Total Nitrogen concentrations increase upstream-to-downstream throughout the watershed, influenced predominantly by treated effluent discharge points. Some increases were seen during higher flows after storm events as well, suggesting Nitrogen also enters these streams via non-point-source runoff.
Results
Nutrients
Key Takeaways
Nutrient levels generally fluctuate inversely with streamflow, measuring lower when flows are high and higher when flows are low. This was seen clearly in the 2023 data, with Nutrients trending lowest in June after weeks of exceptionally high precipitation and runoff of snowpack was also at its peak. Nutrient levels were higher during low flow months, especially the winter. In the St. Vrain, fewer locations were monitored in January and February 2023; these locations are near the WWTP outfall and tend to have higher nutrient concentrations.
Treated wastewater effluent has a larger impact on nutrient levels in the upstream reaches where streamflow is small compared to the effluent discharges. In lower reaches, streamflow is higher and nutrient levels are affected much less.
Total Phosphorus concentrations show similar annual averages when compared to past years across most locations.
Background
Understanding nutrient concentrations in streams is crucial due to their significant impact on water quality and aquatic ecosystems. Excessive nutrients, such as nitrogen and phosphorus, can lead to eutrophication, a process where waterbodies become overly enriched with nutrients, stimulating excessive growth of algae and other aquatic plants.
Elevated nutrient levels can also cause imbalances in the ecosystem, affecting the diversity and health of aquatic species. Monitoring and managing nutrient concentrations is essential to maintaining healthy watershed ecosystems and ensuring safe water quality for various uses.
Learn more in the Keep It Clean Partnership’s Nutrients Fact Sheet! (PDF)
Analysis
The sections below describe Nutrient monitoring in 2023 throughout the basin:
TIP: Jump to the parameter sections below using this linked list!
Total Phosphorus (TP)
Phosphorus concentrations in freshwater arise from erosion of soil particles from steep slopes, disturbed ground, surface runoff containing phosphorus from fertilizers, wastewater effluent, and decaying organic matter. Total Phosphorus (TP) is the measure of all phosphorus in a sample and includes inorganic, oxidizable organic, and polyphosphates.
Seasonally, TP in Boulder and St. Vrain Creeks tends to be elevated during low flow periods and lower during months with higher flows. This suggests that sediment from the stream banks and stream bottom are not significant sources of phosphorus in these streams and the higher flows dilute TP already in the stream from other sources.
Coal Creek TP concentrations on the other hand tends to be fairly consistent year-round. Instead of diluting TP concentrations, this suggests that the higher flows themselves already have similar levels of TP as the baseflows. Given that all the KICP monitoring on Coal Creek are in the lower reaches, downstream of the urbanized areas, it is likely that additional TP enters this stream during high flow periods, however whether this influx of TP is from surface runoff or in-stream sources is not currently known.
Looking at upstream-to-downstream changes, it is evident that effluent discharge impacts TP concentrations in Boulder, Coal and St Vrain Creeks. It can be seen in the upstream-to-downstream charts as well as box plots that those impacts are of varying degrees, depending on the location. This is primarily due to the relative flow levels for the stream at that point versus the effluent flow. Larger WWTPs, such as the Boulder and Longmont WWTPs, have a larger impact on stream nutrients concentrations due the volume of effluent and number of people served in the community compared to smaller WWTPs that serve Louisville, Superior and Lafayette.
Total Nitrogen
Total Nitrogen (TN) is the sum of all nitrogen in the water and is calculated by adding together the measured forms of organic nitrogen, nitrate, nitrite, and ammonia. Similar to total phosphorus, mean monthly total nitrogen (TN) concentrations in all streams tend to be higher during low flow periods and lower during high flow periods, due to dilution by the influx of snow-melt during the spring and early summer.
TN concentrations upstream-to-downstream follow the same pattern as TP with respect to the impacts of WWTP discharge on elevated concentrations of TN below the discharge points.
Nitrate and Nitrite
Nitrates and Nitrites (NO3 - + NO2 -) are oxidized forms of nitrogen and provide essential nutrients for plants. Excess amounts, accompanied by elevated phosphorus concentrations, can cause dramatic increases in aquatic plant growth and have a negative impact on the stream ecosystem. These constituents can enter the stream through the natural oxidation process of nitrogen already in the stream or stream bed, and through runoff that brings human-made sources such as fertilizers, animal feedlots, and urban stormwater.
Like TN and TP, NO3- + NO2- concentrations in 2023 and in the previous 5-year period were higher on Coal Creek in most months than they were on the St. Vrain and Boulder Creek.
Looking at NO3- + NO2- concentrations tend to be higher when streamflow is low and lower during the high flow months in the spring, particularly June, when snowmelt dilutes the base flows. The influence of WWTP effluent discharge can also be readily seen on all streams.
Ammonia
Ammonia (NH3) is a reduced form of dissolved nitrogen that is readily available for phytoplankton uptake. NH3 is often found in low-oxygen environment. Ammonia can enter the aquatic environment via direct means such as municipal effluent discharges and the excretion of nitrogenous wastes from animals, and indirect means such as nitrogen fixation, air deposition, and runoff from urbanized and agricultural lands.