Developing a fate and transport model for arsenic in estuaries

A water quality model was developed for the Patuxent Estuary to simulate the fate and transport of four species of arsenic: arsenate (As(V)), arsenate (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, settling/deposition/resuspension of particulate arsenic, and fluxes of inorganic arsenic from the sediment bed. Modeling work was performed in three main stages: (1) development of a eutrophication and sediment diagenesis model, (2) development of suspended solids and solid-liquid partitioning models and application to single species metals, copper and cadmium, and (3) development of a model for arsenic, requiring combination of routines from the eutrophication and solid-liquid partitioning models with the addition of kinetic routines specific to arsenic. The arsenic model was constructed and calibrated using ambient water quality data for four forms of arsenic provided by Dr. Gerhardt Riedel of the Smithsonian Environmental Research Center (SERC). Nine samples were collected from 11 monitoring stations over a period ranging from May 24, 1995 to October 29, 1997. Overall, results of the calibrated model compared well to observed data and reproduced certain trends identified in the data for all four forms of arsenic. The developed model was utilized to investigate the validity of two hypotheses concerning the behavior of arsenic in the Patuxent Estuary: (1) transformation of arsenic by phytoplankton is responsible for higher levels of DMA in the lower Patuxent Estuary during the winter, and (2) higher levels of As(V) in the lower estuary during the summer are due to flux of arsenic from anaerobic sediments. For the time period considered by the study, 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 from 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.