Fate and toxicity of endosulfan in Namoi River water and bottom sediment

Endosulfan (6,7,8,9,10,10,-hexachloro-1,5,5a,6,9,9a-hexahydro-6,9-methano-2,4,3-benzodioxathiepine-3 oxide) sorption (standardized to 1% total organic carbon and dry weight) was significantly (P < 0.05) more concentrated on the large (> 63 mum) particle fraction compared with smaller size fractions (<5 mum and 5-24 mum) of bottom sediments from the Namoi River, Australia. Following completion of the particle size fractionation (6 to 12 wk) and a sediment toxicity assessment (2 wk), the sediments showed large decreases in concentrations of a-endosulfan that coincided with an increase in endosulfan sulfate concentrations and minimal changes in P-endosulfan concentrations. In the Namoi River, similar patterns were observed in the composition of total endosulfan in monthly measurements of bottom sediments and in passive samplers placed in the water column following runoff from cotton (Gossypium hirsutum L.) fields. The toxicity of endosulfan sulfate in river water indicated by the nymphs of the epibenthic mayfly Jappa kutera, was more persistent than the alpha- and beta -endosulfan parent isomers due to its longer half-life. This suggests that endosulfan sulfate would contribute most to previously observed changes in population densities of aquatic biota. Measured concentrations of total endosulfan in river water of up to 4 mug L-1 following storm runoff, exceed the range of the 96-h median lethal concentration (LC50) values in river water for both a-endosulfan (LC50 = 0.7 mug L-1; 95% confidence interval [CI] = 0.5 to 1.1) and endosulfan sulfate (LC50 = 1.2 mug L-1; 95% CI = 0.4 to 3.3). In contrast, the 10-d LC50 value for total endosulfan in the sediment toxicity test (LC50 = 162 mug kg(-1); 95% CI = 120 to 218 mug kg(-1)) was more than threefold higher than the highest measured concentration of total endosulfan in field samples of bottom sediment (48 mug kg(-1)). This suggests that pulse exposures of endosulfan in the water column following storm runoff may be more acutely toxic to riverine biota than in contaminated bottom sediment.