National Water-Quality Assessment (NAWQA) Program
A total of 128 wells in 4 well networks the ACFB were sampled in 1993-1995 (Frick and others, 1998) and resampled in 2002-2005 (Dalton and Frick, 2008). Two aquifers were investigated, a shallow surficial aquifer and the deeper, highly productive Upper Floridan carbonate aquifer. There were six compounds—five herbicides and one degradate—that were detected in more than 20 percent of samples: alachlor, atrazine, DEA, floumeturon, metolachlor, and tebuthiuron (Dalton and Frick, 2008). Tebuthiuron, used on roadways and rights of way and not used agriculturally in the study area, was detected in almost 20 percent of samples from the carbonate aquifer. Although pesticide use in the study area is generally higher than use nationally, the median concentrations for these six pesticides were at or near their respective laboratory reporting limits.
Location of the Apalachicola-Chattahoochee-Flint River basin (ACFB) study area (from Dalton and Frick, 2008).
There are few indications of pesticide concentrations either increasing or decreasing on a decadal basis in the ACFB; of the 128 wells sampled during this study, there were only significant changes in 7 wells, and DEA was the only compound that had a significant change (increase) in concentration (Dalton and Frick, 2008). Because this study focused on nationally significant pesticides, changes of concentrations of compounds of local importance, such as glyphosate, could not be determined. The general lack of upward changes in pesticide concentrations suggests that groundwater sources of these compounds, although present, are not providing an increasing source of pesticides to surface waters or wells over the time scale represented in this study. Nitrate was not investigated in this study, but Rupert (2008) reported statistically significant increases of nitrate concentrations in two of the well networks from 1993 to 2002.