The charts bring together two bacteria: Escherichia coli (EC), which reacts quickly to fresh sewage discharges, and Intestinal enterococci (IE), which moves more slowly and reflects background contamination. WaterTrends smooths the Environment Agency sample records into four-year rolling averages so short-term spikes do not dominate. We then compare the earliest dependable period with the latest one to show the percentage change above the charts: a negative number means cleaner water, while a positive number means more contamination. Together these indicators show both recent incidents and longer-term trends.
Historic microbiological samples flow through the WaterTrends data layer and are harmonized by DataProcessor before any visualisation.
Water quality trend chart
Official years display the Environment Agency results directly, while earlier or missing seasons are simulated by using bacterial determinands data and BWD classification criteria.
Each simulation uses 4-year rolling windows, log10 95th/90th percentiles and needs at least eight samples per indicator, applying the stricter of the E. coli and intestinal enterococci outcomes in line with the EU Bathing Water Directive 2006/7/EC.
Bacterial indicator charts
Historical sampling data uses 3 different schemes and we harmonize them using data from overlapping periods. We merge sampling programmes by setting ec_unified = EC or 0.95 * faecal coliform and ie_unified = IE or 0.85 * faecal streptococci. Pre-1999 membrane filtration methods could report counts as low as 1 CFU/100ml, while the modern methods phased in during the late 1990s use a 10 CFU/100ml detection limit. All pre-1999 EC, FC, FS and unified values below 10 are floored to 10 to prevent sub-detection artefacts from distorting rolling averages and trend calculations.
