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RFP No. 15-043 Task 4 — Groundwater Monitoring Al - Environmental Site Assessment According with the RWQCB determination of eligibility for closure letter, the existing monitoring wells at the site (19 wells) will be monitored and sampled on an annual basis pending site closure in accordance with the RWQCB requirements. During each monitoring event, general water quality parameters, including dissolved oxygen (DO) concentration, ORP, specific conductivity, pH, temperature, and turbidity, will be measured in the field using a portable water quality meter. Depth to water, total well depth, and the thickness of free product, if present, will also be measured and recorded. Groundwater samples will be delivered to a California -certified laboratory under chain -of -custody protocol for analysis of VOCs by EPA Method 8260B and TPHg by EPA Method 80158. The number of groundwater sampling events will depend on the RWQCB's ability to close the site. Task 5 — Report Preparation Ninyo & Moore will prepare at least two reports for the site. One report will be a summary of the Phase 11 ESA and soil vapor survey. Following completion of the Phase 11 ESA, a Human Health Risk Evaluation (HHRE) will be performed to: determine whether COPCs at the site represent a threat to receptors; pro- vide a basis for determining residual chemical levels that can be left in place; and determine the cancer risk to future occupants of the site. Results of human health risk assessment and regulatory agency input will dictate establishment of clean- up criteria and regulatory negotiations. A conceptual site model (CSM) will be developed which will show potential routes of exposure and receptors. The CSM will be used to guide the HHRE. The maximum reported concentrations of VOCs in soil vapor will be compared to "modified" soil gas screening levels (Modified SSLs), in accordance with EPA or DTSC guidelines from acceptable indoor air concentrations published by the EPA or DTSC. The Modified SSLs will be calculated assuming a resi- dential scenario (as a conservative, health -protective measure) using EPA's Regional Screening Levels (RSLs; EPA, 2017) or DTSC's Human Health Risk Assessment Note Number 3 (California DTSC, 2017) concentrations. Other COPCs will be compared with the EPA RSLs for residential land use. A report de- tailing the results of the HHRE will be prepared and included in the Phase II ESA report. The second report will be the second quarter groundwater monitoring report, which will summarize findings and conclusions on how the dissolved -phase TPHg and BTEX concentrations are increasing or decreasing in concentration. Task 6 — Responsible Parties Determination and Estimate of Cost -to -Closure Potential environmental liability exists for the site and may include the need for additional soil remediation in the vicinity of the former gasoline USTs, monitoring and sampling of the existing on-site groundwater monitoring wells to evaluate for natural attenuation (via monitored natural attenuation [MNA]), and vapor mitigation measures for future site structures. Ninyo & Moore's estimated path to closure of the site is as followed: • Based on our Phase I and Phase II ESAs, Ninyo & Moore will obtain a good understanding of the site history and establish environmental baseline conditions for the site. Ninyo & Moore will provide the City management and legal counsel a defensible "cost -to -closure" to be used during negotiations for the property in the event an escrow account is required to be funded to address environmental monitoring and cleanup costs. • Ninyo & Moore will assist the City legal counsel with negotiations with the PRP for restitution for damages and for an indemnification for environmental impact at the site. • The RWQCB will be notified of the results of the Phase II ESA, and Ninyo & Moore will negotiate cleanup levels of behalf of the City. Ninyo & Moore assumes the RWQCB will allow the residual impact to the site groundwater to be remediated via MINA if concentrations of VOCs in soil and soil vapor are below site screening levels. If the RWQCB determines that soil contamination is still at unacceptable levels, costs for cleanup via targeted soil excavation will be discussed with the PRP as mentioned above. • TPHg remains in the bioattenuation zone at elevated concentrations at the site, with a concentration of 2,600 mg/kg reported in the sample CSB -1 at approximately 10 feet bgs ® Ninyo & Moore I City of Santa Ana - (All Environmental Site Assessment 1 05-00713 1 June 5, 2018