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. SAN LUIS VALLEY CONFINED AQUIFER STUDY Newsletter No. 4 JulylQ87 INTRODUCTION. The purpose of this new' letter is to update the Advisory Committee and the public regarding the progress of the San Luis Valley Confined Aquifer Study. This study was initiated in late March 1986 under the auspices of the Colorado Water Resources and Power Development Authority and will be completed in August, 1987. The'local sponsor of the project is the San Luis Valley Water Conservancy District. The consulting firm is HRS Water Consultants, Inc. The purpose of this study is to assess the physical, institutional, and economic feasibility of water development from the deep confined aquifer system of the San Luis Valley. Following are the conclusions and recommendations from the Phase I investiga- tions. CONCLUSIONS Early in Phase I it was concluded that these studies should focus first on the physical and economic feasibility of ground water development and should lay aside the institutional issues until more physical data could be assembled for the confined aqUifer. This led to the decision to concentrate initially on the confined aquifer system which lies below a depth of 3000 feet, where institutional questions might be of less concern. Reliable data available for characterizing the deep confined aquifer system are primar- ily geophysical logs and other geophysical information from oil and gas exploration efforts. Data from water wells in the deep confined aquifer are very sparse and are highly site- specific, yielding information primarily on the rift-related fault zones in the Alamosa Horst/Baca Graben areas. In this study, only one water well deeper than 3000 feet was successfully tested and logged. Water wells shallower than 2000 feet in the Valley yield usable information for characterizing the upper confined aquifer, but the depths from which the wells draw water are often not known. There are 11,881 wells in the Valley which are registered with the State Engineer's Office, but only 28 were found to be deeper than 2000 feet, and only 6 were found to be deeper than 3000 feet. Virtually all potential demand for water in the San Luis Valley is for irrigation - it has been estimated that some 300,000 acre-feet (af) alone could be used either as supple- mental water or for new lands. Under present market conditions. $15 to $20 per acre-foot per year ($/af/yr) is the average which farmers can afford to pay for supplemental irriga- tion water in the Valley. Under the most optimistic market conditions. the agricultural sector could possibly afford to pay up to $50/af/yr. . .... ,: " . ,';\ .' The cost to develop 100,000 af/yr of ground water from the deep confined aq'~'lfer is . estimated at $21O/af/yr pumping twelve months each year, and $370/af/yr for pumping four months each yeaG This cost estimate is based on development of ground water from aquifers below 2500 feet, having transmissivities averaging 10,000 gallons per day per foot (gpd!ft). ,.