Most Metal Concentrations show similar patterns across most locations, increasing with low flows and geographically from upstream-to-downstream. This suggests that metals in the watershed are predominantly influenced by underlying geology. Effluent discharge does not tend to affect levels of metals in the streams and all metals are well within safe levels throughout the basin.
Results
Metals
Key Takeaways
Metal concentrations are variable but at generally healthy levels throughout the KICP watershed. Metals commonly found in municipal effluents are predictably elevated downstream of treated wastewater effluent outfalls, but not to a harmful degree. Most metal concentrations also trend upward slightly from upstream to downstream, likely due to mineralization of the streambed.
Arsenic tends to increase upstream-to-downstream like other metals but has also been steadily decreasing across most Boulder Creek sites over the past decade. Reasons for this observed decreased are being investigated.View Arsenic Graphs
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 plants (WWTPs) is 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
Arsenic concentrations throughout the watershed are variable, typically elevated, and increase from upstream to downstream. Likely influenced by natural (hydrogeologic) conditions, several streams in this watershed are listed for Arsenic impairments. The highest recorded value in 2024 was 2.25 ug/L which occurred in September on Lefthand Creek at 95th Street. For comparison, dissolved arsenic standards for these streams are defined in Colorado Regulation No. 31 as low as 0.02 ug/L for Water + Fish ingestion, 10 ug/L for domestic water supply and a chronic standard of 150 ug/L for aquatic life. It is notable that the 0.02 ug/L standard is a temporary modification slated to expire in 2029, and 0.02 ug/L is below the detection limits associated with testing methods used in this basin.
Copper
Dissolved Copper concentrations throughout the watershed are also variable but consistently well below stream standards. In 2024 the highest value recorded was 3.8 ug/L in November, 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 November sample was taken, the calculated acute stream standard for that location was 13.55 ug/L, and the chronic standard was 10.84 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 Boulder Creek, and while more variable and higher in the St. Vrain, still consistently below stream standards. Selenium increases upstream to downstream in both Boulder and St. Vrain Creeks, likely due to hydrogeologic influences, such as groundwater. Dissolved selenium standards for these streams are defined in Colorado Regulation No. 31 and set at 4.6 ug/L.
Selenium was measured on Coal Creek in 2022, 2023, and 2024 only at station CC-Ken (at the mouth). Measurements were taken in 2017 - 2020 at other stations upstream as far as the Lafayette WWTP. These sporadic measurements suggest 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, including one in 2024 in November, at CC-Ken (5.6 ug/L). Additional monitoring of selenium in Coal Creek is recommended to better understand these trends.
In Boulder Creek, selenium levels in the Boulder WWTP discharge are higher than those in the stream, suggesting that this source may be a 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, stream concentrations were higher than the WWTP discharge and were minimally affected by the effluent.
Silver
Concentrations of silver tend to be low throughout the watershed, primarily below method detection limits. It should be noted, however, that the applicable stream standard for dissolved silver is calculated based on hardness and is 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 in some cases, hardness values drop low enough that the calculated standard falls below typical method detection limits. In both Boulder and St. Vrain Creeks, however, dissolved silver was not detected in any month of 2024, and in St. Vrain Creek it has not been detected since monitoring was initiated in 2020.
There has been limited sampling over the last 5 years in Coal Creek, although it has been routinely monitored
since 2022 at CC-Ken. There are not, however, accompanying hardness values also measured at this location.
Hardness upstream from CC-Ken tends to be consistent, and around 300 mg/L. Assuming 300 mg/L for calculating
the hardness-based standards for this location, acute standards for dissolved silver in Coal Creek would be
around 2 ug/L and chronic standards around 1.6 ug/L. Most samples in 2024 did not detect dissolved silver,
except in May when a value of 0.411 ug/L was reported, well below the likely standard at that time.
It should be noted that in 2024, method detection limits were adjusted for analyzing the presence of dissolved and total silver, for both Coal Creek and Boulder Creek monitoring programs. Method detection limits are routinely re-calculated as part of the laboratory certification process. The change in method detection limits creates a misleading visual on the historical comparison graphs, suggesting that values have suddenly increased this year, when in fact most measurements were non-detects. The time series graph for location CC-Ken below shows an example of how this change in methodology is evident in the monthly results for an individual location.