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Limited Groundwater Investigation 9 5.0 DISCUSSION OF RESULTS This section presents a discussion of the results and how they compare to the water quality goals. 5.1 Lathrop Facility Groundwater samples collected from two of the five sample locations at the Lathrop facility contained detectable levels of dissolved Cr VI (above the method detection limit [MDL]) at concentrations greater than the PHG of 0.2 µg/L. The highest dissolved Cr VI concentration (10 µg/L) found at LP-G2 is less than the concentrations reported previously for the stockpile extracts DIWET and storm water samples (Brown and Caldwell,April 2001) with the exception of one stockpile extract (9.5 µg/L) out of a total of 20 stockpiles samples analyzed at Lathrop. A summary of these data are provided below: EPA 6010 dissolved Range in /L EPA 7199 dissolved Cr VI EPA 6010 total Cr Cr Grab Groundwater <0.2—10(') 55—860(6)w/one @ 2100 2.3J—16(b) Stockpile DIWET Extracts (March 2001)(`) 9.5-39.8 12-49 ---(d) Storm Water(February&March,2001)() 28-144 44-150 28- 150 (') Two detections out of five samples.Duplicate sample results are included. (b) Detected in all five samples. (`) Brown and Caldwell,April 2001. (d) 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 Lathrop, due to the higher MDLs (8 µg/L) reported for Test Method 7196. Total Cr and dissolved Cr were detected in all samples collect at Lathrop at concentrations above the PHG of 2.5 µg/L, except for dissolved Cr at two sample locations (1'P-G3 and LP-G5). In most cases, the total Cr concentrations were several orders of magnitude greater than the dissolved Cr concentrations (except at LP-G5). The highest concentration of total and dissolved Cr was detected in the duplicate sample of LP-G1, located west of the southern stockpile between the southern stockpile and the fire retention pond (Figure 3). The total Cr levels reported in groundwater were several orders of magnitude greater than the previously reported stockpile extracts DIWET and storm water samples (Brown and Caldwell,April 2001). The dissolved Cr levels found in groundwater were lower than the previous storm water sample results (Brown and Caldwell,April 2001). The high total Cr levels observed in the grab groundwater samples (up to 2,100 µg/L at Lathrop) may be attributed to the suspended soil observed in the samples at the time of collection (samples were turbid at the time of collection). In San Joaquin County soil background levels for total Cr have been reported between 26 and 49 mg/kg (Bradford, et al., March 1996). Typically a 10 times (10X) and/or 20 times (20X) rule thumb is applied to total metal concentrations to estimate the expected leachate or extract concentration. To make a comparison between the total Cr groundwater concentrations observed during this investigation and the reported background values (mg/kg), the 10X and 20X rule of thumb was applied to the maximum total Cr concentration detected at the facility. As such, the estimated soil concentrations may be up to 21 mg/kg using the PAwp\jobs\20587\LP_FCG1X'.doc—DCN 102300 June 2001