Description
An investigation of the hydrogeology of the San Diego Formation was undertaken for the Sweetwater Authority of San Diego County, California, to determine if the San Diego Formation within the Chula Vista and National City city limits is a viable source of potable water and/or a potential storage aquifer. To evaluate these characteristics, four monitoring wells were completed to a depth of 800 feet below ground surface. These wells, along with four previously installed wells, were used to evaluate the hydrogeologic and geologic characteristics of the aquifer. Fossil shells and other species identified from wells in the study area suggest that the age of the San Diego Formation in Chula Vista and National City, California, is Pliocene/Pleistocene and younger, younger than previously determined by other authors. The deposition of the San Diego Formation, in the area investigated, appears to be controlled by the La Nacion fault zone. Lithologic logs, electric logs, and gravity survey data demonstrate that the formation is at least 800-feet thick in the study area. There is little lateral lithologic correlation between monitoring wells as the lithology is vertically and horizontally heterogenous. Depositional environments include non-marine and marine deposits. Sea-level fluctuation and/or fault activity is responsible for the marine/non-marine sequential deposition. The dominant direction of groundwater flow is west to northwest, the hydraulic gradient is 0.0013 in the production interval and 0.001 at the water table. Measured water levels are generally above sea-level suggesting that the current rate of production is not inducing large-scale seawater intrusion The rate of groundwater flow through the San Diego Formation within the study area is between 566 and 1,743 acre-feet/year. Water quality (TDS and chloride concentrations) is worse south of the Sweetwater River and towards the coast and better in the upper zones than in the lower zones. The water quality in the upper zones of monitoring wells SDSW-1 and SDSW-4 is near drinking water quality. Geochemistry analysis of water from monitoring wells within the study indicates evaporation/precipitation cycles, ion exchange, and groundwater mixing (between seawater and groundwater) are influencing the groundwater chemistry. Aquifer transmissivity, determined from developmental pump tests, indicates a range from 1,720 to 5,400 ft2/day, similar to those previously measured in the study area. Hydraulic conductivities estimated from grain size analysis and determined using vertical flowmeter surveys conducted on SDSW-2 and SDSW-4 show that the formation is vertically heterogeneous. Groundwater in storage in the San Diego Formation is 890,000 acre-feet in the study area. Approximately 120,000 to 240,000 acre-feet may be usable without adverse consequences to water quality and overlying structures. The lateral and vertical heterogeneity of the San Diego Formation, together with the poor water quality within the study area, implies that seasonal injection and removal of fresh water will require treatment of the output water. Direct development of shallow groundwater is a possibility, but further studies are required to verify that upward migration of poorer water quality will not degrade the shallow water.
