Monitoring groundwater for pesticides at selected mixing/loading sites in Arkansas

Groundwater monitoring studies have been conducted in recent years to survey contamination due to pesticides, yet few have addressed wells where pesticides are mixed, loaded, or rinsed. Beginning in 1990, a monitoring study conducted over a 2-year period included five collections at each of 16 mixer/loader locations to assess any pesticide and nitrate contamination. At sites in 11 counties, samples for pesticide analysis were extracted with solid-phase extraction (SPE) disks. Samples were screened using gas chromatography-electron capture detection (ECD) and high-performance liquid chromatography-UV detection (LCUV) for 17 pesticides commonly used in Arkansas. Detections were confirmed by gas chromatography-mass spectroscopy (MS) or co-chromatography. Fourteen samples revealed atrazine (1 detection), cyanazine (4), parathion-methyl (2), metolachlor (2), norflurazon (1), pendimethalin (1), propanil (2), or trifluralin (1) at eight locations during the 2-year study. Two detections of parathion-methyl and one detection of trifluralin were above the Lifetime Health Advisory Level (LHAL) of 2 mu g L(-1). Data suggested a high correlation between pesticide used and pesticide detected at sites sampled. Three wells contained NO3-N concentrations of 10 mg L(-1) or higher, but these did not correlate with pesticide concentrations. The pesticide's proximity to the wells during mixing, rinsing, or loading was considered to be a greater influence on temporary contamination of groundwater than chemical or site-specific characteristics.