The Reston Groundwater Dating Laboratory

Dissolved Gases N2 / Ar Background

The N2 / Ar Laboratory provides analytical services for the determination of the permanent gas composition of air and water samples. The primary analyses include dissolved nitrogen, argon, methane, carbon dioxide, oxygen, and helium-4. In addition to providing gas concentrations, excess air concentrations are provided, and the recharge temperature and the amount of denitrification, if applicable, are estimated. These data are regularly used in various USGS hydrologic studies to define recharge temperatures for GW dating, to reconstruct initial nitrate concentrations based on the amount of denitrification that has occurred, and to trace sources of ground-water recharge.

Increasingly, geochemists and hydrologists are including measurements of concentrations of dissolved gases in hydrologic investigations. These include suites of noble gases (Ar, Ne, Kr, Xe, He) to study recharge conditions and deep groundwater circulation, gases that are indicators of microbial processes (O2, CH4, CO2, H2S, N2O, N2, etc.), and gases that can be used to interpret groundwater age (CFCs, SF6, 4He, 85Kr, 39Ar, 81Kr, etc.).

Thousands of measurements of N2 and Ar in groundwater have been made at the USGS over the past decades in order to: