The most recent water quality tests in Medina Lake and in area rivers produced low concentrations of E. coli bacteria in each of the 25 areas tested except one on Seco Creek in far southwest Bandera County that exceeded the hazardous level for E. coli in early May only to fall well under acceptable concentration a week later.
The testing was conducted by the Bandera County River Authority and Groundwater District at first on May 6 and 7 and then again on May 14 at Seco Creek and Farm Road 470 where the lone violation of E. coli concentration limits was discovered.
The concentrations are measured as an MPN, or most probable number, per 100 milliliters of water, and anything exceeding a 399 MPN count is considered excessive by the Texas Commission on Environmental Quality.
At the Seco Creek location this month, the first test produced a 461 MPN reading, but a week later the count had fallen to 37 MPN.
In waters with E. coli at levels higher than 399 MPN, the environmental commission says no “primary contact recreation should take place.” Primary contact recreation is any activity that would cause someone’s head to be fully submerged under water.
David Mauk, the river authority’s general manager, said he would need to examine circumstances in the area of Seco Creek and Farm Road 470 more closely to be certain about what caused a spike in the E. coli concentration.
Typically, however, heavy rains in an area can cause E. coli to rise dramatically because the bacteria get washed into the waterway from the land.
E. coli is a bacteria found in the gut of warm-blooded animals. It is known to potentially cause humans to get sick if it is ingested, a news release from the river authority said.
The river authority regularly evaluates water qualify in the Medina and Sabinal rivers and their watershed and in Medina Lake to see what the E. coli readings are.
They can at times reach excessive levels in different areas, and BCRAGD has issued warnings about the safety of the water when that occurs.
Of the other 24 sites tested by the river authority this month, only four had readings above 100 MPN, and the highest one of those other than the Seco Creek site was at Moffett Park in Medina at 117 MPN.
Many sites had readings at 70 MPN or below, including the West Verde Creek in the Hill Country State Natural Area, which had a reading of 1 MPN, and Medina Lake at the county park on Park Road 37, whose count was under 1 MPN.
Still, officials said swimmers always face a risk of E. coli exposure no matter what the reading is.
Those using the river should never drink river water without proper disinfection and should always recognize that they are swimming at their own risk, officials said.