E. coli is a bacteria found in animal and human poop. If it shows up in bathing water, it's usually from land run-off or sewage. Higher levels mean swimmers could be at risk of getting sick. The UK Environment Agency classifies inland bathing waters as Excellent, Good, Sufficient, or Poor, based on two to four years of testing rather than a single sample. So one reading can't tell you how a bathing water is officially classified, but it can show you how today's water compares to their thresholds. We've used 1,000 CFU/100 mL as the cut-off, which is the upper limit of the Good classification (if over 95% samples are under 1,000 CFU/100mL) and the most common interpretation for single samples.