State-of California Memorandum to Sacramento Environmental

Potential Application to Studies of Pesticide Transport to California Ground Water. The purpose of this memo is to briefly summarize (i) methods for distinguishing different source contributions to shallow ground water using stable isotope markers, and (ii) methods for age-dating shallow ground waters using various environmental tracers. I. INTRODUCTION The Pesticide Contamination Prevention Act (AB2021) assumes that leaching is the mechanism responsible for non-point source pesticide pollution of ground water in California. However, studies by the Environmental Monitoring and Pest Management Branch of the California Department of Pesticide Regulation have raised important questions about the significance of other transport mechanisms. Observations of herbicides in runoff entering dry wells in Tulare County suggest that dry wells may play a significant role in transporting pesticides to ground water(Braun and Hawkins, 1991). Meanwhile in Glenn County, herbicide detections in ground water have led to the establishment of several pesticide management zones (PMZs). However, many Glenn County soils are high in swelling clay and impermeable once wetted, these being unfavorable conditions for transport by'leaching. It has been suggested that preferential flow through large cracks may be a significant transport mechanism for herbicides in these soils (Graham et al., 1992). Dry wells throughout the state may provide a direct conduit to ground water for herbicide-containing runoff from highway rights-of-way (Powell et al., 1995). Age-dating of ground water parcels containing pesticides