Influence of Hydrogeologic Factors

In areas where pesticides are used, hydrogeologic factors influence theirmovement to ground water mainly by controlling the movement of water (see Figure7). Pesticide detections in shallow ground water are generally more commonin areas with permeable soils than in areas covered by glacial tills, clays,and other low-permeability geologic materials. Detections are also more commonin unconsolidated and solution-weathered bedrock (karst) aquifers than in otherbedrock aquifers. In addition, unconfined aquifers are more susceptible to contaminationthan those that are confined. Pesticide contamination is generally more likelyin shallow ground water than in deep ground water, and where well screens arelocated close to the water table, but such relations are not always clear cut.Temporal variations in pesticide concentrations decrease with increasing depthand are generally larger in unconsolidated deposits than in bedrock.

High concentrations of pesticide contaminants in rivers may lead to contamination of shallow ground waters in agricultural areas during periods of extensive seepage of river water into underlying "alluvial aquifers," particularly following spring applications, when pesticide loads and river flows reach maximum levels. Conversely, pesticides in alluvial aquifers may flow into adjoining rivers during periods of low runoff. In many areas, "bank filtration" by alluvial aquifers has been found to be ineffective in removing pesticides from water drawn from pesticide-contaminated rivers into adjacent supply wells.