Hypolimnetic Oxygenation System (HOS) Pilot Program Lakebed Encroachment Permit (PL-25-311)

Hypolimnetic Oxygenation System (HOS) Pilot Program Lakebed Encroachment Permit (PL-25-311)

Clear Lake has long suffered from environmental issues associated with mercury contamination, and harmful algal blooms (HABs). These conditions threaten aquatic life and the community’s cultural, recreational, and economic activities. In support of the Blue-Ribbon Committee for the Rehabilitation of Clear Lake (BRC), University of California Davis (UC Davis), Tahoe Environmental Research Center (TERC) researchers have shown that low-oxygen (hypoxic) conditions in deeper waters are significant contributors to the development of HABs and potentially mercury bioaccumulation. Hypoxic conditions are created during periods of stratification (warm water on the lake surface cuts off oxygen re-supply to the lake's bottom). TERC has recommended implementation of a pilot program to address the HABs with remediation technology called Hypolimnetic Oxygenation System (HOS) in Oaks Arm of Clear Lake. The project is funded by several grants and funding sources, including the California Natural Resources Agency and the US Environmental Protection Agency. The pilot project in Oaks arm proposes to add dissolved oxygen to bottom waters (the hypolimnion). Oaks Arm was selected because it is the smallest arm of the lake, frequently impacted by HABs and affected by mercury contamination. This project is specifically intended to reduce nutrient-driven algae blooms and mercury bioaccumulation. The knowledge gained will allow the BRC to evaluate upscaling the technology application to the full lake. The HOS system includes land and lake components. The land components include a liquid 15,000 gallon oxygen tank on a curbed concrete pad (25 x 25 ft), refill hose and splash pad, evaporator, controls, piping, and discharge manifold. Land components of the HOS will be installed at the Clearlake Oaks County Water District property adjacent to the lake. A pipeline will deliver the compressed oxygen gas along the District’s pier paralleling the plant’s intake pipes into the lake. The site would also include security fencing, lighting and solar panels to power the system. The in-lake components include two pipelines of approximately 20,000 linear feet reaching from the pier around the west end of Rattlesnake Island and into the deepest part of Oaks Arm where the diffusion takes place. There are no pumps or moving parts, and the only power use is for the control system. Construction is anticipated to occur during the period February-May 2027. Operation is planned for May-November 2027. UC Davis will request temporary use of space in the County Park across the street from the water treatment plant for stockpiling materials. The land components will be constructed within the water treatment plant property, on previously disturbed land. There will be no shoreline excavation or construction. The onshore pipeline will likely run from the storage tank along the sea wall to the District’s pier, and along the pier parallel to the water plant’s intake pipe into the lake. The excavation for footings and foundations are not expected to extend beyond the existing fill material at the plant. The in-lake components will be assembled along the beach and the County Park between the water treatment plant property and the park pier. The in-lake supply lines include a pair of HDPE plastic pipes. The pipe will be assembled in sections and floated from the beach into the lake until the entire pipeline is assembled and ready to float by boat into position in the lake. The in-lake pipeline will be connected to the pipeline at the water treatment plant pier, avoiding any construction impacts to the lake shore. The in-lake pipeline will then be sunk to the bottom. The in-lake pipeline will be anchored with approximately 2,000 concrete blocks, weighing about 120 pounds each, typically spaced about 15 feet apart. The concrete anchors for the in-lake pipeline will rest on the top of the lakebed, with the diffuser line suspended about 1-3 feet above the lakebed during operation, which avoids disturbing the lakebed sediments. The buoyancy pipes are used for deployment and retrieval (for repairs or removal) of the entire pipeline. Operation. The project is a short-term research project intended to complete in one season, or possibly two seasons depending on weather conditions that would impact installation and monitoring. The oxygen for the pilot project will be supplied as liquid oxygen delivered by tanker truck approximately 2 or 3 times per week during the operational months. The HOS design is largely self-cleaning, and the whole system needs only to be floated to the surface for repairs in the unlikely event of a line break. Hazard buoys will be deployed with one near the beginning of in-lake piping, and two more at the start and end of the area of diffuser lines where the lines are suspended. Hazard buoys will include a QR links to information about the HOS and may be fitted with a light for visibility to boaters in low light hours. The pilot program would evaluate the effectiveness of increased dissolved oxygen (DO) near the lakebed as a strategy to mitigate HABs, mercury contamination and other water quality issues. The project is funded and scheduled for all equipment to be removed, leaving behind the concrete pads for future use, if the County chooses to pursue installation of permanent system in the future.

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