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Work Plan for Additional Subsurface Investigation Former Helena Chemical Facility July 13,2020 Page 5 groundwater would be impacted based on the analytical result of the surface water sample collected at the Site. Therefore, a boring in the stormwater basis at the Site is unwarranted. Instead, Condor recommends collecting one additional retention basin sample this winter for analysis. 4.5 POTENTIAL FOLLOW-UP INVESTIGATIONS ...on-site data needs to be collected to define the onsite stratigraphy and sample at least the next lower water bearing zone for nitrates and 112,3-TCP...idents all groundwater bearing zones present at the Site, delineate the vertical and lateral extent of the contaminant plumes in each of the groundwater zones, and install groundwater monitoring wells at appropriate locations and depths to monitor the deeper groundwater zones, if warranted...monitoring wells may need to be installed at deeper depths to provide a complete verticalprofile of the extent of contamination and to monitor concentrations of nitrate and 1,2,3- TCP over time. Out of an abundance of caution,Condor does not recommend drilling into deeper water-bearing zones until the lateral extent of impact by 1,2,3-TCP is known because 1,2,3-TCP is denser that water and may readily migrate downward through vertical conduits introduced during drilling. In the interim, Condor is working to locate nearby deeper monitoring wells and/or water supply wells to determine available information regarding deeper stratigraphy and groundwater. Three deeper monitoring wells have been installed at the 1904 Charter Way property located south of the subject Site and at least one known industrial water supply well has been identified northeast of the Site(see Figure 4 in the Sensitive Receptor Survey,Appendix Q. Condor has contacted the Central Valley Water Board and EHD to obtain the associated well logs. In addition,analytical results of groundwater samples collected from the monitoring wells at 1904 Charter Way suggest that deeper water-bearing zones are likely not significantly impacted. Based on historical analytical results at 1904 Charter Way(Geosyntec,2020),vertical attenuation from the shallow monitoring well (MW-5B)to the intermediate zone monitoring well (MW-15)near the source of the plume is greater than 99 percent. By comparison, analytical results of"grab" groundwater samples collected from the Site (Table B-3,Appendix B)suggest a similar attenuation from samples collected at approximately 15 feet bgs to samples collected at approximately 30 feet bgs of between 70 percent and 99 percent. Further investigation of deeper stratigraphy and water-bearing zones will be re-evaluated after off-site well logs are obtained and pending the results of installation and sampling of the proposed additional shallow monitoring wells at the Site. 5.0 PROPOSED METHODOLOGY The purpose of the work is to install and sample three groundwater monitoring wells (MW-6,MW-7, and MW-8) to further evaluate the extent of pollutants associated with the historical storage of agricultural chemicals at the Site. All work conducted by Condor will be under the supervision of a California Professional Geologist. 5.1 SITE PREPARATION Condor will obtain the necessary drilling permits from the EHD prior to scheduling field activities with the sub-contractors and notifying Underground Services Alert(USA). One monitoring well will be installed on the adjacent property to the north, so fieldwork will be coordinated with the adjacent property owner. Condor will mark the drilling locations on the ground with white paint and will contact USA at least 48 hours prior to initiating drilling, as required by law. Condor will obtain the services of an underground utility locating sub-contractor to supplement the locations provided by public utilities during the USA ��i CONDOR