Bacteria

Key Takeaways

Background

Escherichia coli (E. coli) are a large and diverse group of bacteria that typically live in the digestive tracks of warm-blooded animals. Most strains of E. coli are harmless, but some can make you sick. Measuring for E. coli is easier than measuring for all possible pathogens in waterbodies, so it is widely used as an indicator of the presence of bacteria and viruses that can make people sick.

E. coli concentrations provide information about the possible presence of pathogens but not about where the E. coli is coming from. E. coli impairments are commonly found in urban streams along the Front Range, Colorado, and across the country. Urban areas tend to have higher frequency and intensity of E. coli contamination for a variety of reasons that include stormwater runoff, increased populations, pets, and leaking wastewater infrastructure.

Geometric means, also referred to as "geomeans," are used to report E. coli trends as a more accurate representation of central tendency for skewed data, which is common in bacterial counts. Geometric means reduce the impact of extreme values or outliers, offering a more reliable measure of typical contamination levels over time which is particularly useful for assessing long-term water quality trends and compliance with standards.

Learn More in the Keep It Clean Partnership's E. coli Fact Sheet

Analysis

Bacteria concentrations can vary dramatically within short periods. Monthly means by stream reach indicate that they tend to be highest in late summer and fall, which is supported by the fact that warmer temperatures and lower flows present during this period are more favorable for E. coli growth. Analysis of 2023 E. Coli conditions included review of monthly averages across stream reaches and annual averages by location, which are both useful for identifying trends, but due to the potential for highly fluctuating concentrations from location to location and from month to month, time series data for each monitoring location was also considered.

The Interactive Map tool in this report provides access to time series graphs for all monitoring locations and parameters. For this section, in addition to the interactive graphs below showing monthly and annual trends, direct links have been included to time series graphs that help to illustrate key findings.

Coal Creek

Stream to stream comparisons of monthly E. coli geomean concentrations show that Coal Creek reached higher summer levels in 2023 than Boulder Creek or the St. Vrain, rising to a stream-wide geomean of 830 CFU/100ml in July. For comparison, Colorado Department of Public Health and Environment’s (CDPHE) recreational E. coli standard for primary contact waters is 126 #CFU/100ml based on a 2-month static geomean.

This peak in 2023 was similar to 5-year historical patterns for Coal Creek, but the peak in 2023 was much higher the previous 5 years' geomean. Considering that this peak is an average across all the Coal Creek monitoring locations, this study also looked at individual monitoring locations. This revealed that the July 2023 high concentrations were not present throughout the Creek:

• July 2023 concentrations at the downstream-most monitoring location, CC-Ken at Kenosha Rd measured just 224.7 CFU/100ml.

• Further upstream, E. coli concentrations in July 2023 were above the maximum detection limit of 2419.6 CFU/100mL.

• All Rock Creek stations also measured concentrations above the maximum detection limit in July 2023, including the uppermost point 4-RC above the Superior WWTP. Urban runoff along Rock Creek is likely the primary contributor to these elevated levels.

• Coal Creek station 3-CC, above Rock Creek also measured above the maximum detection limit in July 2023, indicating that this stretch of Coal Creek is also influenced by urban runoff and the high levels of E. coli in Coal Creek below Rock Creek are not due just to Rock Creek tributary flows.

• The upper-most stations on Coal Creek near Empire Dr, which are located above (1-CC) and below (2-CC) the Louisville WWTP, show only slightly elevated concentrations of bacteria in July 2023. This further indicates that treated effluent discharge is not contributing to the elevated bacteria levels in Coal Creek and also that the primary urban runoff contributors to Coal Creek enter the stream Empire Drive.

St. Vrain

Concentrations of E. coli in the St. Vrain also vary considerably but do not tend to measure as high as the peaks on Coal Creek. Monthly peaks also occurred in July, averaged across all St. Vrain monitoring locations, and higher annual mean concentrations can be observed at upstream locations vs. downstream locations.

• The highest measured concentration was 1119.9 CFU/100ml in July of 2023, at one of the uppermost stations, M8.9-SV at Golden Ponds.

• St. Vrain station M8-SV just above the confluence with Left Hand Creek measured only 123.6 CFU/100ml in July 2023. Looking at the surrounding areas, it can be observed that the St. Vrain flows through a greenbelt the entire stretch from M8.9-SV to the Left Hand Creek confluence near M8-SV, including the Dickens Farm Nature area, a total distance of approximately 3.5 miles. This length of riparian buffers will slow the streamflow and trap bacteria, allowing soil microbes to absorb and break down the bacteria.

• Left Hand Creek tributary inflows on the other hand, at station T11-LH just above the confluence with the St. Vrain, measured 517.2 CFU/100ml during the same sampling event in July 2023. It appears that Left Hand Creek is influenced by urban runoff conditions, however; only the lower mile or so flows through areas with substantial riparian areas along the banks.

• When looking at the upstream-to-downstream annual geomean plots, it appears that E. coli in the St. Vrain rises again at station M8-SV, however this is misleading. there was a spike in April at that location which pushes up the annual geomean. This April spike was not seen at any other location and therefore appears to be caused by a very localized source.

• Below Left Hand Creek, the St. Vrain flows through primarily non-urbanized areas. The levels of bacterial measured during 2023 continue to fluctuate from station to station and month to month but remain low throughout the reach east of Longmont.

Boulder Creek

Boulder Creek E. coli levels show a distinctly different pattern over time than Coal and St. Vrain Creeks. Average monthly concentrations tend to peak in May and again in October to a lesser degree. In 2023, the May peak was higher than the previous 5-year average, but the October peak was lower.

Looking at individual station data, it can be seen that the average May peak in 2023 was driven by two locations in particular: the downstream-most location at the mouth of Boulder Creek, 11-BC and the station in Boulder below the University campus at the Footbridge by Folsom Field (BC-CU). The highest measurement in 2023 was at 11-BC in May, measuring above the detection limit. The increase at BC-CU follows historical trends along Boulder Creek where E. coli concentrations increase through the urbanized stretch of stream (BC-CU) before decreasing downstream of Boulder (BC-61). More detailed information on E. coli in Boulder Creek can be found on the City of Boulder’s E. coli in Boulder Creek Webpage.

Other stations show much smaller peaks in February, July and September but these have little influence on the annual station geomeans and the monthly stream geomeans. The boxplot for Boulder Creek is very helpful in identifying the other locations where there were higher measurements in 2023. The geographic and temporal variations on Boulder Creek suggest that E. coli in Boulder Creek is also influenced by urban runoff but that these areas of influence and the events that create the runoff are more localized.