A CROSS-SITE STUDY OF PROCESSES CONTROLLING SECONDARY WATER QUALITY IMPACTS OF ANAEROBIC BIODEGRADATION

Electron donor addition can be an effective remediation strategy for stimulating anaerobic biodegradation of groundwater contaminants. However, there is growing concern surrounding the resulting “secondary” plumes, which can consist of elevated levels of dissolved organic carbon, Mn(II), Fe(II), CH4, salts, and As(V/III). Although these secondary water quality impacts are monitored at many individual remediation sites, guidelines are needed for designing improved electron donor addition practices. This necessitates a better understanding, across different geochemical and electron donor-loading conditions, of the extent of these impacts and the processes driving them. This talk includes three parts, which together provide insights into the range of secondary water quality impacts and their control factors. First, a compilation of >155,000 well data points, from 47 electron donor addition remediation sites across the U.S., demonstrates that dissolved Mn, Fe, CH4, and As are commonly elevated, with Mn and Fe levels often exceeding allowable drinking water limits. In the other two parts of this talk, reactive transport modeling of two intensively monitored research sites affected by organic carbon inputs provide more in-depth process understanding. The first is a crude oil spill site near Bemidji, Minnesota, which represents a highly reducing and methanogenic setting more than 30 years following the spill. The second is a former wastewater disposal site in Cape Cod, Massachusetts, which shows rebounding oxygen levels more than 15 years after disposal cessation. At both sites, iron sorption is found to play a major role in attenuating plume growth, retaining electron donors near the contaminant source, and buffering pH. Overall, results for the two sites demonstrate that the strongest influences on secondary plume evolution are characteristics of bioavailable organic carbon, mineral phases, and gas sinks. These findings highlight the importance of assessing organic compound and solid phase properties at electron donor addition remediation sites, such as those considered for our data compilation, for understanding the full range of water quality impacts.