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Kansas Geological Survey, Current Research in Earth Sciences, Bulletin 253, part 3
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The use of the natural abundances of 15N and 13C to determine sources of nitrate-N contamination in the Hays, Kansas, well field indicates the possibility that human and/or animal waste is a source of contamination to the ground water in this location. Well C-27 is nearest to the closed sewage-treatment plant and is near farmland that may have been fertilized with manure in the past. This well has the highest nitrate-N and δ15N values. Wells C-28A, C-19, and C-31 have lower nitrate-N values, but their δ15N values are similar to the enriched values measured at C-27.

Lower nitrate-N values may indicate possible denitrification reduction of the nitrate-N concentration and/or dilution by mixing with other lower nitrate-N water. Occurrence of higher nitrate-N and enriched δ15N values in the ground water suggests that possible macropore flow in the unsaturated zone or poor well construction may provide avenues for flow of contaminated water to ground water somewhere in the city area. The field descriptions of the soils indicate the presence of continuous tubular pores and fine roots throughout the core. These pores and root channels may permit some macropore flow without the retention time needed to decrease the nitrate-N concentrations. Further study is needed to verify this. Also, a survey of wells in the city would be helpful to determine if there are possible avenues for flow due to poor well construction.

Soil chemical profiles and water chemistry indicate that denitrification and volatilization enrichment may be occurring both in the unsaturated zone and in ground water. Presence of abundant calcium carbonate in the soils could permit the changes in the pH of the water necessary for volatilization to occur. The δ15N values generally increase with depth in two of the cores. The soils contain sufficient nitrogen and carbon to permit denitrification to occur in the profile with the resulting enriched δ15N values. However, both the nitrate-N and δ15N values are much lower than in the ground-water samples, suggesting that the soils are not a primary source of nitrogen for the ground water.

The δ13C values are enriched in the soils indicating that C4 plants, such as warm-weather grasses, and other plants, such as sorghum, may be a primary source of the carbon. The values in the ground water are in the range for C3 plants and particularly the range common for animal and human waste, as observed in other studies.

Testing both soil and ground-water samples for 13C and 15N values and nitrogen and carbon contents provides a wider context for evaluation of nitrogen sources. Land use, plant types and classification as C3 or C4 carbon sources, location of potential human- and animal-waste sources, and soil descriptions in terms of caliche and carbonate zones all provide essential information to help define nitrate sources. The results from this study are encouraging for the use of both 13C and 15N for source identification of nitrate-N in ground water.

Acknowledgments-The authors wish to thank personnel from the City of Hays for their assistance in sampling wells, the soil scientists from the northwest Kansas NRCS for their help in collecting and describing soil cores in the area, and the staff at the Kansas Geological Survey for editing, graphic design, and review of this paper.


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