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Foothill Sanitary Landfill 14 Department of Public Works/Solid Waste <br />First Semiannual 2025 Groundwater Monitoring ©2025 County of San Joaquin. All rights reserved. <br /> <br />6.0 MONITORING RESULTS <br />6.1 Statistical Analysis <br />The FSL has been approved to use interwell tolerance limits methods, with an up-gradient well <br />providing background data (Tables 2 and 3). <br />MW-4 is located directly up-gradient of the landfill based on groundwater elevations from <br />previously installed wells. This well was sampled 20 times from installation through July 2015. <br />Monitoring well MW-4 has been reported as dry since the first quarter of 2016. CLs were <br />established based on Sanitas software calculations using data for MW-4. Although MW-4 has <br />been dry since the first quarter 2016 sampling event, the CLs developed from the previous <br />samples collected from this well are believed to be valid representations of up-gradient water <br />quality. In response to the June 6, 2018 NOV described above, the County replaced MW-4 with <br />a deeper well, MW-4A, in December 2018. Updated CLs were prepared using data through <br />2SA22 from MW-4A for use starting in 2023. Where CLs could not be updated due to insufficient <br />data, the previous MW-4 CL will be used until the limit can be updated with data from MW-4A. <br />6.2 Groundwater <br />Sampling and monitoring events during 2025 occurred as follows: <br /> First Quarter 2025 conducted on January 13 through 14, 2025. All wells were sampled <br />except MW-3 and MW-7R which had insufficient water to collect a sample and MW-5 <br />which was dry. <br /> Second Quarter 2025 conducted on April 10, 2025. Only MW-3 was scheduled to be <br />sampled, however, insufficient water was present to collect a sample. <br />6.2.1 Monitoring and Field Parameters <br />Groundwater monitoring results for the first semiannual monitoring and sampling event (Table 2) <br />indicates that the concentrations of field and monitoring parameters are mostly below <br />established CLs. Exceedances of turbidity, sodium, potassium, and sulfate were present in some <br />samples. Bicarbonate alkalinity concentrations were at the CL in three samples. Appendix C <br />includes the laboratory certified analytical reports for the First Quarter 2025 sampling event. A <br />summary of 2025 annual CL exceedances are discussed below. Data for the first semiannual <br />period are available in the historical groundwater monitoring data tables presented in <br />Appendix E. <br /> MW-1A <br />o First Quarter – The CL for turbidity was exceeded however, the concentration <br />remained within historical range. <br /> MW-2R