Tap water in CM21, East HertfordshireSafe to drink.Very hard.
Supplied by Affinity Water. 379 samples, 24 substances, latest on 18 December 2025. Everything is inside the legal limit, and nothing comes close to it.
9.4 / 10
The tank is as full as the score. The specks are the minerals that make water hard.
What to do about it
Hard water: compare softener quotes
At 339 mg/L a softener is the only thing that stops limescale. Filters do not soften water.
It pays for itself in most homes at this hardness, through a boiler that stays efficient and appliances that last.
ZeroWater 12-Cup Ready-Pour
£40typical price
Households concerned about PFAS and heavy metals
Waterdrop 10UA Under Sink
£70typical price
High-volume under-sink filtration without RO complexity
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What's in a glass of CM21 water
Every bubble is a substance the lab measured. The higher it floats, the closer it came to its legal limit, the dashed line at the top.
Nitrate came closest to a limit, at 42% of it. 20 of 21 substances used under a quarter of theirs.
- Nitrate42%
- Bromate15%
- Fluoride13%
- Ammonia12%
- Nickel11%
- Colour10%
- Copper8%
- Lead7%
- Boron5%
- Iron4%
- Arsenic4%
- Trihalomethanes3%
- Turbidity3%
- Mercury2%
- Nitrite2%
- Antimony2%
- Selenium2%
- Cadmium1%
- Aluminium<1%
- Chromium<1%
- Manganese<1%
- E. coli none found
- Coliform Bacteria none found
- Conductivity 574 µS/cm
All 24 results as a table
| Substance | Result | Legal limit | Share of limit |
|---|---|---|---|
| Antimony | 0.000091 mg/L | 0.005 mg/L | 2% |
| E. coli | 0 count/100ml | Must be zero | None found |
| Colour | 2 mg/L Pt/Co | 20 mg/L Pt/Co | 10% |
| Iron | 0.00897 mg/L | 0.2 mg/L | 4% |
| Coliform Bacteria | 0 count/100ml | Must be zero | None found |
| Nitrite | 0.01 mg/L | 0.5 mg/L | 2% |
| Copper | 0.164 mg/L | 2 mg/L | 8% |
| Ammonia | 0.06 mg/L | 0.5 mg/L | 12% |
| Aluminium | 0.00199 mg/L | 0.2 mg/L | <1% |
| Boron | 0.046 mg/L | 1 mg/L | 5% |
| Turbidity | 0.1 NTU | 4 NTU | 3% |
| Trihalomethanes | 0.00334 mg/L | 0.1 mg/L | 3% |
| Chromium | 0.000383 mg/L | 0.05 mg/L | <1% |
| Conductivity | 574 µS/cm | No legal limit | – |
| Bromate | 0.00147 mg/L | 0.01 mg/L | 15% |
| Fluoride | 0.2 mg/L | 1.5 mg/L | 13% |
| Selenium | 0.000643 mg/L | 0.04 mg/L | 2% |
| Manganese | 0.000363 mg/L | 0.05 mg/L | <1% |
| Arsenic | 0.000375 mg/L | 0.01 mg/L | 4% |
| Cadmium | 0.000054 mg/L | 0.005 mg/L | 1% |
| Mercury | 0.000023 mg/L | 0.001 mg/L | 2% |
| Nitrate | 21.1 mg/L | 50 mg/L | 42% |
| Lead | 0.000673 mg/L | 0.01 mg/L | 7% |
| Nickel | 0.00222 mg/L | 0.02 mg/L | 11% |
339mg of limescale minerals in every litre
This is the water that furs up kettles
Very hard on the UK scale. It is fine to drink. It is hard on boilers, showers, skin and anything that heats water, and a jug or tap filter will not change that.
Softener quotes for CM21
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Your river is the Stort and Navigation, B Stortford to Harlow, rated moderate
The Environment Agency rated the Stort and Navigation, B Stortford to Harlow as moderate for ecological health in its 2025 assessment, meaning the river is noticeably changed by pollution or engineering. It is held back by fish and phosphate. It fails the chemical test, as every river, estuary and stretch of coast in England does, because of long-lasting pollutants such as mercury, PFOS (a forever chemical) and old flame retardants.
None of this is a test of your tap water. Environment Agency record · River health across England
CM21 in short
Tap water in CM21 (East Hertfordshire) is supplied by Affinity Water in the East Hertfordshire Central zone and scores 9.4 out of 10, against an England and Wales average of 9.0. None of the 24 tested substances came close to a limit. The latest sample is from 18 December 2025.
- The cleanest tap water of the 8 areas tested in East HertfordshireCM21 scores 9.4 out of 10, ahead of SG11 in second place on 9.4.
- Among the hardest water in the country at 339 mg/LVery hard starts at 250 mg/L. At this level kettles scale up within weeks and a softener is a normal purchase in CM21. How hardness is measured
- Nitrate is the reading closest to its limit, at 42% of the 50 mg/L maximumIt passed with room to spare, and everything else tested sat further below its limit. See every reading
Questions about CM21 water
Is CM21 tap water safe to drink?
Based on the latest tests, CM21 (East Hertfordshire) water scores 9.4/10, which is safe. 0 of 24 tested contaminants exceeded recommended levels. Water is supplied by Affinity Water.
What contaminants are in CM21 water?
We tested 24 contaminants in CM21 water. None exceeded recommended safe levels.
Who supplies water in CM21?
CM21 (East Hertfordshire) is supplied by Affinity Water. You can view their full profile and compliance data on our supplier page.
Is CM21 water hard or soft?
Water in CM21 (East Hertfordshire) has a hardness of 339 mg/L CaCO3, which is classified as very hard. Hard water can cause limescale buildup. A water softener or filter jug may help.
Should I use a water filter in CM21?
CM21 water scored 9.4/10 with no contaminants above recommended levels. A filter is optional but can improve taste, especially if you notice a chlorine flavour. See our filter recommendations for your area.
Are there PFAS forever chemicals in CM21 water?
No PFAS (forever chemicals) result is attached to the tap-water tests for CM21, and the Environment Agency has no PFAS detections in rivers or groundwater within 10 km of it. That reflects where sampling has happened as much as the water itself.
How clean is the river near CM21?
The Environment Agency rated the Stort and Navigation, B Stortford to Harlow as moderate for ecological health in its 2025 assessment, meaning the river is noticeably changed by pollution or engineering. It is held back by fish and phosphate. It fails the chemical test, as every river, estuary and stretch of coast in England does, because of long-lasting pollutants such as mercury, PFOS (a forever chemical) and old flame retardants. This is a rating of the river, not of tap water in CM21, which is treated before it reaches homes.
Keep reading
- Best Water Softener UKHarvey, Kinetico, BWT, Monarch and Water2Buy compared, with real costs
- Do I Need a Water Softener?The hardness bands, the symptoms, and a postcode check
- UK Water Hardness MapIs your water hard or soft, and does it matter?
- Is UK Tap Water Safe?Everything you need to know about tap water safety in the UK
- Do I need a water softener?The hardness bands, the symptoms, and when the answer is no
- Best water softener UKHarvey, Kinetico, BWT, Monarch and Water2Buy compared
- Water softener costUnit, fitting and salt, and how long the payback takes
- Installation explainedWhere it goes, the bypass tap, the drain, DIY or plumber
- Harvey vs KineticoThe two premium twin-cylinder brands, side by side
- Softener salt guideTablet, block or granular, and how much you get through
- Salt-free softenersConditioners and electronic descalers, honestly assessed
How to test your waterBest water filtersFilter shower headsWater testing kitsUK water rankingsWater hardness checkerWater hardness map
River and groundwater readings near CM21
Environment Agency samples from rivers, groundwater and reservoirs nearby. Not your tap water. For scale they are shown against drinking water limits; rivers are judged against much stricter environmental standards, and on those every river in England fails for chemicals.
Get alerts for your area
We'll email you when water quality data changes for CM21.