University of California Water Resources Center

The California Department of Water Resources (2006) estimated a rise in mean sea level along California’s coastline ranging from 10 to 90 cm over the 21st century due to rising global mean surface temperature. This range of sea-level rise is consistent with the Intergovernmental Panel for Climate Change (2007) estimates. The rise in sea level threatens coastal aquifers by exacerbating the risk of saline intrusion. This study simulated the effect of sea-level rise on the Seaside Area sub-basin near the City of Monterey, California. The simulation was carried out with a state-of-art, finite-element, variable-density, numerical model that accounts for the effects of salinity on groundwater density and viscosity. Seawater intrusion was simulated for various scenarios of sea-level rise, varying from 0 m to 1 m assuming a linear increase of sea level through the 21st century. Each scenario contemplated the same level of predicted groundwater extraction through the 21st century in the study aquifers. The numerical simulations of seawater intrusion indicate that one meter of sea-level rise would contribute an additional 10 to 15 meters of inland spread of the 1,000 mg/L saline front and 20 to 30 meters of the 10,000 mg/L saline front. The effect of sea-level rise on seawater intrusion in the Seaside Area sub-basin, therefore, appears minor when compared with historical measurements of seawater intrusion caused primarily by groundwater pumping since the early 1900s. Other aquifers with less topographical relief and more complex hydrostratigraphy could be more vulnerable to sea-level rise, however. One such possibility is posed by the Oxnard Plain groundwater sub-basin, in Ventura County, California. This study compiled a hydrogeologic database and structured the basic elements of a sea-water intrusion numerical simulation model for the Oxnard Plain aquifer. The Oxnard Plain sub-basin is a complex, multi-formation, aquifer that has undergone several decades of groundwater extraction, and, which is known to experience seawater intrusion in several of its coastal areas. The Oxnard Plain sub-basin features an important offshore hydrogeologic section that encompasses its boundary under seawater. Time limitations prevented calibration and validation of the numerical simulation model for seawater intrusion in the Oxnard Plain sub-basin. Nevertheless, the elements needed to complete the Oxnard Plain sub-basin’s seawater-intrusion simulation model in the near future (with additional funding) have been assembled and are presented in this report. The approach presented in this report for the relatively assessment of groundwater extraction and sea-level rise effects on seawater intrusion into coastal aquifers holds potential for wide-ranging applicability in a variety of hydrogeologic settings. In particular, the finite-element spatial grid provides distinct capabilities to represent accurately the geographical layout of an aquifer.