Agrichemical Transport to Groundwater through Coastal Plain Soils

A 1–ha field with Pine Flat loamy sand (coarse–loamy, siliceous, thermic Typic Paleudult) and Troup loamy sand (loamy, siliceous, thermic Grossarenic Kandiudult) surface soils, located near Plains, Georgia, was studied for four years (1993 to 1996) to evaluate potential agrichemical transport to groundwater. The field was managed to produce summer corn and winter wheat. Commercial fertilizer, the herbicide atrazine, and the insecticide carbofuran were applied to the field in 1993, 1994, and 1995. Average annual application rates were 266 kg nitrogen ha–1, 2.5 kg atrazine ha–1, and 2.4 kg carbofuran ha–1. Monthly soil–water and groundwater samples were collected. The samples were analyzed for nitrate nitrogen (NO3(–)–N), chloride, atrazine, carbofuran, and deethylatrazine (DEA). Soil–water and groundwater samples indicated elevated NO3(–)–N concentrations (>5 ppm) in the vadose zone at 4.3 m and in the aquifer at 10 m (>4 ppm). Of the studied pesticides, carbofuran and DEA were observed at the greatest concentrations in groundwater. Both NO3(–)–N and pesticides were transported during groundwater recharge following periods of excess precipitation. Peak pesticide concentrations in groundwater were observed in late 1994, driven by a large precipitation event in July of 1994 when 565 mm of rain fell over a 4–day period. Atrazine and carbofuran concentrations in groundwater did not exceed the EPA maximum contaminant levels of 3 ppb and 40 ppb, respectively. Spatially averaged concentrations observed in monthly groundwater collected directly below the field were well below these standards. Concentrations of NO3(–)–N, atrazine, DEA, and carbofuran observed in groundwater from the on–field wells were significantly different from up–gradient and down–gradient concentrations (p = 0.05). These data indicate a significant impact to the local groundwater. Nitrate N was transported down–gradient from the field at the largest concentrations. Peak concentrations of atrazine and DEA were simultaneously observed in the groundwater, indicating similar transport rates for both compounds and rapid transformation from atrazine into DEA in the root–zone.