COMPLIANCE INFO - Laserfiche WebLink

Home Browse Search

McBillin Real Estate Investment - 2 6 September 2016 2154 S. El Dorado Street Stockton, San Joaquin County transition seal of dry granular bentonite above the sand pack. Please include these changes in a Work Plan Second Addendum (Second Addendum), due 21 October 2016. The soil gas wells are currently proposed to be constructed with the vapor probe set at the bottom of the borehole between 4.5 and 5 feet below ground surface (bgs), with sand from 4 to 5 feet bgs. DTSC guidance indicates that soil gas samples should be collected no shallower than 5 feet, and the vapor probe should be centered vertically in the middle of the sand pack, with at least six inches of sand above and below the probe. As such, the top of the sand pack should be at 5 feet bgs. Please include these changes in the Second Addendum, due 31 October 2016. Currently two water-bearing zones have been monitored as part of this case: shallow groundwater with wells screened between 20-45 feet below the ground surface (bgs), and deep groundwater with wells screened between 90-95 feet bgs. However, an intermediate groundwater zone is present and is described as a wet/saturated sandy zone observed between 55-65 feet bgs during drilling of deep Site wells and borings. No groundwater samples have been collected from this depth. The shallow groundwater flow direction is generally toward the east, though no deep groundwater flow direction can determined from the single existing deep well. In the Monitoring Well Work Plan, AGE proposes the installation of two deep monitoring wells (MW-11 and MW-12) down-gradient, to the east of the Site property. MW-11 is proposed to be located in the gravel road on the eastern edge of the property, approximately 150 feet from the Site source area, and is to be screened from 90-100 feet bgs. MW-12 is proposed to be located approximately 600 feet east of the Site source area, and is to be screened from 90-100 feet bgs. While I concur with the proposed installation of these wells, an accompanying shallow well within 10 feet of each of these deep wells is needed. A shallow well near proposed well MW-11 is needed to delineate shallow groundwater impacts between two of the highest concentrations wells (MW-3 and MW-4), which are separated laterally by a distance of approximately 145 feet. Existing down-gradient shallow wells MW-8 and MW-9 are approximately 300 feet southeast and 625 feet northeast of the Site source area. However, there is an approximately 480-foot lateral distance between these two wells. As proposed MW-12 is in between MW-8 and MW-9, a shallow well in this vicinity is needed to delineate shallow groundwater between these two wells. MW-1 (screened 90-95 feet bgs) appears to delineate petroleum impacts at depth under the source area, with recent concentrations of benzene below the Water Quality Objective of 1 ug/L (the release at this Site appears to predate MTBE). However, hydrocarbon impacts in the intermediate water-bearing zone (between 45 and 90 feet bgs has not been assessed. The intermediate water-bearing zone should be investigated.