Is It Safe to Swim? After a Rain Event?

Among our most common questions are, "Is it safe to swim? What about after a rain?" The questions are particularly relevant given the recent bacteriological update from Charlotte-Mecklenburg Storm Water Services. And the answer... requires the precise context we'll provide here!

The lakes in the Catawba basin are generally in good shape for swimming. However, they are also the destination for all the surrounding streams that drain the basin. In our diverse basin, streams drain both rural and urban areas, but as different as a farm can be from a downtown, the ultimate consequences for lakes can be very similar.

In rural environments, fecal material accumulates on land in between rain events. In urban environments, there is sewage infrastructure. Older segments might be failing, though it might be subtle and unnoticeable when conditions are dry.

"A fecal coliform at rest tends to stay at rest until acted upon by rain."
-Newton's First Law of Fecal Transport

But a good rain flushes the system! The turbidity seen up to a couple days after a rain is indicative of this. Accumulated fecal material on land will suspend in the water and surf its way into a stream, where it is joined by more and more water from other streams on its way to the lake. And then be it a city's sewer system or a residential septic tank, sewage infrastructure failures become exposed and very efficiently connected to creeks and to... [let's see, multiply by three, carry the one]... LAKES!

Below is a recent copy of the monthly CharMeck report on fecal coliform (also known as E. coli -- the bacteria found in feces). The day prior (July 16) had seen rain, and as a result, two sites (Latta Park Canoe Launch and Nance Cove) revealed elevated levels of fecal coliform.

Typically, agencies try to test during baseflow. This is not meant to cover up a problem by avoiding testing that reveals problems. Rather, we know this consistently happens. When 'everyone' jumps into the pool at once, a point source can be difficult to identify. During baseflow, without a vast majority of fecal coliform sources contributing, the point sources that are contributing and that are more problematic are more likely to be identifiable.

In the case of this sampling, a day had passed since the rain, and they decided to test anyways. Re-testing later revealed no problems.

So should you swim after a rain? Likely not. Wait for the water to clear up. Have patience. Perhaps get your fix playing in the rain itself when it has fallen clean from the sky!

But this 'now-you-see-it-but-most-of-the-time-you-don't' does highlight a situation we rarely notice and deal with. This is a problem, being humans, that we have created, that exists and that we must deal with by not swimming after rain events and by chlorinating our water. However, this problem we can ameliorate by reducing runoff from our own property and, especially if you have a farm, ensuring that feces are not directly reaching streams. And never hesitate to report anything that looks -- or certainly smells -- wrong!