U.S. Geological Survey Toxic Substances Hydrology Program--Proceedings of the Technical Meeting, Colorado Springs, Colorado, September 20-24, 1993, Water-Resources Investigations Report 94-4015

Two-Dimensional Distribution of a Bromide Tracer in the Unsaturated Zone at the Plains, Georgia, Research Site

Abstract

A cooperative research investigation was initiated in 1986 near Plains, Georgia, by the U.S. Geological Survey; and the U.S. Department of Agriculture, Agricultural Research Service, to describe the processes that affect the movement and fate of nitrogen fertilizers; the pesticides atrazine, alachlor, and carbofuran; and a potassium bromide tracer in the plant-root, unsaturated, and saturated zones. As a result of ongoing research, in 1991 emphasis was placed on field-scale research to develop a better understanding of the physical and chemical processes that occur as water flows through variably saturated or unsaturated porous media. Data collected along a farmed, 420-foot-long transect were used to define and evaluate the factors that control the rate of movement and distribution of agricultural chemicals in the unsaturated zone. Lateral (nonvertical) migration of soil water and chemicals at the interface between permeable and less permeable material is hypothesized to account for a significant part of the dispersion of agricultural chemicals in the unsaturated zone.

Preliminary results of this investigation indicate that the fluctuations of soil water and concentrations of bromide at specific depths and locations along the transect are a function of the relative vertical hydraulic conductivities of the adjacent soil layers. Soil heterogeneity substantially affects the migration rates and distribution of the bromide tracer in the unsaturated zone. Lateral flow along the interface of permeable and less permeable soil may account for the rapid fluctuations observed in matric suction and the distribution of the tracer in the unsaturated zone. For this reason, the deterministic methods that are routinely used in numerical simulation of fate and transport are not valid in the vast majority of field conditions when applied in sloping, structured, variably saturated porous media.