MODELING THE EFFECT OF TEMPERATURE VARIABILITY ON GROUNDWATER ARSENIC PLUMES DOWNGRADIENT OF EUTROPHIC LAKES (Invited Presentation)

Lakes serve as organic carbon sources to downgradient groundwater when nutrients from, e.g., residential septic systems produce (hyper)eutrophic conditions. Lakes also impart temperature perturbations to adjoining aquifers because surface waters strongly fluctuate in temperature. Organic carbon input at the surface water-groundwater interface is known to mobilize naturally occurring arsenic in aquifers. We test the potential for temperature variability to also influence arsenic release through its control on microbial activity. We implement a reactive-transport model for the downgradient end of a hypereutrophic lake (e.g., Santuit Pond, Cape Cod, Massachusetts), where organic carbon from lake-water recharge likely mobilizes arsenic at concentrations exceeding the drinking water limit (0.13 μM) by triggering reductive dissolution of arsenic-sorbing Fe(III) oxyhydroxides. Simulations include the surface complexation models of Dzombak and Morel (1990) and Appelo et al. (2002) and a site-calibrated cation exchange model. The model also incorporates a new temperature-dependent parameterization of microbial redox rates based on incubation experiments. Over the typical annual temperature range of 2°C to 25°C, aerobic degradation rates slowed by a factor of 2.5, and denitrification rates (assumed to generally represent anaerobic degradation) slowed by a factor of 5 for a 10°C decrease in the lab. The model shows that at 25°C, degradation activity is sufficient to desorb observed levels of As within months. At 2°C, negligible As release occurs. Winter slow-down in microbial respiration rates may allow dissolved oxygen to penetrate into previously Fe-reducing regions of the aquifer, potentially sequestering arsenic on freshly oxidized Fe(III) oxyhydroxides. Thus, seasonal changes in lake temperature may produce fluctuations between aerobic and anaerobic conditions in the downgradient aquifer that can drive the spatial and temporal partitioning of arsenic between groundwater and aquifer sediments.