E. Coli Loads Measured in Four Arcata Creeks

July, 2014






A rainy February enabled Humboldt Baykeeper to augment our 2013 flow study with wet weather data to calculate the mass volume of E. coli in four Humboldt Bay tributaries (Janes, Jolly Giant, Jacoby, and Campbell Creeks).

On February 13, we found that after more than one inch of rain, Janes Creek carried more than 75% of the E. coli delivered to the Bay by these four streams—375 lbs. per day! In technical terms, that’s a concentration of 3,890 MPN/100 mL (a measure of the number of bacterial colonies) combined with a flow rate of 18 cubic feet per second.

The goal of this study was to identify which creek discharges the highest mass volume of  E. coli into the Bay. Thanks to support from the Cereus Fund in 2013, we consulted with hydrologists at Pacific Watershed Associates to train our staff to measure stream flow rates and develop hydrologic rating curves for these four creeks. We measured low flows throughout the exceptionally dry autumn, and shared our October flow measurements with hydrologist Randy Klein to add to the low-flow hydrograph for Jacoby Creek, which is monitored during higher flows at a gaging station in Bayside.

Previous monitoring events only measured concentrations of E. coli—a measure of grab samples taken during dry weather and “First Flush,” the first major rainstorm of the water year. Over seven years, we detected and identified concentrations of E. coli that exceeded Water Board standards in most local streams. The highest levels were found during the First Flush. During low flow, illicit detections can help to better pinpoint pollutant source locations from recent discharges. Monthly sampling is necessary to better understand the pollutant-flow regime.

During our dry weather monitoring on Oct. 3, Jolly Giant Creek carried close to 50% of the total E. coli measured, suggesting a different source of bacterial pollution during dry versus wet weather.

We will use these findings to develop a monitoring plan to more closely examine these creeks using molecular techniques to identify the source animals with the goal of identifying and reducing or eliminating a major source of anthropogenic water pollution in Humboldt Bay. The ultimate goal is to develop strategies for reducing or eliminating this type of pollution and to educate local residents, sewer providers, and land managers about how we can all help improve water quality throughout the Humboldt Bay watershed.

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