Assessing Current and Future Performance of the Alamitos Gap Seawater Intrusion Barrier with a New Flow and Transport Model, Los Angeles and Orange Counties, California

To evaluate the Alamitos Gap seawater intrusion barrier, INTERA created a new flow and solute transport model for permitting, management, and performance assessment of the injection barrier on behalf of the Orange County Water District, the Water Replenishment District of Southern California, and the Los Angeles Department of Public Works.

We devised a new conceptual model, water balance, geologic model, and numerical flow and solute transport models. Developing a hydrogeologic framework for the complex aquifer system was challenging, because erosion and deposition on the uplifted transgression-regression system of aquitards and aquifers created pathways within the Gap for seawater to travel inland.  The pathways, called mergence zones, connect the seawater-intruded Recent Aquifer with several of the deeper aquifers used for water supply. 

Using the new conceptual and geologic models as a foundation, INTERA constructed a three-dimensional transient groundwater flow model using the MODFLOW 2000 code.  Over 19,000 head measurements in nearly 200 monitoring wells well were used for the transient calibration.  INTERA also constructed a three-dimensional transient transport model using the MT3DMS transport code.  The calibrated models were used to assess the performance of four different barrier configurations: a baseline (existing) and three future expansion options. We also investigated the impact of variable-density flow on Alamitos Barrier performance by simulating chloride migration through a sub-domain model that was run using SEAWAT. INTERA handed over the completed modeling tools to the three client agencies in autumn of 2010.