Thesis (M.Sc.). - Asian Institute of Technology, 2002

A thesis submitted in partia fulfillment of the requirments for the degree of Master of Science. Inter-University Program on Environmental Toxicology, Technology and Management. School of Environment, Resources and Development

Paraquat is known as a synthetic herbicide more often applies to control weeds in upland agriculture. This chemical was first introduced since 1962 and being produced in several countries. It has been used worldwide covering over 130 countries. Likewise methods for determination of paraquat residues in plants as well as in environmental media such as water, sediment and soil have been successfully developed to monitor its accumulation and potential harmful to the environment. This study had designed to find out the current situation of the paraquat usage and distribution including contamination into the environment. Moreover, its toxicity to aquatic animals especially commercial fishes at their younger stage of growth were conducted. The selected study areas cover two major criteria, one lake serves as a water catchment for one river that flows to it. The Yorn River and Boraphet Lake in Nakomsawan province fall in to such categories. Field survey and sampling for water and sediments from the lower parts of the Yorn River and Lake were done in early dry season. Paraquat residues from those samples were determined. Acute toxicity tests of paraquat on commercial fish (larval Puntius gonionotus (Bleeker)) had been conducted on site at the lake. Results indicated that paraquat residue concentrations from the Y om River (water and sediment) were much lower than those in the Boraphet Lake. The highest value of paraquat in the lake water was 0.30 mg/L with the average value at 0.22 mg/L. For the lake sediment, paraquat tended to bulid up with the values ranging between 0.13-0.60 mg/L. In toxicity testing the safety of paraquat was obtained. The LC5o of paraquat is 9 .65 mg/L using the larval fish Puntius gonionotus (Bleeker) at 96 h. In general, both lake water and river water including sediments are not close to highly contaminated and considered safe for aquatic life. However, many developed countries were banned of using the chemical, not from the environmental reason, but they greatly concern on the occupational hygienist of the farmer.