Abstract

The population of Delta County, Colorado, like that in much of the Western United States, is forecast to increase substantially in the next few decades. A substantial portion of the increased population likely will reside in rural subdivisions and use residential wells for domestic water supplies. In Colorado, a subdivision developer is required to submit a water-supply plan through the county for approval by the Colorado Division of Water Resources. If the water supply is to be provided by wells, the water-supply plan must include a water-supply report. The water-supply report demonstrates the availability, sustainability, and suitability of the water supply for the proposed subdivision. During 2006, the U.S. Geological Survey, in cooperation with Delta County, Colorado, began a study to develop criteria that the Delta County Land Use Department can use to evaluate water-supply reports for proposed subdivisions.

A table was prepared that lists the types of analyses and data that may be needed in a water-supply report for a water-supply plan that proposes the use of ground water. A preliminary analysis of the availability, sustainability, and suitability of the ground-water resources of Rogers Mesa, Delta County, Colorado, was prepared for a hypothetical subdivision to demonstrate hydrologic analyses and data that may be needed for water-supply reports for proposed subdivisions.

Rogers Mesa is a 12-square-mile upland mesa located along the north side of the North Fork Gunnison River about 15 miles east of Delta, Colorado. The principal land use on Rogers Mesa is irrigated agriculture, with about 5,651 acres of irrigated cropland, grass pasture, and orchards. The principal source of irrigation water is surface water diverted from the North Fork Gunnison River and Leroux Creek. The estimated area of platted subdivisions on or partially on Rogers Mesa in 2007 was about 4,792 acres of which about 2,756 acres was irrigated land in 2000.

The principal aquifer on Rogers Mesa consists of alluvial-fan deposits that overlie shale and, locally, sandstone. Maps of the base of the aquifer, the water table, and the saturated thickness of the aquifer were prepared from data from the well files of the Colorado Division of Water Resources. The base of the aquifer generally is topographically higher than the valleys of the North Fork Gunnison River and Leroux Creek, and direct hydraulic connection of the aquifer to North Fork Gunnison River and Leroux Creek is limited. The aquifer is recharged primarily by infiltration of surface water diverted for irrigation. Ground water discharges to seeps and springs and through slope deposits at the boundaries of the aquifer. Data from the well files also were used to estimate the specific capacity of wells and to estimate the transmissivity and hydraulic conductivity of the aquifer.

A water budget was used to estimate recharge to and discharge from the aquifer. Although storage within the aquifer likely varies seasonally and from year to year, it was assumed that there were no long-term changes in ground-water storage. Estimated average annual recharge to and discharge from the aquifer during November 1998 through October 2006 were about 30,767 acre-feet per year. Although sufficient ground water is available on Rogers Mesa for additional domestic water supplies, conversion of irrigated land to residential land use likely would reduce recharge to the aquifer, affecting the sustainability of ground-water supplies on Rogers Mesa. Stream-depletion analyses indicate that the ground water in the aquifer likely would be considered tributary ground water and additional uses of ground water to supply new subdivisions likely would require implementation of augmentation plans.

Although sufficient ground water is available on Rogers Mesa for additional domestic water supplies, conversion of irrigated land to residential land use likely would reduce recharge to the aquifer, affecting the sustainability of ground-water supplies on Rogers Mesa. Stream-depletion analyses indicate that the ground water in the aquifer likely would be considered tributary ground water and additional uses of ground water to supply new subdivisions likely would require implementation of augmentation plans.

Although the dissolved solids and dissolved sulfate concentrations in ground water from Rogers Mesa aquifer commonly exceeded the U.S. Environmental Protection Agency Secondary Maximum Contaminant Levels for drinking-water supplies, the quality of ground water from the aquifer generally is suitable for residential use. Concentrations of total nitrogen (nitrite plus nitrate, as nitrogen) in ground water ranged from 0.38 to 3.2 milligrams per liter and were less than the State of Colorado maximum contaminant level of 10 milligrams per liter. Concentrations of selenium from seeps and springs at the boundaries of the aquifer commonly exceeded 50 micrograms per liter, the State of Colorado maximum contaminant level for drinking-water supplies.

This preliminary evaluation of ground-water supplies on Rogers Mesa could be improved with the collection of additional data including: additional mapping of hydrogeologic features; more accurate locations and altitudes of wells; accurate estimates of water-budget components; measurements of ground-water levels; and collection and analyses of ground-water samples. The use of numerical models of ground-water flow could improve evaluations of the potential effects of changes in land and water use on the water budget, aquifer storage, stream depletion, and well interference.