Abstract
A study designed to provide a better understanding of the mechanisms of agrichemical transport togroundwater was conducted on a 0.81-ha agricultural corn field near Plains, Georgia. The objectives were to: (1)characterize vadose zone flow paths of water and agrichemicals under normal climatic and management conditions andevaluate their spatial and temporal variability; and (2) relate spatial and temporal transport patterns to geophysicalproperties of the soil and climatic conditions. Agrichemical transport was assessed over a five-year period from 1989 to1994 through analysis of collected soil and groundwater samples. A bromide (Br ) tracer was applied at 78 kg ha1 in1989 and at 105 kg ha1 in 1991. Chloride (Cl) and nitrogen were applied with fertilizer each year except 1994. Soilcharacterization tests indicated a dramatic decrease in the saturated hydraulic conductivity associated with a largeincrease in clay content in a zone from 1 to 4 m below the soil surface. As a result of this soil feature, Br concentrationsin the vadose zone below 4 m were normally less than 2 mg kg1 throughout the study. Aquifer chemical concentrationsindicated nitrate nitrogen (NO3 N) and Cl applied to the soil surface in the spring were transported to the groundwaterat 9 m by that same fall. Bromide concentrations in ground water peaked at 0.65 mg L1 while NO3 N concentrationspeaked at 6.9 mg L1 and Cl at 4.0 mg L1. Agrichemical transport and variability were controlled by climatic patternsand soil hydraulic characteristics. Transport to groundwater increased when precipitation and irrigation volumes in thefirst 30 days after spring fertilization and planting exceeded twice the normal precipitation. If large spring thunderstormsoccur soon after chemical application, the likelihood of groundwater contamination by agrichemicals is substantiallyincreased. These data provide the means to relate transport of agrichemicals in and through the vadose zone togeophysical characteristics and irrigation and precipitation inputs.
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