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(CV o • A S S O C I A T E S I N C The concentrations of petroleum hydrocarbons in groundwater slowly decreased from the first groundwater monitoring event at the site in April 1991 to September 1995, when no petroleum hydrocarbons were detected in any of the water samples analyzed In addition, no petroleum hydrocarbons were detected in any of the water samples analyzed in the October 1995 sampling event Results of laboratory analyses of groundwater samples are included in Table 2 Following the discovery of petroleum hydrocarbons in the groundwater beneath the levee at the site in 1996, groundwater monitoring and sampling were resumed in August 1997 with fuel oxygenates included in the sample analysis suite TPHg, benzene, and total xylenes were detected in well MW-5 in August 1997 BTEX constituents have been periodically detected in the water samples collected from MW-5 from the March 1998 to November 2002 sampling events MTBE was detected in the water samples collected from wells MW-5 and MW-6 in August 1997 (MTBE is further discussed in the next paragraph ) TPHd has been detected in water samples collected from wells MW-5 and MW-6 from the March 1998 to April 2001 sampling events (TPHd was not analyzed during the August 1997 sampling event) TPHd was sporadically detected at low concentrations in wells MW-2, MW-3, and MW-4R from December 1998 to April 2001 TPHg and BTEX constituents have not been detected in any of the samples collected from wells MW-1 through MW-4R and MW-6 from September 1995 to November 2002,the most recent sampling event as of the date of this report While the groundwater was sampled for petroleum hydrocarbons since 1991, quarterly groundwater • sampling for MTBE was not initiated until August 1997 MTBE was detected at 4,100 ug/l in the water sample collected from well MW-5 during the August 1997 sampling event The MTBE concentrations detected in the water samples collected from monitoring well MW-5 have shown a general increasing trend from August 1997 to June 2000 and have generally shown a decreasing trend since the June 2000 peak concentration MTBE concentrations detected in the water samples collected from well MW-7 have ranged from 97,300 to 150,000 ug/l The MTBE concentrations detected in the water samples collected from well MW-7 initially increased between the April 2001 and July 2001 sampling events but have gradually decreased since the July 2001 sampling event SITE GEOLOGY The following summary of the site geology is based on the boring logs contained in ATC's Summary Report of Subsurface Investigation, December 1999 and Summary Report of Subsurface Investigation, October 2001 The geologic materials encountered beneath the site primarily consist of organic clay, silt, and peat with minor amounts of sand There is also an elevated amount of natural organic material (NOM) in the materials beneath the site The heterogeneous subsurface geology is composed of alternating layers of lean clay, inorganic silt, organic clay, peat, clayey sand, and poorly grade sand The lean clay and inorganic silt were encountered in the upper portion of the borings located on the levee The low permeability of these materials is useful in the construction of levees Clayey sand was typically encountered in the upper portions of the borings drilled away from the levee Organic clay and/or peat was encountered beneath the lean clay, inorganic silt, or clayey sand Peat is primarily composed of NOM A stringer of poorly graded sand was encountered at 10 feet bgs in boring SB30 whereas poorly graded sand was encountered from approximately 24 feet bgs to the total depth of the boring (31 5 feet bgs) in monitoring well MW-7 The types of subsurface materials encountered are indicative of deltaic and overbank deposits w 1625141reportslstatusreport doe 4