Groundwater contamination by nonaqueous phase liquids (NAPLs) and increasing salinity in the Central Valley are two challenges facing groundwater remediators in California. Two studies investigating methods for modeling the spread of these contaminants are presented. The first deals with a technique for modeling and characterizing NAPL infiltration into water-saturated sediments at the micro-scale level. The second investigation presents a framework to model regional impacts of waste-water discharge from food processors in the Central Valley. The nature of the different contaminant transport processes and the vastly different spatial scales of the two investigations lead to the selection of very different modeling strategies.

Understanding the behavior and geometry of multi-phase immiscible displacement fronts is essential to characterizing NAPL infiltration. Depending on the flow conditions, the presence of gradients, such as those caused by gravity forces can lead to stabilized (piston-like) or unstable (fingering) fronts. Studies have found the resulting displacement fronts have patterns with percolation, or fractal, characteristics not adequately described by continuum equations. Numerical pore-network models incorporating the appropriate physics provide a framework to study these characteristics. Simulations based on the method of invasion percolation in a gradient were run over a range of Bond Numbers, N_Bo (the ratio of buoyancy and capillary forces), allowing for the identification of stability regimes and scaling laws for the front geometry, the fluid saturations, and the capillary pressure with N_Bo.

Hundreds of food processors in the Central Valley discharge their waste-water by land application, in which ideally, the waste-water both irrigates and fertilizes crops, while biogeochemical activity attenuates the waste components before they reach the water table. Groundwater monitoring has shown significant water quality impacts leading to elevated salinity levels. To study the impacts on a regional scale a 3D groundwater flow and salt transport model was developed for a region of the Lower San Joaquin River basin. The net impact of land discharge from multiple sites over a 30 year period was simulated using a probabilistic framework, indicating that while the waste discharge results in highly elevated salinity levels in exceedence of water quality standards, the spatial extent of contamination is localized rather than regional.