Circulation system moves water to disrupt algae proliferation. The Tahoe Keys was constructed with a water circulation system to keep water in the lagoons as clean as possible. This system used coagulation to reduce Total Dissolved Solids (TDS) and decrease turbidity helping keep the lagoons relatively clear.
The figure below shows the culvert pumps, highlighted in yellow, and the section of the circulation system that was activated in 2020, also highlighted in yellow. Pumps C4-C6 and the smaller 300 HP pump in the pumphouse were utilized to create circulation within the West Lagoon. Water taken in from the intake at W was discharged at 1. All other valves for other discharge areas along the system were closed while the system was active. Water samples were taken from sampling sites marked with red circles.
Tahoe Keys and Lake Tahoe Flow Term Model
The below photo represents both the surface and groundwater natural flow between the Tahoe Keys, Lake Tahoe, Lake Tallac and natural marsh lands around the Tahoe Basin. The circulation system is essential in creating flow of the waters within the Keys lagoons thus eliminating a stagnant environment which allows easy growth of auqatic invasive species due to nutrient dense, shallow, and warm waters.
Circulation Treatment System Design
•Entire lagoon turnover is around 3x per month •Circulation Flow Capacity: 40M gallons per day •Treatment Capacity: 10M gallons per day
Pre-/Post-Keys/Pope Beach Development
With the development of Pope Beach, the road that was created, distrupted the natural ecosystem and generated an issue with the filtration system of Pope Marsh.
1952 Pre-Pope Development
Source: Green, 1998
1971 Pope Development
Source: Green, 1998
Circulation System Objectives
1. Support restoration of native aquatic plant and fish habitats
To filter and remove aquatic weed fragments, algae and nutrients
2. Mitigate effects of increasing frequency/severity of drought cycles
Reduce harmful algal blooms and stagnant waters
3. Improve recreation, aesthetics, and other benefitical use
Lahontan- designated water quality objectives for Lake Tahoe
Why is the Circulation System not currently operating?
It was shut down by Lahontan Water Board per 1996 Cease and Desist, 1999 Clean-up, and 2014 Waste Discharge Orders
The pipes have weak spots, separation at joints, extensive corrosion, biological growth and sediment build up
The pumps, motors and electrical control panels need to be replaced and the pump intakes need to be lowered by 3 ft
Treatment process needs new technology to remove nutrients and improve the water quality
Technical Obstacles to Overcome
Tahoe Keys wells are drying up Pope Marsh
AIS fragments from Lake Tallac are infesting Pope Marsh
Lake Tallac Lagoon water quality is better than the West Lagoon
Tahoe Keys housing development and PopeBeach recreation area cut off hydrological functions of Pope Marsh
The Circulation and Treatment System, even if redesigned, cannot improve the water quality of the lagoons
Pumping treated PCE groundwater into Lake Tallac and Pope Marsh would damage the marsh and risk chemical spills
The weir at the west end of Lake Tallac does not aide Lake Tallac or Pope Marsh
The diversion structure on Lake Tallac causes flooding of State Streets
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