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URS . Mr Harlin Knoll Page 4 San Joaquin County Environmental Health Department ARCO 2093 February 24, 2005 Vault boxes and horizontali in p p g needed for the planned remediation system were installed concurrently with the installation of the new product tanks and piping Groundwater extraction, air- sparging, and vapor extraction piping was installed in subsurface trenches connecting existing and proposed well locations to the proposed remediation compound A piping trench figure is provided in Attachment F Vertical Flow Assessment(July 2003 A vertical flow assessment was conducted at the site on July 23, 2003 This assessment was conducted as a result of discussions with the State Water Resources Control Board (SWRCB) regarding the potential for vertical flow within site groundwater monitoring or remediation wells URS conducted vertical flow assessments on wells ATR-2 and TEX-1 and well condition assessments on wells ATR-7 and ATR-8 Wells ATR-2 and TEX-1 showed no evidence of vertical flow This suggests that the ARCO wells are not acting as vertical conduits for contaminant migration Video inspection of wells ATR-7 and ATR-8 showed some algae on the well screens, and well ATR-7 had roots entering the screen The vertical flow assessment results are presented as Attachment G To remove the possibility that well TEX-1 could act as a vertical conduit in the future, this well was properly abandoned at the request of the SJCEHD on January 26, 2004 Trend Analysis of Contaminant_Concentrations Over Time (March 2004) In March 2004, URS completed a trend analysis of contaminant concentrations over time for monitoring wells ATR-1 through ATR-8 Welts ATR-9 through ATR-12 and TEX-1 never produced samples containing detectable concentrations of contaminants and therefore were not evaluated In addition, well SMW-1 is an air sparging well that has not been sampled since September 1998, therefore, SMW-1 also was not included in the trend analyses The trend analyses showed that contaminant concentrations at ARCO Station 2093 wells have either consistently declined over the period of sample collection or have increased before beginning a steady decrease over the period of sample collection Both remediation efforts and natural attenuation have resulted in these reductions of contaminant concentrations to levels that no longer pose a threat to human health and the environment For those wells that still have detectable concentrations of contaminants, the concentration trends indicate that these concentrations also will decrease to levels that are no longer detectable The trend analysis figures are provided as Attachment H REMEDIATION A combination SVE/AS system was installed at the site to remediate THPg and benzene contamination in the soil and groundwater The SVE system began operation in December 1995, AS system operation began in June 1997 The system has 11 SVE wells and 11 AS wells The SVE system was operated K IWprocessU5 BP ARCa1020931Closure Report\Draft of 2093 Closure Report doc