The High Plains aquifer is a nationally important water resource that underlies about 174,000 square miles in parts of eight western states. The aquifer serves as the primary source of drinking water for most residents of the region, and it also sustains more than one-fourth of the nation’s agricul-tural production. Understanding water-quality conditions and the natural and human factors that control water quality in this aquifer is important because of the implications for human health, the sustainability of water supply and rural agricultural economies, and the substantial costs associated with land and water management, conservation and regulation.

During the 1980s, the U.S. Geological Survey’s (USGS) Regional Aquifer-System Analysis (RASA) program char-acterized the hydrogeology of the High Plains aquifer and developed numerical models of ground water flow to fore-cast changes in water storage caused by pumping. That work established the benchmark for water quantity and water supply in this important aquifer against which regional changes could be compared, and the USGS continues to monitor the amount of water stored in the aquifer. Information on the quality of water within an aquifer system is complementary to understanding the quantity of available water. It is the combination of water quantity and quality in an aquifer that determines what uses the ground water resource can sustain. Prior to this study, there had been no comprehensive assessment of water quality in the aquifer that could serve as a baseline against which regional changes in quality could be compared.

The USGS National Water-Quality Assessment (NAWQA) Program is designed to provide a comprehensive assessment of the status, trends and factors controlling water-quality conditions in the nation’s streams and aquifers. Full implementation of the NAWQA Program began in 1991. During its first cycle of studies, NAWQA delineated study-area boundaries primarily on the basis of surface-water drainages. That approach was effective for assessing surface-water resources and relatively small aquifers, but it was not necessarily suitable for under-standing the water quality in regional aquifer systems that underlie multiple surface-water drainages, such as the High Plains aquifer. Understanding water-quality conditions in regional aquifer systems is complicated by spatial variability in the controlling factors inherent in regional studies and by temporal variability in recharge that may span thousands of years. Studies designed to assess water quality in regional aquifers need to account for that variability. In 1998, the NAWQA Program selected the High Plains aquifer as a pilot for assessing water quality based on a regional aquifer system approach. The effort is referred to as the High Plains Regional Ground-Water (HPGW) study. The foundation for the HPGW study was a 6-year, high-intensity phase of water-quality data collection that occurred from 1999 to 2004.

The HPGW studies were designed to assess linkages between the quality of water recharging the aqui-fer, the effect of transport through the hydrologic system on water quality, and the quality of the used resource as represented by water pumped from domestic, public-supply and irrigation wells. When these study components are combined, they form what is known as a Source-Transport-Receptor (STR) model for water-quality assessments. Within this model, studies were designed to address the NAWQA goals of water-quality assessment and process understanding. The process component included the study of critical processes or factors of regional importance such as recharge, ground-water flow directions and ages, and gradients in land use/land cover and climate that helped to explain baseline conditions. A stratified, nested well network was designed within the STR model. It facilitated up-scaling of monitoring (assessment) results to unmonitored areas of the aquifer, as well as up-scaling of process understanding from local to regional scales. 
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This article is provided through the courtesy of the U.S. Geological Survey, and is excerpted from its professional paper, “Water-Quality Assessment of the High Plains Aquifer.” The paper was written by Peter McMahon, Kevin Dennehy, Breton Bruce, Jason Gurdak and Sharon Qi. To see the entire report, go to www.usgs.gov.