To quantify the environmental risks caused through the use of pesticides, combined environmental and artificial isotopes as well as dye tracer techniques are used in an irrigated 10 hectar market-near study basin in the Middle Mountains of Nepal. Based on a conceptual hydrological model, experiments were designed to generate information about the water-bound pesticide transport through the investigated agro-ecological system. Major results refer to the development of natural deuterium and oxygen-18 contents in soil water, groundwater, precipitation and stream water, and the isotopic information content concerning hydraulic systems and the origin of water. Artificial deuterium and a dye tracer (Vitasin Blue FCF) application in test plots allow to identify preferential flow pathways of water in the unsaturated zones. Following the hypothesis and hydraulic interpretations, the environmental risk of pesticides on environment seems to be low as far as underground water is concerned.
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