Relevance - This indicator measures the total suspended sediment load (measured as fine sediment particle load) delivered to Lake Tahoe via direct surface runoff. Fine suspended sediment (fine sediment particles) are the primary driver of the decline in lake clarity (responsible for 55 to 60 percent of the decline) and urban upland areas are the dominate source of fine sediment particle in the basin (Lahontan and NDEP, 2010a). The TMDL estimated that 72 percent of the fine sediment particle load originates from urban sources. The TMDL estimated that a 71 percent reduction (from 2004 loading estimate) in fine sediment particle load from urban upland sources over 65 years would be required to achieve the pelagic clarity standard.
Human and Environmental Drivers - 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. Sediment and nutrient load in stormwater runoff is influenced by the type, magnitude, and location of landscape modifications and the extent to which practices to mitigate potential impacts are in place. A variety of natural factors also influence the load of sediment and nutrients in stormwater including climate, weather, landscape topography, and vegetation. The TMDL estimated that 72 percent of total fine sediment particle load basin wide originates from urban upland sources.
- Insufficient Data to Determine Status or No Target Established
- Insufficient data to determine status. Data reported in this assessment is load as measured at specific catchments and no overall estimate of load was available at this time. Load reduction estimates and condition assessment commitments are documented in the credit accounting platform of the TMDL. A more robust picture of load in stormwater will be available in March 2017 after credit declaration and associated verification associated with the first TMDL milestone is complete. Fine sediment particle load at all monitoring location outfalls was 3,004 kilograms for water year 2014 and 6,894 kilograms for water year 2015. The two years are not comparable because three additional sites were added in 2015 and total surface volume was much greater in 2014.
- Insufficient Data to Determine Trend
- Insufficient data to determine trend. Stormwater monitoring under the IMP component of RSWMP began in 2014. The IMP of RSWMP is currently funded through 2019 by SNPLMA, and trend assessments are expected to be included in future evaluations.
Fine Sediment Particle load measured at outfalls around the Tahoe Basin. Total for “consistent sites” refers to sites that were monitored in both 2014 and 2015. Sites that have no data for 2014 were only installed for water year 2015. Source: Tahoe Resource Conservation District
Urban growth control limits, best management practices (BMPs) to reduce nutrient and sediment discharge from disturbed soils, BMP retrofit regulations for developed properties, reducing private automobile use through improvements to public transit and alternative transportation modes (with the goal of reducing air pollution and the subsequent deposition of nitrogen and fine sediment), and ongoing allocation of water quality mitigation funds to support erosion control and storm water pollution control projects. Projects completed by EIP partners since between 2009 and 2015 have:
The Regional Plan requires the use of best management practices (BMPs) for new residential and commercial development, and BMP retrofit regulations for developed properties. For example, section 60.4.6.A.1 of TRPA Code requires properties be able to infiltrate the 20-year, one-hour storm into groundwater. The Regional Plan is also designed to limit growth and shift development from sensitive to less sensitive lands. All of these requirements contribute to reducing fine sediment and nutrient runoff from developed areas. The Regional Transportation Plan complements these by encouraging use of public transit and alternative transportation modes, and reducing reliance on private automobile. Water quality mitigation fees, collected on projects that create new cover, support erosion and storm water pollution control projects. Projects completed by EIP partners since between 2009 and 2015 have: • Retrofitted 120.55 miles of road and decommissioned an additional 7.4 miles of road. • Inspected 108.72 miles of unpaved non-urban roads and maintained 98.2 miles. • Issued 18,076 BMP certificates to developed commercial, multifamily and single family residential properties. • TRPA’s grant funded Stormwater Management Program (SMP) focuses compliance and maintenance verification activities on priority commercial and large multi-family residential properties in coordination with local jurisdictions. In 2015, the SMP notified 2,441 parcel owners with BMP Certificates issued more than five years ago that maintenance was due and re-issued 186 BMP Certificates following maintenance verification. • Completed street sweeping on 24,644 miles of roads.
The TRPA Stormwater Management Program leads broad professional and public education including annual BMP trainings for contractors, local jurisdictions and real estate professionals, articles in “Tahoe In-Depth” mailed to all property owners, and public workshops and events to increase BMP awareness and promote proper design, installation and maintenance. Public outreach and educational campaigns (such as the “Take Care” campaign) highlight for residents and visitors what they can do to maintain a healthy environment including BMP completion. Between 2012 and 2015 the South Tahoe Environmental Education Coalition delivered 36 educational programs and reached nearly 30,000 individuals.
The Lake-Friendly Business Program highlights and encourages patrons to visit businesses that are doing their part to help protect Lake Tahoe by installing and maintaining their water quality BMPs. There are currently over fifty Lake-Friendly businesses in the Region.
The TMDL Management System Handbook guides the actions of agencies in the Region to reduce inputs of nutrients and sediments into Lake Tahoe (Lahontan and NDEP, 2014). As part of the TMDL implementation, each jurisdiction in the Region prepares a load reduction plan (pollutant load reduction plans in California and stormwater load reduction plans in Nevada) that detail the steps to achieve the specified load reductions. The Lake Tahoe TMDL estimated that a 50 percent reduction in nitrogen load from urban sources (8 percent of the total nitrogen load) would be required to achieve lake clarity standards (Lahontan and NDEP, 2010a).
The 2015 TMDL Findings and Recommendations memo identified wintertime traction abrasives as a primary source of ultra-fine sediment particles (less than 16 microns in stormwater runoff) (Larsen and Kuchnicki, 2015a). Managers and heavy equipment operators in the Tahoe Region continue to adaptively manage wintertime traction application practices to reduce adverse environmental impacts while ensuring safe roads. In the 2015/2016 winter season this included treating roadways with brine solution prior to storm events, which prevents ice from developing on roads and can reduce prior dry salt applications by as much as 86 percent (Wigart and Ferry, 2015b). El Dorado County, the California Department of Transportation and the City of South Lake Tahoe are utilizing new wintertime traction abrasives that contain 90 percent less ultra-fine particles compared to previously used materials and also break down less into fine fractions from vehicle traffic. This new abrasive is sourced from a native granite material rather than the previously imported non-native volcanic cinders (Wigart and Ferry, 2015a).
Each year the actions of the TMDL implementation partners are summarized and evaluated in the TMDL Performance Report. The pollutant tracking system for urban stormwater was being refined during the reporting period. Future evaluations will use the estimated reductions in urban source pollutants to assess the effectiveness of programs and actions implemented to reduce pollutant load from urban sources (Larsen and Kuchnicki, 2015b).
TRPA infiltration requirements were designed to strike a balance between environmental benefit and cost. A 2011 synthesis of existing knowledge found diminishing returns from increasing storm retention capacity beyond the 20-year, one-hour storm. The synthesis found that doubling retention capacity required to handle the 20-year, one-hour storm would only increase annual retention by seven percent (2ndNature and NHC, 2011). TRPA Code Section 60.4.6.A.1 further requires a one-foot separation between seasonal high groundwater and the bottom of an infiltration system to protect groundwater resources.
In the long term, partners will be able to measure overall load reductions from surface runoff. However, with currently available data, the primary way to measure the effectiveness of programs and actions is to assess the effectiveness of BMP’s at reducing sediment and nutrient loads. Suspended sediments in tributaries and overall lake clarity are secondary indictors of the effectives of reducing suspended sediments in runoff and can be viewed in their respective indicator sheets.
The following results on the effectiveness of BMP’s were reported in the 2014-2015 Stormwater Monitoring Report (Tahoe Resource Conservation District, 2015): • Selected BMP catchment basins (Pasadena, Rubicon, SR 431 Contech, SR 431 Jellyfish) decreased annual fine sediment particle load by 43 percent in water year 2014 and 20 percent in water year 2015
Due to limited data duration, an interim target cannot be set.
Due to limited data duration, a target attainment date cannot be set.
- Reduce total annual nutrient and suspended sediment loads as necessary to achieve loading thresholds for tributaries and littoral and pelagic Lake Tahoe.
- Reduce loads of fine sediment particles, total nitrogen, and total phosphorus as established by Lake Tahoe TMDL
Monitoring is guided by the RSWMP Framework and Implementation Guidance document. During water year 2014 five catchments were monitored for continuous flow and turbidity and sampled for water quality at eleven monitoring stations: the outfalls of the five selected catchments, and the inflows to and outflows from selected BMPs located in three of those catchments. Three additional catchment outfalls were monitored in water year 2015. The catchments were chosen because of their direct hydrologic connectivity to Lake Tahoe, diversity of urban land uses, range of sizes, and a reasonably equitable distribution among the participating jurisdictions. BMP effectiveness sites were selected because of their potential efficacy in treating storm water runoff characteristic of the Lake Tahoe Basin, and the broad interest in, and lack of conclusive data regarding the efficiency of the selected BMPs in reducing runoff volumes and pollutant loads.
- – Low. Where insufficient data exists to determine status, confidence in the status determination is low. There is moderate confidence in the data because it is collected using widely recognized, standardized national protocols (see monitoring approach) with quality assurance/quality control procedures. Only a small proportion of outflows are sampled and not all runoff events are sampled. Regional estimates of overall load and load reduction are not available at this time.
- Low. No trend assessment was performed because both the nature and limited duration of the data preclude trend assessment.
The Lake Tahoe TMDL requires urban jurisdictions to report pollutant load estimates using the Lake Clarity Crediting Program and associated tools. The Pollutant Load Reduction Model (PLRM) provides a consistent method for evaluating both baseline and expected conditions associated with pollutant load reduction actions. The model provides estimates for the expected benefits of actions and on-the-ground field verification methods confirm treatment facility and roadway conditions are consistent with modelled parameters. Load reduction estimates and condition assessment commitments are documented in the credit accounting platform. Credit declaration and associated verification to document the first TMDL milestone is expected in March 2017. The raw RSWMP data detailed in this assessment will be used to help calibrate stormwater treatment BMP performance assumptions in PLRM. RSWMP sites will also provide data to aid in verifying the PLRM estimated changes in load are consistent with observed changes.
No changes recommended
Objective determination of “attainment” status for standards without a specific target is a recurrent challenge both in the Region and in the larger field of monitoring and evaluation (M&E). The standard should be assessed against best practice for the establishment of standards and indicators for M&E, and amended as necessary to improve the evaluability of the standard and the information it provides for management. Development of any new standards should also consider the benefits of alignment with the standards and management strategies implemented through the Lake Tahoe TMDL program. The Lake Tahoe TMDL estimated that a 72 percent reduction in fine sediment particle load (from 3.5E+20 particles/year to 1.015E+20 particles/year) from urban sources would be required to achieve lake clarity standards (Lahontan and NDEP, 2010a). Standard revision should also consider simplification of the text of the existing standards to ensure that the desired outcomes are readily apparent to most readers. The construction of the current standard, which references load reduction “as necessary to achieve loading thresholds for tributaries and littoral and pelagic lake Tahoe” as the target for the standard is confusing and requires readers to look up other standards to understand the standard’s objective.
No changes recommended. The 2015 Findings & Program Recommendation Memo for the TMDL reported that no new findings relative to urban stormwater were reported in previous calendar year (Larsen and Kuchnicki, 2015b).