Water Sampling Results - Rain and Bacteria

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Precipitation is typically associated with an increase in fecal-related bacteria in surface waters. There are several reasons for this. Runoff can pick up fecal matter from wild and domestic animals and transport it to the nearest stream, lake or estuary. Also, stormwater runoff often is connected into wastewater treatment systems, and a heavy rain can cause the total flow into wastewater plants to exceed capacity, resulting in overflow of untreated sewage into the enviroment. It is also possible that heavy rains may increase leakage directly from sewer pipes themselves into ground or surface waters.

Our project has tested for bacteria in dry and increased runoff conditions in the Webhannet watershed. In this small, coastal watershed, we observed numbers of bacteria increase significantly within about 6 hours of rainfall. Due to the unusually dry conditions in 2002, we tested only a few times under these conditions. Here are three graphs that show fecal bacteria counts (note: on these graphs, fecal coliform is measured in CFU/1mL, instead of the typical CFU/100mL), temperature and precipitation for dry conditions, heavy rain 24 hours before sampling, and moderate rain about 6 hours before sampling. See all weather and fecal coliform charts here.