Watermark Scoring
How the Home Water Exposure Score works.
Every Watermark is anchored to a Home Water Exposure Score. It’s a 0–100 composite calculated from nine specific parameters, weighted by health significance, and benchmarked against UK and WHO drinking-water standards. The methodology is independent, peer-reviewable, and versioned.
Three verdicts. One number behind each.
The Home Water Exposure Score is a 0–100 composite. It collapses nine separate parameter readings into a single, comparable number. That number translates into a verdict.
Three consumer verdicts. Each maps to a band of the underlying score.
Clean, drinkable water. No regulatory exceedances.
Aesthetic or economic concerns. No health risk at current readings.
Something to fix. Regulatory threshold exceeded, or a critical health parameter is positive.
For audit and technical review, every score also carries a five-band methodology classification (A · 90–100, B · 80–89, C · 65–79, D · 40–64, F · 0–39) and a methodology version stamp.
Nine parameters. Each one specific.
Each parameter is a measurable, regulated indicator of drinking-water quality. We test for the nine that matter most for household exposure, scored against current UK regulatory thresholds.
Several of these parameters also appear at watershed level on the UK water quality map, where you can see how your area performs before you book a Watermark.
- LeadCritical10 µg/LNo safe lower bound. UK limit tightening from 10 to 5 µg/L by 2036. Detection at any level warrants attention.
- BacteriaE. coli & coliformsCritical0 CFU / 100 mLMicrobial contamination indicator. Any presence requires investigation.
- NitrateSignificant50 mg/LAgricultural runoff indicator. Particular concern for infants and rural areas.
- NitriteSignificant0.1 mg/LHealth concern via methaemoglobinaemia in infants. Often co-occurs with nitrate.
- FluorideSignificant1.5 mg/LMonitored where fluoridation is active or naturally elevated in groundwater.
- Free chlorineModerate0.2–0.5 mg/L at tapDisinfection residual. Too low signals distribution issues; too high affects taste.
- pHModerate6.5–9.5Affects metal leaching from pipes. Low pH increases lead dissolution.
- Total hardnessContextualNo UK limitAffects appliances, taste, soap performance. Baseline for area comparison.
- Total alkalinityContextualNo UK limitBuffering capacity. Affects corrosion control and water stability.
This is the current canonical set (WES v0.3). Earlier versions tested up to 13 parameters. Future versions may add microplastics, PFAS, and oestrogen as sensor technology matures. Historical scores remain valid under their original methodology version.
Weighted, normalised, and capped.
Each raw reading maps to a 0–100 sub-score. Numeric regulatory parameters use a piecewise-linear curve: 100 at zero, 60 at the regulatory limit, 0 at five times the limit. Binary parameters (lead, bacteria) score 100 if not detected, or a fixed penalty if detected (lead 10, coliform 20). Aesthetic parameters score against UK population medians.
The composite is then a weighted average across four tiers.
- Lead · 0.222
- Bacteria · 0.178
- Nitrate · 0.129
- Nitrite · 0.086
- Fluoride · 0.086
- Free chlorine · 0.10
- pH · 0.10
- Total alkalinity · 0.05
- Total hardness · 0.05
If lead OR bacteria is detected, the composite is capped at 39 (Action Required). If both are detected, it’s capped at 25.
A single binary positive on a critical health parameter cannot be averaged away by good readings elsewhere. This override is an extension on the published methodology, signed off in April 2026 and approved via academic review.
Independent. Ranked. Reasoned.
Every Watermark includes ranked recommendations: what to fix, what to monitor, what to skip. Each one is tied to a specific parameter and to the dominant driver in the composite score.
Our recommendations come from independent research, not commercial relationships.
We don’t sell filters. We don’t take affiliate revenue from the brands we recommend. The Watermark is the product; the recommendations are independent.
When we suggest a specific product, treatment, or supplier, we cite the study or regulatory data that supports the recommendation. Each recommendation is ranked by expected impact on the score and by the directness of the link to the flagged parameter.
What we measure against, and who signs it off.
We benchmark against the Drinking Water Inspectorate (DWI), the body that sets current UK statutory limits. The DWI’s lead-limit reduction from 10 to 5 µg/L by 2036 is reflected in our scoring.
For parameters where UK guidance is silent or conservative, we reference the World Health Organization Guidelines for Drinking-water Quality.
The methodology is peer-reviewable. We publish the calculation logic, the parameter weights, the normalisation curves, and the critical override thresholds. Any score can be reproduced from its raw inputs.
The methodology is versioned. Historical scores remain valid under their original methodology version. We do not retroactively re-score.
A score, not a diagnosis.
The Home Water Exposure Score is exposure context. It tells you where your household’s water exposure sits relative to regulatory thresholds and historical baselines.
It is not a medical diagnosis. It does not certify your water as safe or unsafe. It does not replace statutory compliance testing by your water company or the Drinking Water Inspectorate.
We provide measurement. We do not provide medical advice or prescribe action. For regulatory-grade confirmation, accredited laboratory testing is the next step.
Watermark your home.
£29 for two tests, six months apart. Personalised advice after each one. New Forest properties only.
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