Orange County Branch Newsletter

January 2012

Hydraulics and Hydrology Technical Group

Use of a Flow and Transport Model to Evaluate Current and Future Performance of the Alamitos Gap Seawater Intrusion Barrier, Los Angeles, CA


By Portia Gonzalez

ASCE HHTG December speaker was Mr. David Jordan.  Mr. Jordan is a Principal Hydrogeologist with INTERA.  He is a registered professional engineer with over 20 years of experience in quantitative hydrogeology, engineering, numerical modeling, GIS, data management, and project management. He received a BS in Geophysics from Virginia Tech and an MS in Geophysics from New Mexico Tech.  Mr. Jordan provides management and technical guidance to a variety of projects in water resources, GIS, and contaminant hydrogeology.  He has led technical teams in the development and application of regional groundwater flow and transport models for evaluating water availability, water rights, mine dewatering, and seawater intrusion for private, state, municipal, and water district clients throughout the Western United States. 

Seawater intrusion has threatened aquifers that supply Los Angeles residents since the 1920s, but now millions depend on effective resource management, especially for the four injection barriers. Even with approximately 6,000 acre-ft/yr injected through the Alamitos Gap barrier, which straddles the San Gabriel River, chloride concentrations have continued to increase in several aquifers inland of the barrier. On behalf of the Orange County Water District, the Water Replenishment District of Southern California, and the Los Angeles Department of Public Works, INTERA created a new flow and solute transport model for permitting, management, and performance assessment of the injection barrier.

We devised a new conceptual model, water balance, geologic model, and numerical flow and solute transport models in five months to meet a regulatory deadline. Quickly developing a hydrogeologic framework 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. Capturing their locations and geometries was critical to effectively simulating past and future seawater intrusion. We extended the new hydrogeologic framework beyond and below previous frameworks by combining traditional geologic interpretation, GIS analysis, and an innovative geologic modeling software tool.

Using the new conceptual and geologic models as a foundation, INTERA constructed a three-dimensional transient groundwater flow model, the Alamitos Barrier Flow Model (ABFM), using the MODFLOW 2000 code. The ABFM was calibrated in three ways: (1) a steady-state calibration to average heads for the 1999-2009 period, (2) transient calibration to the heads observed from 1999 through 2009, and (3) final flow calibration adjustments based on the chloride transport simulations. INTERA also constructed a three-dimensional transient Alamitos Barrier Transport Model (ABTM) using the conceptual model, the ABFM, and the MT3DMS transport code. 

Ten years of monthly measurements in nearly 200 monitoring wells were used to calibrate heads and chloride. The calibrated ABFM and ABTM models were used to assess the performance of four different barrier configurations: a baseline (existing) and three future expansion options. The third option appeared to be most effective at reducing chloride migration through the barrier and around the barrier to the northwest and southeast. For all three options, the head increase caused by the proposed wells created a more northerly flow direction and thus more potential for slight intrusion north through the existing Los Angeles County portion of the barrier. Careful management of the existing barrier and slight reduction of the targeted average heads at the proposed wells should help reduce this effect. The modeling tools were recently handed over to the three agencies

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