The Annual Cycle of Arsenic in a Temperate Estuary

Dissolved concentrations of four forms of arsenic: arsenite, arsenate, monomethylarsenic, and dimethylarsenic, were measured in the Patuxent River Estuary near Benedict, Maryland, over two annual cycles. In each year, total arsenic concentrations peaked in the summer, in lateJuly and August, while minimum concentrations occurred in winter. Except in late winter, arsenate was a predominant form of arsenic present. In late winter and spring, dimethylarsenic was also a predominant form. A period of monomethylarsenic abundance occurred in summer, following the predominance of dimethylarsenic. Arsenite occurred irregularly in spring. Concurrent temperature and salinity measurements indicate that total arsenic concentrations rose before the summer increase in salinity, suggesting an arsenic source other than the end members, the Patuxent River or Chesapeake Bay.