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Modeling of ground-water flow and surface water/ground-water interactions of the San Pedro River Basin, Cochise County, ArizonaGround-water exploitation in the Upper San Pedro Basin has produced the formation of a cone of depression around the Sierra Vista-Fort Huachuca area. A portion of the mountain front recharge that otherwise would reach the San Pedro River is being intercepted by pumping, and portions of baseflow are being captured by pumping. The purpose of this study is to construct a simulation model capable of simulating the ground-water system as well as the ground-water-surface water interactions. The flow simulation was done by a three-dimensional, finite-difference ground-water flow model (MODFLOW) that incorporates a new stream-aquifer interaction package. Steady state simulations were performed to represent mean annual conditions. Transient simulations cover a 48 year period, starting in 1940 and ending in 1988. A sensitivity analysis of the steady state model was also performed.
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Flow and water quality relations between surface water and ground water in the Puerco River basin near Chambers, ArizonaThe Puerco River is an ephemeral stream that received effluent from uranium-mine dewatering operations from the 1950's until 1962 and from 1968 until mining ceased in 1986. Flow and water-quality relations between the Puerco River and the alluvial aquifer underlying it were investigated at a site near Chambers. Data collection included installing and sampling nine monitor wells and two drive points; monitoring stage and sampling surface water; and slug testing wells. The stream recharges the alluvial aquifer during periods of flow and the streambed is a location of ground-water discharge by evapotranspiration during periods of no flow. Discharge by evapotranspiration may exceed recharge thus reducing the potential for contaminant movement away from the river by advective transport. Geochemical modeling indicates that uranium minerals are undersaturated in the range in Eh observed. A +0.84 correlation was calculated relating dissolved uranium concentration to depth in monitor wells suggesting the stream is a source of uranium to the alluvial aquifer. (Abstract shortened with permission of author.)
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A GEOCHEMICAL APPROACH TO DETERMINE GROUND-WATER FLOW PATTERNS IN THE SIERRA VISTA BASIN, ARIZONA, WITH SPECIAL EMPHASIS ON GROUND-WATER/SURFACE-WATER INTERACTIONWater quality in the Sierra Vista Ground-Water Basin is of extreme importance due to the basin's unique ecosystem and predicted future population growth. Portions of the Upper San Pedro River, flowing through the Sierra Vista Basin, contain some of the few remaining perennial streamflows in the southwest. Baseflow in the perennial reaches of the river are maintained almost entirely by the regional and floodplain aquifer systems. A population increase is predicted for the Sierra Vista Basin, and an impact on groundwater quality and availability can be expected. Due to the closely linked hydrologic systems within the basin, contamination or depletion of the regional aquifer could have direct implications for the San Pedro River. Water samples were collected within the study area from the regional and floodplain aquifers, the San Pedro River, and a bedrock spring in the Huachuca Mountains. Samples were analyzed for field parameters, major-ions, and stable isotopes to describe the main chemical characteristics of the hydrologic systems within the basin. Analysis of regional aquifer geochemistry indicates a ground-water system strongly controlled by calcite precipitation. Specific conductance, deuterium and oxygen-18 values indicate a mixing of regional-aquifer ground water and San Pedro River surface water within the floodplain aquifer. Estimates of inflow to perennial reaches of the floodplain aquifer from the regional aquifer vary from 50 to 80%, depending on location. Inflow to the San Pedro River at Charleston from the regional aquifer is estimated to be about 50 to 70% of the stream discharge.
