Screening of pesticide toxicity in surface water from an agricultural area at Phuket Island (Thailand)

A screening study of the toxicity of expected pesticide-contaminated surface water from an agricultural area at Phuket Island, Thailand, was carried out using standardized bioassays in combination with pre-concentration by solid phase extraction (SPE). The bioassays were an algal growth inhibition test (Selenastrum capricornutum) and a Daphnia immobilization test (Daphnia magna). Tests were run on both filtered water samples and samples pre-concentrated by SPE. Toxicity could be detected in non-concentrated samples originating from areas where pesticides were applied, but pre-concentration was necessary in order to obtain full concentration-response relationships. Water collected at vegetable fields was very toxic. Lower toxicity was found in stream water, but the toxicity increased as the stream passed the vegetable fields. No toxicity was detected in an unpolluted reference sample pre-concentrated 100 times. In general, algal tests proved to be more sensitive than Daphnia tests for monitoring toxicity. It was concluded that pesticides are likely to cause toxic effects in the stream, but due to great dilution by the tide, it is not likely that the current water-borne pesticide pollution will affect the marine ecosystem. Bioassays combined with pre-concentration proved to be useful screening and monitoring tools for initial assessment of water pollution by pesticides. (C) 1998 Elsevier Science Ltd. All rights reserved.