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Description
Borrego Valley is located in Northeast San Diego County, California, near the western border of Imperial County, California. This area is within the Colorado Desert geomorphic province of California. The study area includes the alluvium and sedimentary-rock filled valley floor, and the crystalline-rock mountains of the surrounding watersheds. Development of Borrego Valley began in the early part of the 20th century, and increased in the period following World War 2. Groundwater has been used extensively to support agricultural, municipal, and recreational water demand. Extensive use of groundwater has lead to overdraft of the aquifer system. Groundwater levels in the basin decreased as much as 140 feet during the period 1945 through 2000. The cumulative depletion of groundwater from the aquifer system for that period was over 500,000 acre-feet. The overuse of groundwater has led to adverse impacts including wells going dry, increased operational costs, and deterioration of water quality. Previous studies understated the impacts of historic and future groundwater use in excess of recharge. The objectives of this study were to describe the hydro geology of the Borrego Valley aquifer system, develop and calibrate a groundwater flow model, and use the model to predict future conditions based on anticipated use scenarios. The Borrego Valley aquifer system is comprised of four hydrostratigraphic units, including young, intermediate, and old Quaternary alluvium, and the Tertiary Palm Spring Formation. Groundwater is contained in, and flows through each of these units. The alluvial hydrostratigraphic units are comprised of alluvial fan, intermittent-stream, and lacustrine sediments. The Palm Spring Formation includes sediments deposited in a shallow-lake or delta. Groundwater flow is from the recharge areas around the margin of the aquifer system to both the local and regional topographic lows. Groundwater flow is generally horizontal with an absence of vertical hydraulic gradients. A three-dimensional finite-difference model, MODFLOW, was used to simulate and evaluate groundwater flow in the aquifer system for the period 1945 through 2000. The aquifer system was simulated as six-layers. Calibration of the model was completed to provide estimates of the physical properties of soil textures in each model layer. The quality of calibration of the numerical model was evaluated by comparison with observed water level data from twenty wells in Borrego Valley. The calibrated model was utilized to quantify the impacts of future groundwater utilization in the aquifer system. A thirty-year hydrologic cycle was used to represent climatic conditions during the predictive modeling. Future groundwater utilization was estimated based on two scenarios. Both scenarios considered agricultural, recreational, and municipal groundwater demand for the period 2001 through 2030. Maximum future declines in groundwater levels for the scenarios range from 105 to 170 feet for the predictive modeling period. Observed groundwater levels in the Borrego Valley aquifer system indicate a condition of overdraft. Numerical modeling of the aquifer system predicts continued overdraft of the system under two use scenarios. Overdraft to the aquifer system is expected to exacerbate existing adverse impacts, and lead to new adverse impacts such as intrusion of poor-quality connate water from formational deposits.
