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. .' '" r-. if) - N o Q ~ Development costs for ground water from the upper confined aquifer in a depth range of 1000 feet to 2500 feet are estimated at $50/af/yr pumping twelve months, and up to $80/af/yr for pumping four. months each year, based on an average transmissivity of 250,000 gpd/f~ How- ever, these water development costs are highly dependent on aquifer transmissivity, which in .' turn, has been estimated for the San Luis Valley from minimal amounts of site-specific data. .' .t:.~ 0", ./u Ground water in the deep confined aquifer, below a depth of about 2500 feet. in the central portion of the Valley, is expected to show total dissolved solids (TDS) concentra- tions greater than 3000 mg/l. Above a depth of 2500 feet, ground water is expected to show TDS averaging less than 500 mg/l. Near the rift-related fault zones which form the east and west edges of the Alamosa Horst, and in several other places in the Valley, high heat content and low to moderate production rates (up to 100 gpm) appear to be feasible from wells drilled deeper than approximate ly 3000 fee~ It is expected, however, that water quality from the rift-related geothermal system would be quite poor (TDS ranging from 3000 mg/l to as high as 10,000 mg/l). In the foothills of the San Juan mountain range, the upper 2000 feet of the Conejos Formation constitutes a thick, near-surface, and probably unconfined aquifer. It is prob- able that wells having acceptable water quality (less than 1000 mg/l) can be constructed in this aquifer with production rates of at least 500 gpm. In the Rio Grande fan area, it appears likely that wells could be constructed with higher production rates (up to 2000 gpm) and acceptable water quality in the upper confined aquifer above a depth of about 2000 fee~ Institutional aspects of developing either of these two aqUifer locations would be a primary concern. __Compilation and. synthesis of existing geologic and geophysical data, together with analysis of new satellite imagery, has made possible one of the most extensive evaluations of Jhe geologic structure that has been performed by any entity for the San Luis Valley aquifers. This evaluation shows a complex layered sequence of unconsolidated sediment deposits, extensive faulting criss-crossing the Valley, and possible wide shear zones cross- ing the Valley from the San Juan Mountains to the Sangre de Cristos. The complexity of the geologic structure has led to the conclusion that any modeling of the Valley would depend on estimates of physical aquifer parameters that are highly subjectiv~ Thus, any Valley-wide modeling results with existing data may be subject to significant changes in the futur~ Several locations within the Valley have been identified where there is upward leakage within the deep confined aquifer and the shallow confined aqUifer. If this leakage between the aquifers occurs throughout most of the San Luis Valley, this may indicate a tributary condition for the shallow confined aquifer (above a depth of 2500 feet), and possibly for the deep confined aquifer which lies beneath most of the Valle~ In summary, the Phase I studies conclude that it is not economically feasible to develop water for irrigation from the deep (below 3000 feet) confined aquifer due to rela- tively low transmissivity and poor water qualit~ For the shallow confined aquifer between 1000 and 2500 feet, it is also concluded that development of water for irrigation would not be economically feasible at this time. For the confined aquifer, no definite conclusions can be reached from Phase I studies regarding the institutional issues of tributariness of the aquifer and potential harm to existing water rights.