By Steve Peck
Photo by Stormwater Management Inc
A passive stormwater treatment system, installed two years ago to treat runoff from the Ski Run Marina in South Lake Tahoe, California and resolve a cease-and-desist order issued by the Lahontan Regional Water Quality Control Board, is meeting one of the toughest water quality standards in the USA.
Ski Run Marina is an integral component of a public/private redevelopment project coordinated by the South Tahoe Redevelopment Agency in the northeast portion of the City of South Lake Tahoe.
Initially, Marina property owners had a lined detention basin fabricated to treat stormwater runoff from the site. Due to site constraints, however, the detention basin was built too deep with side slopes that were too steep, which prohibited growth of vegetation in the basin and, in turn, prevented the basin from providing the required level of treatment for stormwater runoff. The basin became an eyesore due to algae growth caused by standing water and the lack of vegetation.
The Lahontan Regional Water Quality Control Board (LRWQCB) determined that the basin was not meeting water quality permit objectives, and it issued a cease-and-desist order requiring that the problem be fixed.
Lake Tahoe has one of the highest effluent standards in the country; for instance, total nitrogen cannot exceed 0.5 mg/l; total phosphates 0.1 mg/l. At the Ski Run Marina, the primary pollutants of concern were phosphorus, nitrogen, and sediment, all found to promote algal blooms in Lake Tahoe and result in a loss of water clarity. Additional pollutants included oil, grease, and metals from parking lot runoff.
The City of South Lake Tahoe Engineering Division researched new and innovative ways to treat the water. The only feasible location was closer to the lake in one of the only remaining open areas at the Marina site, but the city also planned to develop this open space into a plaza-like setting. The new treatment facility would have to treat water to the levels required by the Board, and also limit the area disturbed in order to maximise the plaza area and meet the physical constraints of the site-high groundwater, a complex network of existing pipes, and extremely flat terrain. The Engineering Division contacted the US company Stormwater Management Inc. about its passive filtration product, The Stormwater Management StormFilter®, which could meet their strict criteria.
The StormFilter is a siphonic, flow-through filtration system consisting of an underground concrete structure that houses rechargeable, media-filled filter cartridges. It works by passing polluted water through the cartridges, which trap particulates and adsorb pollutants such as dissolved metals, nutrients, and hydrocarbons.
The City recognised that a filtration system that could be customised to target site-specific pollutants made sense for the area, particularly because this would be the only system directly discharging to the lake. Stormwater Management's team worked closely with city engineers to develop the right solution for water quality treatment to meet Lake Tahoe's high standards. The customised system was installed completely underground, using up no land area and allowing parking above, mitigating any concerns about physical site constraints.
At Ski Run Marina, the StormFilter system contains 50 filter media cartridges in a customised vault with interior dimensions of 36 ft. by 10 ft. The system is designed to treat a water quality design flow rate of 1.67 cfs (750 gpm). Using a layered zeolite-perlite media in its filter cartridges, the system offers physical filtration of sediment, oil and grease, and associated metals and nutrients (perlite) and removal of dissolved metals through surface area contact and cation exchange (zeolite).
The treatment structure was comprised of four three-sided, precast, concrete sections to accommodate shallow depth requirements and minimise onsite construction. The contractor, White Rock Construction, Gardnerville, Nevada, USA, assembled the structure on-site, and then poured a secondary floor over a prefabricated under-drain system to support the filter cartridges. Once installation was completed, the structure was capped with traffic-bearing lids, which included large doors for easy access during future maintenance activities. A StormGate™ high flow bypass, another product by Stormwater Management Inc., was installed upstream of the filtration system to bypass any flows that exceeded the design water quality flow rate to help prevent the re-suspension of collected sediments and oil and grease.
The City regularly monitors the treatment system since its installation. Monitoring data collected from eleven events shows that the StormFilter is meeting the City's expectations, according to Russ Wigart, the water quality specialist for the City of South Lake Tahoe.
Steve Peck is the principal civil engineer for the City of South Lake Tahoe, California, USA.