Phosphorus Concentration (Surface Runoff)
This indicator measures the concentration of phosphorus in surface runoff. In an undisturbed watershed, the majority of stormwater is captured by vegetation and absorbed and filtered through the soil. Development, such as roads, driveways, and rooftops alter the watershed by creating impervious surfaces that prevent stormwater from infiltrating. Instead, stormwater runs over impervious surfaces, collecting pollutants such as phosphorus as it travels, enters the nearest storm drain or stream, and ultimately ends up in Lake Tahoe. Pollutants, including phosphorus, contribute to the decline in lake clarity. Landscape modification (e.g., impervious cover such as roads or residential and commercial development) influences the volume of runoff, erosion rates, and the ability of the watershed to retain sediment and nutrients. Urban growth control limits, best management practices (BMPs) to reduce nutrient and sediment discharge from disturbed soils, BMP retrofit regulations for developed properties, and limits on coverage all help to reduce phosphorus from surface runoff. The Lake Tahoe TMDL is the Region’s science-based strategy to reduce pollutant loading to Lake Tahoe and restore the historic clarity of the lake.
Status
Phosphorus load reduction.
Evaluation Map
Lake Clarity Tracker Projects between 2020 and 2023
2023 Evaluation
- Status
- Insufficient Data to Determine Status or No Target Established
- Trend
- Rapid Improvement
- Confidence
- Moderate
Applicable Standard
WQ20: Achieve a 90 percentile concentration value for dissolved phosphorus of 0.1 milligrams/liter in surface runoff directly discharged to a surface water body in the Basin.
Key Points
- Between 2020-2023, TMDL implementors (CalTrans, CLST, Douglas, El Dorado, NDOT, Placer, Washoe) collectively prevented 6550 pounds of phosphorus from reaching the lake, an average of 1638 pounds a year.
- The Winter of 2022 - 2023 was a historic year in terms of snowfall and low temperatures, which led to long and continuous periods of deep snow and cold temperatures that prevented full inspections necessary to award credits (2024 TMDL Performance Report).
- Urban land uses account for over 38 percent of total phosphorus loading to the lake (Lake Tahoe TMDL). The goal of the TMDL is nearly 100 feet by 2076, with an interim target of the achieve an interim target of 78 feet of clarity by 2031. Achieving the interim target requires reducing phosphorus by the load in 2026 by 17% from the TMDL baseline while achieving 100 ft of clarity will require a 35% reduction.
- The TRPA Code of Ordinances provides specific direction to ensure that activities and development in the Region are compatible with the Regional Plan and support the attainment and maintenance of the Region’s shared goals for restoration and environmental quality as expressed in the threshold standards.
- The Tahoe Science Advisory Council recommended that the discharge standards WQ19-WQ32 are not suitable for threshold standards, but should be retained in the TRPA Code (currently at Chapter 60) as management restrictions on discharge
About the Threshold
- Increased nutrient input is primarily responsible for the cultural eutrophication of Lake Tahoe. Increased organic particles in the lake are also estimated to be responsible for approximately 25 percent of the loss of clarity (Lahontan and NDEP, 2010b). The challenge is not unique to Lake Tahoe, as increasing input of nutrients (nitrogen and phosphorus) is considered a main cause of increasing primary productivity in temperate lakes (Conley et al., 2009). Lake Tahoe is an ultraoligotrophic lake and management goals include maintaining this status due to its historic, cultural, economic, and aesthetic value.
- Landscape modification (e.g. impervious cover such as roads or residential and commercial development or logging) influences the volume of runoff, erosion rates, and the ability of the watershed to retain sediment and nutrients. The concentration of sediment and nutrients in stormwater runoff is influenced by the type, magnitude, and location of landscape modifications. Concentration is further mediated by the extent to which practices to mitigate potential impacts are in place. A variety of natural factors also influence the concentration of sediment and nutrients in stormwater concentrations including climate, weather, landscape topography, and vegetation. The Lake Tahoe TMDL estimated that urban upland areas contributed 18 metric tons of total phosphorus a year to the lake, 38 percent of the annual total (Lahontan and NDEP, 2010b).
Delivering and Measuring Success
EIP Action Priorities
EIP Indicators
Example EIP Projects
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Area Wide Stormwater Infrastructure
TRPA assists local jurisdictions to plan and design area-wide infrastructure within key watersheds to enable more efficient and effective stormwater treatment for local neighborhoods.
Lake Clarity Indicators
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Phosphorous Load Reduction
TMDL urban implementing partners have acheived greater than total phosphorus targets required by permits and agreements.
Monitoring Programs
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Regional Stormwater Monitoring
The Regional Stormwater Monitoring Program measures pollutants in urban runoff to evaluate the effectiveness of pollutant control measures and track and report monitoring findings.
Rationale Details
- Insufficient Data to Determine Status or No Target Established. The concentration required to achieve all water quality targets has not been specified. Assessed against the target of the TMDL, the standard would be assessed as somewhat better than the target. The estimated total phosphorus load reduction was 18.5 percent, higher than the 17 percent load reduction target for 2026 (2023 Findings & Program Recommendations Memo). The sampling design of the Regional Stormwater Monitoring Program supports. Load reduction tracking of the TMDL is estimated based on an annual average water year, which means that an 18.5 percent load reduction is equivalent to an 18.5 percent reduction in average concentration. Assessment of the status of the threshold standard is not possible with the available information.
- Rapid Improvement. The Lake Clarity Crediting Program was designed to assess TMDL implementation by tracking pollutant load reduction in urbanized areas. Annual loading is driven by weather and hydrology. To evaluate the effectiveness of management and track load changes over time, the Crediting Program modeling tools provide average annual load estimates that are linked with on-the-ground field conditions to smooth the interannual variation associated with varying weather and hydrological conditions.
In 2023, local government and state highway departments reduced basin-wide phosphorus loading by 18.5% from 2004 baseline levels. Load is estimated as a function of concentration for an annual average water year, so the estimated annual average load reduction can be interpreted as an 18.5% reduction in the average annual stormwater runoff concentration.
Confidence Details
- Moderate. The load reduction targets of the TMDL are designed to restore the long-term clarity of the lake. The TMDL crediting program is well-documented and science-based. Full details on implementation and crediting are available on the program dashboard: https://clarity.laketahoeinfo.org/Home/ProgramManagement
- Moderate. TMDL load reduction targets accelerate each year of implementation, building towards 17% phosphorus load reduction in 2026 from the 2004 baseline.
- Moderate.
Additional Figures and Resources
No photos available.
No documents available.