U.S. GEOLOGICAL SURVEY
WaterResources Investigations Report 02-4264
Sacramento, California 2003
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Historical data show that ground-water-level declines in the Irwin Basin between 1941 and 1996, caused the formation of a pumping depression near the pumped wells, and that recharge from the wastewater-treatment facility and disposal area caused the formation of a recharge mound. There have been two periods of water-level recovery in the Irwin Basin since the development of ground water in this basin; these periods coincide with a period of decreased pumpage from the basin and a period of increased recharge of water imported from the Bicycle Basin beginning in 1967 and from the Langford Basin beginning in 1992. Since 1992, artificial recharge has exceeded pumpage in the Irwin Basin and has stabilized water-level declines.
A two-layer ground-water flow model was developed to help better understand the aquifer system, assess the long-term availability and quality of ground water, and evaluate ground-water conditions owing to current pumping and to plan for future water needs at the base. Boundary conditions, hydraulic conductivity, altitude of the bottom of the layers, vertical conductance, storage coefficient, recharge, and discharge were determined using existing geohydrologic data. Rates and distribution of recharge and discharge were determined from existing data and estimated when unavailable.
Results of predictive simulations indicate that in 50 years, if artificial recharge continues to exceed pumpage in Irwin Basin, water levels could rise as much as 65 feet beneath the pumping depression, and as much as 10 feet in the wastewater-treatment facility and disposal area.
Abstract
Introduction
Location and Description of Study Area
Acknowledgments
Geohydrology
Geologic Description of Aquifer System
Faults and Ground-Water Boundaries
Aquifer Properties
Hydraulic Conductivity and Transmissivity
Storage Coefficient
Natural Recharge and Discharge
Ground-Water Pumpage, Water Use, and Artificial Recharge
Ground-Water Levels and Movement
Ground-Water Quality
Ground-Water Flow Model
Model Grid
Model Boundaries
Aquifer Properties
Hydraulic Conductivity and Transmissivity
Vertical Leakance
Storage Coefficient
Simulation of Recharge
Natural Recharge
Artificial Recharge
Simulation of Discharge
Model Calibration
Steady-State Model
Transient-State Model
Model Sensitivity
Simulated Effects of Future Pumpage
Ground-Water Flow Directions and Traveltimes
Limitations of the Model
Summary and Conclusions
Selected references
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