ARCHIVED REPORTS_2013 ANNUAL REPORT

Home Browse Search

Nestl6 USA, Inc., Ripon, California March 26, 2014 2013 Annual Report cis-1,2-DCE concentrations in well M-25C2 equal historical maximum concentrations. TCE concentrations in well M-10C1 increased from levels below the RL in 2000 to a peak of 8.5 pg/L in 2010, but decreased to 4.5 pg/L in 2013. • In the area west and north of the Stockton Avenue and east of municipal well MW-3, COC concentrations in wells M-13A, M-24C1, M-28A/C, and M-28C1 continued to increase in 2013. TCE concentrations in well M-13A increased from 7.1 pg/L in First Quarter 2013 to 9.8 pg/L in Fourth Quarter 2013; cis-1,2-DCE concentrations increased from 13 pg/L to 18 pg/L during this period. TCE and cis-1,2-DCE concentrations in well M- 24C1 have shown an increasing trend since 2006. The TCE concentration in well M-24C1 increased from below the RL to 15 pg/L in 2013 and the cis-1,2-DCE concentration increased from 0.91 pg/L to 54 pg/L in 2013. Concentrations of TCE and cis-1,2-DCE in well M-28C1 began increasing from below the RL in 2009, and are currently 4.2 pg/L and 7.9 pg/L, respectively. Concentrations of TCE and cis-1,2-DCE in well M-28A/C began increasing from below the RL in 2012 to 0.85 pg/L and 1.4 pg/L, respectively in Fourth Quarter 2013. Lower Aquifer • Seventy-one percent of the Lower Aquifer wells with historical COC detections have had decreasing or relatively constant COC concentration trends over the last five years of monitoring. Sixty-three percent of the Lower Aquifer wells have consistently had COC concentrations below the laboratory RLs since initial monitoring in each well (Appendix C). • TCE and cis-1,2-DCE concentrations in well M-6D increased from below the RLs in 2002 to 35 pg/L and 12 pg/L, respectively, in 2004. The apparent increasing trend continued until April 2008, when TCE concentrations reached approximately 62 pg/L. This peak was followed by a general decrease in COC concentrations, with a TCE concentration of 27 pg/L reported in October 2013 (Table 5.3). This variation in TCE concentrations is consistent with conduit-related impacts from downward migration of groundwater from the Upper and Intermediate aquifers to the Lower Aquifer. • Concentrations of TCE in wells M-34D and M-32D, located south of the WWTP, have begun to decrease. Since Third Quarter 2009, cis-1,2-DCE concentrations in well M-34D decreased from 6.2 pg/L to 4.0 pg/L. Similarly, since Third Quarter 2010, cis-1,2-DCE concentrations in well M-32D decreased from 6.1 to 4.4 pg/L in Fourth Quarter 2013. An irrigation well (Fiscallini #25) located adjacent to the Modesto Irrigation District Canal between the M-34 well nest and Hall Road operated intermittently until it was grouted in May 2008. This irrigation well was screened in all three aquifers and may have drawn CDCs from the WWTP lagoon area south of the Stanislaus River. Although concentrations continue to decline in these Lower Aquifer wells, the concentrations are low and the decreasing trend is gradual (less than 1 pg/L per year); therefore, monitoring over an additional time period is needed to confirm the relationship between decommissioning of the irrigation well and COC concentrations. 12