ARCHIVED REPORTS_XR0007764 - Laserfiche WebLink

Home Browse Search

i 07 November 2003 . AGE-NC Project No 98-0492 IPage 5 of 9 MTBE has been detected each quarter in the samples collected from wells MW4 and MW5 at concentrations ranging from 76 µg11 to 12,000 µg11 However the average concentration of MTBE �Ueach quarter in the water samples collected from wells MW4 and MW5 is approximately_3,000µg/1 MTBE has also been detected each quarter in samples collected from wells MW6 and MW9 at concentrations ranging from 1 0 µg/1 to 79 µg/1 The only other fuel additive to be detected in the ground water samples collected from the site between April 2001 and August 2003 is TBA TBA was detected this quarter at a concentration of 10 µg/1 in well MW9 Analytical results of ground water samples are summarized in Tables 8 and 9 Cross-sections depicting the general lithology in the soil beneath the site is Illustrated on Figu4,4,"$ and761 A map depicting the approximate distribution of dissolved MTBE is illustrated on Figure 8 TPH-g and benzene were not detected in any of the soil samples collected and analyzed at the site since assessment activities began Only toluene and MTBE in very low concentrations were detected in a total of fourteen soil samples collected at the site The lateral and vertical extent of impacted soil has been defined ' Based on the investigation to date, ground water at the site is only impacted with MTBE at concentrations requiring active site remediation The vertical extent of MTBE-impacted ground �• water has been defined and the lateral extent of MTBE-impacted ground water has generally been defined, to levels required for site assessment and closure Further assessment is required to fully define the MTBE-impacted ground water towards the west beyond well MW9 4.0. POTENTIAL IMPACT ON NEARBY SURFACE AND GROUND WATER i Typically, ground water would represent the migration medium for contaminants over significant ' horizontal distances However, sites located in up-gradient or cross-gradient directions from the subject property are not likely to be negatively impacted by the subject property In addition, first encountered ground water is not used for dnnking or irrigation, although all ground water in the IState of California is considered to be of potential beneficial use Drinking water at the site is supplied by the City of Manteca Municipal Water Well Network, City I of Manteca Water Well Number 13 is closest to the site (2,000 feet south) Municipal water well number 13 poses minimal concern because the well is located up-gradient and cross-gradient of the former UST location Based on analytical data collected from the cross-gradient wells MW3 and ' MW7, the MTBE plume does not appear to have migrated beyond MW3 or MW7 The nearest surface water feature to the site is the Stanislaus River approximately five miles south ' of the site It is very unlikely that impacted ground water at the site represents a significant pollution threat to w ater in the Stanislaus River,based upon the distance and direction of the Stanislaus River from the site ' Advanced GeoEnvironmental,Inc I