A 50-page study that looks at the ground water resources on Rogers Mesa concludes in part that the water table accessed by domestic wells is principally recharged by open irrigation, and also that pumping from domestic wells is a small component of the overall Rogers Mesa aquifer water budget equation.
According to the report, current pumping from permitted wells is not a major factor in the mesa’s water equation. The report states, “In 2004, 98 domestic wells, 40 residential wells, 21 irrigation wells, three commercial wells, and one livestock well had permits to pump water from the Rogers Mesa Aquifer. Ground water withdrawals are not routinely monitored or reported. Estimated ground water withdrawal by permitted wells from the Rogers Mesa aquifer was about 1,290 acre-feet per year.”
Storage capacity of the Rogers Mesa aquifer is estimated at 107,000 acre-feet, not all of which is accessible by well pumping. The annual surface water inflow to the aquifer is estimated at 42,228 acre feet, according to the report.
The study has been completed by the U.S. Geological Survey. Some of the study’s findings will be used by the county planning commission and the Leroux Creek APC in reviews of rural subdivision applications.
The study drew some criticism during a presentation at the Hotchkiss Fire Station on Aug. 12. Data used in the study was limited to what was already existing — very little new data sampling was conducted. Some of the data are 25 years old, and current sampling information from water sources is not comprehensive.
About 25 people including water users from Rogers Mesa attended the presentation. They questioned how the study would be used, and whether and how its methodology and database would be expanded and built upon.
There is a bigger question also: Does the study really give an accurate enough picture of the immensely complicated subject of geology and water resources to serve as a specific policy or planning tool?
The study deals with “big picture” issues, and its findings do not always apply to specific, localized conditions. For instance, while domestic well pumping may not be a significant factor in the mesa’s overall water equation, well pumping can change the performance of other nearby wells significantly.
Among statements in the study’s summary that could have an influence on land use policy decisions are the following:
• The potential effects to groundwater recharge and storage from changes in historic land and water use from irrigated agriculture to residential use (including changes from open irrigation to sprinkler systems) may be of greater concern than the small withdrawals of ground water by residential wells. In other words, because the aquifer is shallow and charged by open irrigation inflows, any change from open irrigation practices to piped or sprinkler systems would have an affect on the aquifer’s recharge rates, as would water use changes from ag to domestic.
• Although ground water is physically available in the Rogers Mesa aquifer to supply additional residential wells, its use may be limited by administrative rules of the Colorado Division of Water Resources.
• Relatively large concentrations of dissolved solids and sulfate and hardness in the water may limit its for residential supplies and/or require treatment.
Other local areas in Delta County have very hard water due to the mancos shale that underlies shallow aquifers. Water hardness can be dealt with by installing commercial “water softening” equipment
• Water in the Rogers Mesa aquifer is (probably) tributary water, thus augmentation plans would be required for the development of ground water from nonexempt wells for new subdivisions.
The Rogers Mesa groundwater aquifer is comprised of a loose accumulation of sand to large cobble-sized components including crushed shale and other clays and silt.
Analyzed samples showed concentrations of dissolved total nitrogen in the ground water are far below maximum allowed levels, indicating little if any contact with ISDS effluent, animal waste, or chemical fertilizers.
County administrator Susan Hansen said the study was commissioned partly in response to questions on the impact of domestic water wells on groundwater resources. Results of the study may be used in evaluating residential growth and water in other areas of the county.
Bruce Bertram, county solid waste coordinator, explained that Rogers Mesa was chosen as the study area for several reasons: it is a clearly defined geographic area; there was already some historical data on water resources available for the study; and it is believed to be similar to other areas of the county in the composition of its ground water system.
Hansen described the study as “a first step” towards developing a database on ground water resources for local policy makers. Bertram said the study will help establish a baseline of water data that will be interfaced with other water data – including the Wright Water study, Colorado School of Mines database effort, state engineer’s office, NRCS data gathering, Southern Piceance Basin study, information from well drillers, and other sources.
Of particular interest to planners and water users are the maps of Rogers Mesa and its water table showing elements of the system that include type of irrigated crop use, altitude of the water table and control points including springs and seeps, and the estimated saturated thickness of the aquifer across the mesa.
That saturated thickenss is estimated to range from 18 feet in the west to more than 100 feet in two small areas of the south central mesa.
The study was criticized by some for the sparseness and age of actual data used. But suggestion for collection and analysis of additional data to extend and complement the study are included. There are six specific suggestions offered that planners, water policy officials, and various commercial interests may be interested in pursuing in future.
• Errors that exist in hydrologic maps can be decreased by detailed field mapping to define the actual boundaries of the aquifer, to identify ground water discharge areas, and to precisely determine the locations and elevations of existing springs and wells.
• Estimates of the aquifer’s total water budget, of its water storage properties, and of the aquifer’s recharge capabilities could be improved by detailed measurement of water application and actual crop use.
• Water levels need to be measures for seasonal variations of the water storage of the aquifer.
• Additional tests of the aquifer and wells are needed to “decrease uncertainty” of estimates used in the study
• More data is needed on the chemical properties and changes in those properties as water flows through the aquifer.
• More data is needed to construct accurate numerical models of the aquifer.
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