ORGANOCHLORINE PESTICIDES IN STREAM MAYFLIES AND TERRESTRIAL VEGETATION OF UNDISTURBED TROPICAL CATCHMENTS EXPOSED TO LONG-RANGE ATMOSPHERIC TRANSPORT

We investigated whether biota and sediments in river catchments consis ting of primary forest in northwestern Costa Rica are contaminated by atmospheric transport of organochlorine pesticides from nearby regions where their use is intense. We measured organochlorine residues in st ream mayflies, as well as in the bark and leaves of trees in catchment s of an undisturbed dry tropical forest west of Volcan Orosi and Cerro Cacao (i.e., western catchments), and in tree leaves and bark in part ially disturbed rain forest catchments east of Cerro Orosi (i.e., east ern catchments) in a region where agricultural activity is intense. Sa mples were solvent extracted, purified by silica gel clean-up, and con centrated before analysis by gas chromatography/mass spectrometry. Twe lve organochlorine pesticides were detected in samples, including hexa chlorocyclohexanes, heptachlor epoxide, endosulfans, DDT, DDE, dieldri n, endrin, endrin aldehyde, and aldrin. Endosulfans dominated the orga nochlorine signature of mayflies collected in the western catchments a nd of leaves and bark collected from eastern catchments. Leaves and ba rk collected in the undisturbed western catchments contained ten-fold lower concentrations of the endosulfans than those collected from the eastern catchments. Hexachlorocyclohexane isomers were dominated by th e gamma isomer and were present at comparable levels in samples of lea ves and bark collected from both sides of the volcanic ridge. An excep tion was high residues in bark collected from one eastern catchment, s uggesting a local source. Stream mayflies and terrestrial vegetation i n the undisturbed dry tropical forest contained substantial quantities of organochlorine pesticides, the most likely source being long- and short-range atmospheric transport.