E. coli Bacteria and Beaches: What is In Your Sand?

The lake water is not the only place where E. coli exists at a beach. E. coli is also found in the sand and groundwater along the shoreline. Scientists in Canada and the United States have been investigating E. coli in the beach sand during the past few years, and are starting to get a picture of just how much of it is in the sand.

To start off with, don’t let this send you running away from the beach! Although high levels of E. coli are present in the sand along the shore where children typically play and build sand castles, these typically do not present a major health risk. However, you should make sure to wash your hands and face with soap and clean water after you or your children play in the sand.

So how does it get there you ask? We know that E. coli is always found in the beach sand and groundwater along the shoreline at all beaches of the Great Lakes, including Lake Huron. And we know that E. coli concentrations here are much, much higher than E. coli levels in the lake water – up to 1000 times higher that the provincial Recreational Water Quality Guidelines for a beach posting!

Most of the E. coli in the beach sand along the shoreline comes from the lake. During storms, lake water will infiltrate into the sand as waves run up the shore. Because lake water typically contains E. coli, the infiltrating lake water will also carry E. coli into the sand. Most of the infiltrated lake water will quickly head back into the lake but the sand will act to filter the E. coli and retain it in the sand. Each subsequent storm will add more E. coli to the sand. The high levels of E. coli in the sand are limited to the swash zone, which includes all of the area that waves extend to before receding back into the lake.

We know that the E. coli in the swash zone can return to the lake, and return in two ways. First, groundwater below a beach always flows towards and discharges into the lake at the shoreline. E. coli will be transported with the groundwater and discharged right at the shoreline (i.e., less than 2 metres from the shoreline). This process could be partly responsible for why E. coli levels in ankle-depth water are higher than knee-depth water and much higher than chest-depth water. Secondly, E. coli can also re-enter the lake as the sand along the shoreline erodes. Beaches are very dynamic environments with waves and currents continually eroding and depositing sand along the shoreline. Any sand that is eroded into the lake will also deposit E. coli that was in the swash zone into the lake. It is suspected that this process is the major reason why beach postings sometimes occur even though there does not appear to be any reason for high levels of E. coli.

So remember, after you or your children play in the sand, wash everyone’s hands and face with soap and clean water!