Tap water in AL9, Welwyn HatfieldToo few tap tests to call it.Very hard.
Supplied by Thames Water. 10 substances, latest on 4 September 2026. These are river and groundwater samples near AL9, not tests of treated tap water, so treat the score as a rough guide.
8.6 / 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 279 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.
Waterdrop G3P600 Reverse Osmosis
£550typical price
Maximum contaminant removal including PFAS and fluoride
Waterdrop 10UA Under Sink
£70typical price
High-volume under-sink filtration without RO complexity
We earn a commission if you buy through these links, at no extra cost to you. It never changes which product we put first. How we make money
What's in a glass of AL9 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.
Iron came closest to a limit, at 100% of it. 4 of 5 substances used under a quarter of theirs.
- Iron100%
- Ammonia10%
- Nitrate1%
- Nitrite1%
- Chromium1%
- pH 7.85 pH
- Temperature 17.7 °C
- Conductivity 949 µS/cm
- Phosphate 0.28 mg/L
- Dissolved Oxygen 7.56 mg/L
All 10 results as a table
| Substance | Result | Legal limit | Share of limit |
|---|---|---|---|
| Iron | 0.2 mg/L | 0.2 mg/L | 100% |
| pH | 7.85 pH | No legal limit | – |
| Temperature | 17.7 °C | No legal limit | – |
| Conductivity | 949 µS/cm | No legal limit | – |
| Nitrate | 0.66 mg/L | 50 mg/L | 1% |
| Nitrite | 0.0066 mg/L | 0.5 mg/L | 1% |
| Phosphate | 0.28 mg/L | No legal limit | – |
| Dissolved Oxygen | 7.56 mg/L | No legal limit | – |
| Chromium | 0.0006 mg/L | 0.05 mg/L | 1% |
| Ammonia | 0.05 mg/L | 0.5 mg/L | 10% |
279mg 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 AL9
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Your river is the Colne (upper east arm including Mimshall Brook), rated bad
The Environment Agency rated the Colne (upper east arm including Mimshall Brook) as bad for ecological health in its 2025 assessment, meaning the river is severely damaged, with most of the expected wildlife missing. It is held back by fish, insects and other small river life and water plants and algae. 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
AL9 in short
Tap water in AL9 (Welwyn Hatfield) is supplied by Thames Water in the Welwyn Hatfield Central zone and scores 8.6 out of 10, against an England and Wales average of 9.0. 1 of 10 tested substances was close to or over a limit. The latest sample is from 4 September 2026.
- Iron is within its legal limit but flagged, at 100% of the 0.2 mg/L maximumAnything over three quarters of a limit is flagged here as worth watching. It has not gone over. See every reading
- Very hard water at 279 mg/LThe threshold for very hard is 250 mg/L. Expect visible limescale on taps and in kettles in AL9. How hardness is measured
- 3rd cleanest of 5 areas tested in Welwyn HatfieldSquarely mid-table for Welwyn Hatfield, between AL6 on 9 and AL8 on 8.3.
Questions about AL9 water
Is AL9 tap water safe to drink?
Based on the latest tests, AL9 (Welwyn Hatfield) water scores 8.6/10, which is safe. 1 of 10 tested contaminants exceeded recommended levels. Water is supplied by Thames Water.
What contaminants are in AL9 water?
We tested 10 contaminants in AL9 water. 1 were flagged, including Iron.
Who supplies water in AL9?
AL9 (Welwyn Hatfield) is supplied by Thames Water. You can view their full profile and compliance data on our supplier page.
Is AL9 water hard or soft?
Water in AL9 (Welwyn Hatfield) has a hardness of 279 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 AL9?
With 1 contaminant above recommended levels in AL9, a water filter could help. A filter jug or under-sink filter can reduce most common contaminants. See our filter recommendations for your area.
Are there PFAS forever chemicals in AL9 water?
No PFAS (forever chemicals) result is attached to the tap-water tests for AL9, 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 AL9?
The Environment Agency rated the Colne (upper east arm including Mimshall Brook) as bad for ecological health in its 2025 assessment, meaning the river is severely damaged, with most of the expected wildlife missing. It is held back by fish, insects and other small river life and water plants and algae. 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 AL9, which is treated before it reaches homes.
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