U.S. Geological Survey Toxic Substances Hydrology Program--Proceedings of the Technical Meeting Charleston South Carolina March 8-12,1999--Volume 2 of 3--Contamination of Hydrologic Systems and Related Ecosystems, Water-Resources Investigation Report 99-4018B

Finding Minimal Herbicide Concentrations in Ground Water? Try Looking for the Degradates

By Dana W. Kolpin, E. Michael Thurman, and S. Michael Linhart

This report is available in pdf format: Kolpin.pdf

ABSTRACT

Extensive research has been conducted regarding the occurrence of herbicides in the hydrologic system, their fate, and their effects on human health and the environment. Few studies, however, have considered the degradates of herbicides. In this study of Iowa ground water, herbicide degradates were frequently detected. In fact, herbicide degradates were 8 of the 10 most frequently detected compounds. The total residue concentration (sum of parent compound plus degradates) was substantially greater than the concentration of just the parent compound for every herbicide examined.

The frequencies of herbicide detection and concentrations varied substantially among the major aquifer types sampled. These differences, however, were much more pronounced when herbicide degradates were included. Aquifer types having the most rapid recharge rates (alluvial and bedrock/karst region aquifers) were those most likely to contain detectable concentrations of herbicide compounds.

Two indirect estimates of ground-water age (depth of well completion and dissolved-oxygen concentration) were used to separate the sampled wells into general vulnerability classes (low, intermediate, and high). The results show that the frequencies of herbicide detection and concentrations varied substantially among the vulnerability classes regardless of whether or not herbicide degradates were considered. Nevertheless, when herbicide degradates were included, the frequency of herbicide compound detection within the highest vulnerability class approached 90 percent, and the median total herbicide residue concentration increased over an order of magnitude to 2 micrograms per liter. The results of this study indicated that obtaining data on herbicide degradates is critical in attempting to understand not only the fate of a specific herbicide but also its overall effect on human health and the environment.