How to read sea-water quality results
SeaCheck tests the sea for the bacteria that signal sewage pollution, and occasionally runs DNA tests to trace where that pollution comes from. This guide explains every number you will see.
What SeaCheck measures
The Environment Agency judges bathing water on two faecal-indicator bacteria: Escherichia coli (E. coli) and intestinal enterococci (IE). Finding them in the sea means faecal contamination has reached the water.
cfu — colony-forming units
Bacteria are counted as colony-forming units per 100 mL (cfu/100 mL). Lower is cleaner. The EA rates ≤250 as Excellent and ≤500 as Good.
DNA source tracking
A bacteria count tells you the water is contaminated, but not by what. Microbial source tracking looks for species-specific DNA to trace the source: human (sewage), ruminant (farm run-off) or gull (seabirds). Markers are measured in genomic copies per 100 mL (gc/100 mL).
Frequently asked questions
What does cfu/100 mL mean?
Colony-forming units per 100 millilitres — how many live E. coli bacteria a lab grew from the sample. Lower is cleaner. The Environment Agency rates ≤250 cfu/100 mL as Excellent and ≤500 as Good.
What does it mean when human DNA is found in the water?
DNA source tracking looks for species-specific genetic markers. A human marker points to sewage or wastewater as the source of contamination, rather than livestock or seabirds. Human sewage carries the widest range of pathogens that affect people, so it is the most important source to identify.
What is 'below the limit of quantification'?
A faint DNA signal that sits below the level the lab can confidently measure. It is a trace, not a positive result. A confident positive typically needs two of three replicates to amplify above the threshold.
Is a single SeaCheck reading the same as the beach's official rating?
No. The Environment Agency's official bathing-water class is calculated over four years of samples using statistical percentiles. A single SeaCheck sample is an indicative snapshot of one spot at one moment, useful for awareness, not an official classification.
What is the difference between E. coli and intestinal enterococci?
Both are faecal-indicator bacteria the EA uses to classify bathing water. E. coli is the headline count; intestinal enterococci (IE) is a second indicator. Excellent water is ≤250 E. coli and ≤100 IE per 100 mL.