Dissolved Gases N2 / Ar Background
The Dissolved Gas Laboratory of the U.S. Geological Survey 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. Dissolved gas data are regularly used in various USGS hydrologic studies to define recharge temperatures for CFC dating, to reconstruct initial nitrate concentrations based on the amount of denitrification that has occurred, and to trace sources of ground-water recharge. The laboratory is located in the USGS Headquarters in Reston, Virginia.
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 decade in order to:
- estimate recharge temperature and excess air for CFC/SF6 dating studies
- estimate quantities of denitrification in agricultural studies
- reconstruct paleoenvironmental conditions
- identify and trace sources of water in aquifers
- interpret recharge mechanisms, and
- estimate altitude or depth of recharge.