Numerical Simulation of Groundwater Conditions in a Coastal Aquifer, Southern California, USA

Abstract:

Development of local groundwater resources in coastal areas is influenced by freshwater recharge and discharge, topography, geologic structure, and changes in sea level. For the coastal San Diego area, a density-dependent cross-sectional groundwater flow model was constructed to examine the aquifer and flow characteristics. The model domain represents generalized conditions along an east-west transect that is described by data from two multiple-depth monitoring-well sites; the domain extends into the Pacific Ocean to the 120 meter (m) bathymetric contour. Vertically, the model was discretized into four zones that represent geologic formations; each zone was assigned a horizontal permeability based on aquifer testing. Temporally, the model was divided into two stress periods. The first stress period simulates pre-development conditions, with an instantaneous sea-level rise of 120 m starting 6,000 years ago. This is a simplified representation of transient conditions since the last glacial maximum. The second stress period simulates 60 years of groundwater development, which is represented in terms of net fresh water flow through the domain. Near the coast, observed water-quality data indicate (1) brackish-to-hypersaline groundwater at shallow depths, (2) fresh continental recharge at intermediate depths, and (3) seawater intrusion at depths greater than 300 m. In order to simulate these general groundwater conditions, vertical anisotropy of the upper permeability zone was increased, freshwater discharge was constrained to the seafloor (no discharge along the coast), and groundwater development was simulated as exceeding freshwater recharge (additional water is provided by depletion of freshwater reserves). This numerical testing identifies specific factors that influence current conditions and provides an initial assessment of resource management alternatives for the San Diego coastal aquifer.