Title: Influence of Subsurface Anomalies on Vadose Zone Transport

Cooperators: D.D. Bosch, C. Truman, and F. Davis

Problem:

Transport of agricultural chemicals from the root zone results in decreased effectiveness, economic losses, and adverse environmental impacts. This transport depends on rainfall amount, intensity, and duration; chemical solubility and degradation properties; soil properties; biological processes; and management practices. An additional characteristic which has been found to affect this transport process is soil variability. Soil and geologic heterogeneity play a significant role in groundwater contamination. Horizontally layered areas of reduced conductivity restrict vertical transport and concentrate moisture and solutes, funneling the solute plume. It is believed that preferential pathways in the vadose zone lead to unexpected groundwater contamination in areas where matrix flow, or flow through the bulk soil, would not.

Approach:

A procedure to identify and sample vadose zone preferential pathways using non-destructive methods has been established. Detailed investigations were conducted on a 1 ha conventionally managed corn field near Tifton, Georgia. Soil-solution samples collected using suction lysimeters from January 1993 to September 1996 following the application of commercial fertilizer, the herbicide atrazine, and the insecticide carbofuran indicate significant vadose zone transport. Because of the difficulty associated with collecting the types of field data necessary to quantify vadose zone mass transport, and in particular those related to preferential pathways, a 2-D simulation model ( VS2DT) was used to further analyze the process.

Results:

Samples indicate high pesticide concentrations (296 ppb of carbofuran) in the vadose zone and significant concentrations in the ground water at 12 m (7 ppb of carbofuran). Data and simulation results indicate clay lenses induce ponded water in the vadose zone and impact flow and transport. Considerable variability was found in the concentrations of soil water delivered to the water table. This variability is a direct effect of the clay lense in the vadose zone. While point observations of solute concentration of flows to ground water increase in some positions under the clay lens, the composite effect may decrease overall groundwater contamination. Simulations indicate that clay lenses increase the period over which the solutes are delivered to the water table, effectively decreasing the concentration in ground water under the plot at any one time.