Hydrogeology and water quality of the shallow unconsolidated-rock aquifers in the Upper Arkansas River Basin from Buena Vista to Salida, Colorado

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Population growth near Buena Vista and Salida in Chaffee County has caused concern for the long-term availability and quality of groundwater . Groundwater withdrawals from deep wells that are completed in the basin-fill deposits and designed to supply multiple-dwelling developments could cause drawdowns in water levels of the overlying aquifers. Continued development of groundwater supplies in other parts of the county also could affect groundwater quality in the aquifers and cause drawdown of water levels in nearby existing wells.

The U.S. Geological Survey is assessing the possible effects of increased groundwater withdrawals on the availability and quality of groundwater.

The upper Arkansas River Basin between Buena Vista and Salida, Colorado, is a downfaulted basin, the Buena Vista-Salida structural basin, located between the Sawatch and Mosquito Ranges. The primary aquifers in the Buena Vista-Salida structural basin consist of poorly consolidated to unconsolidated Quaternary-age alluvial and glacial deposits and Tertiary-age basin-fill deposits. Maximum thickness of the alluvial, glacial, and basin-fill deposits is about 5,000 feet, but 95 percent of the water-supply wells in Chaffee County are no more than 300 feet deep. Hydrologic conditions in the 149-square mile study area are described on the basis of hydrologic and geologic data compiled and collected during September 2000 through September 2003. The principal aquifers described in this report are the alluvial-outwash and basin-fill aquifers.

An estimated 3,443 wells pumped about 690 to 1,240 acre-feet for domestic and household use in Chaffee County during 2003. By 2030, projected increases in the population of Chaffee County, Colorado, may require use of an additional 4,000 to 5,000 wells to supply an additional 800 to 1,800 acre-feet per year of groundwater for domestic and household supply.

The estimated specific yield of the upper 300 feet of the alluvial-outwash and basin-fill aquifers ranged from about 0.02 to 0.2. Current (2003) and projected (2030) groundwater withdrawals by domestic and household wells are less than 1 percent of the estimated 472,000 acre-feet of drainable groundwater in the upper 300 feet of the subsurface. Locally, little water is available in the upper 300 feet. In densely populated areas, well interference could result in decreased water levels and well yields, which may require deepening or replacement of wells.

Infiltration of surface water diverted for irrigation and from losing streams is the primary source of groundwater recharge in the semiarid basin. Groundwater levels in the alluvial-outwash and basin-fill aquifers vary seasonally with maximum water levels occurring in the early summer after snowmelt runoff peaks. Because of the drought during 2002, relatively large declines in groundwater levels occurred in about one-half of the monitored wells. Differences in water-level altitudes in shallow and deep wells indicate the potential for downward flow in upland areas and support results of preliminary cross-sectional models of groundwater flow. The apparent mean age of groundwater recharge ranged from about 1 to more than 48 years before 2001. The older (pre-1953) water was from wells that were located in groundwater discharge areas. Groundwater flow in the Buena Vista-Salida structural basin drains eastward toward the Arkansas River and, locally, toward the South Arkansas River.

Groundwater in the alluvial-outwash and basin-fill aquifers generally is calcium-bicarbonate water type with less than 250 milligrams per liter dissolved solids. Nitrate concentrations generally were less than 1 to 2 milligrams per liter and do not indicate widespread contamination of groundwater from surface sources.