Chlorides in water: origin, effects, standards, and treatment

Chlorides are mineral ions naturally present in water. Their concentration can increase due to salt dissolution, saltwater intrusion, road de-icing salts, wastewater, or industrial discharge. They must not be confused with the chlorine used to disinfect water. At typical concentrations, they pose no recognized direct health risk, but they do influence taste, salinity, corrosion, and the durability of infrastructure.

Formula: Cl⁻
Common origin: Salts and saline intrusion
Main effect: Taste and corrosion
EU value: 250 mg/L

What are chlorides in water?

Chlorides are ions with the formula Cl⁻. They represent the stable ionic form of chlorine when associated with cations such as sodium, calcium, magnesium, or potassium.

They are naturally present in oceans, saline rocks, soil, and many groundwater sources. Their concentration can also increase due to saltwater intrusion, road de-icing salts, wastewater, agriculture, and certain industrial discharges.

Chlorides should not be confused with the chlorine used to disinfect water. Free chlorine is a reactive oxidant, while the chloride ion is a much more stable and non-disinfecting form.

Key takeaway: Chlorides are primarily a parameter for taste, salinity, and corrosion. In the European Union, the parametric value is set at 250 mg/L.

Chemical properties of chlorides

Characteristic	Information	Importance for drinking water
Formula	Cl⁻	Highly soluble monovalent ion.
Molar mass	35.45 g/mol	Used for conversions between chloride and salts.
Charge	−1	Allows transfer via membranes and anionic resins.
Solubility	Very high for many salts	Mechanical filtration does not retain them.
Volatility	None in ionic form	Boiling does not remove them.
Reactivity	Low under ordinary conditions	They persist in water and track salinity changes.

What is the difference between chlorides, chlorine, and chloramines?

Substance	Form	Role in water
Chloride	Cl⁻	Stable mineral ion, no disinfecting effect.
Free chlorine	HOCl / OCl⁻	Oxidizing disinfectant used in distribution systems.
Monochloramine	NH₂Cl	More stable disinfectant formed with ammonia.
Chlorite	ClO₂⁻	By-product related to chlorine dioxide.
Chlorate	ClO₃⁻	By-product or distinct contaminant.

A chloride analysis therefore measures neither residual chlorine, nor chloramines, nor disinfection by-products.

What are the natural sources of chlorides?

Seawater and sea spray

Oceans are the primary natural reservoir of chlorides. Sea spray and marine aerosols deposit salt on soil and watersheds.

Dissolution of saline rocks

Halite deposits and other evaporites can significantly enrich groundwater.

Saline intrusion

In coastal aquifers, excessive pumping can cause saltwater to move into freshwater sources.

Deep and ancient water

Some confined aquifers contain naturally mineralized water or relics of ancient marine water.

Volcanism and thermal springs

Geothermal fluids can contain high concentrations of chlorides.

What human activities increase chlorides?

De-icing salts

Sodium chloride and calcium chloride used on roads can contaminate surface water and groundwater.

Domestic wastewater

Domestic discharge contains chlorides from food, household products, water softeners, and resin regeneration.

Agriculture

Certain fertilizers, irrigation, and livestock effluent can increase salinity and chloride levels.

Industry

Chemical, mining, oil, textile, paper, and food industries can produce chloride-rich effluent.

Water softeners and brine

Sodium-based water softeners use sodium chloride brine to regenerate their resins. The discharge of this brine contributes to the chloride load in wastewater.

How to identify saline intrusion?

Saline intrusion often leads to a simultaneous rise in chlorides, sodium, and conductivity. The ratio between different ions can help distinguish seawater from other salinity sources.

Indicator	Typical trend	Interpretation
Chlorides	Increase	Classic tracer of salinization.
Sodium	Increase	Often associated with sodium chloride.
Conductivity	Increase	Overall rise in dissolved ions.
Bromides	Possible increase	Can help identify marine influence.
Hardness	Variable	Depends on geochemical exchanges.

At what concentration do chlorides affect taste?

The taste threshold depends on the associated cation. Sodium chloride gives a salty taste, whereas calcium or magnesium chloride may be perceived differently.

The WHO does not establish a health-based guideline value for chlorides but indicates that concentrations above approximately 250 mg/L may be detectable by taste. The European value of 250 mg/L is therefore based primarily on acceptability and the protection of facilities.

Some individuals perceive the taste before this threshold, while highly mineralized water can mask or alter the perception.

What are the health effects of chlorides?

Chloride is an essential electrolyte. It participates in water balance, acid-base balance, and the formation of gastric hydrochloric acid.

At the concentrations normally encountered in drinking water, chlorides do not pose a recognized direct health risk for the general population. The WHO does not establish a health-based guideline value.

High salt intake

When chlorides are associated with sodium, they contribute to total sodium chloride intake. The cardiovascular risk mainly concerns excess dietary sodium.

Digestive effects

Highly saline water containing certain chloride salts, particularly magnesium, can cause an unpleasant taste and sometimes laxative effects.

People on medical diets

Water rich in sodium chloride may be a factor for people on strict sodium-restricted diets, but the sodium concentration must be measured directly.

Why do chlorides promote corrosion?

Chlorides increase water conductivity and can penetrate protective layers on metal surfaces. They notably promote pitting corrosion in certain stainless steels and alloys.

Stainless steel

At high concentrations, especially with high temperatures and stagnant water, chlorides can promote localized pitting.

Copper

They can contribute to corrosion mechanisms depending on pH, alkalinity, sulfates, temperature, and stagnation time.

Lead and other metals

More corrosive water can indirectly increase the release of metals present in pipes and fittings.

Chloride concentration alone is not sufficient to predict corrosion. pH, alkalinity, sulfates, conductivity, temperature, and materials must also be considered.

What role does the chloride/sulfate ratio play?

In distribution systems, the mass ratio between chlorides and sulfates can influence certain forms of galvanic corrosion and lead release. It is sometimes used as a complementary indicator.

This ratio does not in itself constitute a health standard. Its interpretation requires knowledge of materials, anti-corrosion treatment, and the overall water chemistry.

Effects on concrete and infrastructure

Chlorides can penetrate concrete and reach metal reinforcements. They then destabilize the protective layer of the steel and promote corrosion.

In water networks, reservoirs, and hydraulic structures, high salinity can therefore reduce the durability of materials if they are not specifically adapted.

Why are chlorides important for irrigation?

Chloride is a micronutrient for plants, but an excess can become toxic to sensitive crops. It can accumulate in leaves and cause burning, edge drying, and yield reduction.

The risk depends on concentration, crop type, climate, drainage, and irrigation method. Overhead irrigation on foliage can increase sensitivity.

What are the reference values for chlorides?

Organization or regulation	Value	Type of value
World Health Organization	No health-based guideline value	Taste may become perceptible above approximately 250 mg/L.
European Union – Directive (EU) 2020/2184	250 mg/L	Indicator parameter value.
France	250 mg/L	Drinking water quality reference.
Canada	≤ 250 mg/L	Aesthetic objective.
United States – EPA	250 mg/L	Secondary Maximum Contaminant Level, not based on primary health risk.

The 250 mg/L value is primarily related to taste, salinity, and technical effects. An exceedance should nevertheless be investigated, as it may indicate saline intrusion or contamination.

How to analyze chlorides in water?

Ion chromatography

It separates and quantifies chlorides along with other anions such as nitrates, sulfates, fluorides, and bromides.

Argentometric titration

Chloride is titrated with a silver nitrate solution. Mohr or potentiometric methods are widely used.

Selective electrode

A chloride-sensitive electrode can be used within certain concentration ranges and matrices.

Conductivity

It allows for the monitoring of overall salinity, but it is not specific to chlorides.

How to interpret a chloride result?

Result	General interpretation	Possible action
Less than 50 mg/L	Low to moderate content	Common situation in many fresh waters.
50 to 150 mg/L	Significant mineralization	Interpret with sodium and conductivity.
150 to 250 mg/L	High content	Monitor taste, corrosion, and trends.
Greater than 250 mg/L	Exceeds European indicative value	Investigate saline intrusion, road salts, or discharges.
Seasonal increase	De-icing salts or possible hydrological variation	Compare several analysis campaigns.
Increase with sodium and bromide	Possible marine influence	Hydrogeological study of the catchment.

What technologies reduce chlorides?

Reverse osmosis

A suitable reverse osmosis membrane can significantly reduce dissolved chlorides. Performance depends on pressure, temperature, recovery rate, and membrane condition.

Electrodialysis

Ion-exchange membranes and an electric field allow for the transfer of chlorides into a concentrated compartment.

Distillation

Chlorides are not volatile. Properly designed distillation retains them in the concentrated residue.

Deionization

Anionic resins combined with cationic resins can remove chlorides and associated ions.

Nanofiltration

Nanofiltration generally retains monovalent ions like chloride less effectively than reverse osmosis. Performance must be verified for each membrane.

Activated carbon

Standard activated carbon is not a reliable technology for removing dissolved chlorides.

Mechanical or ceramic filtration

Chloride ions pass through ordinary sediment filters and microporous barriers.

Conventional softening

A sodium softener does not remove chlorides. It can even contribute indirectly to saline discharges during regeneration.

Boiling

Boiling does not remove chlorides. Evaporation increases their concentration in the remaining water.

Comparison of treatment technologies

Technology	Reduction potential	Main limitations
Reverse osmosis	High	Pressure, maintenance, water discharge, and membrane control.
Electrodialysis	High	Cost and management of concentrate.
Distillation	High	Energy and low flow rate.
Deionization	High	Saturation, regeneration, and microbiological quality.
Nanofiltration	Variable	Limited rejection of monovalent ions possible.
Activated carbon	Ineffective	Chloride is a highly soluble mineral ion.
Mechanical filtration	Ineffective	Retains only particles.
Sodium softener	Ineffective	Does not demineralize water.
Boiling	Ineffective	Concentrates dissolved ions.

How to evaluate a filter against chlorides?

Performance against chlorides must be demonstrated by a specific test. Reduction of free chlorine does not prove any reduction of chlorides.

Element to check	Importance
Chlorides explicitly analyzed	Avoids confusion with chlorine.
Input concentration	Allows judging the severity of the test.
Output concentration	Shows the actual reduction obtained.
Total volume treated	Verifies performance stability.
Pressure and temperature	Influence membrane processes.
Recovery rate	Modifies salt discharge.
Conductivity	Overall indicator, but non-specific.
End-of-life test	Initial performance alone is insufficient.

The performance of a filtration system must be evaluated based on specific tests conducted by its manufacturer. In the absence of published results regarding this contaminant, no numerical reduction can be claimed.

View Monderma certifications and analyses

How to prevent excessive chloride concentration?

monitor coastal intakes exposed to saline intrusion;
avoid over-pumping of aquifers;
reduce and optimize the use of road salts;
control industrial discharges and brines;
maintain sanitation systems;
monitor chlorides, sodium, and conductivity simultaneously;
protect wells against contaminated runoff;
track trends over several years.

What to do in case of high chlorides in a well?

Confirm the result and then look for the source. Additional analysis may include sodium, conductivity, bromides, sulfates, nitrates, calcium, and magnesium.

check proximity to the sea or an estuary;
look into the local use of de-icing salts;
inspect salt storage and brine discharges;
evaluate seasonal and historical trends;
check the pumping flow rate of the catchment;
have a hydrogeological study performed in case of persistent increases.

Frequently asked questions about chlorides in water

What is the chloride value in water in France?

The quality reference is 250 mg/L.

Does the WHO set a health limit?

No. The WHO does not set a health-based guideline value for chlorides.

Why is the 250 mg/L value used?

It mainly corresponds to taste, salinity, and technical effects.

Are chlorides chlorine?

No. Chlorides are stable mineral ions, whereas free chlorine is a disinfectant.

Do chlorides disinfect water?

No. The chloride ion has no disinfectant effect.

Are chlorides dangerous?

At usual concentrations, they do not present any recognized direct risk to the general population.

Why do they cause a salty taste?

The taste depends on the salt formed, particularly sodium chloride.

Do chlorides promote corrosion?

Yes, they can increase conductivity and promote certain types of localized corrosion.

What parameters should be analyzed with chlorides?

Sodium, conductivity, bromides, sulfates, calcium, and magnesium.

Does an increase in chlorides indicate marine intrusion?

This is a possibility, especially if sodium and conductivity also increase.

Do road salts contaminate aquifers?

Yes. Sodium chloride used for de-icing can reach groundwater.

Does activated carbon remove chlorides?

No. Standard activated carbon is not effective against dissolved chlorides.

Does a ceramic filter remove chlorides?

No. Dissolved ions pass through a simple microporous barrier.

Does reverse osmosis remove chlorides?

A suitable and maintained membrane can significantly reduce them.

Is nanofiltration sufficient?

Its performance on monovalent ions is variable and must be verified.

Does boiling water remove chlorides?

No. Evaporation concentrates them in the remaining water.

Does distillation remove chlorides?

Yes, with a properly designed device.

Does a softener remove chlorides?

No. Conventional softening does not demineralize water.

Are chlorides problematic for irrigation?

Yes, certain crops are sensitive to their accumulation.

What to do in case of an exceedance?

Confirm the analysis, search for the source, and choose an appropriate demineralization treatment if necessary.

Scientific and administrative sources

World Health Organization – Chemical fact sheet: Chloride
Consult the WHO chemical fact sheet
World Health Organization – Chloride in drinking-water
Consult WHO resources
European Union – Directive (EU) 2020/2184
Consult the directive on EUR-Lex
Health Canada – Chloride in drinking water
Consult the technical document
Environmental Protection Agency – Secondary Drinking Water Standards
Consult EPA secondary standards
Monderma – Certifications and laboratory analyses
Consult Monderma certifications and analyses

Contaminants and associated parameters

Sodium Sulfates Nitrates Ammonium Copper Lead Iron Manganese

Health warning: this sheet is provided for informational purposes only. It does not replace an analysis performed by a laboratory, the recommendations of the competent health authority, or a corrosion or hydrogeological study. A persistent increase in chlorides must be investigated.