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Description
This study evaluates using naturally occurring chloride to determine groundwater recharge from precipitation to fractured rock aquifers in eight field areas located in the mountains of central San Diego County. Water supply in this area largely comes from groundwater, and recharge regularly factors into land use and development decisions. The chloride mass balance (CMB) method has been successful in recharge analysis in semi-arid regions like San Diego. The individual field areas ("catchments") span most of the range of annual precipitation that occurs in the study area. The ratio of wet and dry ("bulk") atmospheric chloride (Cl[subscript p]) to chloride in groundwater (Cl[subscript gw]) represents the fraction of precipitation that recharges a catchment. Groundwater was collected from active domestic wells completed in the fractured rock aquifers in the upland zone of each catchment, considered the primary recharge zone and upstream of anthropogenic activity. Recharge was also assessed within each catchment using the program Recharg2, which is a water budget based on the Thornthwaite Method, and referred to as the soil moisture budget (SMB) method. Within each catchment Cl[subscript gw]) s mostly within a 90% confidence interval. There is a generally direct relationship between Cl[subscript gw]) and precipitation. One average value of Cl[subscript p]) was used. A general trend of increased recharge with increased precipitation was shown. Recharge was anomalously high in Warner catchment where precipitation was highest, possibly illustrating a constraint on settings in which the method would work. The SMB method results were comparable to the CMB method, except for Warner catchment, with a higher correlation to precipitation. A runoff sensitivity analysis showed the CMB method is not as sensitive to uncertainty in runoff compared to the SMB. Results of this study indicate the CMB method may be useful in assessing recharge in central San Diego. However, the Cl[subscript p] value(s) should be based on more localized data, and expanded groundwater sampling efforts would broaden the characterization of chloride in the aquifers. Soil properties and Recharg2 output data are included on a separate DVD. The DVD, an appendix to the thesis, is available for viewing at the SDSU Media Center of Library & Information Access.
