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Mr. Jeffrey Wingfield field - 2 - 40 18 April 2007 p Port of Stockton be developed that depicts a single water bearing zone over the entire interval from 2 to 35 feet bgs. The soil boring logs and cross sections presented in the Report suggest that the sand materials within each of these zones are interconnected across zones. The fact that methyl tertiary butyl ether (MTBE) concentrations in groundwater within zones are similar also suggests interconnection. 2. The down-gradient direction of groundwater, based on water table elevation data collected from four monitoring wells screened to a maximum depth of 10 feet, has been towards the northwest. However, the down-gradient direction of groundwater, based on MTBE groundwater concentration data collected from cone penetrometer test borings pushed into the upper 35 feet of the subsurface, appears to have been towards the west. Down-gradient wells have not been installed to the west of the former Building 16 UST site. 3. Groundwater samples have been collected after purging wells utilizing low-flow methods. However, the Port (or the Navy) has not established whether low-flow purging at this site yields samples that are representative of groundwater quality. For example, the maximum concentration of total petroleum hydrocarbons (TPH) in the gasoline range in groundwater samples collected from the monitoring wells in August of 1999 was 13,000 micrograms per liter (I.Lg/L), whereas the maximum concentration in samples collected between November 1999 and January 2007 was 1 ,300 µg/L. The August 1999 samples were collected after the wells had been purged of at least three well volumes, the November 1999 and later samples, utilizing low-flow methods in which much less water was purged from each well. 4. Laboratory chemical analyses reports from three laboratory companies are included in Appendix C of the Report: Analytical Sciences, Mobil Chem Labs Inc., and Severn Trent Laboratories, Inc. The reporting of TPH in the gasoline range concentrations has varied in the reports prepared by these labs. Analytical Sciences includes the concentration of MTBE in the concentration of TPH in the gasoline range; Mobil Chem Labs Inc. does not. Also, it appears that Severn Trent Laboratories, Inc. included MTBE up until January 2007 at which time they did not. Laboratory chemical analyses concentration data for TPH in the gasoline range presented in report tables should be footnoted to indicate whether or not each concentration value includes the concentration of MTBE. For State of California UST proiects, chemical analysis laboratories commonly exclude MTBE from the concentrations reported for TPH in the gasoline range. For future sampling events, the Port should instruct their contract laboratories to exclude MTBE from the TPH in the gasoline range concentrations they report. MTBE should be reported separately. 5. Beginning in the first quarter of 2006, the Port resumed quarterly groundwater monitoring and sampling of Building 16 site monitoring wells. Work Plan Approval The Port plans to develop a new entry road for access to Rough and Ready Island. Their plan requires the removal of the four monitoring wells at the Building 16 site. Regional Water Board staff concurs with the Port's proposal to destroy monitoring wells MW-1 through MW-4 and install replacement wells, provided the following comments are implemented: 1. The wells must be placed in the approximate locations proposed in the Work Plan.