General Water Quality

Key Takeaways

Key influences on general water quality parameters include high flows due to spring snow runoff and storm events. High flow conditions tend to dilute certain parameters, including Hardness, Alkalinity and Conductivity. Otherwise, the watershed has fairly elevated levels of these parameters due to natural geologic conditions. High-flow events can also transport sediment, stir up stream beds, and erode stream banks, leading to increased Suspended Solids.

Background

Water quality parameters include a combination of both physical characteristics (e.g., temperature, pH, suspended solids, etc.) and general chemistry (e.g., alkalinity, conductivity, dissolved organic carbon, etc.) of the water. Many of these components are also related to other parameters and can help determine watershed patterns and activities that impact stream water quality.

Analysis

The sections below graphically illustrate water quality highlights for the key parameters listed below. 2023 monthly and annual averages are compared to the previous 5-year period (2018 through 2022).

View more graph options in the Explore section

Access graphs for individual monitoring locations on the Map

Navigation Tip: Jump to the parameter sections below using this linked list

• Temperature
• pH
• Dissolved Oxygen
• Conductivity
• Alkalinity
• Hardness
• Magnesium
• Chloride
• Sulfate
• Dissolved Organic Carbon
• Total Suspended Solids

Water temperature affects levels of dissolved oxygen, chemical reactions, and the health of aquatic species. High temperatures can stress organisms and reduce oxygen levels. Temperature is primarily controlled by weather but can also be influenced by human activities, streamside vegetation, and discharges (e.g. stormwater and wastewater effluent).

In the graphs below water temperatures closely follow the patterns observed in previous years and mimic seasonal changes in air temperature throughout the watershed. Maximum monthly temperatures of approximately 22°C are typically observed between July and August when air temperatures are typically highest.

Water temperatures typically increase from upstream-to-downstream in the Boulder/Saint Vrain basin as the streams transition from mountain to plains streams and flow through urbanized areas that receive municipal discharges (e.g. stormwater and wastewater effluent).

Typically, the largest increases in stream temperatures are occurring downstream of municipal Wastewater Treatment Plant (WWTP) effluent outfalls. This is a common pattern because effluent discharge is often warmer than the receiving stream during winter months.

The pH scale is used to specify how acidic or basic water is. A pH of 7 is considered neutral, values below 7 are acidic, and above 7 are basic. The acceptable range outlined in Reg 38 is 6.5 to 9.0 to support healthy aquatic life. Since the pH scale is logarithmic, small changes reflect significant shifts; for example, a pH of 6 is 10 times more acidic than a pH of 7.

Monthly mean pH values on Boulder, Coal, and St. Vrain Creek can be seen to range between 7 and 8.4 during 2023 as well as during the last 5 years. All monthly mean pH values recorded met the Reg 38 stream standards for aquatic life (6.5-9). pH tended to be higher in streams than in effluent discharge, however the difference was not significant enough to alter stream chemistry downstream of effluent discharges.

Periodic exceedances of the pH standard did occur in some locations in 2023. These exceedances (pH >9) were typically paired with high dissolved oxygen concentrations - both of which are likely a result of increased photosynthesis. Increased photosynthesis can make a stream less acidic (increase pH) as carbon dioxide and carbonic acid are removed from the water column during the process.

Dissolved oxygen (DO) is essential for aquatic life and is affected by temperature, organic matter, and primary productivity. Pollution can lower DO levels, and low levels can stress fish and other organisms. DO can be expressed as concentration (mg/L) or as percent saturation (%) which is directly related to temperature, since the capacity of water to absorb oxygen decreases as temperature increases. Algae and aquatic plants have the ability to increase oxygen in streams through photosynthesis.

Streams designated as cold water aquatic life should maintain DO concentrations above 6 mg/L (>/= 7 mg/ during spawning) and warm water to remain above 5.0 mg/L.

Monthly average Dissolved Oxygen (DO) concentrations varied between 7.2 to values over 15 mg/L in April 2023 and to nearly 18 mg/L in December. All streams maintained monthly DO level that support healthy aquatic life.

Conductivity measures the water’s ability to carry an electrical current, indicating the presence of dissolved minerals or pollutants. It’s influenced by the geology with typical values between 50-1500 µS/cm. Higher temperatures increase conductivity, so measurements are adjusted to standardize at 25°C and reported as specific conductance (µmhos/cm @ 25 ⁰C). For the sake of simplicity, specific conductance is referred to as “conductivity” in this report.

In 2023 conductivity values largely mirrored the seasonal trends observed in the 5-year averages, except January in St. Vrain Creek and December in Coal Creek and Boulder Creek, where conductivity was higher than the 5-year means. Looking at conductivity values upstream-to-downstream, levels can be seen 4 times higher in the lower reaches of St. Vrain Creek than upstream, and more than 6 times higher downstream on Boulder Creek. These increases are expected as dissolved substances accumulate from urbanization, including stormwater and wastewater effluent inputs, as the water flows downstream.

Overall, conductivity in Coal Creek tends to be higher when compared to Boulder Creek and St. Vrain Creek, likely due to surrounding geology. The upstream reaches of Coal Creek, 2023 conductivity did not follow the 5-year averages. Conductivity was above average from CC-1, furthest upstream, to 3-CC, which are all above Rock Creek. The cause of this phenomenon is currently not known but these three monitoring sites are immediately below residential and light industrial areas and a major transportation corridor (US Highway 36), any of which could have influenced the 2023 water chemistry in Coal Creek.

Alkalinity, expressed as mg of CaCO3/L, represents the presence of bicarbonates and carbonates in water and indicates the buffering capacity or ability to neutralize acids. A higher buffering capacity can reduce the potential for pH swings during photosynthesis (removing CO2) by primary producers (algae) and plant growth. A minimum alkalinity of 20 mg/L is the aquatic life criteria recommended by the EPA.

Average monthly alkalinity was consistently within an acceptable range on in all three streams. St Vrain Creek tends to demonstrate lower alkalinity than Boulder Creek or Coal Creek, but only marginally. All three streams had decreased alkalinity in June 2023 which is common following periods of high precipitation as was present in May and June.

Alkalinity monitoring has been limited on Boulder Creek with measurements taken only at 9-BC and 11-BC, river mile 17.69 to 22.42 respectively, and E-BC (the WWTF effluent) since 2019. In 2018, Alkalinity was measured at all locations which is why the 5-year mean values are present on the graphs.

Water hardness refers to the concentration of dissolved minerals in the water, primarily calcium and magnesium, but also in some instances also chloride and sulfate. Higher Hardness levels tend to reduce toxicity by binding metals and making the metals less bioavailable or harmful to aquatic species.

For most of the year, hardness in Boulder, Coal and St. Vrain Creeks tends to be somewhat hard, above 150 mg/L, which protects aquatic life from metals. Levels drop in the spring during the period of snow runoff.

In 2023, hardness can be seen dropping to the lowest points of the year across all streams in June, during spring runoff. This was later in 2023 than was typical in the previous 5 years, due to a delayed runoff period, and also high rains in May and June, which extended the period during which the streams were diluted with soft water from snow and rain.

During these periods, the hardness-based regulatory standards for many metals also drop to much lower values. Read more about the relationship between hardness and the standards for metals in the Metals section.

Magnesium, Chloride, and Sulfate are components known to affect conductivity and hardness. Monitoring these parameters can help shed light on sources of pollutants and the reasons for elevated Hardness and Conductivity levels.

Magnesium typically originates from the natural weathering of rocks and minerals within a watershed. Additional urban sources that may contribute to magnesium concentrations include fertilizers, road dust, and wastewater effluent.

Magnesium did not demonstrate clear patterns through 2023, although overall values were lower on Boulder Creek than St. Vrain Creek or Coal Creek.

Chloride concentrations in the St. Vrain demonstrate an up-to-downstream pattern that increases towards M8-SV, and then drops lower just below the Longmont effluent outfall and Left Hand ditch. Higher chloride levels are observed in several tributaries to the St Vrain, possibly due to natural hydrogeology. These tributaries also convey stormwater and may be impacted by non-point sources.

Chloride was not measured in Boulder or Coal Creeks.

Sulfate concentrations in the St. Vrain demonstrate the same up-to-downstream pattern that Chloride does, increasing sharply through the same stream reach, likely for the same reason mentioned above.

Sulfate was not measured in Boulder or Coal Creeks.

Dissolved Organic Carbon (DOC) plays a significant role in stream and river ecosystems, affecting processes such as light penetration, nutrient uptake, bioavailability of toxic compounds, and carbon cycling.

The 2023 seasonal patterns for DOC were similar to the prior 5-years. On the Boulder and St. Vrain Creeks, concentrations increase below effluent discharge points, but drop down to levels only slightly elevated compared to above the discharge.

DOC is not measured in Coal Creek.

Total Suspended Solids (TSS) quantifies concentrations of suspended sediment and other particulates in water. Suspended solids in streams are primarily influenced by suspended silts, clays, and coarser particles suspended in the stream. TSS in streams generally increases during spring and summer months, affected by increased streamflows associated with spring runoff and storm events.

Elevated TSS concentrations throughout the watershed 2023 peaked later than historically, due to the season's delayed runoff cycle and higher than average precipitation in June.

Rock Creek and Left Hand Creek Tributaries (to Coal Creek and St. Vrain Creek respectively) in particular measured significantly higher than the mainstems, bringing suspended solids in from surrounding areas, however flow in these tributaries is relatively small and did not affect TSS levels in the mainstems to any noticeable degree.