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Ms. Beth Thayer Central Valley Regional Water Quality Control Board Page 2 Our response to these issues raised by SJEHD are as follows: 1. Best available technology for treating groundwater has typically not been successful at reaching MCLS, much less ND, for aromatic hydrocarbons in groundwater. Many sites have been closed with levels higher than those recorded at Nor-Cal. Leaving these levels in place in not unprotective of public health or beneficial uses of groundwater in this area. It would appear that SJEHD recognized these arguments previously since they requested your concurrence in closing the site, stating "all appropriate response actions have been completed, that all acceptable remedial practices were implemented, and that no further investigation, remedial action, or monitoring" was required. We note that on page 10-11 of the State Board Resolution 92-49 it states that the degree of threat to water quality and beneficial uses as well as the financial burden , whether to the discharger or the State, should be considered when prescribing investigative and clean--up action levels. 2 . We have reviewed the County file for the site referenced by SJEHD with respect to the divergent gradient. This site is the Stockton Metropolitan Transit (SMART) property located approximately 1500 feet west of Nor-Cal. The file does indicate a SE-SW gradient over approximately one year of monitoring, however, the gradient magnitude is consistently very low, around . 001 (5 feet per mile) , and the three wells on which this is based are all within approximately 40 feet of each other. This low gradient calculated from a tight well cluster means that small errors in surveying or well elevation measurement will drastically affect gradient calculations. More importantly, the quarterly reports repeatedly note that wellheads were submerged at the time of monitoring and the reports show the bus wash area and associated water sump is immediately (less than 15 feet) north-northwest of the monitoring wells, again suggesting site specific reasons for the calculated gradient. Although only small amounts of dissolved contamination have been recorded at the SMART site, it is important to note that well MW-3 (to the west of the tank pit and ostensibly the upgradient well) had equivalent benzene concentrations and higher toluene, ethylbenzene and xylene concentrations than did 'downgradient' well MW-2 . This suggests that historically, at least on average, the gradient has a component to the west or northwest. As an additional check on our original work we obtained another nearby site file (Union Ice) located at the northwest corner of Lindsay and Union Streets (approximately 2500 feet due west of the Nor-Cal site) . The gradient at Union Ice is . 0025 to the northwest, also consistent with a convergence of the local gradient • in this part of Stockton toward Ragu Foods located 3500 feet due north of the Nor-Cal site where large volumes of water are pumped