Environmental Risk Assessment of pesticide use based on the modelling of the environmental fate of pesticides in soil

Based on laboratory and field experiments the environmental fate of four selected pesticides (Dimethoate, Fenvalerate, Malathion, and Metalaxyl) in the Jhikhu-Khola watershed, Nepal was investigated, both on soils representative for irrigated (khet) and rainfed (bari) conditions. The degradation, sorption, and transport behaviour were described with deterministic mathematical models. Based on these models a risk assessment of the accumulation of the pesticides in soil and groundwater was conducted. In order to achieve this standard, worst and best case scenarios were developed. The framework and conditions for the scenarios were set by the physical and socio-economic environment in the watershed, which was investigated by a socio-economic survey. With this method it was possible to assess the risk of groundwater contamination and residue formation in soil on a quantitative basis for parts of the watershed. According to this the risks of severe, actual residue formation are very low, but chances for long term residue accumulation are present. The likelihood of groundwater contamination is also low, but more uncertainties remain as compared to residue formation, because the transport mechanisms are not completely understood and because of insufficient data.