NATIONAL STATUS AND TRENDS MUSSEL WATCH PROGRAM - CHLORDANE-RELATED COMPOUNDS IN GULF-OF-MEXICO OYSTERS, 1986-90

The National Oceanic and Atmospheric Administration's National Status and Trends (NS&T) Program has been monitoring the chemical contaminati on in bivalve tissues from the US coastal waters since 1986. Alpha-Chl ordane, trans-nonachlor, heptachlor and heptachlor epoxide, components of technical chlordane, are among the chlorinated pesticides measured . The geographical distribution of these chlordane compounds in oyster samples from the US Gulf of Mexico is well-established. For example, highest residue levels, predominantly alpha-chlordane and trans-nonach lor, were encountered in samples collected near highly populated areas in contrast with the concentrations measured in predominantly agricul tural areas. Data collected during 5 years of bivalve sampling are use d to evaluate temporal trends in residue concentrations at most NS&T s ites. Minor decreases can be observed in the concentrations of alpha-c hlordane and trans-nonachlor. Heptachlor and its epoxide concentration s, in contrast, have been increasing since 1987.