From Bureau of Economic Geology, The
University of Texas at Austin (www.beg.utexas.edu).
Bureau Seminar, January 18, 2008
Using Basic Hydrochemical and Radio-isotopic Data
Bureau of Economic Geology
The Edwards aquifer consists of Cretaceous carbonates in semi-arid to arid West Texas that has poorly understood recharge areas and flow paths. Carbon-14 (half-life = 5,300 y) analyses of groundwater from Edwards carbonate in the central part of the Edwards Plateau, Texas, indicate apparent ages of less than 2000 y to almost 25,000 y. Significant quantities (0.1-3.5 T.U.) of tritium (half-life = 12.4 y) occur in water with carbon-14 ages of less than 10,000 years. Explanations for the occurrence of tritium, whose presence indicates modern water, in waters whose carbon-14 values suggest old age include 1) mixing of waters that range widely in age and 2) mixing of waters whose carbon-12 contents ("dead carbon") vary. In the second case, all the waters in the mixture could be modern, although higher values of tritium indicate younger effective ages. Waters without measurable tritium have probable effective ages of at least 50 y. Correlations between carbon-14 contents, tritium contents, and Mg/Ca values allow use of readily available Mg/Ca data to infer apparent ages. Maps of Mg/Ca are integrated with topographic relief maps to indicate preferred areas of recharge are in the upper reaches of losing streams and not along topographic drainage divides. Areas of low inferred groundwater ages show prominent northwest-to-southeast and southwest-to-northeast alignments that are similar to those seen on maps of fracture-controlled cavern passages in the same areas. Regional maps of Mg/Ca and isotope data are interpreted as delineating the more highly evolved areas of a subsurface karst system. This investigation demonstrates that, in some cases, relatively inexpensive and readily available hydrochemical data can be used as proxies for more expensive groundwater-age data where sufficient data are available to define relationships between apparent ages and hydrochemical data. Recharge volumes may be estimated by a chloride mass-balance approach once groundwater chloride concentrations have been corrected for non-precipitation chloride contributions.