Occurrence of pesticides in groundwater underlying areas of high-density row-crop production in Alabama, 2009-2013

The U.S. Geological Survey, in cooperation with the Alabama Department of Agriculture and Industries, sampled a network of 15 wells for up to 167 pesticides and pesticide degradates from 2009 through 2013 in three areas of high-density row-crop agriculture in Alabama. Eighteen herbicides, 2 fungicides, and 9 degradates were detected in water from the sampled wells. The highest concentration of a detected pesticide was 4.49 micrograms per liter of bentazon in Baldwin County, Alabama, which was well below the lifetime health advisory level of 200 micrograms per liter. None of the measured pesticide concentrations exceeded a human-health benchmark. Insecticides were not detected.

Relatively flat land and permeable soils prevalent in each of the three areas facilitate the transport of pesticides through the unsaturated zone into the underlying aquifers. Pesticides and the degradate, deethylatrazine, were more frequently detected in groundwater from wells located in northern Alabama than in southeastern Alabama and Baldwin County, Alabama. Greater amounts of pesticide usage and shallow well depths in northern Alabama likely explain the detection of pesticides in that area. Pesticides were detected in two of the shallowest sampled wells in southeastern Alabama, and the detected pesticides have been extensively used on the crops grown in this area. Total pesticide use among the three areas was lowest in Baldwin County; however, fungicides were detected more often in Baldwin County, which is indicative of peanut crops planted in that area.

Concentrations of metolachlor and atrazine have substantially decreased in the northern Alabama wells since 2000. A decline in use of metolachlor and atrazine from a high in the late-1990s and a high in 2004, respectively, in northern Alabama could account for the lower concentrations. Fluometuron use has also declined since 1998, but the relation between time and concentrations differed in the five northern Alabama wells. Fluometuron concentrations in three of the five wells have been decreasing over time, while concentrations in the remaining two wells have been increasing.