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U.S. GEOLOGICAL SURVEY

Numerical Simulation of Ground-Water Flow and Assessment of the Effects of Artificial Recharge in the Rialto–Colton Basin, San Bernardino County, California

Prepared in cooperation with the
SAN BERNARDINO VALLEY MUNICIPAL WATER DISTRICT

By Linda R. Woolfenden and Kathryn M. Koczot

Abstract

The Rialto–Colton Basin, in western San Bernardino County, California, was chosen for storage of imported water because of the good quality of native ground water, the known storage capacity for additional ground-water storage in the basin, and the availability of imported water. To supplement native ground-water resources and offset overdraft conditions in the basin during dry periods, artificial-recharge operations during wet periods in the Rialto–Colton Basin were begun in 1982 to store surplus imported water. Local water purveyors recognized that determining the movement and ultimate disposition of the artificially recharged imported water would require a better understanding of the ground-water flow system.

In this study, a finite-difference model was used to simulate ground-water flow in the Rialto–Colton Basin to gain a better understanding of the ground-water flow system and to evaluate the hydraulic effects of artificial recharge of imported water. The ground-water basin was simulated as four horizontal layers representing the river- channel deposits and the upper, middle, and lower water-bearing units. Several flow barriers bordering and internal to the Rialto–Colton Basin influence the direction of ground-water flow. Ground water may flow relatively unrestricted in the shallow parts of the flow system; however, the faults generally become more restrictive at depth. A particle-tracking model was used to simulate advective transport of imported water within the ground-water flow system and to evaluate three artificial-recharge alternatives.

The ground-water flow model was calibrated to transient conditions for 1945–96. Initial conditions for the transient-state simulation were established by using 1945 recharge and discharge rates, and assuming no change in storage in the basin. Average hydrologic conditions for 1945–96 were used for the predictive simulations (1997–2027). Ground-water-level measurements made during 1945 were used for comparison with the initial-conditions simulation to determine if there was a reasonable match, and thus reasonable starting heads, for the transient simulation. The comparison between simulated head and measured water levels indicates that, overall, the simulated heads match measured water levels well; the goodness-of-fit value is 0.99. The largest differences between simulated head and measured water level occurred between Barrier H and the Rialto–Colton Fault. Simulated heads near the Santa Ana River and Warm Creek, and simulated heads northwest of Barrier J, generally are within 30 feet of measured water levels and five are within 20 feet.

Model-simulated heads were compared with measured long-term changes in hydrographs of composite water levels in selected wells, and with measured short-term changes in hydrographs of water levels in multiple-depth observation wells installed for this project. Simulated hydraulic heads generally matched measured water levels in wells northwest of Barrier J (in the northwestern part of the basin) and in the central part of the basin during 1945–96. In addition, the model adequately simulated water levels in the southeastern part of the basin near the Santa Ana River and Warm Creek and east of an unnamed fault that subparallels the San Jacinto Fault. Simulated heads and measured water levels in the central part of the basin generally are within 10 feet until about 1982–85 when differences become greater. In the northwestern part of the basin southeast of Barrier J, simulated heads were as much as 50 feet higher than measured water levels during 1945–82 but matched measured water levels well after 1982. In the compartment between Barrier H and the Rialto–Colton Fault, simulated heads match well during 1945–82 but are comparatively low during 1982–96. Near the Santa Ana River and Warm Creek, simulated heads generally rose above measured water levels except during 1965–72 when simulated heads compared well with measured water levels.

Average annual total recharge calculated by the model during 1945–96 was about 33,620 acre-feet and average total discharge was about 35,220 acre-feet. Underflow from the Bunker Hill and Lytle Basins accounted for about 59 percent of total recharge. Seepage loss from the Santa Ana River and Warm Creek accounted for about 15 percent of total recharge. Underflow to the Chino and North Riverside Basins accounted for about 65 percent of total discharge. Model results for 1945–96 indicate that the quantity of water removed from storage was about 7,350 acre-feet, and the quantity going into storage was about 5,960 acre-feet, resulting in a net average storage depletion of about 1,390 acre-ft/yr.

A sensitivity analysis was done to determine the model inputs that were most important in affecting model-generated hydraulic heads at the calibration wells. In layers 1, 2, and 3, hydraulic heads were most sensitive to an increase in total recharge. Hydraulic heads in layer 1 also are sensitive to an increase in the streambed conductance. Hydraulic heads in layers 2 and 3 also are sensitive to removal of all internal barrier conditions. In layer 4, hydraulic heads were most sensitive to the removal of all internal barriers and removal of the unnamed fault. Hydraulic heads were least sensitive in layer 1 to a reduction in the primary storage coefficient and the removal of Barrier H; in layer 2, to an increase in the primary storage coefficient and the removal of Barrier H; in layer 3, to a decrease in the primary storage coefficient and an increase in the vertical conductance; and in layer 4 to an increase in the primary storage coefficient and a decrease in the primary storage coefficient.

Predictive simulations were made for three artificial-recharge alternatives, including artificial recharge at Linden Ponds, discontinued artificial recharge, and artificial recharge at Cactus Basin. To extend the ground-water flow model beyond 1996, average hydrologic conditions (natural recharge and discharge) for 1945–96 were used for the predictive simulation period, 1997–2027. Results of the predictive simulations indicate that artificial recharge at Linden Ponds causes water levels to rise northeast of the unnamed fault (which isolates the recharge ponds from the main water-producing part of the basin) but has little influence on water levels southwest of the unnamed fault. The water-level response to artificial recharge at Cactus Basin indicates that wells northwest of Barrier J and northeast of the unnamed fault are sufficiently isolated by the faults to be affected. Water levels southeast of Barrier J and north of the Santa Ana River and Warm Creek rose as much as 50 feet higher than discontinued recharge levels. Water levels near the Santa Ana River and Warm Creek were not affected with artificial recharge at either Linden Ponds or Cactus Basin, indicating that a longer period of recharge would be needed to raise water levels in this area.

Results of the particle-tracking simulations indicate that the imported water would reach production wells only with artificial recharge at Cactus Basin, suggesting that artificial recharge at Cactus Basin may make the imported water more available to production wells than recharge at Linden Ponds. The imported-water particles move only about a mile farther with artificial recharge at Linden Ponds than with discontinued recharge in the basin, indicating that artificial recharge may be only slightly more effective than the absence artificial recharge in the movement of imported water.

Contents

Abstract
Introduction
Geohydrology
Numerical Simulation of Ground-Water Flow
Simulated Effects of Artificial-Recharge Alternatives on Ground-Water Levels and Movement
Discussion of Results of Transient and Predictive Simulations
Use and Limitations of Ground-Water Flow and Particle-Tracking Models
Summary and Conclusions
References Cited
Appendix A

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Send questions or comments about this report to the author, L.R. Woolfenden , (916) 278-3014.