3815_PGSF WMVD Pkg 2_Vol 4-2_03.03.2026_BXWA_Certified

ASPECT CONSULTING 74 FINAL PROJECT NO. AS190583A-08  MAY 21, 2025 on average across the Upland Area, and as expected, it has declined further following completion of the CM removal. It is well documented that, over time, exposed cement “carbonates” naturally—converting calcium hydroxide to calcium carbonate [CaCO3]—as it is exposed to carbon dioxide [CO2] in air or water via the reaction: Ca(OH)2 + CO2 => CaCO3 + H2O Exposed cement surfaces become mineralized with calcium carbonate, preventing leaching of hydroxide ions that result in increased pH. Therefore, the potential for CM, or any cementitious material, to produce high pH infiltration gradually dissipates over time. Table 6-5 presents a comparison of the average pH for the wet season and for the dry season between 2016 and 2017, when measurements were collected from numerous wells across the Upland Area in both seasonal conditions. Note that the 2017 measurements were collected during a year of record-setting precipitation (2016-2017 water year). Table 6-5 also includes dry-season measurements collected between June and October 2020 for the CM removal project. Of the 59 wells measured across the Site in March 2017, 35 were also measured in February 2016 (22 inland, 13 shoreline wells26). Of the 22 inland wells with both sets of wet-season measurements, eight wells had a higher pH and 14 wells had a lower pH; the average pH for the 22 inland wells declined 0.9 pH units 27. The wet-season pH measured at 10 of 13 shoreline wells also declined between the two monitoring events, with an average decline of 0.8 pH units (Table 6-5). Of the 26 inland wells with pH measurements in August 2016 and September 2017, 10 wells had a higher pH and 16 had a lower pH in 2017 than in 2016. The magnitude of pH declines was generally larger than the pH increases at individual wells, such that the average dry-season pH for the 26 inland wells declined approximately 0.2 pH units over the 1-year period. The average pH in the 13 shoreline wells measured in both events declined 0.3 pH units (Table 6-5). The data indicate that, despite the record-setting amount of precipitation falling in the 2016–2017 water year, the average groundwater pH across the Site was gradually declining due to carbonation of the CM. The lower portion of Table 6-5 compares dry-season measurements collected in 2016, 2017, and 2020 from 12 inland wells. For this subset of wells, which includes many of the highest-pH wells on Site, the average pH declined 0.2 pH units between 2016 and 2017 and an additional 0.2 pH units between 2017 and 2020. The diminishing extent of higher-pH groundwater between 2017 and 2020 is also apparent from comparing Figures 6-P7 (2017 average pH) and 6-P8 (2020 average pH)—notably the extents of pH 26 Designated as ‘shoreline wells’ if located within 200 feet of the shoreline. 27 Because pH is a logarithmic term, the pH values were converted to hydrogen ion concentration (= 10^[-pH]), the geometric mean calculated (data span orders of magnitude), and then that mean value converted back to pH units.