Seasonal Distribution

Seasonal Distribution of E. coli

For each sampling date, the arithmetic mean for E. coli concentrations for the sampling period May 2, 1998-November 3, 1998 was calculated based on all 15 sites (except for 5/2/98 which does not include sites 13-15). As indicated in the figure below, mean E. coli was lowest in May and increased slightly through the summer. A few high peaks occurred in August and September while smaller peaks occurred in June, July, and October. When compared to streamflow, all peaks (except 7/20/98 and 10/29/98) coincide with significant runoff events (6/16, 8/11, 8/25, 9/16) as evident in the figure below.

Data and analysis by Deb Sargent under the direction of Dr. Leslie Morrissey, School of Natural Resources, University of Vermont. Cooperator: Friends of the Mad River.

For the 1998 sampling period, the concentrations of E. coli for all 15 sites are shown in the figure below . On most dates the E. coli concentrations for each site follow the same pattern, that is, consistently low values through July, highest concentrations in August and September, and decreasing concentrations in October. Similar to the previous figure, high E. coli concentrations coincide with storm events with a few exceptions; these exceptions provide insight into processes which affect E. coli concentrations in streams. Three anomalous events occur which do not coincide with storm events; site 11 is exceptionally high on 9/22 and 9/29 and site 13 is unusually high on 10/20. These episodic events are further analyzed in relation to land use and land cover.

Data and analysis by Deb Sargent under the direction of Dr. Leslie Morrissey, School of Natural Resources, University of Vermont. Cooperator: Friends of the Mad River.

STREAMFLOW EFFECT

River discharge has a significant effect on E. coli concentrations. The fact that the highest E. coli concentrations occurred in August and September is due in large part to the number of samples acquired at peak stream flows. The arithmetic mean for E. coli concentrations for the Mad River sites in 1998 is plotted with the daily mean discharge for the Mad River gauging station. High mean E. coli concentrations occurred on peak runoff sampling dates (6/16, 8/11, 8/25, 9/16). A major flood occurred on 6/25/98 but was not sampled.

Data and analysis by Deb Sargent under the direction of Dr. Leslie Morrissey, School of Natural Resources, University of Vermont. Cooperator: Friends of the Mad River. Streamflow data: USGS, Mad River gauging station.

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