Spatial and temporal variation in hexachlorocyclohexane isomers in a temperate estuary.

Hexachlorocylcohexanes (HCHs) are pesticides that persist in air and water of the Northern hemisphere. To understand the spatial and temporal variability in HCH levels in estuarine surface waters we measured concentrations of two HCH isomers (alpha-HCH and gamma-HCH) at six sites in the York River estuary at bimonthly intervals for a year. Bacterial abundance and activity were also monitored using acridine orange direct counts and uptake of tritiated substrates, respectively. Alpha-HCH was consistently observed to be significantly higher in marine water compared to river water entering the estuary, suggesting that the Chesapeake Bay or Atlantic Ocean is a larger source of this compound to the York River estuary compared to riverine input. Moreover, following periods of high freshwater flow into the estuary during spring and early summer, both alpha- and gamma-HCH mixing curves indicated an additional source of these pollutants to the estuary such as land-derived runoff or groundwater discharge. In contrast, during low freshwater flow (late summer and fall) the estuary was a sink for HCHs, with y-HCH more rapidly removed from the estuary than alpha-HCH. During the period of low freshwater flow, concentrations of both alpha- and gamma-HCH were negatively correlated with bacterial activity. Bacterial activity as opposed to abundance appears to control HCH degradation in estuarine surface waters.