Lake limits

It’s a given that in a lake there will always be some E. coli bacteria.

The real issue is in what concentration.

Starting in mid-June 2019, swimming was permitted in Purcell Lake. And since then, the City of Purcell has pulled weekly water samples which are sent to Accurate Laboratories in Stillwater for analysis.

Samples are taken on Tuesdays and results are available by the end of the week.

The testing runs from mid-April through September.

Results are reported in the number of colony forming units of E. coli per 100 ml of water.

The Environmental Protection Agency has designated a value of 410 CFU as the threshold for placing an advisory on recreational waters. However, the city’s self imposed threshold is 320 CFU.

According to the city’s website, the E. coli concentration came close to that threshold just once in 2019 when testing showed 307.6 CFU.

This year on July 28, the concentration was 1,120 CFU, but there was a delay in the city receiving that result from the lab. The reading was a huge spike from the previous week’s 52.9 CFU.

Numbers dropped the following week, but not enough to lift the advisory against going in the water. The August 4 result was 727 CFU.

Bunn said E. coli numbers typically spike if the weekly test is taken on a really windy day following rain.

The lowest concentration in 2019 was 8.6 CFU on September 24. The low point in 2020 was 2.0 on May 5.