Technical Announcement on Water Quality Studies of Southwest Basin-fill Aquifers - October 5, 2011

Recent studies by the National Water-Quality Assessment Program improve our understanding of the hydrogeology and groundwater quality of unconsolidated basin-fill aquifers in the arid to semiarid Southwest, and of the natural and human factors that affect the water quality in these aquifers. Results are available in two reports describing (1) hydrologic and groundwater-quality conditions in 15 Southwest basins (available at http://pubs.usgs.gov/pp/1781/), and (2) regional conceptual models of the natural and human factors affecting groundwater quality across these basins (available at http://pubs.usgs.gov/sir/2011/5020/). Study basins are located in Arizona, California, Colorado, Nevada, New Mexico, and Utah.

Water development, such as importation of surface water and redistribution of within-basin water, has changed saturated thicknesses, and altered groundwater flow velocities and directions. In some basins, development has caused recharge of excess irrigation water and discharge of groundwater from pumping to exceed natural rates of recharge and discharge by up to six or seven times. Development also has increased the susceptibility and vulnerability of aquifers to anthropogenic and natural contamination. Groundwater-quality issues that can be affected by development include elevated concentrations of dissolved solids, nitrate, arsenic, and uranium, and the presence of human-related organic compounds, such as pesticides and volatile organic compounds (VOCs).

Conceptual models indicate which sources and controls are most likely to influence concentrations of water-quality constituents. Natural factors, such as aquifer geologic composition, have important effects on dissolved solids, arsenic, and uranium. Flushing of natural accumulations in the soil zone to the water table is important for nitrate. Among human sources, infiltration of excess irrigation water is an important agricultural source of dissolved solids, nitrate, and pesticides. Point sources, irrigat­ed turf, and leakage from distribution pipes and sewer systems can be important urban sources of dissolved solids, nitrate, pesticides, and (or) VOCs. Factors unrelated to sources also influence susceptibility and vulnerability of aquifers to contamination. Important natural factors include presence of confining units, direction of vertical hydraulic gradients, concentrating effects of evapotranspiration, and aquifer geochemical conditions (such as redox and pH). Human-related factors include depth to water (in areas of artificial recharge), contribution of artificial recharge, pumping, well depth, preferential groundwater flow along wellbores, and changing land use, such as from agricultural to urban.

Information presented in the reports serves as a resource and foundation for those interested in the hydrogeology and groundwater quality of basin-fill aquifers in the southwestern United States. A related report describing statistical models of nitrate and arsenic occurrence is planned for release in 2012.

If you have any questions, please contact Susan Thiros (sthiros@usgs.gov; 801-908-5063).  Please feel free to distribute this information to your colleagues and members.