General Water Quality

Key Takeaways

Key influences on water quality include high flows due to spring rain, snowmelt runoff, and storm events. High flow conditions tend to dilute certain parameters, including hardness, alkalinity, and conductivity, which are all fairly elevated in this watershed 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

General 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. The current year monthly and annual averages are compared to the previous 5-year period.

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 2024 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.

Most monthly measured pH values fell within the acceptable range to support aquatic life. Occasionally measurements outside this range did occur in some locations, to a small enough extent that they do not show up in the mean values illustrated in the graphs below. Elevated pH above the aquatic life threshold (9.0) 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.

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 the temperature increases. Algae and aquatic plants can 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.

DO concentrations measured in 2024 were typically well above thresholds 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 2024 conductivity largely mirrored the seasonal trends observed in the 5-year averages. Conductivity values increase upstream-to-downstream in all streams, likely due primarily to hydrogeology and groundwater. Geology and groundwater naturally elevate concentrations of metals in streams within the watershed. Urbanization can potentially contribute dissolved substances through stormwater which can elevate conductivity, however, current monitoring does not focus on storm events, and relationships in this watershed between conductivity and runoff in urbanized areas cannot be determined without further study. Treated wastewater effluent discharge does not appear to be an influence on conductivity in this watershed.

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 and Coal Creeks, but only marginally. The high value seen in December on Coal Creek is due to a spike in alkalinity measured at station 7-CC, below the Lafayette Wastewater Treatment Plant.

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 WWTP effluent) since 2019.

Water hardness refers to the concentration of dissolved minerals in the water, primarily calcium and magnesium, but also in some instances 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 2024, hardness can be seen dropping to the lowest points of the year June (Boulder and St Vrain Creeks) and May (Coal Creek), during spring runoff. 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.

Patterns of Magnesium concentration in all streams can be seen to closely track with Hardness (see previous section), suggesting that Magnesium is a primary component of water hardness in this watershed.

Chloride concentrations in the St. Vrain demonstrate an up-to-downstream pattern that increases towards stream mile 6 (station M8-SV) and then drops or levels off below the Longmont effluent outfall and Left Hand Creek. Higher chloride levels are observed in Left Hand Creek, possibly due to natural hydrogeology. This Saint Vrain Creek tributary also conveys stormwater and may be impacted by non-point sources. Chloride was not historically measured in Boulder or Coal Creeks however Chloride was measured in the lower portion of Boulder Creek in 2023 and 2024 (the “5 years” line for Chloride in Boulder Creek includes only 2023).

Sulfate concentrations in the St. Vrain demonstrate a similar up-to-downstream pattern to chloride, increasing through the same stream reach, likely for the same reasons. 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.

Seasonal patterns for DOC have been very consistent over the full period of record in Boulder and St. Vrain Creeks. Concentrations increase below effluent discharge points but drop down to levels only slightly elevated compared to above the discharge, although the influence of effluent on Boulder Creek appears to have decreased over time.

DOC is not monitored 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 stream flows associated with spring runoff and storm events.

TSS concentrations throughout the watershed in 2024 peaked between April and June (depending on the stream), tracking with early spring rains and spring runoff cycles. Rock Creek (tributary to Coal Creek) measured significantly higher in April than Coal Creek itself, bringing suspended solids in from surrounding areas and likely causing the elevated TSS levels in lower Coal Creek. This phenomenon is also evident in prior years, although not to as great an extent as 2024, and usually in May. Early rains appear to have forced an early spring runoff cycle in Coal Creek, possibly bringing with it more sediment than usual. TSS spikes seen later in the year in Coal Creek align with heavy rain events.