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Multiple sampling approaches were utilized to assess the occurrence and variability of selected pesticides and degradation products in ground water and surface water within the Great Valley Carbonate subunit of the Potomac River Basin as part of the National WaterQuality Assessment Program. Pesticide compounds (pesticides and degradation products) are present at low concentrations (generally less than 1 microgram per liter) in ground water and surface water in the Great Valley Carbonate subunit. Of the 47 pesticide compounds for which water samples were analyzed, 17 compounds were detected in ground-water samples and 24 compounds were detected in surface-water samples. Federal Maximum Contaminant Levels have been established for alachlor, atrazine, carbofuran, lindane, and simazine. No ground-water sample contained pesticide compounds in concentrations above the Maximum Contaminant Levels. Concentrations of atrazine were found above the Maximum Contaminant Level of 3 micrograms per liter in two surface-water samples, and concentrations of simazine were found above the Maximum Contaminant Level of 4 micrograms per liter in a single surface-water sample. Atrazine was the most commonly detected compound in ground-water and surface-water samples, followed by desethylatrazine (a degradation product of atrazine), simazine, metolachlor, and prometon. No other compound was detected in more than 25 percent of the samples. Concentrations of atrazine, desethylatrazine, and simazine in ground-water and surface-water samples from subunit-scale studies indicate a general north-south variation, with the higher concentrations in the northern part of the Great Valley Carbonate subunit, which also has a higher percentage of cropland. Concentrations of atrazine, desethylatrazine, and simazine in ground-water and surface-water samples all show a significant positive correlation to the percentage of cropland. In addition, concentrations of atrazine in the water samples have a significant positive correlation to the estimated amounts of atrazine used. As part of an assessment of local patterns in pesticide occurrence and distribution, additional water samples were collected from a shallow ground-water-flow system at Muddy Creek and from domestic wells within the Muddy Creek watershed. Except for prometon, pesticide compounds that commonly occurred in samples from the flow-system study and watershed survey were similar to those that were commonly detected in samples from the subunit survey. The highest concentrations of pesticide compounds in the flow-system study were detected in samples from shallow regolith wells near cropped fields. Atrazine concentrations were significantly lower in samples from bedrock wells in the flowsystem study area than in samples from associated regolith wells. The occurrence and variability in concentration of pesticides at two fixed sites in the Great Valley are dependent upon scale. Throughout the period of sampling at Muddy Creek and Shenandoah River, concentrations of atrazine, desethylatrazine, metolachlor, prometon, and simazine were detected at low levels (generally less than 1 microgram per liter). In general, commonly detected pesticides that are applied to cropland appear to reach their highest concentrations in surface water soon after application, which typically occurs in the spring to early summer each year. Streamflow at the Muddy Creek fixed site and ground-water levels from one of the regolith-bedrock well pairs from the flow-system study area fluctuate in concert, indicating that there are good connections between the ground-water and surface-water flow systems. In this setting however, concentrations of atrazine were significantly higher in surface-water samples than in ground-water samples. Pesticides in the Great Valley Carbonate Subunit

Water-quality assessment of the Potomac River Basin: Occurrence of pesticides in the Great Valley Carbonate subunit