WORK PLANS - Laserfiche WebLink

Home Browse Search

Pilot Study Workplan November 7, 2014 Project#54.62583.0001 Page 5 lithology consists of sand and gravelly sand followed by silt to a depth of 139 feet bgs. The maximum depth explored during any of the investigations at the site is 139 feet. 3.3 HYDROGEOLOGY The site overlies the Eastern San Joaquin Subbasin of the San Joaquin Valley Groundwater Basin. The subbasin is bounded by the Mokelumne River to the north and northwest; San Joaquin River on the west; Stanislaus River on the south; and consolidated bedrock on the east. Groundwater occurs in the Stockton area and at the site as an unconfined shallow aquifer and a confined deeper aquifer. Static groundwater depths measured in the shallow aquifer after the initiation of groundwater monitoring through February 2014 have progressively increased over the period of record. Specifically, in February 1991 the depth to groundwater in monitoring wells MW-1 through MW-3 was measured at an average depth of 70.28 feet, while in February 2014 the depth to groundwater in the eight shallow monitoring wells was measured at an average of 42.85 feet. These data represent an increase in groundwater of approximately 27 feet in the period between 1991 and 2014. The depth to water measured in the deeper aquifer has ranged from 56.30 feet bgs in October 2004 to 40.86 feet bgs in February 2014. Historic groundwater elevation data indicate groundwater in the shallow aquifer beneath the site flows generally toward the east with variations ranging from northeast to southeast. Groundwater flow at the site in February 2014 was toward the northeast with an average gradient of 0.001 ft/ft or 5.3 feet per mile (Figure 3). This flow direction and gradient are generally consistent with previous monitoring events. Only one well is screened in the deeper aquifer so the gradient direction and magnitude cannot be determined. 3.4 SURFACE WATER The closest surface water body to the site is the Mormon Slough which lies on the southern edge of the site. The slough flows to the west and drains into the San Joaquin River over two and a half miles to the west. Based on the latest depth to groundwater measurement at MW1, the closest monitoring well to the slough, the slough is likely a losing stream. Based on the well log for MW1, the soil is classified as silty clay, clayey silt, and silt for the top 14 feet, then classified as alternately, silty sand, sandy silt, and sand to 90 feet bgs. Due to the limited thickness of clay and silt in the top 14 feet and extensive sand and silty sand below suggests that water from the Mormon Slough potentially impacts the aquifer. However, due to limited flows especially during the dry summer months, this impact may be negligible. In addition there appears to be a cement lined holding basin located on the south side of the main building at the site. As a lined basin it does not appear to likely impact the aquifer. 3.5 LOCAL LAND USE The site is used as a walnut processing and packaging facility. Adjacent properties include commercial/industrial (west and south), railroad yard (north), agricultural (southeast) and some residential (south and northeast beyond the railroad yard).