TRACER STUDIES OF SUBSURFACE FLOW PATTERNS IN A SANDY LOAM PROFILE

A study designed to provide a better understanding of the mechanisms of agrichemical transport to groundwater 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 and evaluate their spatial and temporal variability; and (2) relate spatial and temporal transport patterns to geophysical properties of the soil and climatic conditions. Agrichemical transport was assessed over a five-year period from 1989 to 1994 through analysis of collected soil and groundwater samples. A bromide (Br –) tracer was applied at 78 kg ha–1 in 1989 and at 105 kg ha–1 in 1991. Chloride (Cl–) and nitrogen were applied with fertilizer each year except 1994. Soil characterization tests indicated a dramatic decrease in the saturated hydraulic conductivity associated with a large increase in clay content in a zone from 1 to 4 m below the soil surface. As a result of this soil feature, Br – concentrations in the vadose zone below 4 m were normally less than 2 mg kg–1 throughout the study. Aquifer chemical concentrations indicated nitrate nitrogen (NO3 – N) and Cl– applied to the soil surface in the spring were transported to the groundwater at 9 m by that same fall. Bromide concentrations in ground water peaked at 0.65 mg L–1 while NO3 – N concentrations peaked at 6.9 mg L–1 and Cl– at 4.0 mg L–1. Agrichemical transport and variability were controlled by climatic patterns and soil hydraulic characteristics. Transport to groundwater increased when precipitation and irrigation volumes in the first 30 days after spring fertilization and planting exceeded twice the normal precipitation. If large spring thunderstorms occur soon after chemical application, the likelihood of groundwater contamination by agrichemicals is substantially increased. These data provide the means to relate transport of agrichemicals in and through the vadose zone to geophysical characteristics and irrigation and precipitation inputs.

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