ASSESSING MTBE CONTAMINATION IN AN EPHEMERALLY-SATURATED AQUIFER

Quaternary stratigraphy in the Newhall area of the Santa Clarita Valley, Los Angeles County, California, is comprised predominantly of alluvium and the underlying Saugus formation. Aquifers within these formations provide drinking water supplies for residents within the valley area. In portions of the groundwater basin, where the alluvium is adequately deep, the basal sands and gravels are saturated and yield water to supply wells; in other areas the base of the alluvium may be dry or only ephemerally saturated. Releases of gasoline from underground storage tanks at a former service station resulted in the contamination of a large volume of alluvial soils beneath the property, primarily by methyl tertiary butyl ether (MTBE) and benzene. Assessment of this contaminant plume in soil determined that the entire column of alluvium beneath the property, ranging from 85 to 115 feet below grade, was affected. During the dry seasons and drought years, the basal alluvial section is not saturated at this location. However, during wet seasons and years with above-average rainfall, infiltrating and percolating groundwater ephemerally saturates the deepest few feet of alluvium. As this ephemeral alluvial groundwater migrates laterally and downward along the contact with the underlying Saugus formation, it preferentially flows toward paleochannels and buried canyons until reaching areas of perennial saturation. Dissolved phase migration of MTBE within the ephemeral groundwater is a significant transport mechanism for the contaminant. Additionally, some degree of percolation into the Saugus formation is evident based on concentrations of contaminants found therein and ephemeral saturation of sands and gravels that overlie siltstone and claystone beds within the Saugus formation. Assessment has involved the installation of monitoring wells at the base of the alluvium, which may also be used as vapor extraction wells for remediation, and deeper wells that monitor the shallowest aquifers within the Saugus formation. Determination of the orientation of paleochannels and buried canyons, detailed correlations of lithologic and geophysical logs, and an understanding of regional geology and its relation to contaminant transport and water supplies have been key to the assessment of contaminants beneath the site.