Analytic Element Domain Boundary Conditions for Site-Scale Groundwater Flow Modeling Los Angeles Basin

Abstract

Physics-based groundwater flow modeling is a useful tool for the design and optimization of pump-and-treat systems for groundwater site cleanup. Numerical methods like finite differences and finite elements, and hybrid analytic elements, require boundary conditions (BC) to be assigned to the outer domain of the grid, mesh, or line elements. These outer BC do not always correspond with hydrogeologic features. Common practice in model setup is to either: (1) extend the model domain boundary outward such that introduced artificial outer BCs (e.g., first type head specified, second type flux specified) do not have undue influence on near-field scale simulations; or (2) assign outer BCs to capture the effective far-field influence (e.g., third type head-dependent flux). Groundwater flow modeling options for assigning BCs were demonstrated for the extensively documented Dual Site Superfund cleanup in Torrance, California. The existing MODFLOW models for the Dual Site scale and the Los Angeles basin scale document the current hydrogeologic conceptual site model. Simplified analytic element AnAqSim models at the LA Basin scale, West Coast Subbasin scale, and Dual Site scale, were used for mapping near-field domain velocity vector fields and pathline envelopes. The pump-treat-inject system demonstrated hydraulic containment and showed pathline envelopes relatively insensitive to BC choices. However, the near-field domain boundary groundwater flow fields were sensitive to BC choices. The Los Angeles basin case study demonstrated the use of analytic element groundwater modeling for testing stress dependent boundaries during site pump-treat-inject design.