Risk Assessment of Three Herbicides Used in Rice Field in Californian Watersheds on Freshwater Fishes

Abstract:   Molinate, glyphosate and thiobencarb are three commonly used herbicides in rice fields. The purpose of this study is using the exposure:effect ratio (RQ value) to quantify the risk of these herbicides to fish in some Californian watersheds. The risk assessment process follows the framework suggested by US-EPA. For the exposure assessment, data reported by California Department of Pesticide Regulation was used. The largest surface water exposure concentrations of glyphosate, molinate and thiobencarb in San Joaquin and Sacramento Basins were 39.0000μg/L, 44.0900μg/L and 16.9000μg/L respectively. For the effect assessment toxicity data for fish from laboratory studies was retrieved from databases and the open scientific literature. The most sensitive fish to glyphosate was Oreochromis sp. (LC50 value 3.5mg/L); the most sensitive fish to molinate was Cyprinus carpio L.(LC50 value 0.21mg/L); and the most sensitive fish to thiobencarb was Lepomis macrochirus (LC50 value 0.33mg/L). A comparison of RQ values of the herbicide in counties with high usage in San Joaquin and Sacramento Basins, Colusa shows a relatively high RQ value of 5.12 for thiobencarb while the RQ value for molinate in Colusa, Sutter and Yolo are 21.00, 11.29 and 9.14 respectively. Glyphosate has a lower average risk, and the highest RQ value is 1.11 in Fresno.