Tracer studies of subsurface flow patterns in a sandy loam profile

A study designed to provide a better understanding of the mechanisms of agr ichemical transport to ground,vater 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 prop erties of the soil and climatic conditions. Agrichemical transport was asse ssed over a five-year period from 1989 to 1994 through analysis of collecte d 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 w ere applied with fertilizer each year except 1994. Soil characterization te sts indicated a dramatic decrease in the saturated hydraulic conductivity a ssociated with a large increase in clay content in a zone from 1 to 4 m bel ow the soil surface. As a result of this soil feature, Br- concentrations i n 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 t he groundwater at 9 m by that same fall. Bromide concentrations in ground w ater 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 control led by climatic patterns and soil hydraulic characteristics. Transport to g roundwater 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 appl ication, the likelihood of groundwater contamination by agrichemicals is su bstantially increased These data provide the means to relate transport of a grichemicals in and through the vadose zone to geophysical characteristics and irrigation and precipitation inputs.