|Nitrogen Concentration (Surface Runoff)|
|Nitrogen Concentration in Runoff|
|Achieve a 90 percentile concentration value for dissolved inorganic nitrogen of 0.5 milligrams/liter.|
|:||Milligrams of nitrogen in each liter of stormwater. The proportion of individual measurements that exceed 0.5 milligrams/liter is measured.|
No subcategories for this indicator.
Approach 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.
Associated Programs data not provided.
Increased nutrient input is primarily responsible for 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).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. 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).
Achieve a 90 percentile concentration value for dissolved inorganic nitrogen of 0.5 mg/1, for dissolved phosphorus of 0.1 mg/1, and for dissolved iron of 0.5 mg/1 in surface runoff directly discharged to a surface water body in the Basin.
Insufficient data exists to determine status. The sampling design and the small fraction of the outflows points and storm events sampled preclude a status determination on the concentration of stormwater inflow points. Further discussion is included in the recommendations sections. Of the small fraction of outflow points samples in water years 2014 and 2015, 8.3 percent of outflow samples met or were below the threshold standard of 0.5 milligrams/liter for total nitrogen and 91.7 percent of samples exceeded the 0.5 milligrams/liter concentration(Tahoe Resource Conservation District, 2015).
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.
Status: 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 concentration are not available at this time.
Trend: Low. No trend assessment was performed because both the nature and limited duration of the data preclude trend assessment.
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, 2010b).
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.
The 2014-2015 Stormwater Monitoring Report analyzed the effectiveness of BMPs in the Region (Tahoe Resource Conservation District, 2015). At BMP’s where both inflow and outflow were monitored, reported results included:
• The Central Incline Village Phase II Project, which included three upstream infiltration basins, two small roadside infiltration pools, 450 linear feet of roadside infiltration channel and a Jellyfish treatment vault, effectively eliminated flows and pollutant loads reaching Lake Tahoe (Tahoe Resource Conservation District, 2015).
• 11 percent of inflow samples (seven of 64) and 31 percent (20 of 64) of outflow samples met the TRPA nitrogen concentration threshold of 0.5 milligrams/liter. However, 19 of the 20 outflow samples that met the standard were dry samples (dry samples indicate that BMP was effective in infiltrating the storm event).
• Average nitrogen concentrations at measured inflows was 2.39 milligrams/liter while average nitrogen concentrations at outflows was 1.20 milligrams/liter, a reduction of 50 percent. Average outflow concentrations includes values of 0 mg/L where there was no measured flow leaving the BMP.
An interim target cannot be set at this time, due to the duration of the data.Target Attainment Date
A target attainment date cannot be set at this time, due to the duration of the data.
No changes recommendedMonitoring Approach
IMP was designed primarily to meet jurisdiction’s reporting requirements under California National Pollutant Discharge Elimination System (NPDES) permits and Nevada Interlocal Agreement commitments. The focus of both is on load reaching the lake, not concentration. The primary objectives of the RSMWP design are (a) the status and trend of catchments with respect to load estimates, and (b) the evaluation of BMP effectiveness (Tahoe RCD, 2015a). The sampling design of RSWMP reflects these objectives and poses a challenge to evaluation of TRPA concentration based standards. For example, the success of the Central Incline Village Phase II project has resulted in the site being removed from the 2016 monitoring plan, and replaced by a site where load is likely to be higher. The result of this change as described in the monitoring plan, “will skew long-term status and trends data and may encourage a false perception that erosion control projects do not have a measureable benefit” (Tahoe Resource Conservation District, 2015).
Recommendations below are summarized from the IMP (Tahoe Resource Conservation District, 2015). The Tahoe Resource Conservation District suggested the IMP discontinue monitoring of the two Rubicon monitoring stations (RI and RO) and the catchment outfall station at State Route 431 (S5) due to extremely low flow volumes. The very small pollutant loads from these catchments and the relative difficulty of monitoring sites with low flow did not warrant the effort and cost required to continue monitoring these stations. The IMP agreed with this assessment and the request was brought to Lahontan and Nevada Division of Environmental Protection who approved the change for water year 2016. In addition, the Tahoe Resource Conservation District suggested removing the first flush sample requirement for the next permit term (beginning water year 2017) for three reasons; 1) the first flush sample does not represent a consistent percentage of the total runoff volume for an event and is difficult to compare the results across events or sites, 2) first flush samples generally represents a very small portion of the total runoff volume and therefore its contribution to the calculation of the flow-weighted event mean concentration is often negligible, and, 3) Continuous turbidimeters are installed at all monitoring sites allowing questions most pertinent to jurisdictions to be answered more effectively.
Standard revision should consider consistency with the pollutant load-based standards and management strategies implemented through the Lake Tahoe Total Maximum Daily Load (TMDL) program per water quality policy 1.5 of the TRPA Regional Plan.Attain or Maintain Threshold
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, 2015a). Continue to implement the programs of the TMDL.
Map description not provided.
No figures available.
The 2011 Evaluation Report has not yet be uploaded or is unavailable.