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Nitrate in Groundwater in the Great Valley Carbonate Subunit of the Potomac River Basin

By Matthew J. Ferrari and Scott W. Ator

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Agriculture is the major land use in the carbonate part of the Great Valley of the Potomac River Basin. Applied fertilizer and manure are potential sources of nitrate that can contaminate Groundwater. Nitrate concentrations in Groundwater increased with increasing percentage of cropland but did not decrease with increasing well depth, as has been found in previous studies elsewhere. Samples from 28 wells contained nitrate concentrations ranging from 0.29 to 29 mg/L (milligrams per liter) as nitrogen, with a median concentration of 4.55 mg/L, compared to a median of 1.8 mg/L for 1,056 Groundwater samples from the entire Potomac River Basin. Median nitrate concentrations in Groundwater samples were higher in the Maryland, Pennsylvania, and West Virginia part of the valley, and were lower in the Virginia part (7.45 and 2.95 mg/L, respectively), probably as a result of differences in agricultural land-use patterns; this geographical difference was also noted in surface-water samples (6.65 and 2.3 mg/L, respectively). The area of contribution to each well could not be delineated by surface topography or distance to the well, because Groundwater flow and nitrate transport can be unpredictable in the carbonate region because of fractures and solution channels present.

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