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Description
San Diego County contains many small alluvial basins which have potential to be used for small scale conjunctive use of surface and ground water. A numerical model was constructed in order to test this potential. San Dieguito basin, located on the coast near Del Mar, was used for the study because it is relatively large and can be recharged from Lake Hodges, which has overflowed 23 times since 1920. The model was used to determine potential long-term basin yield, evaluate the anger of sea water intrusion and ways of reducing the danger, and test the ability of the basin to accept recharge. Geology and hydrology of the basin were studied before the model was constructed. The basin is surrounded by marine sedimentary rocks of Eocene age underlain by Mesozoic crystalline rocks, and is filled with recent alluvium. Lithology of the alluvium was mapped using water well logs supplemented with a resistivity survey. Lithologies found suggest depositional environments of fluvial sands in the upper basin, beach sands near the coast, lagoonal clays and silts in the rest of the lower basin, and floodplain silts containing sand-filled buried channels in the remaining areas. Most of the alluvial aquifer is unconfined, but a portion of the narrow middle basin appears to be a multi-layered confined aquifer. Except for spill from Lake Hodges, only small amounts of local runoff water act as surface recharge to the basin. Subsurface recharge consists of poor quality water which enters the basin from surrounding Eocene sediments, and sea-water which enters during periods of reversed water table gradients. Pinder's finite-element model, ISOQUAD II, was used after modification for water table conditions. The model was designed with a constant-head oundary at the ocean and at the head of Ozuna Valley, and constant flux boundaries in areas representing Eocene rocks. San Dieguito River was modeled as a constant-head boundary when flowing, and disregarded at other times. Steady state calibration was made to Spring, 1982, and October, 1961, water tables. Transient calibration was made using .the period October, 1961, to April, 1962. After calibration, the model was used to determine storage, yield, and rates of recharge. Useful storage in the basin was found to be 7,770 acre-feet, yield during a single nine-month pumping season to be greater than 5,500 acre-feet, and maximum recharge to be 4 , 010 acre-feet during the first month, 593 in the second and 378 in the third, with smaller amounts in the following months. At least 79 additional acre-feet can be recharged per month with aggressive recharging techniques. Using historical data, long-term yield in the basin was determined to be 1,650 acre-feet per year. Sea water intrusion appears to not be a problem if pumping is confined to the upper basin, but the results are not conclusive.
