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r <br /> Mr. Carl Navarra <br /> 7500 W. Eleventh Street <br /> Page 2 of 3 <br /> are currently impacted by an unidentified substance that is detected in the diesel <br /> hydrocarbon range, but is `cleaned up' with silica gel treatment. The substance(s) <br /> must be identified before EHD will concur with destruction of monitoring well MW- <br /> 12A and before implementing the modified sampling schedule detailed below. <br /> EHD sees no need to destroy monitoring well MW-8 at this time. EHD believes the <br /> well provides useful water level data and poses minimal risk as a vertical conduit. <br /> EHD will approve obtaining quarterly depth to water measurements from the well <br /> with annual sampling for laboratory analysis. <br /> EHD agrees that the sampling frequency of many of the wells can be reduced to a <br /> semiannual or annual event. However, as your plume contains free product and <br /> significant concentrations of methyl tert-butyl ether (MTBE), and is on the order of <br /> 900 feet long, semi-annual sampling is .not appropriate for all monitoring wells. All <br /> wells should have water levels monitored quarterly. EHD will approve semi-annual <br /> sampling of monitoring wells MW-2, MW-3A, MW-9 and MW-12, and annual <br /> sampling of monitoring wells MW-1, MW-313, MW-8 and MW-12A. All other <br /> monitoring wells should be sampled quarterly. <br /> EHD agrees that removal of free product from MW-7 should commence <br /> immediately. TSJCI's proposal to open an excavation approximately 100 feet long, <br /> 10 feet wide and 15 to 20 feet deep on the shoulder of West Eleventh Street to <br /> collect and remove free product cannot be approved by this office at this time. <br /> There is no justification for the design based on an assessment of the free product <br /> plume extent, distribution and mass or feasibility assessment for the method. You <br /> are directed to immediately initiate and maintain free product removal from <br /> monitoring well MW-7 and to prepare a work plan to assess the free product <br /> plume. The frequency of free product removal should be set to optimize the <br /> recovery rate. You may want to consider an automated recovery method. <br /> EHD notes that the western margin of the plume of impacted ground water has not <br /> been delineated through recent sampling and is not currently being monitored. <br /> Please prepare work plans to complete delineation of the plume of impacted <br /> ground water and to install ground water monitoring wells to monitor the western <br /> margin of the plume. As the plume is currently modeled to be at least 900 feet <br /> long, EHD believes that one or two additional wells along the plume axis would be <br /> beneficial for more fully characterizing the plume and monitoring its migration <br /> and/or response to remediation efforts. Also, cross-section A-A' depicts the clay <br /> interval at approximately 25 feet bsg to potentially be discontinuous, therefore at <br /> some point north-northeast of MW-7, the vertical extent of impacted ground water <br /> should be delineated to demonstrate that impacted ground water is not below the <br /> clay interval in which MW-11 bottoms. <br /> EHD agrees that the water supply wells at 7601 W. Eleventh . Street, <br /> 7500 W. Eleventh Street, 23950 S. Chrisman Road, 23901 S. Chrisman Road and <br />