Relevance

Excessive nitrate discharge into the air and subsequent deposition into water can negatively impact air and water quality. Atmospheric sources of biologically available forms of nitrogen have been linked to declines in lake transparency because different forms, such as nitrate and ammonium, provide nutrients for attached and free-floating algae (TERC 2011). Because both gaseous nitric acid and particulate ammonium nitrate particles are small, less than 2.5 microns in diameter, they do not easily settle out of the air or water. For this reason, they have also been linked to decreased visibility within the basin.

Human and Environmental Drivers

Natural sources of oxides of nitrogen, nitrate, and dissolved inorganic nitrogen include wildfire and transformation of nitrogen resulting from sunlight and electrical storms. Human-generated sources include the combustion of fossil fuels, fertilizers, and industrial emissions from outside of the basin. Atmospheric DIN deposition records begin in 1981 for Lake Tahoe and suggest that deposition peaked in 1989/1990 (Domagalski, 2021). Tailpipe emissions regulations have reduced nitrogen emissions from the transportation sector.

EIP Action Priorities

• Build and Enhance Trail Networks
• Build and Enhance Transit Systems

EIP Indicators

• Miles of Pedestrian and Bicycle Routes Improved or Constructed

  Since 1997 EIP partners have collectively constructed or improved 198 miles of pedestrian and bicycle routes reducing the number of vehicle trips.

Example EIP Projects

• South Tahoe Greenway Shared Use Trail Phases 1b & 2

  This project completed in 2021 expands on the existing bicycle network and connections in South Lake Tahoe.

• Wildfire Smoke Impacts to Lake Tahoe’s Water Quality

  Tahoe Science Advisory Council, Impacts of Smoke-Ash from the 2021 Wildfires to the Ecology of Lake Tahoe

Monitoring Programs

• Pelagic Water Quality (Clarity)

  Atmospheric deposition monitoring at the mid lake station.

• Transit

  TRPA monitors, evaluates, and updates the Region's multi-modal transportation system for outcomes that decrease dependency on the private automobile, support mobility, and appeal to users.

Management Status Rationale

Status – Implemented. Policies, ordinances and environmental improvements have been implemented. TRPA has adopted several policies to encourage reduction in air and water pollutants, including sources of nitrate and dissolved inorganic nitrogen deposition into Lake Tahoe. TRPA has adopted several policies that support the use of alternative modes of transportation to reduce atmospheric sources of air pollutants such as nitrate and improve air quality (e.g., bicycle and pedestrian facilities, public transportation, postal delivery, and waterborne transportation). The TRPA Code of Ordinances includes regulations requiring that combustion appliances and wood heaters meet emission standards. The TRPA Code of Ordinances also requires that potential air quality impacts from a project be addressed as a component of the environmental documentation and permitting process.

For sources of nitrogen pollution associated with surface and groundwater, TRPA has adopted policies and ordinances to reduce nitrogen loads into Lake Tahoe ((TRPA 1986)). For example, TRPA requires that stormwater is infiltrated (treated) on-site for each developed parcel in the basin. Initiatives such as the Lake Tahoe Total Daily Maximum Load program administered by the states of Nevada and California require local jurisdictions to demonstrate pollutant load reductions from various sources, including atmospheric sources. The Environmental Improvement Program (EIP) administered by the TRPA in partnership with state, federal, and local governments, has facilitated a number of projects that were designed to aid in achieving this standard. Numerous projects under the EIP, such as bicycle trails and the Heavenly Gondola Project, have been implemented to reduce dependency on private automobiles, and thus, reduce pollutant loads to Lake Tahoe.

---