Crustal Imaging and Characterization Team
Task Contact: Gary Landis
Isotopically enriched or single isotope xenon can be injected into groundwater systems either through direct injection into wells, or through percolation via surface catchments in recharge areas. As a totally safe, colorless, dissolved gas, its arrival at monitoring wells down hydrologic gradient can be detected with appropriately designed sampling schemes. Sampling and mass spectrometer measurements of dissolved xenon gas is economical and rapid, giving precise information on flow rates. Even with 2x-3x diffusional or dispersive loss, successful detection is possible with a xenon cost of approximately $2 per acre-ft. Groundwater age information is available from the same sampling scheme using tritiumhelium/3, and possibly krypton/81 and for older waters, in-growth accumulation of argon/39 and helium/4. The later noble gas isotope geochronometers are useful at 10^3 to 10^6 years. Combined with water chemistry and stable isotope measurements, and solubility data on noble gases, the xenon measurements can help to better understand groundwater systems; their flow rates, recharge areas, sources and mixing between aquifers, groundwater age(s), and possibly to fingerprint point sources of contaminants.
Emphasis will be on noble gas behavior in the crust and geologic-geochemical implications for ore formation and deep crustal hydrologic processes (connected with ore formation). Originally designed to advance technology of geohydrologic studies in support of groundwater and 3D mapping efforts, this project has crossed a critical juncture in greater understanding of noble gas behavior not only in hydrologic systems but also that of hydrothermal-magmatic volatiles, ore deposits, and processes in the lower to upper crust. This task will concentrate on refining heavy noble gas measurements Kr and Xe from groundwater studies, and continue methods development of UV laser opening of fluid inclusions for noble gas analyses, with emphasis on improving small sample size sensitivity and precision. Analyses of magmatic volatiles, hydrothermal system volatiles, and ground water- geologic structure gases will help to constrain the links between ore-forming systems, meteoric waters, crustal fluids at various positions in the crust, and crust/mantle fluid contributions to zones of partial melting and magma genesis.
The SPMD diffusion samplers have been tested successfully in Texas at Tri-County #4 well to a depth of 1500 feet, and at several points in the effluent from Comal Springs (New Bronfels, TX). A new string of SPMD diffusion samplers are now deployed in wells in the northern recharge zone of the Edwards aquifer, and will be analyzed in August- September. Two Kemmerer downhole samplers are currently being used to sample water with contained dissolved gas at several well transect localities across the freshwater-saline water interface of the Edwards aquifer, TX. Results obtained add substantial insights into the 3-D functioning of the Edwards aquifer, resulting in several completed and planned publications. Finally, successful competetion for USGS Geologic Discipline venture capital funding has enabled us to complete the development and purchase of a discrete downhole sampler system capable of collecting dissolved gas in water to a depth of >3000 ft. in wells. Continued development of this system with Auslog, Ltd (Australia) has resulted in very unique special depth sampling and realtime parameter logging tools critical to success of many groundwater studies.
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Mineral Resources |
Eastern / Central / Western / Alaska / Minerals Information |
| Crustal Imaging & Characterization / Spatial Data |
U.S. Department of the Interior |
U.S. Geological Survey
URL: http://crustal.usgs.gov
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