WORK PLANS FILE 2 - Laserfiche WebLink

Home Browse Search

Gnvi of August 13, 1997 Ms. Mary Meays San Joaquin County Public Health Services Environmental Health Division Site Mitigation Unit P.O. Box 388 Stockton, California 95201-0388 RE: WORK PLAN ADDENDUM Shell Service Station 2575 Country Club Boulevard Stockton, California WIC 204-7524-3505 Dear Ms. Meays: Enviros,Inc. (Enviros)has prepared this Work Plan Addendum to address issues raised in San Joaquin County Public Health Service (SJCPHS) correspondences dated April 7, 1997 and May 28, 1997. These documents request that a deep well be installed as part of assessment activities. As part of previous investigation at the site, boring RW-B 1 was drilled to a total depth of 46.5 feet below grade (fbg). Soil samples were collected at depths of 11, 31.5, 36.5, 41.5, and 46.5 fbg. Concentrations of petroleum hydrocarbons in the samples collected from 31.5 to 46.5 fbg ranged from none detected (ND) to 1.2 parts per million (ppm). Benzene concentrations ranged from 0.014 to 0.87 ppm. As a result of these detections, SJCPHS had requested installation of a deeper monitoring well to verify that the deeper ground water zone has not been impacted by the release. We do not believe that site lithology indicates that a deeper ground water zone has been encountered, or that installation of a deeper ground water monitoring well is appropriate. Logs from installation of ground water monitoring wells and borings to date have shown permeable sand and gravel layers first encountered at depths of approximately 12 - 14 fbg, and extending to depths of approximately 20- 26 fbg. Boring RW-B 1 was drilled to a total depth of 46.5 fbg. In this boring, the same permeable soils are observed to a depth of 20- 25 fbg, below which a clay was encountered. This clay was observed to the total depth explored of 46.5 fbg. Samples collected from depths of 25 -45 fbg are described as being moist to dry. This suggests that this clay layer may be acting as an aquitard between shallow water and deeper water bearing zones. The presence of this clay layer, combined with the fact that the specific gravity of each of the contaminants of concern is less than one (i.e. the contaminants will float rather than sink) make it unlikely that they will migrate to a deeper water bearing zone. We propose to drill two additional soil borings in the locations shown on Plate 1. These borings will be drilled to estimated depths of 30-35 fbg to determine whether this clay layer extends across the site acting as a barrier to vertical migration. Drilling methods and Envlros. Inc. 270 Perkins Street Sonoma, California To/707/935-4850 P.O.Box 259 954760259 Fax 707/935-6649