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l Lincoln Center - 3 - 29 August 2019 San Joaquin County compliance limitations as described in the MRP. Additionally, certain monitoring wells in the treatment zone are designated as sentinel wells for tracking the concentration of hexavalent chromium in groundwater which was seen as a reaction byproduct during the ISCO pilot test. The MRP includes requirements for establishing trigger levels for these sentinel wells for installing ion exchange in the groundwater treatment system to treat any elevated hexavalent chromium that reaches the extraction wells. For the proposed ISCO injections along the sewer line, existing monitoring wells MW-006A, MW-130A-U, MW-130A-L, MW-131A-U, MW-131A-L, PT-8, PTA1, and CHMW-005Awill be monitored in accordance with the MRP and the work plan as treatment zone wells to evaluate performance. Samples will be collected weekly for four weeks, monthly for six months, then return to the previous monitoring schedule for the site-wide groundwater monitoring program. Other monitoring wells already included in the MRP for the existing injections are located downgradient from the proposed source area injections for evaluation of performance and reaction byproducts after the six-month period. Contingency Plan: The General Order requires a contingency plan for corrective actions should water quality exceed the requirements of the Order at the compliance wells. The General Order prohibits concentrations of total dissolved solids (TDS) or dissolved metals more than 20% greater than their respective background levels, or exceedances of water quality limits at the compliance wells. Baseline concentrations of TDS and dissolved metals will be determined prior to the injections following the procedures specified in the attached MRP. If there is an exceedance at a downgradient compliance well, a confirmation sample will be collected within 10 days of learning of the exceedance. If the exceedance is confirmed, the Central Valley Water Board shall be notified in writing within 10 days. If an exceedance is confirmed in a downgradient compliance well, a corrective action work plan shall be submitted to the Central Valley Water Board for approval within 30 days. The work plan shall include one or more of the following: • Increase monitoring frequency; • Expand the monitoring network; • Inject a reducing compound using amendments such as sodium thiosulfate, calcium polysulfide, zero valent iron, ascorbic acid, and/or lactate and ethanol to reduce dissolved metals mobilized by ISCO; or • Induce hydraulic control of amendments in the target treatment area. All contingency measures in the work plan shall be fully implemented within six months of the confirmation of an exceedance in a downgradient compliance well, following approval by the Executive Officer.