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FORMER AMERICAN FOREST PRODUCTS -7.0 Rationale for Site Closure Closure Summary Report July 2009 URS Project 17322736 Page 7-1 7.0 RATIONALE FOR SITE CLOSURE �-� This Closure Summary Report responds to the 20 items listed in the tri-regional guidelines for cases f where concentration of constituents remaining exceed water quality objectives. Those items, and the �- sections of this report that respond to the guidelines, are listed in Section 1.2 of this Closure Summary Report. This section summarizes the three items most relevant to this Site: plume delineation, consistent predictive concentration trends, and water use. Vertical and Lateral Plume_Delineation--The focus of more recent groundwater investigations at the Site were the assessment of�UOCs,.primarily:l,2-DCA.-Following the failure of several direct push investigations to provide adequate vertical and lateral delineation of the extent of 1,2-DCA contamination, five additional groundwater monitoring wells were installed for that specific purpose. Five quarters of groundwater monitoring data collected subsequent to their installation confirms vertical delineation with depth anddateral-&lin on to the west with consistent(non--detect results from DMW! V14 and D W 13Irespectively. Downgradient delineation is provided by DMW=11, where concentrations remain consistently below 1.0 µg/L,and to the cast by concentrations o4 2.4 µg/L or lower in samples collected from DMW-12 (Figure 7). Consistent and Predictive Declining Trends--Plots of 1,2-DCA concentrations versus time for the four l wells in which 1,2-DCA are consistently reported show consistent declining trends (Figures 9 and 10). The calculated trend lines are reasonably predictive of`1,2-DCA concentrations reaching non-detect-byy -approximately 2012.Plots of 1,2-DCA concentrations for plume delineation wells DMW-11 through DWM-14 show consistent low to non-detect results for those wells since their completion in the spring of 2008. Water Use and Proposed Site Use—As part of this Closure Summary Report,URS has updated a survey for water supply wells within 2,000 feet of the Site.No water supply wells are within 2,000 feet downgradient of the former UST locations. Four irrigation wells are located within 2,000 feet upgradient or crossgradient on two cemeteries to the west and northwest of the Site. The AFP property has been purchased by the;,Sa Joagdin:Regional KRITo mission(SJRRC) as part of a property acquisition for a maintenance facility servicing the Altamont Commuter Express (ACE). Since 1998,the ACE has provided rail service between Stockton and the Bay Area.No development of groundwater is planned by SJRRC. The ACE maintenance facility will be constructed to the east of the ! .. - existing Union Pacific Railroad mainline,which bounds the east side of the Site.:No----r-��-constructio-on- by the SJRRC-is-planned-for.this area of the Siie.However, to accommodate the ACE facility the UPRR mainline will be shifted slightly to the west, such that the UPRR casement will include the current location of DMW-12. Mainline relocation and monitoring well destruction would be expedited if site ` closure and destruction of all of the existing monitoring wells could be accomplished prior to the start of the mainline relocation,planned for as early as the end of 2009. KAWproees M7361Amerit Porem\Closttre-St my-Report-Rv)01.doc 1