Modeling Arsenic in the Patuxent Estuary

A water quality model was developed to track the fate and transport of four arsenic species in the Patuxent Estuary: arsenate (As(V)), arsenite (As(III)), methylarsonate (MMA), and dimethylarsinate (DMA). Processes simulated include mass transport, solid–liquid partitioning with suspended solids, uptake and transformation of As(V) by phytoplankton, oxidation of As(III), demethylation of MMA and DMA, and settling/deposition/resuspension of particulate arsenic in the water column. A sediment module was also developed and linked with the water column to generate fluxes of inorganic arsenic from the sediment bed. The arsenic model was calibrated using water quality data from the Patuxent Estuary over a period ranging from May 24, 1995 to October 29, 1997. Model results indicated that transformation of arsenic by phytoplankton is not a significant source of DMA to the lower Patuxent. Instead, results suggested that the primary source of methylated arsenic (DMA and MMA) to the lower estuary is beyond the downstream boundary (Chesapeake Bay). However, model results supported the hypothesis that flux of arsenic from the sediment is a significant source of inorganic arsenic to the lower estuary.