USGS


Simulation of Ground-Water Flow in the Irwin Basin Aquifer System, Fort Irwin National Training Center, California

By Jill N. Densmore

 

U.S. GEOLOGICAL SURVEY

Water–Resources Investigations Report 02-4264

Sacramento, California 2003


Prepared in cooperation with the Fort Irwin National Training Center



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Abstract

Ground-water pumping in the Irwin Basin at Fort Irwin National Training Center, California resulted in water-level declines of about 30 feet from 1941 to 1996. Since 1992, artificial recharge from wastewater-effluent infiltration and irrigation-return flow has stabilized water levels, but there is concern that future water demands associated with expansion of the base may cause a resumption of water-level declines. To address these concerns, a ground-water flow model of the Irwin Basin 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.


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.


Particle-tracking simulations were used to determine the pathlines and the traveltimes of water high in dissolved solids into the main pumping area. The pathlines of particles from two areas with high dissolved-solids concentrations show that in 50 years water from these areas almost reaches the nearest pumped well.


CONTENTS

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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Water Resources of California


U.S. Department of the Interior, U.S. Geological Survey
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