USING A REACTIVE TRANSPORT MODEL TO EVALUATE TEMPERATURE-DEPENDENT ARSENIC RELEASE DOWNGRADIENT OF NUTRIENT-RICH SURFACE WATER SYSTEMS

Arsenic at concentrations well above the drinking water limit has been detected in groundwater downgradient of a hypereutrophic lake on Cape Cod, Massachusetts. Groundwater chemistry downgradient of nearby lakes reveal changes consistent with seasonal variations in the intensity of biogeochemical processes. We hypothesize that these arsenic plumes are associated with warm lake water entering the aquifer during the summer and increasing the reactivity of organic carbon degradation. A reactive-transport model was implemented to test this conceptual model. Simulations include the transport of nutrients, organic carbon, and seasonally variable temperatures from lake water into the aquifer. The model used temperature-dependent organic carbon degradation rate laws based on experimentally observed aerobic degradation and anaerobic degradation coupled with denitrification. Arsenic sorption was represented by surface complexation to both hydrous ferric oxide and bulk sediment surfaces, with the latter calibrated for Cape Cod aquifer sediments. Model simulations show arsenic plumes developing in the summer, when intensive temperature-driven organic carbon degradation consumed oxygen and lowered the pH of aquifer waters. This caused the reductive dissolution of hydrous ferric oxide and the desorption of arsenic from bulk sediment surfaces, both of which acted as release mechanisms of arsenic. Desorption from sediment surfaces accounted for the majority of the simulated arsenic release during these months. During the winter, simulated organic carbon degradation rates decreased, which allowed oxygen to penetrate the aquifer and cause hydrous ferric oxide to re-precipitate and immobilize arsenic. The greater redox potential also oxidized arsenic(III) to more readily sorbed arsenic(V), which increased sorption on aquifer sediment surfaces as well. These results shed light on the often overlooked role of variable temperature in controlling the spatiotemporal rates of biogeochemical processes that control mobilization of arsenic. Our work has important public health implications for arsenic-bearing hydrogeologic settings in temperate climates, where surface water bodies undergo large seasonal fluctuations in temperature.