Creek bacteria test shows human, animal pollution

By: 
Dalton Sweat, Editor

The initial results from the first Bacterial Source Tracking Study on Cypress Creek were released last week and did show at least some sewage related pollution, but the results are far from conclusive. 

The first of three tests was made while it was raining. The next two tests will be during dry weather.

“Lets be cautious at looking at any one sampling event to draw any conclusions,” Nick Dornak, an environmental scientist contracted by the Meadows Center to oversee the project, said. “We need to wait until we have all the data and then we will be able to find out if we have any trends that we need to focus on in the future.”

The bacteria counts were high, but that is expected during a rain event.

“It is also very important to understand that bacteria concentrations tend to be greater and come from more diverse sources during a rain event,” an email from the Cypress Creek Watershed Protection Plan Team said. “This is because bacteria and other pollution is picked up and carried by rainfall flowing across the watershed and into the creek. In addition to bacteria that might normally be in the creek, bacteria from the landscape is added to the water column.”

This testing did not show how much a given bacterial source contributed to the e coli levels.

“Let’s say there were maybe 2,000 colonies in the sample of those 2,000 colonies,” Dornak said. “We took four randomly selected colonies and sourced them. So that is the info we have. So of 2,000 colonies we took four and did the genetic testing on those four. It is a very small sample size of that particular event, but it gives us an idea of what are some of the sources.”

The first test included a location upstream from downtown and below the Ranch Road 12 bridge downtown. The test from the upstream location showed 6,400 colonies of bacteria formed per 100mL of water. The sources were from livestock, domestic animals, non-avian wildlife and feral hogs. The downstream location showed 18,000 CFU/100mL and the pollutants found were raw sewage, cattle and feral hogs. 

“These results are not unusual – multiple sources of pollution were likely carried into the creek by rainfall runoff,’ the email said. “Human bacteria could be coming from saturated aerobic septic systems, leaking tanks (rainfall can leak into tanks) or tanks can collect and release water from saturated substrates, as often is the case during rain events… Another source could be septic leakage directly into the creek.”

While how much a given source contributes to pollution can not be verified by this test, there are some initial takeaways.

“There was more bacteria at the downstream site,” Dornak said. “There was more bacteria coming from downtown...

"The next two events are base-flow (or regular flow) events. We expect bacteria counts to be smaller, so when we pull three or four colonies we will have a greater representation of that sample. When we put all three samples together we will see if there are any trends. If there are, we can say we really need to focus more on wildlife or human contributions or whatever it happens to be. The idea is, don’t take too much from this sampling event. Have some patience and at the end we will try and determine a trend.”

Results from the remaining samples are expected to be released in November and January.

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