ARCHIVED REPORTS_XR0011916 - Laserfiche WebLink

Home Browse Search

. contaminants are contained within this channel deposit at a depth of 35-55 feet, but contamination locally reaches a depth of at least 65 feet where the hardpan is absent north of the UST cavity The volume of contaminated soil remaining in place at this time is estimated to be slightly more than 7,000 cubic yards,with concentrations that range from below 100 mg/kg to 20,000 mg/kg 3)The primary shallow aquifer at the site is the Riverbank Formation, which is present at a depth of 55 or 60 feet Groundwater in this aquifer was as deep as 85 feet in 1990, but has risen as high as 57 feet during this investigation and is presently at approximately 65 feet As a result, it has been impacted by the contaminated soil above the Riverbank Formation At the present time, the groundwater plume covers an area of 1/2 acre and extends to and slightly beyond the southern boundary of the property Six monitoring wells are presently beirig used to monitor the depth of groundwater and the extent of the contaminant plume 4) The large channel deposit that overlies the Riverbank Formation forms a secondary aquifer that is occasionally saturated when groundwater levels are high This aquifer is probably perched above a low- permeability owpermeability snit bed that inhibits but does not completely prevent communication with the Riverbank aquifer Water is present at this tune at the base of this channel deposit and is in contact with highly contaminated soil As a result, concentrations approaching free-phase product currently exist within the groundwater of this upper aquifer 5)No aquifer tests have been performed, but hydraulic conductivity measurements on selected samples indicate that the lower Modesto Formation channel deposit is a lugh-permeability aquifer, the basal portion of the Modesto Formation is a low-permeability aquitard, and the uppermost Riverbank Formation is an mtermediate-permeabihty aquifer 6) Naturarl biodegradation at this site appears to be limited Possible reasons include low oxygen content, limited food source (plant matter and/or petroleum hydrocarbons), and limited inorganic nutrients Moisture content should be sufficient to support microbial growth 7.0 RECOMMENDATIONS Table 1 of Appendix B of the Tri-Regional Water Quality Board Guidelines for the Investigation of Underground Tank Sites lists sixteen items that are required before requests for no further action will be considered Eight of these items have now been completed and are presented or addressed in this report Before proceeding to submitting a request for site closure, we therefore recommend that the following activities be performed 1) Compile a list of all nearby underground utility lines, wells, and surface drainages that could act as potential contaminant receptors within 2000 feet of the site 2) Perform a risk analysis to determine whether the site can be classified as low-nsk The analysis should follow the procedures currently employed in Risk-Based Corrective Action (RBCA) plans, . including computer modeling of contaminant transport 16