CHERVEN, VICTOR B., and CLYDE L. HEBBRON
Upgradient Environmental Consultants, Stockton, California
Abstract: Geometry of Diesel Contamination Plume Controlled by Buried Fluvial Channel, San Joaquin Valley, California
Ten borings were drilled to investigate the subsurface distribution of petroleum contamination from leaking UST's at a trucking company in Stockton, California. Selected samples were analyzed for diesel, gasoline, and other contaminants, and isocontour maps of the laboratory results have been constructed for the soil at various depth intervals, as well as for the shallow groundwater. The samples were also used to develop a detailed stratigraphy, correlate lithologic units between borings, and generate cross sections, structure contour maps, lithofacies maps, and isopach maps.
The geometry of the diesel plume is controlled by a coarse-grained sand body whose narrow, linear geometry; sharp, erosional base; and upward-fining texture imply deposition in a small stream channel. The permeability of the deposit increases downward, which caused the plume to spread laterally with depth and become increasingly elongate parallel to the channel axis. It reaches a length of 150 feet and width of 50 feet at the base of the channel. The UST's were located near the center of the plume, implying that diesel has spread both up and down the hydraulic gradient.
The channel is floored by a low-permeability silt bed and associated but localized cemented soil horizons ("hardpans"). They restricted downward petroleum migration below 55 feet and prevented severe contamination of the underlying aquifer. Recently, however, groundwater rose into the contaminated channel and became impacted.
Bacterial abundance data suggest that naturally occurring hydrocarbon-degraders are not sufficiently abundant to expect that intrinsic bioremediation will lead to natural attenuation of the contaminants. Various methods of actively addressing the contamination have been proposed to the regulatory agency.
AAPG Search and Discovery Article #90920©1999 AAPG Pacific Section Meeting, Monterey, California