USGS_GeophysicalLoggingInSLV_Report

Purpose and Scope confined-aquifer system consists of several alternating aquifer and confining layers rather than a single This report describes the results of geophysical aquifer layer and is thus referred to as the ?confined- logging in 32 wells in the CAWN from September aquifer system? (Hearne and Dewey, 1988). 1998 through May 2000. Logs from one well (SAG.6) The clays of the confining layer are referred to yielded inconclusive information that is not described as the ?blue clay? by local well drillers and as the in this report. Logs recorded or attempted to be ?clay series? by Emery and others (1973). Many litho- recorded in wells were video, caliper, water specific logic logs identify the clay series by its bluish color, conductance, water temperature, and water flow. This although the clays are not necessarily a bluish color in report describes well construction and generalized all locales. The upper confining layer lies at depths lithology, where available. Logs recorded in each well from 20 to greater than 100 ft throughout the central are discussed and correlations between the logs for a part of the San Luis Valley, with the deepest occur- well are made to determine the zones that contribute rences of this layer on the eastern side of the valley water to the well. This report also provides an evalua- and in the vicinity of Center (fig. 1) (Emery and tion of the applicability of water levels measured in others, 1973). each well for understanding local and regional changes The confined-aquifer system consists of varying in the potentiometric surface in the confined-aquifer combinations of mostly interbedded clay, sand, and system of the San Luis Valley in south-central Colo- gravel. Previous studies have not identified a contin- rado. uous confining layer that separates an upper confined aquifer from a lower confined aquifer. The interbedded layers of the confined-aquifer system are limited in Acknowledgments lateral extent and vary considerably in their hydraulic properties (Hearne and Dewey, 1988). The author is grateful to the individual well Generally, recharge to the confined-aquifer owners and the Rio Grande Water Conservation system occurs at the perimeter of the valley and District for allowing and facilitating access to the discharge occurs in the central portion of the valley CAWN wells for geophysical well logging and to (Hearne and Dewey, 1988). Wells completed in the Michael Haley of the USGS for obtaining logs in the confined-aquifer system have a water level that lies wells. Thanks are also given to Ken Watts and Fred above the bottom of the confining unit between the Paillet of the USGS for assistance in data interpreta- unconfined aquifer and the confined-aquifer system. tion. Water levels in some wells completed in the confined- aquifer system are above land surface (flowing), and in other wells the water level is below land surface GEOHYDROLOGIC SETTING (nonflowing). The geothermal gradient in the San Luis Valley The San Luis Valley occupies a structural basin varies throughout the valley. The average geothermal in south-central Colorado bounded by igneous, meta- gradient in the central portion of the valley, where morphic, and sedimentary bedrock. The basin contains most wells are located, is approximately an increase of valley fill that consists of interbedded deposits of sand, o F) per 100-ft increase in 3.17 degrees Fahrenheit ( clay, gravel, and some layers of volcanic rocks depth (Repplier and Fargo, 1981). (Robson and Banta, 1995). The valley fill, which can be as much as about 30,000 ft thick, is thinnest on the margins of the San Luis Valley and thickest in the center of the valley (Gaca and Karig, 1966). Most of WATER LEVELS AND HEADS IN THE the water produced by wells in the San Luis Valley CONFINED-AQUIFER SYSTEM comes from the upper part of the valley fill (Powell, 1958; Hearne and Dewey, 1988). The water level in a well penetrating a confined Two aquifers exist in the valley, the unconfined aquifer defines the elevation of the potentiometric aquifer and the confined-aquifer system, separated by surface at that point. The potentiometric surface repre- a fairly extensive confining unit composed of a series sents the hydrostatic pressure level, or head, in the of clay layers and unfractured volcanic rocks. The aquifer (Todd, 1980). The sum of three head compo- GEOHYDROLOGIC SETTING 3