SITE INFORMATION AND CORRESPONDENCE

Home Browse Search

Former Circle K Store No. 086701 Page 2 of 3 8606 Thornton Road November 3, 2014 Stockton, California 95209 The EHD's analysis of contaminant concentration trends in groundwater leads it to the conclusion that while there is no unimpacted downgradient well , it appears that impacted groundwater may not extend a significant distance downgradient of MW-7. Monitoring well MW-1 , upgradient, but immediately adjacent to the underground storage tank (UST) pit, was the first well impacted by dissolved MTBE concentrations exceeding 100 micrograms per liter (ug/1) and was impacted above that concentration for approximately 10 years, reaching a maximum concentration of 4,400 ug/I before declining ultimately to undetectable concentrations. Monitoring well MW-4, down-gradient of the UST pit and the dispenser islands , was impacted by MTBE to concentrations exceeding 100 ug/1 approximately a year after MWA , reached a maximum concentration of 10 , 000 ug/l MTBE , and declined below 100 ug/I after 8 years. Ten years after MW-1 was impacted to MTBE concentrations exceeding 100 ug/I , down-gradient monitoring well MW-7 (further down-gradient from MW-4) was impacted by MTBE at concentrations exceeding 100 ug/l; MW-7 obtained a maximum concentration of 240 ug/I MTBE and declined below 100 ug/l after only 3 years of exceeding that concentration . This leads the EHD to consider the rise in MW-7 to be analogous to an attenuating ripple migrating outward from the source area of the former UST pit and dispenser islands. The CVRWQCB's concern with vertical delineation is based on the 44400t vertical distance between screen bottoms of shallow wells and screen tops of deep wells and the locations of the deep-screened wells. The shallow wells bottom in the top of a 20-foot sand-rich interval located approximately 42 to 65 feet below surface grade (bsg) , the deep wells are screened in the base of a 35-foot sand-rich interval (approximately 75 to 110 feet bsg) , with a 5- to 10-foot fine- grained interval separating the sand-rich intervals; the fine grained interval may pinch out toward the south. The deep wells are situated obliquely upgradient to cross-gradient from impacted wells MWA , MW4 and MW-5, so are not well situated to intercept a descending, migrating plume of impacted groundwater. With nearly a total of 34 vertical feet of sandy interval between the shallow and deep screened intervals, there is plenty of room for impacted groundwater to migrate undetected. However, the fine-grained interval could be expected to confine moderately impacted groundwater to the 20-foot sandy interval with the groundwater flow direction toward the northeast absent a significant downward vertical gradient, such as may exist near pumping water supply wells. The low mass release inferred by the EHD intuitively seems inadequate to drive a significantly impacted plume of groundwater laterally or vertically over large or unusual distances , but the EHD has concluded that it is not unreasonable to test the model presented above prior to closing the site. By electronic mail dated 15 January 2014, the EHD had directed Cardno to discontinue monitoring and sampling all wells associated with this site, but to test the EHD model to justify closure, you are now directed to sample monitoring wells MWA , MW-4 and MW-7 and analyze the samples for MTBE to confirm the inferred declining trends; also sample MW-8 and MW-9 and analyze for MTBE to document that impacted groundwater is still not present the deep zone in those localities. In addition , the EHDdirects an effort be made to obtain samples from the domestic water supply wells at 8732 and 8 5 t3 vis Road for analysis for MTBE to demonstrate the wells are not impacted . Assuming fav able data is obtained , the EHD will be prepared to move forward with the site closure . d�r rnA) I , ( L Although the EHD anticipates closing the site as soon as these issues are resolved and the closure related activities are completed , this is technically a denial of your closure request at this time. This decision is subject to appeal to the State Water Board , pursuant to California Health and Safety Code Section 25296 .40 (a) (1 ) , (Thompson-Richter Underground Storage Tank Reform Act — Senate Bill 562) . Please contact the State Water Board