ARCHIVED REPORTS XR0011755 - Laserfiche WebLink

Home Browse Search

Evaluation ofNatural Attenuation: 7500 West Eleventh Street, Tracy, CA. Page 28 f' hydrocarbons in the source zone. For example, the linear regression extrapolations for TPHg, TPHd, and MTBE for the source zone that are shown on Figures, 27, 28, and 30, respectively, indicate that complete remediation of diesel will occur within two years of the present time. However, the same condition is not predicted to be achieved for TPHg until 16 years from the present and the currently low concentrations of MTBE in the groundwater in that area may not be fully remediated until a similar amount of time has passed. In addition to the cautions expressed concerning the predicted time required for natural attenuation to completely eliminate analytes of concern from the source area of the primary plume, it is also important to recognize that at the Navarra Site high y concentrations of components of fuel hydrocarbons are present down-gradient from the source in the area around Monitoring Well MW-7. Because, on numerous occasions, LNAPL has been present on the groundwater in that well and that material has periodically been purged from the well, it is not possible to construct a meaningful trendline from the concentration data available from analyses of samples recovered from MW-7. Under those circumstances, it is Iikely that the time required for the processes of l natural attenuation to reduce the concentrations of analytes of concern in groundwater in that area of the site to innocuous amounts will clearly be significantly longer than those predicted by the regression analyses of the data from monitoring well MW-3 - the source area of the plume - where groundwater benefited from the aggressive remediation that was implemented at the time that the tanks were removed from the site. In SJC's opinion, significant reduction in the time required to significantly reduce concentrations of analytes of concern in groundwater in Monitoring Well MW-7 would require = % implementation of agressive remediation, as we previously recommended (The San Joaquin Company Inc. 2004c, 2003e, 1999, Watkins 2002),by excavation of affected soil in that area and removal of groundwater containing high concentrations of components containing fuel hydrocarbons by pumping from the resulting excavation. However, the implementation of that remediation technique was, unfortunately, proscribed by the i- - SJCEHD (San Joaquin County Environmental Health Department 2004). Despite the limitations described above that encumber attempts to make definitive predictions of the time required for natural attenuation to fully remediate the affected groundwater at the 7500 West Eleventh Street site, the evidence that has been developed +_ from the groundwater-quality monitoring program is sufficient for a conclusion to be reached. Natural remediation will produce essentially complete elimination of the components of fuel hydrocarbons within a sufficiently limited time to be considered as a practical corrective action policy for the site. Based upon linear extrapolations of the relationship between concentrations of analytes of concern over time in the source area of the primary plume, the future time required to achieve that goal appears to be on the order of ten to twenty years. 7.1.3 Mass Balance of BTEX in Groundwater j' At sites where contaminant plumes are stable, as has been demonstrated to be the case at the Navarra Site in Sections in 7.1.1 and 7.1.2, an estimated natural attenuation rate can sic