10-10-1988 - Agenda Packet City Council - regular meeting

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s compounds (VOCs) were identified in monitoring well MW-4D. The presence of VOCs in MW-4 prompte installation of three additional wells proposed in the May 24 work plan. 3,0 ADDITIONAL PROJECT RESULTS 3.1 General Three additional monitoring wells were installed during June 1988 to further define the extent of VOC contamination. The locations of the new wells (MW-10, MW-11, and MW-12) are shown in Figure 2 along with the locations of previously installed wells. After installation, the new wells were developed and sampled. 3.2_ Soil Borings and Monitoring Wells Three additional soil borings were advanced at the site and were completed as monitoring wells. The boring well locations are shown in Figure 2. The subsurface geology of the site consists of heterogeneous mixture of clay, silt, sand, and gravel. The stratigraphy consists of silty clay with intermittent medium to coarse grained sand lenses which are generally less than two feet in width. The sand lenses do not appear to be laterally extensive. Complete lithologic boring logs for the three soil borings are included in Appendix A. Borings MW-10 and MW-1 1 were each advanced to a depth of 25 feet below existing grade. Boring MW-12 was advanced to a depth of 55 feet below the existing grade. Each of the three borings were completed as monitoring wells. The wells are constructed of low carbon steel risers with five foot stainless steel screens. Complete well construction details are given in Appendix B. 3.3. Contaminant Conditions 33.ISoil Contamination Soil contamination was pre,. iously identified in the vicinity of MW-4 and MW-4D; therefore, the upper 21 feet of soil was not sampled during the installation of MW-12. Low contaminant concentrations, as detected by the hNu photoionization meter, were detected in an interval between 23 and 36 feet below the ground surface during the drilling of MW -12. A summary of hNu results for borings MW-10, MW- 11, and MW-12 is included in Table 1. Ionizable gases were not detected in any of the samples after opening the split -spoon. Headspace checks revealed some volatiles in borings MW-11 and MW-12. However, the concentrations (Table 1) are at the lowest detection limit for the hNu which is approximately one part per million. At these concentrations, interferences with water vapors are common and the concentrations must be viewed in this context.