Characterization of Potential Transport Pathways and Implications for Groundwater Management Near an Anticline in the Central Basin Area, Los Angeles County, California.

摘要

The Central Groundwater Basin (Central Basin) of southern Los Angeles County includes 280 mi2 of the Los Angeles Coastal Plain and serves as the primary source of water for more than two million residents. In the Santa Fe SpringsWhittierNorwalk area, located in the northeastern part of the basin, several sources of volatile organic compounds have been identified. The volatile organic compunds are thought to have contributed to a large, commingled contaminant plume in groundwater that extends south-southwest downgradient from the Omega Chemical Corporation Superfund Site across folded geologic strata, known as the Santa Fe Springs Anticline. A multifaceted studythat incorporated a three-dimensional sequence-stratigraphic geologic model, two-dimensional groundwater particle-tracking simulations, and new groundwater chemistry datawas conducted to gain insight into the geologic and hydrologic controls on contaminant migration in the study area and to assess the potential for this shallow groundwater contamination to migrate into producing aquifer zones. Conceptual flow models were developed along a flow-parallel cross section based on the modeled stratigraphic architecture, observed geochemistry, and numerical model simulations that generally agree with observed water levels and contaminant distributions. These models predict that contaminants introduced into groundwater at shallow depths near the Omega Chemical Corporation Superfund Site and along the study cross section will likely migrate downgradient to depths intercepted by public supply wells. These conclusions, however, are subject to limitations and simplifications inherent in the modeling approaches used, as well as a significant scarcity of available geologic and hydrogeochemical information at depth and in the downgradient parts of the study area. In this study, a three-dimensional sequence-stratigraphic model of Quaternary and late Plioceneage deposits was developed to identify unconformities (time-horizons) that define the geologic structure and bound chronostratigraphic units (containing genetically related aquifers and aquitards) that can be correlated regionally. This model suggests that syndepositional deformation of the Santa Fe Springs Anticline appears to have caused relative thinning of the stratigraphic units over the crest of the fold and, possibly, preferential erosion of capping aquitards near the anticline crest that could provide pathways for migration of contaminants downward into older units. To the south and southwest of the Santa Fe Springs Anticline crest, the units thicken as they plunge deeper into the Central Basin. Contaminated parts of these units, where they occur near the anticline crest, are likely genetically connected to the aquifers that are being pumped in the Central Basin.