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Home Water Streams and Wetlands

Total Nitrogen Concentration

Status and Trend

Interpretation and Commentary

Status: Considerably Worse Than Target
Trend: Litle or No Change
Confidence: Moderate

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Map


The five California streams monitored for total nitrogen concentration are all on the left-hand side of the state boundary line running through Lake Tahoe. The California streams are Trout, General, Blackwood and Ward creeks, and the Upper Truckee River. The sub-watersheds where stream monitoring occurs are colored in the figure above.
  • Relevance - Nitrogen is a nutrient important to the growth and reproduction of plants, and it is considered a pollutant of concern in the Lake Tahoe Basin (Lahontan and NDEP 2010). The majority of nitrogen entering Lake Tahoe is from atmospheric sources. Nitrogen and phosphorus together support the growth of algae in Lake Tahoe (TERC 2011a). Free-floating algae (i.e., phytoplankton) occur throughout Lake Tahoe and contribute to the decline in water transparency by absorbing light for photosynthesis and scattering light. Attached algae (i.e., periphyton) coat rocks in the near shore, adversely impacting near shore aesthetics. From an ecological perspective, algae are a dominant component of the aquatic food web, providing an important source of energy and nutrients that support other organisms in the food web (e.g., zooplankton and herbivorous fish). Under the Federal Clean Water Act and in California under the Porter-Cologne Act, each state develops a set of water quality standards designed to protect the beneficial uses of a waterbody. In the Tahoe Basin, Lahontan Regional Water Quality Control Board (CA), Nevada Division of Environmental Protection (NV), and TRPA have the individual authority to determine if standards have been violated.
  • Adopted Standards  - Attain applicable state standards for concentrations of dissolved inorganic nitrogen; California does not have a standard for dissolved inorganic nitrogen, but does for Total Nitrogen:0.15 milligrams per liter (mg/L) for General and Ward Creeks, and 0.19 mg/L for Blackwood and Trout Creeks, and the Upper Truckee River. Nevada does not have a numeric TN concentration standard for tributaries to Lake Tahoe.
  • Indicator - The Indicator isaverage total nitrogen (TN) concentration measured over a water year (Oct. 1-Sept. 30). Average TN concentration is based on atotal of 20-35 individual samples currently collected each water year from each of the five monitored streams in California, although the number of individual samples collected each water year has varied during the period of record from three to 91. Total nitrogen concentration is determined by adding the measured concentrations of total Kjeldahl nitrogen and dissolved nitrate plus nitrate (as nitrogen). The annual average TN concentration values presented here differ from the flow-weighted concentrations, in that the former value is the straight arithmetic mean of the measured values within a given water year regardless of inflow. All annual average TN concentrations are reported in milligrams per liter (mg/L).
  • Status – In the table below, scores forannual averageTN concentration status, trend, and confidence were assigned for each of the five regularly monitored California streams in the Lake Tahoe Basin. Assigned scores for individual streams and overall status, trend, and confidence were based on: 1) percent to target calculations, 2) standard exceedance rate (see also Appendix WQ-1), 3) visual inspection of the graphed data record, and 4) the aggregation methods described in the Methodology Chapter of this report. The annual average TN concentrations in water year 2010 for the five regularly monitored streams in California ranged from 0.18 mg/L (General Creek, 20 percent greater than standard) to 0.31 mg/L (Blackwood Creek, 63 percent greater than the standard). The status of annual average tributary TN concentrations were determined to be “considerably worse than the target” because all five monitored streams exceeded the relevant established standard by between 20 percent and 93 percent (Appendix WQ-1 and figure above).

    Stream Status Trend Confidence
    Trout -4 -2 0
    Upper Truckee -4 0 0
    General -2 0 0
    Blackwood -4 2 0
    Ward -4 2 0
    Overall Average -3.6 0.4 0
    Overall Narrative Assessment Considerably Worse Than Target Little or No Change Moderate
       

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  • Trends Between water years 1989 and 2010, the average TN concentrations for the five regularly monitored streams in California ranged from 0.10 (67 percent of the standard) to 0.29 mg/L (93 percent greater than the standard) in General and Ward creeks, respectively and from 0.12 (63 percent of the standard) to 0.63 mg/L (332 percent greater than the standard) in Trout Creek, Upper Truckee River, and Blackwood Creek (see also appendix WQ-1). Annual average Total Nitrogen concentrations relative to the Numerical Standard differed from that observed for total phosphorus (TP). TP in the California streams exceeded the Numerical Standard in all years; however, it was common for the average TN concentration to vary among years above, at, or below the relevant standard. The patterns for TN and TP concentrations are not expected to be the same, since TN is comprised primarily of dissolved organic nitrogen (Coats and Goldman 2001), which is not directly associated with soil erosion, but is associated largely with microbial processing of vegetative biomass. For General Creek, TN was near (within 0.02 mg/L), at, or below the 0.15 mg/L value in about 68 percent of the years for which data are available. Ward Creek, with the same TN standard of 0.15 mg/L, was near, at, or below the standard in 55 percent of the years. Blackwood Creek, Trout Creek, and the Upper Truckee River, all with a TN standard of 0.19 mg/L, were near, at, or below the standard in about 50 percent of the years. There is no discernible long-term trend in average TN concentrations relative to the established standards. This finding is due to the observed variations in average TN concentrations within and among the five monitored streams, not due to insufficient data.

Confidence
  • Status – There is high confidence in the reliability in the TN concentration data because the data collection consistently followed national field monitoring protocols established by the U.S. Geological Survey for stream monitoring (USGS variously dated; Rowe et al. 2002). All field and laboratory data are subject to extensive quality assurance requirements (USGS 2006). Currently, a total of 20-35 individual samples are collected each water year from each of the five monitored streams. This sampling frequency is considered sufficient to characterize different inflow conditions observed during the water year. The sampling frequency has varied over the period of record. The stream monitoring program focuses on both event-based conditions (large runoff events associated with rainfall and snowmelt) and baseline conditions (low inflow during summer when precipitation is negligible). The analytical methods for measuring nutrients have been developed and customized for use in aquatic systems where concentrations can be extremely low (Goldman et al. 2009). Thus, there is high confidence in the status determination.
  • Trend – Confidence in the trend determination is considered low. Although there is high confidence in the data, assessments and interpretations of trend are all based on inspection of graphed data (i.e., qualitative).
  • Overall Confidence – Overall confidence in the status and trend determination is “moderate” given the high confidence in status and the low confidence in trend.
  • Human and Environmental Drivers - Within the Tahoe Basin, all the tributaries deliver sediment and nutrients to a single downstream water body, Lake Tahoe. Lake Tahoe has 63 individual tributaries and associated watersheds, each with their own drainage area, slope, geology, and land-use characteristics. Furthermore, variability in the amount, timing, and type of precipitation strongly influences runoff patterns. A substantial rain shadow exists across the lake from west to east, where precipitation can be twice as high on the West Shore relative to the East Shore. Both new and legacy disturbances to the landscape can affect the volume of runoff, erosion rates, and the ability of the watershed to retain nutrients. Landscape disturbances including, but not exclusive to, impervious road and parking lot surfaces, residential and commercial development, wildfire, and degradation of stream environment zones, can contribute to sediment and nutrient inputs to the Lake or its tributaries. Weather variations and long-term climate change are considered among the most important environmental drivers of tributary runoff.
  • Monitoring Approach – The LTIMP stream monitoring program was first developed in 1979 to provide a Basin-wide evaluation of sediment and nutrient input from tributaries to Lake Tahoe, and to support research efforts that aim to understand the drivers affecting the transparency of Lake Tahoe. Ten streams have been monitored since the early 1990s; five in California (Upper Truckee River, and Trout, General, Blackwood, and Ward creeks) and five in Nevada (Third, Incline, Glenbrook, Logan House, and Edgewood Creeks). Six of these streams have been monitored since water years 1980 or 1981. A few of the ten streams have had multiple monitoring stations along the tributary, and all have primary monitoring stations at or near the point of stream discharge to Lake Tahoe. Currently, a total of 20-35 individual samples are collected each water year from each of the monitoring stations. The tenprimary stations allow for evaluation of the cumulative conditions within the watershed and represent approximately 50 percent of the yearly tributary inflow into Lake Tahoe (Lahontan and NDEP 2010). U.S. Geological Survey gauging stations are located at each of the monitoring stations, where inflow (discharge) measurements are collected and continuous inflow is calculated. Other water quality-related constituents monitored include water and air temperature, pH, specific conductance, and dissolved oxygen.
  • Monitoring Partners – U.S. Geological Survey (Nevada and California Water Science Centers), University of California at Davis (Tahoe Environmental Research Center), Tahoe Regional Planning Agency, and U.S. Forest Service (Lake Tahoe Basin Management Unit).

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Trend Charts

Chart data not available at this time
These plots show how average yearly total nitrogen (TN) concentration measured over a water year (Oct. 1-Sept. 30) compare to the water quality standard in place for each of five regularly monitored California streams in the Lake Tahoe Basin. The individual bars represent the average yearly TN concentration, based on atotal of 3-91 (depending on the water year) individual samples collected from each of the five monitored streams. Total nitrogen concentration is determined by adding the measured concentrations of total Kjeldahl nitrogen and dissolved nitrate plus nitrite (as nitrogen). The horizontal red line represents the California numeric standard for average TN concentration: 0.15 milligrams per liter (mg/L) for General and Ward Creeks, and 0.19 mg/L for Blackwood and Trout Creeks, and the Upper Truckee River. Nevada does not have a numerical TN standard for tributaries to Lake Tahoe. Stream monitoring data used to evaluate TN concentrations are from the sampling locations closest to where the tributaries discharge to Lake Tahoe. Data are from the Lake Tahoe Interagency Monitoring Program (LTIMP).

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References

Additional Information

Last Updated on Tuesday, 13 November 2012 07:15