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Daniel Villanueva From: Buehler, Alan@Waterboards <Alan.Buehler@Waterboards.ca.gov> Sent: Thursday, October 25, 2018 7:36 AM To: Daniel Villanueva Subject: RE: Ralph's Square Attachments: 2510071600_MX-4111 N_20181025_084804.pdf Daniel — Unfortunately we feel that some sort of additional downgradient assessment is necessary at this Site. The lack of impact to on-site wells MW -4 and MW -6 indicates that the plume is likely narrow. While MTBE has not reached furthest downgradient wells MW -15/16, wells MW -12/17 and MW -13/14 are likely outside of the lateral edges of the narrow plume. I do agree that MW -15/16 do delineate the length of the plume to less than 1,000 feet currently. However, another requirement of the LTCP is that the aerial extent of the plume be stable to decreasing. The large distance between MW -5 and MW -15/16 makes this problematic. Additionally, there is a 5 year gap in GW data between Oct 2008 and Mar 2014, during which time MTBE in MW -2 decreased from the Site historical max of 380,000 ug/L to 25,000 ug/L. While TBA production does appear to have occurred in MW -2, TBA concentrations in this well don't really start to increase until several events after the 5 year gap. In MW -3, 1 sample is collected in 2011, with significant MTBE decrease from 2008, but no TBA present until the 2014 sample. Basically, while TBA production is evident, it seems to lag behind the highest MTBE concentrations, and with the 5 year gap, its hard to tell what really happened to the MTBE, if it migrated or degraded. The additional assessment would provide data between MW -5 and MW -15/16. We want to avoid a situation where a huge mass of MTBE has migrated downgradient of MW -5, has threaded the needle between MW -12/17 and MW -13/14, and is waiting to impact MW - 15/16. In the Site map you included, I believe that the southern proposed location to be too far south, given the likely narrow nature of the plume. I have attached a map showing possible well locations. Wells would provide repeat sampling points so stability could be determined after several events. If you would prefer to use borings, then we would need to see some sort of modeling of the fate of MTBE at the Site, with the borings providing confirmation of the model. Chris and I would be willing to have a conference call to discuss if you wish. ALPV�, M. gLCeKeK,P.G. Engineering Geologist I UST Enforcement Unit Central Valley Regional Water Quality Control Board California Professional Geologist #9340 11020 Sun Center Drive, Suite 200, Rancho Cordova, CA 95670 Phone 916.464.4615 1 Fax 916.464.4704 alan.buehler@waterboards.ca.gov