SITE INFORMATION AND CORRESPONDENCE

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Bert E. Van Voris _ 19 - 17 September 2004 Supervising Engineer Groundwater—Offsite Regarding the transition from onsite to offsite groundwater,the RWD states that,based on the site's geomorphology as an alluvial fan, "it is reasonable to conclude that shallow groundwater in the downgradient areas of the site merges with both onsite and offsite deep groundwaters."26 In several meetings with Musco's technical consultants, Regional Board staff indicated that an alternative and more likely explanation for any mixing of groundwater flowing to the valley flow from individual water bearing zones underlying the site was due to wells perforating more than one water bearing zone, and not due to any geological mixing zone per se. The RWD presents no evidence in support of its conclusion. Regarding offsite groundwater quality, the RWD summarizes information obtained from regional studies performed in the vicinity, as well as well quality data for wells in the vicinity drilled to a maximum depth of 200 feet bgs. Figure 21 is a graphical summary of this information. The RWD cites TDS data from four wells (two of which were sampled by Musco in 2004) in the immediate vicinity of the California Aqueduct and Delta-Mendota Canal to conclude groundwater in an elongated area underlying these conveyance facilities contains elevated TDS compared to groundwater farther east(in the Tracy area and beyond). The RWD then concludes, "groundwater conditions migrating off the site and into regional groundwater [as characterized by the now-dry MW-11 and the increasingly saline W-2] are less in TDS concentrations than groundwater offsite and in an area between the Musco Site and the City of Tracy.i27 Comment: Figure 21 also shows 14 wells east of the site and within this elongated area that are labeled "Offsite well—TDS data being researched." Accurately characterizing groundwater immediately offsite will be a critical component of site closure. The conclusion drawn in the RWD is problematic for at least two reasons. First, it assumes that the quality of groundwater flowing through W-2 and offsite will not be influenced by the higher TDS groundwater in the upgradient swale area. Evidence from W-2 suggests this degradation is already occurring. Second, it is based on data from only four wells within a large area. Musco should provide TDS data from the 14 additional wells indicated in Figure 21 (as the data become available), in addition to the well construction details (if available) from the two wells sampled by Musco in 2004. Land Application Management and Site Closure The RWD indicates that the inorganic content of the combined wastewater and RO permeate will be about the same or lower than the TDS of groundwater underlying the proposed use areas. In describing future cropping plans, the RWD acknowledges Musco's limited success with germination and crop growth, and indicates that potable water may be applied to promote successful germination. Once offsite recycling is fully implemented in about five years,the land application area will return to natural vegetative cover. Ultimately, the site will be developed for commercial or industrial purposes and the site"will be left undisturbed and/or capped by paving, further limiting potential for infiltration and leaching of material downward. ,28 The RWD suggests that it is less important to monitor the land application system and the quality of soil-pore liquid, and hence the previous proposal for an extensive system of lysimeters is obsolete. The RWD includes a revised proposal for a modified lysimeter network to evaluate mobility of soil constituents.