due to global warming. Metal removal by sedimentation is often 60–80% effective [6], depending on the type of BMP and water retention time. Maybeck [7], noted that sedimentation can reach 99%; ,
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