EXPLORING THE ROLE OF SUBURBAN DEVELOPMENT AND CONFINED RECHARGE ON CONTAMINANT FLOW IN AN ALLUVIAL FAN: A MODFLOW INVESTIGATION OF THE POMONA VALLEY, CALIFORNIA

Covering 36 square miles of Southern California and containing six groundwater basins, the Pomona Valley is bounded on the north by the San Gabriel Mountains, the east and south by the San Jose fault, and the west by the San Jose Hills. Suburban development has forced the confinement of recharge from the San Gabriel Mountains into controlled spreading grounds. Previous studies have focused primarily on the water balance and sustainable yield within the basin. However, subsurface contamination could be a devastating blow to the short- and long-term water supplies for the many southern Californians using groundwater domestically and industrially. To date, no work has been done exploring the role that confining recharge to these spreading grounds has on contaminant movement despite the strong probability of groundwater contamination (Kelley 1988).

This study examines the effects of confined recharge on contaminant migration by modeling the groundwater in the Pomona Valley study area. Two simulations are conducted: the first in which recharge occurs under current confined conditions, and the second in which assumed natural conditions spread recharge over the entire alluvial fan. The study uses Visual MODFLOW to build a comprehensive, multi-layered model of the Pomona Valley study area. Buried gasoline tanks are used as potential sources of contaminants. Modeled recharge conditions are manipulated to explore the linkages between government-controlled recharge, contaminant migration, and the water table.

The model finds that confinement of recharge to spreading grounds has no discernable effect on contaminant flow within the Pomona Valley study area. However, results indicate that hydraulic conductivity of aquifer materials exerts a strong control over contaminant movement. Future studies using a refined model (containing more detailed spreading and pumping data) would be able to explore the dominant processes controlling contaminant flow in the Pomona Valley study area.