Tap water in SO21, WinchesterSafe to drink.Very hard.
Supplied by Southern Water. 34,396 samples, 25 substances, latest on 30 April 2026. Everything is inside the legal limit. One result, nitrate, sits close to the line.
9.3 / 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 276 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
Removes nitrate, the result that stood out in SO21.
Osmio PRO-III Ultimate Whole House
£499typical price
Removes nitrate, the result that stood out in SO21.
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What's in a glass of SO21 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 81% of it. 19 of 20 substances used under a quarter of theirs.
- Nitrate81%
- Colour15%
- Trihalomethanes10%
- Ammonia8%
- Fluoride7%
- Aluminium6%
- Iron5%
- Mercury4%
- Manganese4%
- Nickel3%
- Turbidity3%
- Lead3%
- Arsenic2%
- Nitrite2%
- Bromate1%
- Selenium1%
- Antimony<1%
- Cadmium<1%
- Copper<1%
- Chromium<1%
- Coliform Bacteria none found
- E. coli none found
- Conductivity 667 µS/cm
- pH 7.3 pH
- PFAS (total) 0.00072 µg/L
All 25 results as a table
| Substance | Result | Legal limit | Share of limit |
|---|---|---|---|
| Nitrate | 40.4 mg/L | 50 mg/L | 81% |
| Turbidity | 0.13 NTU | 4 NTU | 3% |
| Coliform Bacteria | 0 count/100ml | Must be zero | None found |
| E. coli | 0 count/100ml | Must be zero | None found |
| Copper | 0.0084 mg/L | 2 mg/L | <1% |
| Lead | 0.00027 mg/L | 0.01 mg/L | 3% |
| Conductivity | 667 µS/cm | No legal limit | – |
| Iron | 0.00928 mg/L | 0.2 mg/L | 5% |
| Colour | 3 mg/L Pt/Co | 20 mg/L Pt/Co | 15% |
| pH | 7.3 pH | No legal limit | – |
| Ammonia | 0.039 mg/L | 0.5 mg/L | 8% |
| Aluminium | 0.0113 mg/L | 0.2 mg/L | 6% |
| Manganese | 0.00178 mg/L | 0.05 mg/L | 4% |
| Trihalomethanes | 0.0104 mg/L | 0.1 mg/L | 10% |
| Bromate | 0.00013 mg/L | 0.01 mg/L | 1% |
| Antimony | 0.000044 mg/L | 0.005 mg/L | <1% |
| Selenium | 0.00043 mg/L | 0.04 mg/L | 1% |
| Nitrite | 0.009 mg/L | 0.5 mg/L | 2% |
| Arsenic | 0.00024 mg/L | 0.01 mg/L | 2% |
| Cadmium | 0.000031 mg/L | 0.005 mg/L | <1% |
| Chromium | 0.00019 mg/L | 0.05 mg/L | <1% |
| Nickel | 0.00067 mg/L | 0.02 mg/L | 3% |
| Mercury | 0.00004 mg/L | 0.001 mg/L | 4% |
| Fluoride | 0.1 mg/L | 1.5 mg/L | 7% |
| PFAS (total) | 0.00072 µg/L | No legal limit | – |
276mg of limescale minerals in every litre, typical for the SO area. SO21 has no reading of its own.
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 SO21
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Your river is the Nun's Walk Stream, rated moderate
The Environment Agency rated the Nun's Walk Stream 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. 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. Its ecological rating has dropped since the previous assessment.
None of this is a test of your tap water. Environment Agency record · River health across England
18
forever-chemical detections in nearby rivers and groundwater
At 1 site. The highest was PFBA at 0.0018 µg/L, at RSN0121 Monks Brook Downstream of A3090, 9.5 km away, in April 2023. These are river and borehole samples, not tests of your tap water.
Every PFAS sample around Southampton · Which filters remove PFAS · PFAS explained
SO21 in short
Tap water in SO21 (Winchester) is supplied by Southern Water in the Winchester Central zone and scores 9.3 out of 10, against an England and Wales average of 9.0. 1 of 25 tested substances was close to or over a limit. The latest sample is from 30 April 2026.
- Nitrate is within its legal limit but flagged, at 81% of the 50 mg/L maximumAnything over three quarters of a limit is flagged here as worth watching. It has not gone over. See every reading
- The cleanest tap water of the 7 areas tested in WinchesterSO21 scores 9.3 out of 10, ahead of SO22 in second place on 9.3.
- Very hard water at 276 mg/LThe threshold for very hard is 250 mg/L. Expect visible limescale on taps and in kettles across the SO postcode area. How hardness is measured
Questions about SO21 water
Is SO21 tap water safe to drink?
Based on the latest tests, SO21 (Winchester) water scores 9.3/10, which is safe. 1 of 25 tested contaminants exceeded recommended levels. Water is supplied by Southern Water.
What contaminants are in SO21 water?
We tested 25 contaminants in SO21 water. 1 were flagged, including Nitrate. PFAS (forever chemicals) were also detected at 0.00072 µg/L in local drinking water tests.
Who supplies water in SO21?
SO21 (Winchester) is supplied by Southern Water. You can view their full profile and compliance data on our supplier page.
Is SO21 water hard or soft?
No hardness reading has been published for SO21 itself. Across the SO postcode area the typical value is 276 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 SO21?
With 1 contaminant above recommended levels in SO21, a water filter could help. A reverse osmosis or activated carbon filter is recommended for PFAS removal. See our filter recommendations for your area.
Are there PFAS forever chemicals in SO21 water?
Yes, PFAS (per- and polyfluoroalkyl substances) have been detected in SO21 at 0.00072 µg/L from drinking monitoring. The UK currently has no legal limit for PFAS in drinking water. Reverse osmosis and activated carbon filters can reduce PFAS levels.
How clean is the river near SO21?
The Environment Agency rated the Nun's Walk Stream 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. 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. Its ecological rating has dropped since the previous assessment. This is a rating of the river, not of tap water in SO21, which is treated before it reaches homes.
Keep reading
- Best Filter for PFAS RemovalThe filters with independent certification to remove forever chemicals
- PFAS Forever ChemicalsWhat PFAS are and how to reduce your exposure at home
- 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?
- Best Water Filters UKEvery filter type compared on what it actually removes
- 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
PFAS explainedNitrate levelsHow to test your waterBest water filtersFilter shower headsWater testing kitsUK water rankingsWater hardness checkerWater hardness map
River and groundwater readings near SO21
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 SO21.