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Geophysical Logging to Determine Construction, <br />Contributing Zones, and Appropriate Use of Water <br />Levels Measured in Confined-Aquifer Network <br />Wells, San Luis Valley, Colorado, 1998?2000 <br /> Daniel L. Brendle <br />Abstract INTRODUCTION <br />The San Luis Valley in south-central Colorado is <br />Geophysical logs were recorded in 32 wells <br />a high-elevation valley with an average elevation of <br />in the confined-aquifer monitoring well network <br />about 7,700 feet (ft) above sea level and an area of <br />maintained by the Rio Grande Water Conserva- <br />about 3,000 square miles (Hearne and Dewey, 1988). <br />tion District. Logging results were used to deter- <br />Much of the central portion of the valley is used for <br />mine well construction, zones contributing water <br />agriculture, which is irrigated with diverted surface <br />to the wells, and the purposes for which the water and by ground water that comes from an uncon- <br />fined aquifer and a confined-aquifer system. <br />ground-water levels measured in the wells can be <br />Ground-water levels and the potentiometric <br />used. The confined-aquife r well network consists <br />head (head) in the confined-aquifer system are <br />of 42 flowing and nonflowing wells. This network <br />measured in 42 wells in the San Luis Valley (fig. 1) by <br />consists of wells used to supply water for irriga- <br />the Rio Grande Water Conservation District in cooper- <br />tion, household use, wildlife refuge supply, and <br />ation with the U.S.Geological Survey (USGS). This <br />stock use, and wells for water-level monitoring. <br />confined-aquifer well network (CAWN) includes <br />Geophysical logs recorded in the wells included <br />flowing and nonflowing wells and consists of irriga- <br />video, caliper, water specific conductance, water <br />tion, household, wildlife refuge supply, stock use, and <br />monitor wells. Water levels are measured in flowing <br />temperature, and water flow. Most wells in the <br />wells by stopping the flow at the surface (shut-in), <br />confined-aquifer well network yield a composite <br />measuring the pressure in the well with a transducer, <br />water level representing water levels in multiple <br />and converting the pressure measurements to water <br />permeable zones in the confined-aquifer system <br />level above land surface. Water levels are measured <br />of the San Luis Valley. A potentiometric-surface <br />manually in nonflowing wells with an electric or steel <br />map constructed using November 2000 water <br />tape. Many of the wells are open to multiple perme- <br />levels indicates that water levels from most wells <br />able intervals of the confined-aquifer system. <br />in the network are correlated with water levels <br />Because the construction of many of the wells in <br />from nearby network wells. Potentiometric- <br />the CAWN was not known or the integrity of the well <br />casings may have degraded since construction, the <br />surface maps that are constructed from water <br />USGS, in cooperation with the Colorado Division of <br />levels measured in most of the wells in the <br />Water Resources and Colorado Water Conservation <br />network can be used to understand long-term <br />Board, began geophysical logging of the wells to <br />local and regional changes in water levels in the <br />document well construction, determine the zones <br />confined-aquifer system. Water levels measured <br />contributing water to the wells, and determine the <br />in 8 of the 42 wells in the confined-aquifer <br />applicability of water levels measured in each well for <br />network are not representative of water levels in <br />understanding the hydrology of the confined-aquifer <br />the confined-aquifer system. <br />system. <br />Abstract <br />1 <br />