Reactive transport modeling of secondary water quality impacts due to anaerobic bioremediation

Friday, 19 December 2014
Gene-Hua C Ng1, Barbara A Bekins2, Douglas B Kent2, Robert C Borden3 and Jason Tillotson4, (1)University of Minnesota Twin Cities, Minneapolis, MN, United States, (2)USGS California Water Science Center Menlo Park, Menlo Park, CA, United States, (3)North Carolina State University at Raleigh, Civil, Construction, and Environmental Engineering, Raleigh, NC, United States, (4)North Carolina State University at Raleigh, Raleigh, NC, United States
Bioremediation using electron donor addition produces reducing conditions in an aquifer that promote the anaerobic biodegradation of contaminants such as chlorinated solvents. There is growing concern about secondary water quality impacts (SWQIs) triggered by the injection of electron donors, due to redox reactions with electron acceptors other than the target contaminant. Secondary plumes, including those with elevated concentrations of Mn(II), Fe(II), and CH4, may create long-lasting impairment of water quality. Understanding conditions that control the production and attenuation of SWQIs is needed for guiding responsible bioremediation strategies that limit unintended consequences. Using a reactive transport model developed with data from long-term anaerobic biodegradation monitoring sites, we simulate diverse geochemical scenarios to examine the sensitivity of secondary plume extent and persistence to a range of aquifer properties and treatment implementations. Data compiled from anaerobic bioremediation sites, which include variable physical and geochemical relationships, provide the basis for the conditions evaluated. Our simulations show that reduced metal and CH4 plumes may be significantly attenuated due to immobilization (through sorption and/or precipitation) and outgassing, respectively, and that recovery time to background conditions depends strongly on the chemical forms of reduced metals on sediments. Unsurprisingly, scenarios that do not easily allow outgassing (e.g. deeper injections) led to higher CH4 concentrations, and scenarios with higher hydraulic conductivity produced more dilute concentrations of secondary species. Results are sensitive to the assumed capacity for Fe(II) sorption and reductive dissolution rates of Fe(III) oxides, which control Fe(II) concentrations. Simulations also demonstrated the potential importance of chemical reactions between different secondary components. For example, limited CH4 loss from outgassing and Fe(III)-mediated anaerobic CH4 oxidation together generate more extensive Fe(II) plumes. Our work provides ranges for possible SWQIs and highlights the importance of characterizing mineral phases, electron donor injection practices, and transport behavior for understanding coupled SWQIs.