Nitrogen Concentration (Surface Runoff)
This indicator measures the concentration of nitrogen 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 nitrogen as it travels, enters the nearest storm drain or stream, and ultimately ends up in Lake Tahoe. Pollutants, including nitrogen, 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 nitrogen 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
Annual nitrogen load reduction as measured by the clarity crediting program of the TMDL.
Evaluation Map
Lake Clarity Tracker Projects
2023 Evaluation
- Status
- Insufficient Data to Determine Status or No Target Established
- Trend
- Rapid Improvement
- Confidence
- Moderate
Applicable Standard
WQ19: Achieve a 90 percentile concentration value for dissolved inorganic nitrogen of 0.5 mg/liter in surface runoff directly discharged to a surface water body in the Basin.
Key Points
- In 2019, TMDL implementors (CalTrans, CLST, Douglas, El Dorado, NDOT, Placer, Washoe) exceeded their load reduction targets and collectively reduced average nitrogen concentrations in surface runoff by 11.7 percent from baseline levels.
- The targets for surface runoff and groundwater discharge articulated in water quality threshold standards (standards 19-32) were designed to reduce pollutant load and improve ambient water quality. 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 recommends that 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. The challenge is not unique to Lake Tahoe, increasing nutrient (nitrogen and phosphorus) inputs are considered a main cause of increasing primary productivity in temperate lakes globally (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. Increased organic particles in the lake are also estimated to be responsible for approximately 25 percent of the loss of clarity (Lahontan and NDEP, 2010a).
- 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 63 metric tons of total nitrogen a year to the lake, 16 percent of the annual total (Lahontan and NDEP, 2010a).
Delivering and Measuring Success
EIP Action Priorities
- Restore Native Vegetation and Forest Communities
- Restore Priority Meadows, Wetlands, and Lake Tahoe Tributaries
Lake Clarity Indicators
- Forest Roads Decommissioned or Retrofitted
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Nitrogen Load Reduction
TMDL urban implementing partners have acheived greater than total nitrogen targets required by permits and agreements.
- Non-Compliant Wood Stoves Removed or Retrofitted
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 considerably better than the target. The estimated total nitrogen load reduction was 13.5%, higher than the 4% 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 a 13.5% load reduction is equivalent to a 13.5% reduction in average concentration.
- 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 nitrogen loading by 13.5% 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 13.5% 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. See confidence in status.
- Moderate.
Additional Figures and Resources
No photos available.
No documents available.