Geology topics
Publications and source records attributed to H. T. Mitten.
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This report contains listings of the model input and sample output for simulation of the Pajaro Valley aquifer system, Santa Cruz and Monterey Counties, California. The files are contained on a 5 1/4-inch diskette. The decompressed files require approximately 5.3 megabytes of disk space on an IBM-compatible microcomputer. (USGS)
Because of the imbalance between recharge and pumpage, groundwater levels declined as much as 100 ft in some areas of Borrego Valley, California during drinking 1945-80. As an aid to analyzing the effects of pumping on the groundwater system, a three-dimensional finite-element groundwater flow model was developed. The model was calibrated for both steady-state (1945) and transient-state (1946-79) conditions. For the steady-state calibration, hydraulic conductivities of the three aquifers were varied within reasonable limits to obtain an acceptable match between measured and computed hydraulic heads. Recharge from streamflow infiltration (4,800 acre-ft/yr) was balanced by computed evapotranspiration (3,900 acre-ft/yr) and computed subsurface outflow from the model area (930 acre-ft/yr). For the transient state calibration, the volumes and distribution of net groundwater pumpage were estimated from land-use data and estimates of consumptive use for irrigated crops. The pumpage was assigned to the appropriate nodes in the model for each of seventeen 2-year time steps representing the period 1946-79. The specific yields of the three aquifers were varied within reasonable limits to obtain an acceptable match between measured and computed hydraulic heads. Groundwater pumpage input to the model was compensated by declines in both the computed evapotranspiration and the amount of groundwater in storage. (USGS)
Groundwater development has resulted in lowered water levels and seawater intrusion in the Pajaro Valley, California. An investigation was undertaken to describe the geohydrology of the groundwater flow system and to evaluate the response of the system to pumping stresses by using a mathematical model. The aquifer system consists of three aquifers. The lower aquifer is in fluvial sequences of Quaternary Aromas Sand below interbedded clay layers. The middle aquifer is in upper fluvial and lower eolian sequence of Aromas Sand, and in overlying basal gravels in terrace deposits and alluvium. Weathered soil zones in the Aromas Sand, and clay layers in the terrace deposits and alluvium overlie the middle aquifer. The upper aquifer is actually many discontinuous water bearing zones in the Aromas Sand, terrace deposits, alluvium, and dune sand. The three aquifers are represented in the mathematical model by three model layers separated by two confining layers. Model-generated water budgets for the 11-year simulation period show that storage decreased by 23,000 acre-ft, mostly during the 1976-77 drought. The calibrated model can simulate, with acceptable accuracy, both semiannual and long-term trends of potentiometric heads in parts of the lower and middle layers. (USGS)
Municipal pumpage data for 1931-80 is summarized for the Fresno area, California. Pumpage data for 269 wells were compiled for the cities of Fresno and Clovis, Fresno County, and other nearby water districts. Municipal pumpage for wells in the city of Fresno increased from 7,690 million in 1931 to 23,924 million gallons in 1980. The data will be used by Fresno County to assist in the development of water management plan for the Fresno area. (USGS)
The U.S. Geological Survey, in cooperation with Fresno County, is developing a computer model of the unconfined aquifer system in the Fresno area. In the area, consolidated rocks are overlain by unconsolidated deposits. The unconsolidated deposits are divided into a lower fine-grained unit and an overlying coarse-grained unit. The system is assumed to be a single unconfined unit. The top of the system is the water table and the bottom is the base of the coarse-grained deposits. The northeastern boundary is the only natural lateral hydrologic boundary. The other lateral boundaries are placed so that errors along them will not affect heads in the area of interest. Initial heads for 1943, hydraulic conductivity, and bottom elevations were used in a steady-state model that generated net flux values. It is anticipated that most of the remaining effort will center on model calibration. (USGS)
In response to increasing demand on water supplies and declining water levels in San Joaquin County, the U. S. Geological Survey, in cooperation with the San Joaquin County Flood Control and Water Conservation District, is evaluating the potential for artificially recharging the aquifer system in eastern San Joaquin County, California. Through a well canvass and analyses of existing data on geology, soils, and drillers logs, this study (phase one of three phases) resulted in identification of 20 sites for exploratory test drilling in areas potentially favorable for artificial recharge. Ten of the sites are in areas adjacent to the Mokelumne River, six are in areas adjacent to the Calaveras River and Mormon Slough, and four are north of Littlejohns Creek. (USGS)
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