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Limited Groundwater Investigation 10 1 OX rule of thumb and up to 42 mg/kg using the 20X rule of thumb,which are similar in concentration to the background values reported for total Cr in soil in San Joaquin County. Therefore, the elevated levels of total Cr detected in the grab groundwater samples may be related to background levels of Cr in the suspended soil observed in the samples. 5.2 French Camp All five groundwater samples collected at the French Camp facility contained detectable levels of dissolved Cr VI (above the MDL) at concentrations greater than the PHG of 0.2 µg/L,but at similar levels to the Cr VI concentrations reported at Lathrop. Dissolved Cr VI concentrations ranged from trace levels (0.3J µg/L) to 16 µg/L,with the highest report concentration detected at FC-G4 located between the active and eastern stockpiles. Except for FC-G4, the dissolved Cr VI concentrations were less than the lowest concentration found in the stockpile extracts and storm water samples from this facility (Brown and Caldwell,April 2001).A summary of these data are provided below for comparison: EPA 6010 Range in /L EPA 7199 dissolved Cr VI EPA 6010 total Cr dissolved Cr Groundwater 0.3J - 10) 28-620(a)w/one @ 1700 1.8J—50) Stockpile DIWET Extracts (March 2001)(6) 9.7-39.9 12-42 ---(C) Storm Water(February&March,2001)(6) 63-95 64-100 59-91 (a) Five detections out of five samples.Duplicate Sample results are included. (b) Brown and Caldwell,April 2001. (c) Stockpile extracts were not assessed for dissolved Cr. Dissolved Cr VI was not detected by EPA Test Method 7196 in any of the groundwater samples collected at French Camp,except at FC-G4 (10 µg/L). In most cases this was due to higher MDLs reported for Test Method 7196. Similar to Lathrop the total and.dissolved Cr concentrations detected in groundwater were above the PHG of 2.5 µg/L, except for the dissolved Cr concentrations found at FC-G1 (1.8J µg/L). The total Cr groundwater concentrations were several orders of magnitude greater than dissolved Cr. The highest total and dissolved Cr concentration was found at FC-G4. Similar to Lathrop, the total Cr concentrations detected in groundwater were several orders of magnitude greater than the stockpile extracts DIWET and storm water samples,with the exception of FC-G5. The highest concentration of dissolved Cr found in groundwater is less than the previous reported storm water samples (Brown and Caldwell,April 2001). As discussed previously, the high total Cr levels detected in the groundwater grab samples may be attributed to suspended soil observed in the samples at the time of collection. The maximum total Cr level observed in the grab groundwater samples (1,700 µg/L at French Camp) was compared to background soil levels for San Joaquin County (Bradford, et al., March 1996) and using the 1OX and 20X rule of thumb. The estimated soil concentrations may be up to 17 mg/kg (at 10X) and up to 34 mg/kg (at 20X), and are similar in concentration to the range of background values (26 and 49 mg/kg) reported for total Cr in soil in San Joaquin County (Bradford, et al., March 1996). As stated previously, the elevated levels of total Cr detected in the grab groundwater samples at the French Camp facility may be related to background levels of Cr in the suspended soil observed in the samples. P:\wp\jobs\20587\LP—FCG1X'.doc—DCN 102300 June 2001