Dealing with Non-Precipitation Chloride Contributions in the Chloride-Mass Balance Approach to Recharge Calculations in the Plateau Aquifer System, Texas

H. S. Nance¹ and John M. Sharp^2
1Bureau of Economic Geology, Jackson School of Geosciences, The University of Texas at Austin, University Station Box X, Austin, Texas 78713
²Department of Geological Sciences, Jackson School of Geosciences, The University of Texas at Austin, University Station Box X, Austin, Texas 78713

The chloride-mass balance approach offers potential for estimating recharge from hydro-chemical analyses of semi-arid aquifers. Simple applications assume that rainfall provides all chloride to the aquifer after rainwater evaporation at land surface. Recharge efficiency ratios are calculated from rainfall and groundwater chloride concentrations. Geographically variable dry deposition at the surface and other sources also contribute chloride. Data from the Edwards-Trinity aquifer system of Texas suggest that groundwater is a mixture of precipitation-derived recharge and variably saline water present in aquifers from cross-formational exchange between the hydraulically and hydrochemically distinct Edwards and Trinity intervals. Viable application of chloride-mass balance must account for these contributions. Consideration of Br/Cl values in Plateau groundwater suggests that an end-member mixing model can partly explain its hydro-chemistry. If true, then relative proportions of admixed fresh recharge and formation groundwater can be determined. Deviations in presumed groundwater mixtures of other ionic constituent concentrations from simple mixing relationships can be explained by cation exchange and dissolution of sulfates along flow paths. At present data on dry deposition are insufficient for reliable use to correct for its contribution and assumptions must be made to apply the chloride-mass balance approach.

AAPG Search and Discovery Article #90080©2005 GCAGS 55th Annual Convention, New Orleans, Louisiana