Rehabilitate White Wolf Sewer System in Yosemite National Park
The existing White Wolf wastewater system, constructed in the 1970s, serves the campground, housing, and concessionaire operations at White Wolf. An underground gravity collection system conveys wastewater to a central collection manhole. From there the wastewater enters a central main sewer pipe, which runs downhill approximately one mile to an above-ground treatment lagoon. The pipe is buried in some stretches and runs above ground in others, parallel to and above the Middle Fork Tuolumne River. The lagoon covers approximately 1.7 acres and feeds a disinfection system inside a small enclosure. From there water is pumped to a 2.5-acre spray field.
The current system requires significant maintenance efforts to start up and remain functional. Snow loads, rock fall, and downed trees are common occurrences that impact above-ground sections of the sewer main, necessitating repairs prior to opening the system each summer. In addition, the lagoon is typically full and the spray fields saturated due to spring snow runoff. Operators must wait until the spray fields dry out sufficiently to allow infiltration of water before starting to draw down the lagoon. Once the spray fields are operating, it can be weeks before sufficient freeboard (vertical distance between the crest of the embankment and the lagoon water surface) is established in the lagoon to allow activation of the sanitary system. These limitations delay the opening of the White Wolf facilities each year. Breaks in the mile-long sewer main have the potential to allow wastewater spills into the Middle Fork Tuolumne River.
This project proposes to replace the portion of the wastewater system that is downstream of the central collection manhole with a new system which pumps the gravity-collected wastewater from new underground tanks (located adjacent to that manhole) uphill to a new leach field. The proposed 14,000 square foot leach field would be a mound system located in a forested area north of the White Wolf Lodge and south of the employee housing area. The existing gravity collection system upstream of the central collection manhole would be retained as part of the upgraded system.
Please note that an additional replacement leach field area is identified on the schematic drawings (as required by Section VII of the Tuolumne County Guidelines for Design and Evaluation of Special Design On-Site Sewage Treatment and Disposal Systems). This area will not be developed as part of the current undertaking but is required to be identified per regulations. Should the replacement leach field be required due to failure or general age of the system in the distant future, the replacement undertaking will undergo separate consultation.
Installation of the wastewater collection and dosing system will include the following:
• Remove the existing central collection manhole and replace with a new manhole, requiring soil disturbance of 10 feet width and 5 feet depth, to receive wastewater from the existing gravity collection system.
• Trench approximately 90 linear feet at 4 feet width and 5 feet depth to install 4-inch diameter polyvinyl chloride (PVC) pipe for draining inflow and infiltration from the gravity system during non-summer months when new sewer system is not operational.
• Trench approximately 5 linear feet at 4 feet width and 5 feet depth to install 6-inch diameter PVC sewer line to connect collection manhole with septic tank.
• Install two 4,000-gallon underground septic tanks in series (requiring soil disturbance of 12 feet width, 8 feet depth, and 40 feet length) to receive and provide primary treatment. The tanks will be installed under the existing dirt road to minimize new disturbance.
• Install a 1,000-gallon underground dosing tank to receive effluent from the septic tanks, requiring soil disturbance of 11 feet width, 8 feet depth, and 11 feet length.
• Trench approximately 20 linear feet at 4 feet width and 5 feet depth to install 6-inch diameter PVC sewer line to connect dosing tank with overflow septic tank.
• Install a 4,000-gallon underground septic tank (requiring soil disturbance of 8 feet depth, 20 feet length, and 12 feet width to install) to hold overflow as needed. The tank will be installed under the existing dirt road to minimize new disturbance.
• Install two lift pumps inside the dosing tank to convey effluent uphill to the leach field.
• Install pole-mounted above-ground electronic control panel box with alarm and backup generator switch near the existing propane tanks and solar panels.
• Trench approximately 10 linear feet at 3 feet width and 3 feet depth to install 1½ -inch diameter underground electrical conduit between dosing tank pumps and new electrical control panel.
• Trench approximately 90 linear feet at 3 feet width and 3 feet depth to install 1½ -inch diameter underground electrical conduit between dosing tank pumps and existing electrical box near generator building.
Installation of the effluent leach field will include the following:
• Trench approximately 4 feet width and 4 feet depth to install approximately 700 linear feet of 4-inch diameter high-density polyethylene force main from the new dosing tank to the new leach field. Most of the pipe would be installed under an existing utility road (unpaved), with approximately 30 linear feet of trenching in natural, forested area.
• Remove approximately 90 lodgepole pine trees ranging from approximately 6-36 inches diameter at breast height within and around the proposed leach field area, to prevent root intrusion into leach field pipe network.
• Scarify an area of approximately 14,700 square feet (approximately 124 feet by 119 feet) to a depth of approximately two feet. This scarification area includes cutting and removing debris for the central 9,500 square feet required for the leach field, plus a 12-foot border (an additional 5,250 square feet) around the edges to grade the system to match the existing topography.
• Lay edges of rodent exclusion barrier of ¼ inch wire mesh over outer two feet of scarified area.
• Cover the central 9,500 square feet of the scarified area with import sand to a height of approximately two feet, sloping to meet the existing ground surface at the edge of the scarified area at a slope of three (horizontal) to one (vertical).
• Cover the central 9,500 square feet of sand with nine inches of gravel; this is the distribution bed.
• Install leach system on top of gravel, consisting of a grid of approximately 3,000 linear feet of parallel 1.5-inch diameter PVC pipes branching off of a 100-linear-foot 1.5-inch diameter PVC manifold.
• Fill gaps between leach lines and cover with an additional three inches of gravel, so the total depth of the gravel layer is 12 inches.
• Cover gravel distribution bed with fill soil to a height of approximately 1.5 feet above the gravel at the center, with a minimum cover depth of one foot at the edge of the distribution bed. Rodent exclusion layer to be wrapped around the sand and distribution bed at roughly the middle of the fill soil layer.
• Finished leach mound will cover approximately 14,700 square feet, with a height of 4-5 feet at the central 9,500 square feet. The edges will slope to meet the existing ground surface at a slope of three (horizontal) to one (vertical).
• Landscape and initiate revegetation of the mound so that the area blends with the surrounding natural landscape. Revegetation will use native grass and forb seed sources. Place logs on top of mound to blend the area with the forested landscape and discourage visitors from walking on top of the system. Screen mound by planting shallow-rooting native shrubs around edge of leach field to dissuade visitor activities.
Project staging is proposed in paved and graveled areas including the Lodge parking lot, existing utility areas, and access roads.
Due to the need to treat and dispose of snowmelt that accumulates in the lagoon each winter, the components of the existing wastewater system downstream of the central collection manhole (central sewer main, lagoon, pump house, and spray system) will remain in place until a future project is developed to decommission them. This project would lay the essential groundwork for a future project to remove the mile-long sewer main, which is in wilderness, and the lagoon, which is not.
Site investigation and design work began in September 2019 and is anticipated to be complete in spring 2020. Construction would begin in late spring or early summer 2021, once snow has melted.
This project pertains to improvements on the wastewater system serving the White Wolf area of Yosemite National Park.