Copper concentrations increase slightly from upstream-to-downstream both in 2023 and in the 5-year averages. These increases correlate with the locations of effluent outfalls, however concentrations in the effluent are well within standards and effects on the streams is not harmful.
Results
Metals
Key Takeaways
Metal concentrations throughout the KICP watershed are variable and typically below thresholds harmful to aquatic life. Metals commonly found in municipal effluents were predictably elevated downstream of the WWTF effluent locations, but not to any harmful degree. Most metal concentrations also trend upward slightly from upstream-to-downstream, likely due to mineralization of the stream-bed, however this has not been studied in this watershed.
Arsenic tends to increase upstream-to-downstream, likely due to hydrogeologic conditions; treated effluent discharge waters tended to have lower concentrations than the receiving water.
Selenium concentrations in Boulder and St. Vrain Creeks in 2023 were similar to the past 5-years, generally increasing from upstream-to-downstream. Effluent outfalls do not appear to affect these levels and they are likely more influenced by hydrogeologic conditions.
Silver concentrations are low throughout the watershed. Stream standards for dissolved silver are calculated based on hardness and drop significantly when hardness is also low. During snowpack runoff hardness drops low enough that dissolved silver occasionally measures between the chronic and acute standards.
Background
Metal concentrations in streams are important due to their potential toxicity and harmful effects on aquatic ecosystems and human health. Elevated levels of metals including arsenic, copper, and selenium can contaminate water sources, posing risks to both wildlife and people who depend on these waters for drinking, fishing, and recreation. Metals can accumulate in the tissues of aquatic organisms or disrupt aquatic habitats, impairing the growth and reproduction of fish and other organisms.
Effluent from wastewater treatment facilities (WWTFs) are a known to contribute metals to their receiving waters, but advancements in treatment technologies and stricter regulations have improved their ability to reduce metal concentrations in effluent. Understanding the concentrations of metals in waterways helps water providers implement the best treatment practices, ensuring proper removal and minimizing impacts on the receiving water.
Many water quality standards for metals are based on the buffering capacity or hardness of the water, meaning that the actual standard value varies depending on the hardness at any given time.
Monitoring and managing metal concentrations in streams is essential to protect ecological health and ensure the safety of water resources. KICP partners monitor many metals, however just those with water quality standards within the watershed stream reaches are discussed in this section.
Analysis
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Arsenic
Dissolved Arsenic concentrations throughout the watershed are variable but quite low. The highest recorded value recorded in 2023 was 1.35 ug/L which occurred in August at BC-bCC just below East County Line Road. For comparison, dissolved arsenic standards for these streams are defined in Colorado Regulation No. 31 and set at 150 ug/L.
Total and Dissolved Arsenic show a generally increasing pattern from upstream-to-downstream, likely due to geologic influences. Treated effluent discharge does not impact this trend as Arsenic concentrations in the effluent sources were lower than or similar to the receiving water.
Copper
Dissolved Copper concentrations throughout the watershed are also variable but consistently well below stream standard. In 2023 the highest value recorded was 4.58 ug/L in April, at BC-aDC at the Footbridge above Dry Creek. Stream standards for dissolved copper are calculated based on the water hardness at the time the copper sample is taken. At the time the April sample was taken, the calculated acute stream standard for that location was 25.68 ug/L and the chronic standard was 16.11 ug/L.
Treated effluent discharges had higher copper concentrations than their receiving waters. Although instream copper concentrations increased slightly downstream of effluent discharges, the increase was not to a significant or harmful extent. Rather copper concentration fluctuations are likely a result of natural geology and a collection of dispersed non-point sources.
Selenium
Dissolved Selenium concentrations throughout the watershed tend to be consistent and low in the upper reaches of Boulder Creek and the St. Vrain. Concentrations are more variable and at times elevated in the St. Vrain at and below station M8.2-SV at Pratt Parkway and in Boulder Creek at and below station BC-aDC just below the Boulder WWTP outfall. At the CC-Ken station on Coal Creek near the mouth selenium measured above 2.5 ug/L in most months. Dissolved selenium standards for these streams are defined in Colorado Regulation No. 31 and set at 4.6 ug/L.
No selenium samples were taken on Coal Creek in 2023 at stations other than at CC-Ken however measurements taken in 2018, 2019 and 2020 at station 6-CC above the Lafayette WWTP indicate a highly variable and consistently elevated trend in dissolved selenium at least as far upstream as that point. There have been periodic exceedances of the stream standards at this location historically as well, however, there has been a significant downward trend over the previous 6 years and no measured exceedances since 2020 at either location.
In Boulder Creek, selenium levels in the Boulder WWTP discharge are higher than those in the stream, suggesting that this source may be a primary contributor to selenium concentrations downstream. However, as noted above, these levels remain well below stream standards, and comparisons with effluent and stream levels in other parts of the watershed indicate that this apparent relationship could be coincidental.
On the St. Vrain, stream concentrations are already elevated above the Longmont WWTP discharge point and while levels of selenium in the Longmont WWTP discharge are consistently around 1 ug/L and spiked to 3.03 ug/L in June 2023, the stream concentrations were typically higher than the WWTP discharge and did not appear to be affected by the effluent. It is likely that the spike in the WWTP discharge had no effect on the stream during June because snowpack runoff was peaking and high streamflows were diluting concentrations of most constituents, including selenium.
The apparent trend for selenium concentrations to increase upstream-to-downstream suggests that the elevated selenium levels could be a result of natural geology and groundwater influences. Agricultural drainage is also known to be a contributor to selenium in streams and the proximity of stations with elevated measurements to agricultural areas does suggest this could be a factor as well. As time and resources allow, the Partnership should consider additional selenium studies to better characterize the sources and drivers of selenium concentrations in the watershed.
Silver
Concentrations of silver tend to be low throughout the watershed. It should be noted however that the applicable stream standard for dissolved silver is calculated based on hardness, and are very sensitive to low hardness levels. Each June during high snowpack runoff, hardness levels drop to levels low enough that standard exceedances are a potential issue and have occasionally occurred.
In June of 2023, hardness levels fell to just 33.2 mg/L at BC-61 west of the 61st Street bridge. This resulted in a calculated chronic standard of just 0.05 mg/L. the measured concentration of dissolved silver during that same sample event was 0.229 ug/L, which is unusually high at this location. This measurement was in exceedance of the chronic standard, although below the acute standard. Hardness levels in Boulder Creek quickly returned to more typical conditions and reviewing historical data indicates that while this combination of events is rare it has occurred previously.
Due to limited sampling over the last 5 years in Coal Creek there is not enough data to determine trends or the frequency of similar conditions. Dissolved silver has not been detected in the St Vrain Creek since monitoring was initiated in 2020.