SITE INFORMATION AND CORRESPONDENCE

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Mr. Curt M. Richards -2- 24 January 2012 However, Central Valley Water Board staff retained the requirement to record monthly total flow from these wells in the Draft MRP. 4. The MRP Request proposes to abandon upper horizon monitoring well MW-7 because it is located up-gradient of the COC source area and has non-detect concentrations of COCs. Central Valley Water Board staff agrees that sampling of this well is no longer necessary. Furthermore, monitoring of its water level elevation is not necessary for estimating water table surface contours. However, it would be prudent for Olin to retain this well because there are no other up-gradient monitor wells on the Site and it could help to identify off Site sources of COCs and other chemicals in the future. At this time, Central Valley Water Board staff has included MW-7 in the Draft MRP for water table elevation monitoring only. 5. The MRP Request proposes to abandon down-gradient upper horizon monitoring well MW-2, pointing out that down-gradient upper horizon wells MW-3 and MW-6 are sufficient for delineation of COCs. Central Valley Water Board staff agrees and has removed MW-2 from the Draft MRP. 6. The MRP Request proposes to discontinue quarterly measurement of depth to groundwater in all of the monitoring wells, pointing out that historic data have established flow direction trends. Central Valley Water Board staff agrees and has changed the measurement frequency of all monitoring wells in the Draft MRP to semiannual. 7. The MRP Request proposes to reduce sampling for chloride and sodium from semiannual to annual because the historic data show declining or stable concentration trends. Central Valley Water Board staff agrees and has changed the sampling frequency of all monitoring wells in the Draft MRP to annual. 8. The MRP Request proposes to discontinue sampling of the Homestead, Reed, and Medina private water supply wells. The MRP Request presents the following reasons: laboratory chemical analyses have not detected VOCs since first sampled in 2001, the owners use them for non-domestic/residential purposes, the well screens are below the lower horizon, the wells appear to be side-gradient of groundwater flow from the Site, and MW-16 is finished in the lower horizon between them and the Site. However, with the exception of the Reed well, well completion data for these wells are not available. The Reed well is 115 feet deep, but the screen interval data are not available. Annual groundwater sampling and chemical analyses must continue for the Homestead, Reed, and Medina wells. Sampling of the Pombo private water supply well, screened and sealed below the regional Corcoran Clay strata, is no longer required; Central Valley Water Board staff has removed that well from the Draft MRP. 9. The MRP Request proposes to reduce the sampling frequency for monitoring wells MW-9, MW-11, MW-13, MW-14, and MW-16 from semiannual to annual. Central Valley Water Board staff agrees and has incorporated the change in the Draft MRP. 10.The MRP Request proposes to reduce the sampling frequency of the Defense Logistics Agency monitoring wells LM151AB and LM177B from semiannual to annual. Central Valley Water Board staff agrees and has incorporated the change in the Draft MRP. 11.The MRP Request proposes semiannual sampling of wells MW-1, MW-3, MW-6, MW- 10, and MW-15. Central Valley Water Board staff agrees and has incorporated the change in the Draft MRP. In addition, the Draft MRP requires semiannual sampling for well MW-12. 12.The MRP Request proposes to remove nitrate analysis from the current MRP because the concentration has been stable since May 2005, there are no sources on the Site,