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The Carrizo-Wilcox Aquifer of Texas: Groundwater Chemistry, Origin, and Ages
Texas Water Development Board, P.O. Box 13231, Austin, Texas 78711-3231
The Carrizo-Wilcox Aquifer is one of the most prolific groundwater sources in Texas. Sixty Texas counties situated between the Rio Grande and the Red River rely to various degrees on groundwater from this aquifer, predominantly for irrigation and municipal water supply needs.
The Carrizo-Wilcox Aquifer consists primarily of sand interbedded with gravel, silt, clay, and lignite deposited in a fluviodeltaic environment. Groundwater in the aquifer is predominantly fresh to slightly saline. Total dissolved solids (TDS) concentrations are typically below 3 parts per thousand (ppt) (3,000 milligrams per liter [mg/l]). In 2005 and 2006, the Texas Water Development Board (TWDB) analyzed 331 groundwater samples for major and minor ions, trace elements, and radionuclides. Although the quality of the groundwater was generally good, some of the samples exceeded the maximum contaminant levels and secondary standards for nitrate, lead, fluoride, chloride, sulfate, iron, manganese, and dissolved solids.
Groundwater ages progressively increase along flow paths from the recharge areas to the downdip areas of the Carrizo-Wilcox Aquifer. Stable isotope data suggest that most of the samples originated as rainwater and underwent subsequent changes.
For the most part, groundwater TDS showed little change over time in wells from the northern and central sections of the Carrizo-Wilcox Aquifer although some changes were noted in wells in the southern section of the aquifer. Most changes in TDS were moderate, of no more than 0.1 ppt (100 mg/l) over the period of record; however, larger fluctuations were observed at locations in Zavala, Dimmit, and Frio counties. For the most part, the nitrate in groundwater has been within the admissible limits for drinking water.
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