Lucas Valley Gasoline Spill Infrastructure Removal Project (Notification of Lake or Streambed Alteration, No.

Lucas Valley Gasoline Spill Infrastructure Removal Project (Notification of Lake or Streambed Alteration, No.

The Project is limited to decommissioning five groundwater monitoring wells, removing two permeable reactive barrier (PRB) trenches, removing seven piezometers, and removing two monitoring points, all of which were installed in Miller Creek as part of a gasoline spill monitoring and remediation effort under Notification of Emergency Work 1600-2019-0381-R3. The project also includes restoration of all disturbance areas within the stream. The five monitoring wells are designated MW-1 through MW-5. MW-1 is four inches in diameter and 15 feet deep, MW-2 is 2 inches in diameter and 45 feet deep, MW-3 is 5 inches in diameter and 23 feet deep, MW-4 is 4 inches in diameter and 13 feet deep, and MW-5 is 5 inches in diameter and 17 feet deep. Each of the five wells will be decommissioned by grouting the interior of the well to prevent future migration of fluids along the bore path. In each well, grout will be fed into the well and pressurized to 25 psi for a minimum of 10 minutes per well to ensure the grout mixture penetrates and seals the material between the well screen and the well casing. Well MW-03 was built prior to the gasoline spill and the length and composition of the well screen is unknown. Prior to grouting MW-03, the outer edge of the well will be perforated throughout to ensure that grout is able to enter and seal the filter pack surrounding the well casing. A minimum of 12 hours after the pressure grouting is completed, the upper 5 feet of well casing, protective stove pipes, and concrete pads will be removed using a backhoe. The cavity will be filled with native streambed gravel and sand from an existing stockpile, which will be graded to match the existing surface conditions. All waste material from well destruction activities will be disposed at an off-site landfill. Each well will be destroyed by a C-57-licensed driller under the supervision of a California Professional Geologist. Well destruction procedures and backfill materials will conform with the California Department of Water Resources Bulletin 74-90, California Well Standards (California Department of Water Resources, 1991). Two PRB trenches extend approximately 15 feet across Miller Creek at a depth of 11 feet and width of 3 feet. The trenches were excavated and backfilled with a mixture of activated carbon amendment and clean sand underlaid with geotextile fabric. Three piezometers were installed in each of the PRB trenches and a seventh was installed in native material between the two PRB trenches. The trenches and piezometers will be excavated and removed using a small excavator or backhoe. Excavated PRB material will be transported out of Miller Creek by placing it from the backhoe into one-cubic-yard-capacity track-mounted dump trucks, which will transport the non-native material to the staging area using the existing access road. There, it will be temporarily stockpiled for waste disposal. This stockpile will be covered in 6-millimeter plastic sheeting for protection from erosion until removed offsite for disposal. The piezometer installed between trenches will be removed from the streambed using an excavator and disposed of in the same manner. Two monitoring points, each consisting of three-inch PVC pipe, were sunk approximately 3 to 7 feet below ground surface upstream of the PRB trenches. These will be excavated by shovel and removed by hand. The hole will be backfilled using native material and restored to original grade, and the pipe and any screening material will be points will be by foot only. The Project will result in 1,516 square feet and 80 linear feet of temporary impact to streambed and to habitat hydrologically connected with the stream. No in-water work will be conducted. No trees will be removed as part of the Project, however trees may be pruned to facilitate site access.

Unincorporated area

Marin

Northern California, San Francisco Bay Area

Marin