High Plains aquifer water quality varies from state to state
By United States Geological Survey Thursday, April 24, 2008
The U.S. Geological Survey (USGS) released a report March 11 describing water-quality conditions in the High Plains aquifer.
The High Plains aquifer, which encompasses an eight state region from South Dakota to Texas, is the most heavily pumped aquifer in the nation. The report represents the first systematic assessment of water quality in this nationally important aquifer and concludes that water produced by domestic, public supply, and irrigation wells in the aquifer generally was acceptable for most uses, although differences in water quality among aquifer subunits exist.
The Ogallala Formation in the southern High Plains, Texas, had the poorest water quality and the Ogallala Formation in the central and northern High Plains, Colorado, Kansas and Nebraska, had the best water quality. Although the majority of water pumped from the High Plains aquifer is used for irrigation, nearly two million people depend on it for drinking water.
Although the USGS study did not analyze drinking water after treatment, the results from domestic supply wells were compared to federal drinking-water standards. According to USGS scientist Peter McMahon, “Most exceedances of primary and secondary drinking-water standards were for arsenic, dissolved solids, fluoride, iron, manganese, and nitrate. The most frequently detected pesticide compounds were atrazine and deethylatrazine, whereas the most frequently detected volatile organic compound was chloroform. None of the pesticide compounds or volatile organic compounds exceeded a primary drinking-water standard.”
The USGS report is based on analysis of ground-water samples from nearly 500 wells that were collected between 1999 and 2004. The study was designed to broadly characterize water quality throughout the aquifer and to understand the natural and human factors controlling water quality.
“The largest concentrations of human-made contaminants were found in shallow ground water. Although the shallow ground water is not typically used as a drinking-water supply, concern arises where it may be hydrologically linked to deeper parts of the aquifer. The study shows that long-term pumping of irrigation and public supply wells, and leakage down inactive irrigation wells, are considered to be major processes for moving contaminants from near the water table to deeper zones more rapidly than would occur otherwise under natural conditions” said McMahon.
The combination of chemical inputs to the water table from land surface and mixing n the aquifer by pumping has resulted in gradual increases in concentrations of contaminants such as nitrate and dissolved solids in the ground water.
Once contaminated, deep zones in the aquifer where production wells are screened are not likely to be remediated quickly because of long ground-water residence times in the aquifer and slow rates of contaminant degradation. The study found that deep ground water in some parts of the aquifer was about 10,000 years old, indicating that it was recharged near the end of the last Ice Age. That old water still contains small amounts of nitrate of natural origin that was present at the time of recharge.
“The availability and sustainability of water supplies in the High Plains aquifer are influenced by many factors, one of which is water quality. This water-quality assessment establishes a regional baseline against which water-quality conditions can be tracked over time and provides process-level understanding to help explain observed changes,” said McMahon.
The High Plains aquifer encompasses parts of Colorado, Kansas, Nebraska, New Mexico, Oklahoma, South Dakota, Texas, and Wyoming.
The report, is available at http://pubs.usgs.gov/pp/1749/, or by calling 1-888-ASK-USGS. For more information visit www.usgs.gov.
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