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Home Water Streams and Wetlands

Total Nitrate and Nitrite (and Total Nitrogen) Load

Status and Trend

Interpretation and Commentary

Status: Unknown
Trend: Litle or No Change
Confidence: Moderate

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The ten streams routinely monitored for nitrogen load include five streams in Nevada and five streams in California: Ninety percent of the cumulative total inflow from the ten monitored streams is from the five California streams and ten percent is from the five Nevada streams.
  • Relevance - This indicator measures how much nitrogen is delivered to Lake Tahoe via monitoring streams (measured as nitrogen load). 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). 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. 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). Nitrate and nitrite are inorganic forms of nitrogen that are directly available for use by plants, whereas total nitrogen includes all organic and inorganic forms of nitrogen that are directly and indirectly available to plants. Nitrogen occurs naturally in the soils of the Lake Tahoe Basin to some extent, but organic nitrogen primarily comes from the decomposition of plant material. Atmospheric deposition is considered the primary source of inorganic nitrogen to Lake Tahoe (Lahontan and NDEP 2010).
  • Adopted Standards  - 1) Tributaries: reduce total annual nutrient and suspended sediment load to achieve loading thresholds for littoral and pelagic Lake Tahoe; 2) Pelagic and Littoral Zones: a) reduce dissolved inorganic nitrogen (N) loading from all sources by 25 percent of the 1973-81 yearly average; and b) reduce dissolved inorganic nitrogen loads from surface runoff by approximately 50 percent, from groundwater approximately 30 percent, and from atmospheric sources, approximately 20 percent of the 1973-81 annual average.
  • Indicator - The yearly load for each stream is calculated as the sum of daily loads for a given water year. Currently a total of 20-35 individual samples are collected each water year from each of the 10 streams. The combined yearly load represents an estimate of the total mass of the constituent transported by ten streams to Lake Tahoe during a single water year. Nitrate plus nitrite (as nitrogen) values are used as estimates of the combined yearly dissolved inorganic nitrogen load. Indicators measured include total nitrogen (TN) load (expressed as kilograms per year; kg/yr), and nitrate plus nitrite as nitrogen load (expressed as kilograms as N per year; kg as N/yr).
  • Status – The combined yearly load for total nitrogen (TN) was 53,876 kilograms (kg) in water year (WY) 2010. The combined yearly load of nitrate plus nitrite, as nitrogen (N+N) was 5,110 kg as N in WY 2010. The total yearly inflow for all ten streams was 176,000,000 (or 1,760 x 105) cubic meters in WY 2010. The WY 2010 load estimates are similar to the yearly load estimates for WY 2003, which had a similar level of total yearly inflow. TN and N+N loads were largely driven by the yearly loads from the Upper Truckee River, and Trout, Blackwood, and Ward creeks. Together these streams contributed 82 percent and 85 percent of the average yearly TN and N+N loads, respectively, from all ten streams (Appendix WQ-2).

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  • Trends Tributary nitrogen loads are a function of the constituent concentration and amount of inflow. Combined yearly tributary loads of TN and N+N are strongly influenced by inflow, and in the Lake Tahoe Basin there can be considerable year-to-year variation in combined total yearly inflow from the ten monitored streams. Tributary flows depend on the amount and timing of precipitation (snow and rain) that falls in each watershed. Factors such as air temperature and snowmelt, rain-on-snow events, and rain versus snow, all affect the timing of inflow within the water year. Subsurface infiltration and stormwater discharge from developed areas also affect tributary flows; thus, estimates of nitrogen load presented here represent some combination of non-urban and urban sources.

    From water years 1993 to 2010, the total yearly inflow ranged from 772 to 3990 x 105 cubic meters. During the 18-year period, total yearly inflow was highest in water years 1995 through 1998, and 2006, and was above 3000 x 105 cubic meters in those water years. Inflow was extremely high in water year 1997 when a huge rain-on-snow event caused massive flooding, especially on the West Shore. These types of large hydrologic events have the capacity to transport significant amounts of material to the Lake as they are intensive and erosive. For the period 1999-2010, total yearly inflow has been moderate to low, ranging from 827 to 2770 x 105 cubic meters, except for the large total yearly inflow in water year 2006 – the result of a large New Year’s rain-on-snow event. Water Years 1994, 2001, 2002, 2004, and 2007 to 2009, were characterized by low total yearly inflow (less than 1500 x 105 cubic meters) from the 10 monitored streams.

    The combined yearly TN loads for the ten monitored streams during water years 1993-2010 ranged from 13,600 kg in 2001, to 137,000 kg in 1995. Water year 2006 also had a large yearly TN load of 113,000 kg. Combined yearly TN load was highest in water year 1995, followed by 2006, whereas total phosphorus (TP) and suspended sediment (SS) combined yearly loads were highest in water year 1997, followed by 2006. TN yearly load was related to total yearly inflow for the combined streams.

    The evaluation of trend in nitrogen loads from tributaries to Lake Tahoe is complicated by the large inter-annual variability in precipitation and inflow. It would be erroneous to make conclusions based on apparent short-term trends in combined yearly loads, given the known (and sometimes extreme) variability in yearly inflow. Yearly flow-weighted concentrations (FWC) of total phosphorus were calculated to investigate long-term pollutant loading trends in the presence of variable inflows (Appendix WQ-3). Further, a five-year moving average of sediment FWC was constructed to help reveal long-term trends. For water years 1993-2010, combined yearly FWC for TN for all ten streams ranged from 0.16 to 0.34 mg/L. While combined yearly FWC for TN exhibited inter-annual variability, there is no observable overall trend.

    The combined yearly nitrate plus nitrite (N+N) loads for the ten monitored streams during water years 1993-2010 ranged from 1,640 kg as N in 1994, to 15,900 kg as N in 1995. Yearly N+Nload for the 10 monitored streams was related to total yearly inflow, but not as closely as TN, suspended sediment (SS), and total phosphorus (TP) loads. Yearly N+N load peaks occurred in water years 1995 and 1999, whereas the peaks for TN occurred in 1995 and 2006, and TP and SS yearly load peaks occurred in 1997 and 2006. N+N can rapidly leach through soils. It is of biological origin and a by-product of vehicle emissions (NOx). Erosion events are not as likely to be associated with N+N loading as they are with SS and TP loading; although N+N load will be affected by inflow since inflow is used to calculate loads. For water years 1993 to 2010, the ratio of N+N load to TN load ranged from 8 to 24 percent, and averaged 14 percent. Thus, the majority of the TN in the ten monitored streams occurs as organic nitrogen; both dissolved organic nitrogen and particulate organic nitrogen. For water years 1993-2010, combined yearly FWC for N+N for all ten LTIMP streams ranged from 0.017 to 0.052 mg/L as N. There was considerable inter-annual variability in the N+N combined yearly FWC between water years 1993-1999. However, since 2000, the variability has declined, with the five-year moving average dropping from 0.35 mg/L as N prior to 2000 to 0.023 mg/L as N in 2010.
    A determination of "little or no change" in the overall trend of nitrogen load is based on no observable trend in the flow-weighted concentration of TN.

Confidence
  • Status – There is high confidence in the reliability of the data used to calculate yearly loads as 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 ten monitored streams. This sampling frequency is considered sufficient to sample during 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 sediment, fine particles, and nutrients has been developed and customized for use in aquatic systems where concentrations can be extremely low (Goldman et al. 2009).
  • Trend – There is high confidence in the reliability of the data used to calculate yearly flow-weighted concentrations as the data collection followed national and consistent field monitoring protocols established by the U.S. Geological Survey for stream monitoring (USGS variously dated; Rowe et al. 2002). However, visual inspections of the five-year moving average was used to estimate the long-term trend in TP and SRP loads, yielding a “low” confidence in trend assessment.
  • Overall – Because there is high confidence in status and low confidence in trend, the overall confidence was determined to be “moderate.”
  • 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 (Lahontan and NDEP 2010). Atmospheric sources are considered one the most prevalent sources of nitrogen input into Lake Tahoe (Lahontan and NDEP 2010). 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 the 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). Reporting of combined yearly loads begins in water year 1993 because that was when the 10th stream was included in the monitoring program. U.S. Geological Survey gauging stations are located at each monitoring station, 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
Combined yearly total nitrogen loads (provided as nitrate plus nitrite and other nitrogen) and total yearly inflow for tenstreams routinely monitored in the Lake Tahoe Basin. Data are displayed for each water year (Oct. 1 - Sept. 30) from 1993 through 2010. The yearly load for each stream is calculated as the sum of daily loads for a given water year. The total nitrogen load represents an estimate of the total mass of nitrogen transported by ten streams to Lake Tahoe during a single water year. The line on each plot is the combined total yearly inflow from the ten streams, which represents approximately 50 percent of the inflow from all 63 tributaries to Lake Tahoe. Stream monitoring data used to calculate loads are from the sampling locations closest to where the tributaries discharge to Lake Tahoe. Data are from the Lake Tahoe Interagency Monitoring Program (LTIMP).

Additional Info

References

Additional Information

Last Updated on Tuesday, 13 November 2012 07:13