Nutrients

Key Takeaways

Nutrient values are generally lower during periods of high streamflow and higher during periods of low streamflow. This can be seen clearly in the 2024 data, with nutrients trending lowest in May when runoff was at its peak from snowmelt and highest during low flow months.

Effluent from wastewater treatment facilities can have a significant impact on nutrient levels, especially when facilities are treating a large amount of water. Additional nutrient standards are anticipated for streams in this watershed in the next decade, and current conditions may not meet proposed standards. As such, wastewater treatment facilities will likely be required to update or improve treatment processes to further reduce the amount of nutrients discharged to streams. Some wastewater treatment facilities in the watershed have invested significantly in reducing nutrients in treated effluent. We can see positive impacts from these investments in both St. Vrain and Boulder Creeks. Mitigating or preventing inflow of nutrients to the watershed will continue to be a major focus in future years.

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 2024 throughout the basin:

TIP: Jump to the parameter sections below using this linked list!

• Total Phosphorus
• Total Nitrogen
• Nitrate and Nitrite
• Ammonia

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. Total phosphorus in Boulder, Coal, and St. Vrain Creeks is influenced by seasonal changes in streamflow, with higher concentrations during low-flow periods and lower concentrations during high-flow periods when averaged along the stream length.

TP concentrations in the watershed are also influenced by wastewater treatment outfalls, with this influence more pronounced during low flows. This is most evident in Boulder and St Vrain creeks, which receive treated wastewater from Boulder and Longmont respectively. The stream data suggests that Lafayette WWTP outfall has a similar impact on Coal Creek, but outfall data is limited. Obtaining more recent effluent discharge monitoring results for Lafayette WWTP, which have not been included in the KICP database for more than 5 years, would help to fill out this picture for Coal Creek.

The City of Louisville's WWTP discharge TP concentration is notably less than other facilities. As such, its impact on stream concentration is smaller. The Town of Erie's WWTP discharge concentration is close to the level in its receiving stream and showed less impact on stream condition in 2024. However, TP levels in Erie's WWTP outfall have increased over the last two years and should continue to be monitored for impacts in the lower reach of Boulder Creek.

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 snowmelt during the spring and early summer.

TN concentrations upstream-to-downstream follow the same patterns as total phosphorus (TP) does with respect to impacts of WWTP discharge on elevated concentrations of TN below the discharge points. See further discussion of this pattern in the previous section on TP.

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 synthetic sources such as fertilizers, animal feedlots, and urban stormwater and wastewater outfalls. Mean monthly 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 snowmelt during the spring and early summer.

Concentrations upstream-to-downstream also follow the same patterns as total phosphorus (TP) and total nitrogen (TN) with respect to impacts of WWTP discharge on elevated concentrations below the discharge points. See further discussion of this pattern in the previous section on TP.

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.

Like other nutrients, concentrations of Ammonia tend to be higher during low flow periods, typically peaking in December and January. An exception occurred in 2024 on Coal Creek when two separate events, in different sections of the stream, caused elevated monthly means over the total stream reach in August (higher than typical concentrations in the Louisville WWTP effluent which elevated concentrations temporarily in the stream) and September (higher than typical concentrations below the Lafayette WWTP). The Coal Creek upstream-to-downstream graphs indicate that in-stream concentrations remained low and recovered quickly downstream of the event locations.

An additional compelling trend can be seen in the upstream to downstream graphs for all three basins, especially when comparing 2024 to the previous 10 years. 2024 monitoring shows lower Ammonia concentrations when compared to the previous 5 years, and the gap widens when compared to the previous 10 years. In the St. Vrain, a reduction in Ammonia over the past 10 years can be observed for all the monthly means.

To better visualize this, the 10-year comparison graphs below illustrate a gradual reduction in the impacts of effluent discharge over time, in particular in Coal Creek below the Louisville WWTP, in Boulder Creek below the Boulder WWTP, and in St. Vrain Creek below the Longmont WWTP.

NOTE: Erie and Rock Creek WWTP effluent discharge has been excluded from the Upstream-to-Downstream Ammonia graphs because the annual average values are high (2.62 mg/L at Rock Creek and 0.35 mg/L at Erie) and including them on the graphs obscures detail. Please see the time series graphs available on the interactive map to see data for those locations.