Water quality and the bad-water (saline-water) zone of the Edwards (Balcones Fault Zone) Aquifer

John M. Sharp
Brian A. Smith

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The Edwards aquifers are typically faulted, karstified, and transmissive. Water quality is generally excellent; the hydrochemical facies is mostly a calcium bicarbonate water with total dissolved solids (TDS) <500–1000 mg/L. Exceptions to this result from both natural and anthropogenic factors. In the Edwards Plateau, mixing of the formation water with underlying water from the Trinity aquifers or Permian rocks increases salinity to the west. Along the Balcones fault zone, the southern and eastern borders of the Edwards (Balcones Fault Zone) Aquifer are demarcated by a bad-water line where salinity rises to over 1000 mg/L. Detailed studies show that this line is a band, because salinities in the aquifer are not uniform with depth. The bad-water (or saline-water) zone is relatively stable over time, and six hydrochemical facies were identified, which are created by different combinations of dissolution of evaporite and other minerals, mixing with basinal brines, dedolomitization, and cross-formational flow from underlying formations. Flow in this zone is restricted, the waters are reducing, and recent studies suggest that microbes play important chemical and physical roles. The bad-water zone has sufficient water in storage and sufficient permeability so that desalination could be a future water-source option.