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Relating Minimum Soil Infiltration Rate to Nitrate-Nitrogen Concentrations in Shallow Ground Water in the Coastal Plain of the Albemarle-Pamlico Drainage Basin, North Carolina and Virginia

Albemarle-Pamlico Estuarine Study Comprehensive Conservation and Management Plan Implementation Forum, June 5-6.
By Jo Leslie Eimers and Tim Spruill

Abstract

Soils vary in their ability to transmit water, and this variation affects relative amounts of ground-water nitrate contamination that may occur. The Natural Resources Conservation Service has characterized soil series by their minimum infiltration rate when thoroughly wet, and uses a classification system grading from A (well drained) to D (poorly drained). This system also allows for soils to be classified by two labels, such as A/D (well-drained in drained areas, poorly drained in undrained areas). It is in areas of well drained and moderately well-drained soils that shallow ground water is most susceptible to nitrate contamination.

Ninety shallow wells were sampled for this study; 64 wells were sampled in 1993-94 by the U.S. Geological Survey and 26 wells were sampled in 1992-1994 by the North Carolina Division of Water Quality. These wells have a median screen depth of 10 to 15 feet below land surface. Soil types at these wells were grouped into four hydrologic groups according to minimum infiltration rates-- AB, (soils ranging from deep to moderately deep, moderately well-drained soils with moderately fine to moderately coarse texture to excessively-drained sands); C (soils having a layer that impedes the downward movement of water or soils of moderately fine texture or fine texture), D (soils that have a high shrink-swell potential, soils that have a permanent high water table, soils that have a clay layer at or near the surface, some organic soils, and soils that are shallow over nearly impervious material) and oD (hydrologic groups A/D, B/D, and C/D, where the first letter characterizes drained areas and the second letter characterizes undrained areas.)

For data analysis, undetected nitrate-nitrogen in samples were treated as 0 mg/L of nitrate-nitro-gen. Median nitrate-nitrogen values vary among the categories: 3.90 mg/L for AB, 0.06 mg/L for C, 0.05 mg/L for oD, and 0.00 mg/L for D. Tukey's standardized range test on ranked data indicates that nitrate-nitrogen concentration in category AB is larger than nitrate-nitrogen concentration in category D at the alpha =0.05 significance level. A Kruskal-Wallace test was used to determine if nitrate concentrations differ according to soil minimum infiltration rate. This test indicates that nitrate-nitrogen differs by soil infiltration category (p=0.0128).

These data indicate that where a nitrogen source is present, nitrate concentrations in shallow ground water tend to be highest in areas having well-drained and moderately well-drained soils. These nitrate data also imply that other soluble chemicals that are used or generated in areas of well-drained soils could similarly contaminate shallow ground water. Data on soil-infiltration rates is now available for most Coastal Plain counties in North Carolina and Virginia at a map scale of 1:24,000. Soil maps at this scale provide an effective basis for evaluating ground-water vulnerability in the Coastal Plain of the Albemarle-Pamlico drainage basin and provide an effective basis for managing land-use activities that are likely to cause contamination.

Eimers, J.L., and Spruill, T.B., 1997, Relating minimum soil infiltration rate to nitrate-nitrogen concentrations in shallow ground water in the Coastal Plain of the Albemarle-Pamlico Drainage Basin, North Carolina and Virginia: Albemarle-Pamlico Estuarine Study Comprehensive Conservation and Management Plan Implementation Forum, June 5-6.