◉ LIVE · — BST
The ExplainerIssue I · 18 APR 2026

What's really
in the water.

BY
GOOD BUOY
EDITORIAL

ISSUE I
APR 2026

10 MIN READ
Every British beach has a hidden hydrology. Rain falls, sewers fill, overflows open. Within three hours, a plume of untreated wastewater can reach the break. Good Buoy reads seven markers at the shoreline and one — rainfall — inland, to tell you whether you can actually swim today.
In this issue
  1. The seven markers
  2. Modelling the plume
  3. Where we stop short
  4. Sources
The Seven Markers

What each reading means.

01

pH

Acid–alkaline balance. A sudden swing signals contamination or algal activity.

02

Turbidity

Cloudiness of the water. Storm runoff and sewage push this upward within hours.

03

Chlorophyll-a

Algal biomass. A spike above 15 µg/L can mean a bloom capable of producing toxins.

04

Dissolved oxygen

Oxygen available to marine life. Falls below 4 mg/L when organic pollution loads spike.

05

Salinity

A sharp drop means freshwater is pouring in — a proxy for runoff or a combined sewer overflow.

06

Temperature

Warmer water accelerates bacterial growth and signals thermal plumes from outfalls.

07

E. coli & Enterococci

Lab-sampled indicator bacteria. The EA counts these to classify bathing waters.

Modelling

How the plume actually spreads.

A plume does not obey schedules — and it does not spread in a circle. It follows the water. We rasterise the Ordnance Survey tidal and surface-water shapes for the whole South West to a 25-metre grid, then trace contamination outward from each live discharge through water only — it cannot cross land, and narrow channels concentrate it, exactly as in reality. Concentration falls off with distance travelled, not distance as the crow flies.

“We show the plume as a fading wash that follows the real water — never as a hard boundary.”

Rivers carry it too. Contamination runs down the river network from inland outfalls, and where a polluted river meets the sea we hand it off to the coastal model, so a spill miles upstream can still reach the beach at the mouth. How strongly a site weighs is set by its own six-year spill history, not just today’s event.

Three hours is the figure that frames it. After a CSO opens, water reaches most UK bathing beaches within about three hours; by the six-hour mark, tidal flush has usually begun to dilute it. The wash leans with the live tide.

Fig. 3 · Plume propagation
TIDERAINCSOplume, 0–3h
Limits

Where we stop short.

  • Buoy readings refresh every 5 minutes; bacterial lab samples are daily.
  • The plume is an indicative model, not a measurement — read the fading wash, not a hard edge. Outfalls on streams too small to map fall back to a tide-leaning estimate.
  • We do not replace Environment Agency bathing-water classifications. We sharpen them with real-time context.
  • When a buoy is offline, the map shows the last-known reading with a mono “Last seen HH:MM” folio. We never interpolate.
Sources
Updated 18 APR 2026
Environment Agency bathing water API
National Storm Overflow Hub
South West Water · Southern Water · Northumbrian Water EDM
Open-Meteo rainfall + wind
UKHO tidal predictions
OpenStreetMap coastline polygons
GB buoy fleet (in-house)