The Canary Islands monitor the water quality of their beaches at irregular intervals. What makes this necessary are the points where waste water is discharged into the sea. Many of these discharge points are not authorised, and some release partially treated or untreated waste water off the coast. Weeks can pass between two official samples, and with them high bacterial counts during your very visit. We therefore draw on current data, weather data and historical readings to run a calculation in real time. It rests explicitly on probabilities and is not an actual measurement. We want to contribute with it, and at the same time we do not place ourselves above the official samples. That is why those come first, with the date of the most recent one. The official sample is and remains legally binding, but it can be three weeks old. Our calculation estimates whether, on the arithmetic alone, different values could be present today. How we calculate is explained below:
Load = distance to the source x weight of the source x current conditions, then the history factor of the zone.
Distance, straight line
| Distance | Factor |
|---|---|
| under 250 m | 1,00 |
| under 500 m | 0,75 |
| under 1 km | 0,45 |
| under 2 km | 0,20 |
| under 3 km | 0,08 |
| further away | 0 |
Weight of the source, all from official fields
| Feature of the discharge | Factor |
|---|---|
| not authorised | x 1,30 |
| authorisation pending | x 1,15 |
| plant not in operation | 0 |
| no treatment or treatment unknown | x 1,35 |
| screening only | x 1,20 |
| with secondary treatment | x 0,80 |
| with disinfection | x 0,55 |
| pipeline in poor condition | x 1,20 |
| domestic waste water | x 1,15 |
| brine from a desalination plant | x 0,45 |
| continuous discharge | x 1,10 |
Conditions
f = 0.10 + 0.60 x rain + 0.30 x drift. Rain is the sum of the past 48 hours divided by 20 millimetres and capped at 1. Drift is the positive part of the cosine between the current direction and the bearing from the discharge to the beach, scaled with the speed. Several effective sources raise the result by 5 percent each.
Thresholds: from 0.55 red, from 0.22 yellow, below that green.
The current comes from an Open-Meteo model, not from a measurement on site. The model works on a coarse grid and does not resolve small eddies inside a bay.
The history factor
Distance, permit status and weather do not know whether a discharge has ever reached this beach. The official samples do. The model calculation therefore passes through a factor taken from the zone own series, calculated as shares of the respective limit so that E. coli and enterococci are comparable. Damping only happens with evidence: below 20 samples the factor stays 1. A history of bans beats any damping.
| Own series of the zone | Factor |
|---|---|
| five or more bathing bans on record | 1,40 |
| at least one ban or one exceedance | 1,20 |
| unremarkable, p90 above 35 per cent of the limit | 1,00 |
| p90 below 35 per cent | 0,90 |
| p90 below 15 per cent | 0,75 |
| p90 below 5 per cent | 0,55 |
| fewer than 20 samples | 1,00 |
Limits
Legal limit in coastal water under RD 1341/2007, sufficient class: 500 cfu per 100 ml for E. coli, 185 cfu per 100 ml for intestinal enterococci.
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