E. coli concentrations are influenced by a number of factors resulting in hourly and daily changes in concentration. Those factors include:
- Wave height
- Turbidity (water clarity)
- Air temperature
- Water temperature
- Shape of the coast line
- Wind speed and direction
- Relative humidity
- Lake level
- Ultraviolet (UV) light exposure (Francy, 2009)
The way these factors influence E. coli concentrations can be complex. For example,
- Rainfall washes E. coli into storm drains and rivers resulting in higher concentrations in the lake
- High winds create waves that stir up sediment and sand from the beaches and lake bottom – the E. coli that was in the sediment and sand increases the concentration in the lake water
- UV light exposure may kill E. coli at the surface of the water lowering concentrations
Sources of E. coli
Known sources of E. coli in fresh water lakes are:
- Storm drains
- Septic systems
- Environmentally adapted strains of E. coli (Edge & Hill, 2005; Gerba, 2000; Haack, Fogarty, & Wright, 2003; Jarvie et al., 2010; Kon, Weir, Howell, Lee, & Trevors, 2009).
Storm drains and rivers
The source of the E. coli in storm drains and rivers is determined by the watershed land use. In predominantly urban areas, such as Grand Traverse Bay or Lake St. Clair in Michigan, the main source of E. coli is urban sources: residential, commercial, and industrial. In predominantly rural areas, such as Huron County, the main source of E. coli is agricultural.
Septic tanks, particularly those not installed or maintained properly, contribute to E. coli concentrations. Impact of septic tanks is greatest in clay soils and when the water table is high, such as after heavy rainfall.
The contribution of groundwater to E. coli concentrations is not well understood. Groundwater is thought to have the greatest impact on E. coli concentrations in calm, shallow lake water. E. coli may enter the groundwater through leaking sewer lines, septic systems or storm drains.
Birds are considered to be a significant source of E. coli for freshwater beaches. Feces from gulls, geese, pigeons and other birds are deposited directly and indirectly into the lake. Indirect feces deposit occurs when waves wash bird feces off the beaches and when rain washes feces on the ground into storm drains and rivers. In some cases, such as Hamilton Harbour with its abundance of geese, the dominant source of E coli in the lake water is birds.
Swimmers contribute E. coli to lake water through direct deposit of feces and washing bacteria off their skin. Bacteria, including E. coli, can live on skin and when swimmers enter the water the bacteria washes off. When beaches have a high density of swimmers, particularly young children who may deposit feces, E. coli contributed by swimmers is higher.
Environmentally adapted strains of E. coli
Some strains of E. coli have adapted to life outside humans and animals creating another source of E. coli for the Great Lakes. Water samples collected near Point Clark, Ontario demonstrated that E. coli can thrive in beach sand. When waves or rainfall wash over the sand, E. coli is deposited in the water. In the Point Clark study, environmentally adapted E. coli contributed up to 22 per cent of the E. coli found in the lake water samples.
Current E. coli Data Tables
Detailed E. coli results for 2019 are in the 2019 E. coli Data Tables.