Transport and deposition of organochlorine pesticides from farmland to estuary under tropical regime and their potential risk to aquatic biota.

In this study, residues of organochlorine pesticides (OCPs) in water, suspended particulate matter (SPM), sediment and oysters from the Chanthaburi estuary were monitored closely in the wet and dry seasons for determining potential environmental risks. In water samples, out of twenty OCPs measured, endosulfan and hexachlorocyclohexanes (HCHs) were the two predominant compounds ranging and concentrations between 0.6-13 ng/L and 1-12 ng/L, respectively. Both compounds were found in higher concentrations during the wet season. The two insecticides were significantly correlated with total organic carbon (TOC) in the water with r values of 0.571 (P < 0.05) and 0.440 (P < 0.1), for endosulfan and HCHs respectively suggesting that these chemicals are strongly bound and concentrated by water-soluble organic carbon. As expected, high concentrations of endosulfan and HCHs were associated with SPM and in the surface sediment (0-5 cm) with a concentration ranging from 25-1,219 microg/kg dry wt and 19-110 microg/kg dry wt for SPM, 4-70 microg/kg dry wt and 0.5-50 microg/kg dry wt for surface sediment, respectively. Sediment quality of the Chanthaburi estuary was also assessed in terms of risk to aquatic biota using concentration found in the sediment compared to published allowable threshold levels (U.S. EPA and New York guidelines). Results suggested that most of endosulfan and HCHs levels in sediment samples exceeded the threshold values. Thus potential and environmental risks may be anticipated from levels found in sediment. Oyster from selected farms cultured at the study area appeared to take up both endosulfan and HCHs in proportion to level presented in the water and SPM. The range of endosulfan and HCHs observed in oyster tissue were from 4-46 microg/kg wet wt, and from non-detectable to 8 microg/kg wet wt. respectively. Human risk from oyster consumption was considered by comparing the value measured with allowable threshold level of concerned chemical. The levels of endofulfan and HCHs measured in oyster showed that oysters from this estuary area were safe for consumption.