Water is never just water. As it falls as rain, runs over land and soaks through soil and rock, it picks up minerals, gases, metals and sometimes bacteria and pollutants. Some of these are harmless or even beneficial, some are a nuisance, and a few are a genuine health risk. This guide explains the most common contaminants in UK water, how to recognise them and how to treat each one.

Where your water comes from changes what is in it

Mains water is treated and disinfected by your water company and must meet the standards in the Water Supply (Water Quality) Regulations 2016. Problems at the tap usually come from your own pipework, such as lead pipes, or from things the water company adds or cannot remove, such as chlorine and hardness.

Private supplies from boreholes, wells and springs have no treatment unless you fit it. They are where bacteria, nitrate, iron, manganese, acidity and hydrogen sulphide are most often found. Read more about private water supply testing.

For a guide to what to expect from each type of supply, from boreholes and springs to rainwater and caravan tanks, see water sources explained.

Start with what you notice

Brown water or orange stains

Usually iron. Black specks point to manganese.

Rotten egg smell

Hydrogen sulphide, from the supply or your hot water cylinder.

Blue-green stains or pinhole leaks

Acidic water dissolving copper pipes.

Cloudy, milky or gritty water

Often harmless air, sometimes sediment or limescale.

Swimming pool taste

Chlorine added to mains water for disinfection.

Limescale on kettles and taps

Hard water: calcium and magnesium.

Many of the most important contaminants, including bacteria, nitrate, lead and arsenic, give no sign at all. The only way to know is a laboratory water test.

Common water contaminants at a glance

Contaminant Main source Health risk or nuisance? How to treat it
Bacteria and E. coli Septic tanks, livestock, surface water getting into private supplies Health risk Sediment filter then UV disinfection
Nitrate Fertiliser, manure and slurry Health risk, especially for babies Nitrate reduction or reverse osmosis
Lead Old lead pipes and solder Health risk Replace pipes; reverse osmosis meanwhile
Arsenic and other heavy metals Natural geology, old plumbing, industry Health risk Arsenic reduction, reverse osmosis
Iron and manganese Natural minerals in groundwater; old mains Mainly nuisance; manganese a concern at high levels Iron and manganese filters
Acidic water (low pH) Upland, peaty and low-limestone areas Indirect: dissolves copper and lead pH correction
Hydrogen sulphide Low-oxygen groundwater; hot water cylinders Mainly nuisance; corrosive H2S treatment or carbon
Sediment and turbidity Silt and clay, especially after rain Nuisance; shields bacteria from UV Sediment filtration
Chlorine and chloramine Added by water companies Taste and odour; safe at levels used Carbon filtration
Hardness (limescale) Calcium and magnesium from chalk and limestone Nuisance; damages appliances Water softener or scale conditioner
Fluoride Natural, or added in some areas Limit 1.5 mg/l Fluoride reduction
PFAS (forever chemicals) Industry, firefighting foam Emerging health concern Reverse osmosis or an ion exchange PFAS filter

How contaminants are grouped

Microbiological

Bacteria, viruses and parasites. The most important for health, and the reason private supplies need disinfection. Labs test for indicator organisms such as E. coli and coliforms.

Chemical and metals

Dissolved substances such as nitrate, lead, arsenic and other heavy metals, fluoride and PFAS. Most are invisible and only found by testing.

Aesthetic

Things that affect how water looks, smells and tastes, or what it does to your home: iron and manganese, hydrogen sulphide, turbidity, chlorine and hardness. Rarely harmful, but often the first thing people notice.

Corrosive

Acidic water is not usually a problem itself but releases metals from pipes, linking it to lead and copper.

Treatment works in the right order

When a supply has several problems, the order of treatment matters. A typical private supply system runs: a coarse sediment pre-filter → pH correction → iron and manganese removal → softening or nitrate reduction → a fine sediment filter → UV disinfection last. Our guide on how to purify well water explains why. On mains water, the usual answer is point-of-use treatment such as drinking water filters or reverse osmosis, or whole house filtration.

Not sure what is in your water?

A laboratory test is the only way to know for certain. Start with our drinking water testing guide, check what your numbers mean in water test results explained, or ask our team. Wrekin Water is a family run company based in Bettisfield, Shropshire, serving more than 50,000 customers over 15 years and rated Excellent on Trustpilot.

Frequently asked questions

On mains water, the most common issues are hardness, chlorine taste and lead from old pipes. On private supplies such as boreholes, wells and springs, bacteria, nitrate, iron, manganese, acidity and hydrogen sulphide are the most common.

Some contaminants show themselves through staining, smells or taste, but many of the most important, including bacteria, nitrate, lead and arsenic, give no sign at all. A laboratory water test is the only reliable way to know.

Mains water in the UK meets very high standards. Problems usually come from inside the property, such as lead pipes or poorly maintained tanks. Private supplies are not treated by a water company, so their owners need to test them.

No single filter removes everything. Reverse osmosis removes the widest range of dissolved contaminants for drinking water, but bacteria on a private supply need UV disinfection, and iron, acidity and hardness need their own treatment. Systems are built from test results.

A typical private supply runs a coarse sediment pre-filter, then pH correction, then iron and manganese removal, then softening or nitrate removal, then a fine sediment filter, with UV disinfection last so it receives clean, clear water.