The Reston Groundwater Dating Laboratory

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Applications of Dissolved N2 and Ar in Groundwater Presentation

L. Niel Plummer, Eurybiades Busenberg, and Peggy K. Widman

Abstract

Geological Society of America, Annual Meeting & Exposition, Denver, Colorado, November 7-10, 2004, Abstracts with Programs, v. 36, No. 5, p. 468.

Recharge temperatures and concentrations of excess air in ground water are routinely calculated from gas-chromatographic analyses of dissolved N2 and Ar at the US Geological Survey, Reston, VA. The accuracy of the calculated recharge temperature is typically better than 0.5 C for laboratory water standards; field uncertainties can be larger for ground-water samples if the altitude of recharge is uncertain. Though higher precision in recharge temperature is possible from measurement of suites of noble gases, N2-Ar analyses are relatively simple to perform, are cost effective, can be obtained in large numbers, and are usually reliable in oxic waters from areas of relatively low relief. For example, an uncertainty in recharge altitude of 500 m led to uncertainty of 2.3 C in recharge temperature, but if recharge altitude is known within 100 m, the uncertainty in the calculated recharge temperature is within the uncertainty of the analytical method. Denitrification processes can reduce dissolved nitrate to N2 gas in anoxic waters, leading to a warm bias in recharge temperature, an over-estimate of excess air, and under-estimation of total nitrate initially recharged in ground water. The N2 and Ar concentrations are used to (1) interpret apparent age of ground-water based on atmospheric gases such as CFCs and SF6, (2) reconstruct initial nitrate in denitrified samples and obtain local histories of fertilizer application, (3) recognize high and low-altitude sources of recharge, (4) infer mechanism of recharge from excess air concentrations and recharge temperature (diffuse vs focused recharge, and recharge through deep unsaturated zones), (5) recognize degassed samples, and (6) identify paleowaters. Concentrations of excess air determined from mass-spectrometric measurements of Ne and He are the same within the analytical uncertainties, as concentrations of excess air calculated from N2 and Ar concentrations in water from springs and wells in the Blue Ridge Mountains of Virginia, indicating gas confinement. In the Middle Rio Grande Basin, NM, recharge temperatures of 20 ka waters were as low as 3.2C for high-altitude recharge along the flanks of the Jemez Mountains, and 8.1C along the eastern mountain front.