Sources and Movement of Saline Groundwater in a Coastal Aquifer, Southern California, USA

Abstract:

Development of local groundwater resources in coastal areas is limited by the presence of saline groundwater. For a study in the San Diego area, a geochemical approach was used to investigate the sources and movement of saline groundwater in the coastal aquifer. Chemical and isotopic data were collected from multiple-depth monitoring-well sites near the San Diego coastline at discrete intervals to depths of more than 650 meters. The groundwater samples were analyzed for major and minor ions, the stable isotopes of hydrogen, oxygen, and strontium, and radioactive isotopes of tritium and carbon-14. Each chemical and isotopic tracer preserves some aspect of the hydrologic history of the groundwater ranging from the chemical characteristics (major and minor ions), to the source of water (stable isotopes of oxygen and hydrogen), to the types of rock encountered in the groundwater flow system (strontium isotopes), to time-since-recharge (tritium and carbon-14). By using sodium-to-calcium mass ratios, in combination with the isotopic data, the occurrence of saline groundwater as a result of seawater intrusion was distinguishable from groundwater in a previously-saline aquifer that has been “flushed” by fresher continental recharge. The systematic analysis of these tracers indicate that the sources and movement of saline groundwater in the coastal San Diego area are dominated by: 1) regional flow of higher-elevation precipitation that recharged many thousands of years ago along deep flowpaths; 2) recharge of local precipitation in relatively shallower portions of the flow system; and 3) intrusion of seawater that primarily entered the aquifer during pre-modern times. Use of multiple chemical and isotopic tracers provides unique insight regarding the processes affecting groundwater quality, enabling local water agencies to assess the groundwater resources in the coastal aquifer and begin to reduce the area's reliance on imported water.