WORK PLANS - Laserfiche WebLink

Home Browse Search

Groundwater Monitoring Well Construction Workplan February 2024 Monitoring Well Design Justification A four-level nested monitoring well will be constructed at each of the drill sites. The levels (depths) of the screen intervals were selected to provide groundwater levels above and below the Corcoran Clay, groundwater use in the area, potential differences in water quality, and to assess whether the groundwater pumping is raising the base of fresh water. These monitoring wells (four-levels)will also better characterize groundwater conditions by establishing groundwater outflow gradients from or to the East Contra Costa Subbasin into or out of the TSb. The lithologic information will also help to confirm/support Airborne Electromagnetic (commonly known as AEM) survey findings. Figures 9 and 10 shows the preliminary monitoring well designs. The monitoring wells screen locations were developed based on the projected depth and thickness of the projected Corcoran Clay and the base of fresh water. It is likely the Corcoran Clay is present beneath the drill sites. Figure 4 shows the extent and structure of the Corcoran Clay as interpreted by the USGS. The top and thickness of the clay at MW-201 and MW-202 were not defined in the project area. The top of the clay using projections using the USGS data suggests the top of the clay may be 140 feet bgs or based on a nearby borehole possibly 370 feet bgs based on regional projections suggesting the clay may be dipping. The base of fresh water is at elevation-800 feet amsl or about 800 feet bgs (refer to Figure 5). Because the depth and the thickness of the Corcoran Clay is unknown at this location the following description of the well screen placement are based on the deeper projections of the clay. The well screens will be adjusted once the borehole is drilled and a geophysical survey is conducted so that it is above the Corcoran Clay, if the clay is present. The shallowest monitoring wells (named MW-201A or MW-202A)will be constructed in the Upper Aquifer, above the Corcoran Clay. Aquifers above the Corcoran Clay are used by domestic wells that are constructed to depths of 108 to 170 feet bgs. These monitoring wells will be used to monitor groundwater levels in the Upper Aquifer to track changes and inform the GSAs if lowering of groundwaters could affect these domestic well owners. The well screens will be adjusted once the borehole is drilled and a geophysical survey is conducted so that it is above the Corcoran Clay. Six monitoring wells (named MW-201 B, C, D and MW-202B, C, D)will be screened below the Corcoran Clay. The screens for B level monitoring wells will be positioned just below the Corcoran Clay, where low salinity groundwater may be present, based on other occurrences in the Subbasin. The screens for C level monitoring wells will be positioned about half-way between the bottom of the Corcoran Clay and the base of fresh water. Both B and C level screens will be positioned to track groundwater level changes and inform the GSAs if lowering of GEI Consultants, Inc. 17