AP2400573 (4) - Laserfiche WebLink

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ekienvironment & water Relative to the Fall 2022 sampling event, the Spring 2023 NO3-N concentrations decreased in all CLSP compliance wells (CLSP-1, CLSP-8, CLSP-9, CLSP-12, and MWM-19) except for CLSP-11 (duplicate sample concentrations of 16.7 and 16.9 mg/L) and CLSP-3 (not detected). NO3-N concentrations in CLSP-11 have been steadily increasing since Spring 2022; however, the Spring 2023 concentration remains below the historical maximum (29.0 mg/L). Relative to the Spring 2022 sampling event, the Spring 2023 NO3-N concentrations in both River Islands compliance wells MWR-28 and MWR-32 increased by 1.6 mg/L and 0.1 mg/L, respectively. In MWR-28,the NO3-N concentration is the highest recorded (1.6 mg/L) by 0.4 mg/L; however, it remains below the proposed revised groundwater limitation for this well (10 mg/Q. The concentration in MWR-32 remains below the historical maximum and the proposed revised groundwater limitation for this well. Relative to the Fall 2022 sampling event, the Spring 2023 NO3-N concentrations in the CTF compliance wells slightly increased in KMW-8, KMW-9 and KMW-10 and decreased in KMW-2, KMW-4, KMW-6, and KMW-11. The concentrations in KMW-8 and KMW-9 did not exceed historical maximum observed concentrations; however, the sample in KMW-10 is the highest in the historical observed concentrations and exceeds the groundwater limitation for this well (10 mg/Q. In general, NO3-N concentrations in KMW-10 have increased since the Spring 2020 monitoring event. The WDRs report that NO3-N concentrations in groundwater are likely due to historical agricultural practices(see Finding 61 of the WDRs). Moreover,the Groundwater Salinity Mitigation Plan concluded that constituent concentrations beneath the CTF most likely have been affected over time by regional agricultural irrigation, unlined ponds operated by the former Occidental Chemical Company, and effluent from the WWTF. Hence, dissolved constituent concentrations in samples from wells are likely due, at least in part, to background conditions. 4 LABORATORY REPORTS All laboratory analytical reports used to prepare this report will be maintained on file by the Discharger and are available upon request. A discussion of the quality assurance and control assessment of the analytical results is included in Appendix D. City of Lathrop 14 13 July 2023 EKI B90088.03