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WATER RESOURCES RESEARCH, VOL. 44, W04401, doi:10.1029/2007WR006252, 2008

Source and transport controls on the movement of nitrate to public supply wells in selected principal aquifers of the United States

P. B. McMahon

U.S. Geological Survey, Denver, Colorado, USA


J. K. Böhlke

U.S. Geological Survey, Reston, Virginia, USA


L. J. Kauffman

U.S. Geological Survey, West Trenton, New Jersey, USA


K. L. Kipp

U.S. Geological Survey, Denver, Colorado, USA


M. K. Landon

U.S. Geological Survey, San Diego, California, USA


C. A. Crandall

U.S. Geological Survey, Tallahassee, Florida, USA


K. R. Burow

U.S. Geological Survey, Sacramento, California, USA


C. J. Brown

U.S. Geological Survey, Hartford, Connecticut, USA


Abstract

In 2003–2005, systematic studies in four contrasting hydrogeologic settings were undertaken to improve understanding of source and transport controls on nitrate movement to public supply wells (PSW) in principal aquifers of the United States. Chemical, isotopic, and age tracer data show that agricultural fertilizers and urban septic leachate were the primary sources of large nitrate concentrations in PSW capture zones at Modesto, California (Central Valley aquifer system) and York, Nebraska (High Plains aquifer). Urban septic leachate and fertilizer (possibly nonfarm) were the primary sources of large nitrate concentrations in PSW capture zones at Woodbury, Connecticut (glacial aquifer system), and Tampa, Florida (Floridan aquifer system), respectively. Nitrate fluxes to the water table were larger in agricultural settings than urban settings, indicating that it would be beneficial to reduce PSW capture zone areas in agricultural regions. Mixing calculations indicate that about 50 to 85% of the nitrate in water from the PSW could be from those modern anthropogenic sources, with the remainder coming from sources in old (>50 years) recharge or sources in young recharge in undisturbed settings such as forests. Excess N2 concentrations and age tracers showed that denitrification at Modesto occurred gradually (first-order rate constant of 0.02/a) in a thick reaction zone following a ∼30-year lag time after recharge. Denitrification generally was not an important nitrate sink at Woodbury. At York and Tampa, denitrification occurred rapidly (0.5 to 6/a) in thin reaction zones in fine-grained sediments that separated the anoxic PSW producing zones from overlying oxic, high-nitrate ground water. Particle tracking showed that a major pathway by which anthropogenic nitrate reached the York and Tampa PSW was by movement through long well screens crossing multiple hydrogeologic units (York) and by movement through karst features (Tampa), processes which reduced ground water residence times in the denitrifying zones. These results illustrate how PSW vulnerability to nitrate contamination depends on complex variations and interactions between contaminant sources, reaction rates, transit times, mixing, and perturbation of ground water flow in contrasting hydrogeologic settings.

Received 12 June 2007; accepted 7 December 2007; published 4 April 2008.

Keywords: nitrate; public supply wells; denitrification; flow modeling.

Index Terms: 1831 Hydrology: Groundwater quality; 1832 Hydrology: Groundwater transport; 0454 Biogeosciences: Isotopic composition and chemistry (1041, 4870); 0412 Biogeosciences: Biogeochemical kinetics and reaction modeling (0414, 0793, 1615, 4805, 4912).

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Citation: McMahon, P. B., J. K. Böhlke, L. J. Kauffman, K. L. Kipp, M. K. Landon, C. A. Crandall, K. R. Burow, and C. J. Brown (2008), Source and transport controls on the movement of nitrate to public supply wells in selected principal aquifers of the United States, Water Resour. Res., 44, W04401, doi:10.1029/2007WR006252.