SITE INFORMATION AND CORRESPONDENCE

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Mr.Jeffrey D. Wingfield . -2- 29 July 201111 Port of Stockton California calculated arsenic concentration statistics using data from 93 wells across Rough & Ready Island, including all five Site 47 monitoring wells. Therefore, the Port cannot use the Navy background statistics to determine'whether or not Site 47 groundwater has been impacted by arsenic. Response #1: The Port must calculate background statistics for arsenic using a data set that does no include the Site 47 monitoring well concentration data. The Port must then compare the alternate arsenic background data to the arsenic data collected from the five Site 47 monitoring wells to determine if arsenic in soil has impacted groundwater. Comment #2: The Port has designated the uppermost fine-grained water-bearing zone beneath Rough & Ready Jsland as the CA Zone. The CA Zone varies in depth from 0 to 25 feet and is composed of organic matter, clay, and silt. CA Zone water surface contour maps are shown on Figures 10 through 12, and are based on triangulation of the water surface elevations measured in three shallow Monitoring Wells: W282, W284, and W285. However, one of the three (W285) is screened 15 feet deeper than the other two and may not be valid for contouring near-surface groundwater elevations. Furthermore, W284 and W285 are adjacent to each other, further reducing the validity of triangulating the water table. If one excludes the water surface elevation in W285, the actual flow direction during any give event could be interpreted to be any direction from northwest to southwest to southeast. Response #2: The Port must contour the CA Zone water surface elevation using at least three monitoring wells that have been screened between approximately 10 to 15 feet below the ground surface (bgs). Central Valley Water Board staff note that Monitoring Well W312, located north of Site 47, is screened between 10 and 15 feet bgs and could be utilized for this purpose. Based on inferred groundwater flow direction obtained from the CA Zone contours, the Port must install groundwater monitoring wells down-gradient of the three arsenic "hotspots" shown on Draft RI figures and first identified by the Navy in the 1990s. Comment #3: The Port has designated the uppermost coarse-grained water-bearing zone beneath Rough & Ready Island as the WA Zone. The WA Zone varies in depth from 15 to 30 feet and is composed of sand or silty sand. The Draft RI does not provide groundwater flow contour maps for the WA Zone beneath Site 47. Furthermore, the Draft RI does not provide adequate arsenic concentration data from groundwater in the WA Zone. The only monitoring well at Site 47 that is screened in the WA Zone is W280. Response #3: The Port must contour the WA Zone water surface elevation using at least three monitoring wells that have been screened within the coarse-grained material of this zone. Based on inferred groundwater flow direction obtained from the WA Zone contours, the Port must install groundwater monitoring wells down-gradient of the three arsenic "hotspots" shown on Draft RI figures. The Port should place Monitoring Well W280 on cross section D — D' in the Draft RI. Also, the Port should place groundwater arsenic concentration data on all of the cross sections provided in the Draft RI. Comment#4: The concentration of arsenic in WA Zone Monitoring Well W280 has increased between October 1999 and August 2008. The arsenic concentration during the first sampling event, conducted on 21 January 2000, was 228 micrograms per liter (Ng/L); the concentration during the last event, conducted on 13 August 2008, was 400 pg/L. This well is on the western edge of one of the arsenic "hot spots" shown on Draft RI figures. The well is also approximately 200 feet northwest of the Site 47 roadway relocation project constructed by the