ARCHIVED REPORTS XR0007061 - Laserfiche WebLink

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n A S S O C I A T E S I N C The high amount of organics encountered in the geologic materials beneath the site has important implications for subsurface transport of petroleum hydrocarbons NOM can retard the movement of petroleum hydrocarbons in the subsurface through adsorption NOM can have a high affinity to petroleum hydrocarbons causing the petroleum hydrocarbons to be retained by the geologic materials Once adsorbed, the petroleum hydrocarbons can then be slowly released over time by desorbtion The sorption process of NOM can therefore effectively reduce the movement of petroleum hydrocarbons in the subsurface environment DISCUSSION AND RECOMMENDATIONS Elevated concentrations of MTBE have been detected in the water samples collected from well MW-7 since the first sampling of MW-7 in April 2001 The relatively recent discovery of these elevated concentrations has given the appearance that a recent fuel release has occurred at the site However, given the timing of the first appearance of MTBE in the groundwater beneath the site and the properties of the aquifer material that could hinder petroleum hydrocarbon transport in groundwater, the gasoline range hydrocarbon release appears to have occurred from the UST system before August 1997, as further discussed below Both diesel and gasoline fuels have been released to the subsurface beneath the site Diesel fuel was released to the subsurface in 1989 when a fuel line on the ruptured on the levee west of the store The type of contaminants currently detected in the groundwater suggests that another release, of gasoline range hydrocarbons, also occurred at the site The source of the gasoline range petroleum hydrocarbons in groundwater, including MTBE, appears to be the fuel piping of the former UST fuel system based on the following 1)the appearance of MTBE and other petroleum hydrocarbons in the groundwater beneath the site 2 years before the conversion of the fuel system from USTs to an AST in March 1999, 2) releases of petroleum hydrocarbons from the fuel pipelines that cross the levee were confirmed during compliance soil sampling when the fuel system was upgraded to March 1999, and 3) the center of mass for the petroleum hydrocarbon plume in groundwater is located hydraulically downgradient from the former UST pipelines that cross levee Elevated concentrations of MTBE have been detected in water samples collected from monitoring well MW-5 since 1997 However, only low concentrations, or no detectable concentrations, have been measured in the water samples collected from wells located downgradient of MW-5 Given the highly mobile nature of MTBE in the subsurface and a relatively high hydraulic gradient at the site, the MTBE groundwater plume would be expected to travel a significant distance over a period of 5 or more years However, the aerial extent of MTBE in groundwater at the site has been limited to date This retardation of contaminant transport is not unexpected given the low permeability materials and elevated NOM encountered at the site The elevated concentrations of MTBE encountered in monitoring well MW-7 do not necessitate the discovery of a new release ATC's December 1999 subsurface investigation constrained the source of the release to the levee area Well MW-7 was subsequently installed in the area of elevated petroleum hydrocarbons encountered by ATC on the levee Elevated concentrations would be expected near the source of the release As discussed above, the properties of the subsurface materials can commonly hinder the transport of petroleum hydrocarbons in the subsurface The combination of close proximity to f. the petroleum hydrocarbon release and subsurface conditions conducive to slowing the movement of the petroleum hydrocarbons could account for the elevated concentrations of MTBE in groundwater samples w 162514Veportslstatusreport doe 5