Noble gases are excellent tracers for ground-water studies, and commonly can help to recognize ground-water origins and interactions with aquifer rocks and key water/rock processes. Upon entering the ground, water flow is completely dependent on the subsurface lithology and structure. Spatial-temporal variations in noble and other dissolved gases in groundwater commonly provide direct evidence for interpretation of 3-D geology that is on par with well logging and geophysical methods.
The very rapid ground-water movement through the Edwards aquifer is controlled by two major flowpaths, the Western Medina flowpath and the Eastern flowpath. The Western Medina flowpath is the less understood of the two and lies in parts of Uvalde, Medina, and Bexar Counties. Within both flowpaths, the freshwater zone of the aquifer is bounded to the south and southeast by the transitional down-dip saline zone. The up-dip fresh-water zone (FWZ) is recharged with surface water infiltrating into the Edwards aquifer outcrop. The saline-water zone (SWZ) is adjacent to the FWZ and forms an interface at the down-dip limit of the fresh water. This interface, locally referred to as the "bad-water line," is arbitrarily defined where aquifer waters exceed 1,000 mg/L of total dissolved solids (TDS, see below). There is little understanding of the hydraulics, interconnectivity, and structural controls between the fresh and saline zones as the wells that penetrate the Edwards are in excess of 800 ft in depth, with many over 1200 to 1800 ft.
Project geophysical studies in FY-02 to FY-03 were conducted in and around selected transect wells of the fresh-/saline- water interface to provide images in 3-dimensions using audio-magnetotellurics (AMT). Transects include the fresh-/saline-water interface southeast of the Balcones Escarpment, a near-surface volcanic plug along the Frio River, and a water-filled limestone cave associated with a normal fault near Honey Creek. Transects were also completed near newly-drilled fresh-/saline-water interface monitor wells in the Kyle area, near Cibolo Creek, and San Marcos and Comal Springs. Some of these soundings are published in the USGS Open-file Report 2002-118 (http://pubs.usgs.gov/of/2002/of02-118/).
Significant noble gas isotope accomplishments include the development and testing of several noble gas sampling methods that have proven successful in collecting water samples while retaining the total dissolved gas from discrete depths within deep-penetrating water wells to more than 2600 ft. The first is a simple and inexpensive Kemmerer sampler for collection of noble gases at shallow depths. The other is the development and testing of several USGS-Auslog Ltd. Discrete Sample Sondes. These newly-developed down-hole instrument/samplers reach water depths unattainable with previously available equipment. Advanced scientific investigations and future collaborative GD-WRD research opportunities, as well as potential for significant technology/cost sharing, is possible with these tools.
Fluid logging and discrete isotope and noble gas sampling was conducted in four separate well transects across the fresh-/saline- water interface in Uvalde, Medina, Bexar, Guadalupe, and Hayes Counties. The fluid logs obtained from the monitoring well transects clearly shows a distinct (usually encountered within a single well screen) interface in the eastern part of the study area, such as near Kyle (see below).
In the western transect (Uvalde County), the interface appears more gradual and no distinct interface is apparent. Thermal gradients associated with the interface sections also indicate that, in some cases, not only is the fresh-water zone actively flowing, but the saline water is also flowing (thermally connected).
The dissolved gas samples indicate that the saline zone water is much older than previously thought. Waters associated with the fresh-water zone contain tritiogenic 3He (waters < 45 yrs in age) and have a meteoric origin with a recharge temperature that is close to that of average air temperature within the study area. Pre-modern waters in the saline zone have a meteoric origin (as defined by 
18O and D).
The helium isotopes (3He, 4He) show a very interesting story. With increasing helium concentrations, the R/RA (ratio of 3He/4He in the sample normalized to atmospheric 3He/4He ratio) of the saline waters all approached 0.22 +/- 0.007 within all study wells except in Kyle wells 1, 2 and 3. This unique value is indicative of two distinct end members; crustal values from alpha decay reactions (R/RA of 0.02) and a mantle value from trapped/ recycled 3He contained within the upper mantle (R/RA ~ 8.0). The uniform composition of the helium isotopes suggests a source reservoir of helium that is associated with late Cretaceous volcanic intrusions.
This source area is spatially extensive within the Edwards aquifer except within the Kyle cluster where the end member R/RA value measured is 0.13 +/- 0.01 (instrumental measurement error is also 0.01) after being corrected for atmospheric helium. This suggests that the waters associated with the Kyle cluster have a different origin and may be from the underlying Trinity aquifer that could be locally recharging the Edwards near the town of Kyle. Communication with the Trinity is further reflected in the dramatic salinity changes in the Kyle no. 2 well following a major recharge event in the spring of 2003 (shown below).
U.S. Department of the Interior | U.S. Geological Survey
URL: http://esp.cr.usgs.gov/info/edwards/geochem.html
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Page Last Modified: Fri 7-Oct-2005 10:32:05 MDT
