Drinking water parameters

Drinking water parameters are tested for in drinking water samples to ensure that the drinking water quality is of an acceptable standard, as set out in the European Union Drinking Water Regulations 2014 legislation. We carry out regular testing and monitoring of all public water supplies in Ireland in accordance with this legislation which sets out the drinking water parameters that are to be tested, how often they are to be tested for and the acceptable limits for each parameter.

Drinking water parameters are grouped into 3 categories:

1. Indicator parameters
Indicator parameters are usually tested along with microbiological and chemical parameters in a water sample. These parameters may not necessarily pose a risk on their own, but they can indicate the presence of other potentially more harmful parameters in a water sample.

2. Chemical parameters
Chemical parameters are tested in drinking water samples to determine if any chemicals present are within acceptable limits. Chemical parameters that are present in drinking water samples can be caused by chemicals dissolving into water from pipes, chemicals carrying over from water treatment processes, chemical reactions that can occur between different materials in the water, or even runoff from farms.

3. Microbiological parameters
Microbiological parameters are microorganisms (i.e. bacteria) that are tested in drinking water. The presence of these parameters in a drinking water sample may indicate that contamination may have occurred either at a water source or within the water distribution network.

Drinking water exceedances

A drinking water exceedance is a result from a drinking water sample which is above the acceptable limit, as set out in the European Union Drinking Water Regulations 2014 legislation.

When a drinking water exceedance occurs, Irish Water carries out an assessment and where necessary, consults with the Health Service Executive (HSE) to seek their advice in the interest of protecting public health. Where the HSE have been consulted and it is deemed that a particular exceedance poses a risk to public health, the customers within the affected Water Supply Zone (or a smaller area within an affected WSZ) are notified, usually in the form of a Water Restriction Notice or Boil Water Notice.

Where required, Irish Water also notifies the Environmental Protection Agency (EPA) in relation to drinking water exceedances. All drinking water exceedances are fully investigated to determine the cause and to ensure that corrective actions are carried out.


You can click on any of the drinking water parameter names listed below to see an explanation and the typical causes of exceedances for each of the parameters.

What is aluminium?
Aluminium is a metallic element and is one of the most common metals found on earth. It is an indicator parameter for drinking water which means that it may indicate the presence of other parameters in a water sample.
Aluminium occurs naturally in small amounts in both surface and ground waters (e.g. rivers or natural wells).

What is an aluminium exceedance?
A drinking water exceedance is a result from a drinking water sample which is above the acceptable limit.
The limit for aluminium in drinking water is 200µg/l, therefore any result that is greater than this is seen as an aluminium exceedance.

What can cause an aluminium exceedance?
Aluminium can be used in surface water treatment plants (usually in the form of aluminium sulphate) to remove impurities from raw water. Exceedances for aluminium can occur where small quantities of aluminium remain in the water following the treatment process.

How are exceedances for aluminium dealt with?
All exceedances for aluminium are notified to the HSE and Irish Water consults with them to seek their advice in relation to the protection of public health. Aluminium exceedances are also notified to the EPA where there may be a risk to public health, or where the exceedance indicates a persistent issue.
Aluminium exceedances are fully investigated to determine the cause and suitable corrective actions are taken.

What is clostridium perfringens?
Clostridium perfringens is a bacteria found in humans. It is an indicator parameter for drinking water which means that it may indicate the presence of other parameters in a water sample.

What is a clostridium perfringens exceedance?
A drinking water exceedance is a result from a drinking water sample which is above the acceptable limit.
The limit for clostridium perfringens is 0 per 100ml, therefore any result that is greater than this is seen as a clostridium perfringens exceedance.

What can cause a clostridium perfringens exceedance?
Exceedances for clostridium perfringens can occur following a contamination incident or from a failure in the treatment process to remove or inactivate it.

How are exceedances of clostridium perfringens dealt with?
All exceedances for clostridium perfringens are notified to the HSE and Irish Water consults with them to seek their advice in relation to the protection of public health. Clostridium perfringens exceedances are also notified to the EPA where there may be a risk to public health, or where the exceedance indicates a persistent issue. Clostridium perfringens exceedances are fully investigated to determine the cause and corrective actions are taken to ensure the water supply is adequately disinfected.

What is cryptosporidium?
Cryptosporidium is a microorganism that can cause illness (called cryptosporidiosis) in humans. Those particularly at risk from this illness are people with weak or compromised immune systems, the elderly and infants.These microorganisms can be found in the faecal matter of humans and animals and can survive in the environment for several months. Not all species or types of cryptosporidium are harmful to humans; the main species that can cause illness are Cryptosporidium parvum and Cryptosporidium hominis.

What is a cryptosporidium exceedance?
A drinking water exceedance is a result from a drinking water sample which is above the acceptable limit.
The cryptosporidium test typically involves pumping approx. 1,000 litres of drinking water through a special filter to check for and trap any cryptosporidium. Any detection of cryptosporidium in a drinking water sample is regarded as an exceedance.

What can cause a cryptosporidium exceedance?
Exceedances for cryptosporidium can occur following a contamination incident or a failure in the treatment process to remove or inactivate it.

How are cryptosporidium exceedances dealt with?
Irish Water reports all detections of cryptosporidium in drinking water to the HSE and the EPA. All information regarding a cryptosporidium exceedance is provided to the HSE, who are the experts in public health and a decision is made following a consultation if a Boil Water Notice is required. The HSE also provides regular updates to both Irish Water and Local Authorities on any reported incidences of cryptosporidiosis. All cryptosporidium exceedances are fully investigated to determine the cause, with suitable corrective actions (e.g. UV treatment) being identified and implemented.

For more information on the possible health implications of a cryptosporidium exceedance in drinking water, please visit www.hse.ie/water

What is E. coli?
The full name for E. coli is Escherichia coli. It is a type of bacteria found in humans and some animals. It is a microbiological parameter for drinking water. Certain strains of E. coli (e.g. O157:H7) can cause illness in humans.

What is an exceedance for E. coli?
A drinking water exceedance is a result from a drinking water sample which is above the acceptable limit.
Any detection of E. coli in a drinking water sample is seen as an exceedance.

What can cause an E. coli exceedance?
Exceedances for E. coli can be due to a contamination incident either at a water source or through the water network, for example at a burst mains or at an illegal connection etc. Exceedances of E. coli can also be due to a failure in the disinfection treatment process.

How are exceedances for E. coli dealt with?
Irish water reports all exceedances for E. coli to the HSE and EPA. All information regarding an E. coli exceedance is provided to the HSE, who are the experts in public health and a decision is made following consultation if a Boil Water Notice is required. All exceedances for E. coli are fully investigated to determine the cause and to ensure that suitable corrective actions are taken to ensure the water supply is adequately disinfected.

For more information on the possible health implications of an E coli exceedance in drinking water, please visit www.hse.ie/water

What is enterococci?
Enterococci is a type of bacteria found in humans and some animals. It is a microbiological parameter for drinking water. Certain strains of enterococci (e.g. Enterococcus faecalis) can cause illness in humans.

What is an exceedance for enterococci?
A drinking water exceedance is a result from a drinking water sample which is above the acceptable limit. Any detection of enterococci is in a drinking water sample is seen as an exceedance.

What can cause an enterococci exceedance?
Exceedances for enterococci can be due a contamination incident either at source or through the water network, for example at a burst mains or at an illegal connection etc. Exceedances of enterococci can also be due to a failure in the disinfection treatment process.

How are exceedances for enterococci dealt with?
Irish water reports all exceedances for enterococci to the HSE and EPA. All information regarding an enterococci exceedance is provided to the HSE, who are the experts in public health and a decision is made following consultation if a Boil Water Notice is required. All exceedances for enterococci are fully investigated to determine the cause and to ensure that suitable corrective actions are taken to ensure the water supply is adequately disinfected.

For more information on the possible health implications of an enterococci exceedance in drinking water, please visit www.hse.ie/water

What is iron?
Iron is a metallic element and is naturally present in small amounts in most surface and ground waters (e.g. rivers or natural wells). It is an indicator parameter for drinking water which means that it may indicate the presence of other parameters in a water sample.

What is an iron exceedance?
A drinking water exceedance is a result from a drinking water sample which is above the acceptable limit.
The limit for iron in drinking water is 200µg/l, therefore any result that is greater than this is seen as an iron exceedance.

What causes an iron exceedance?
The main cause of iron exceedances in drinking water is due to disruption of iron sediment in old cast iron mains. Iron exceedances can also occur when small amounts of iron-based coagulants (e.g. Ferric Chloride or Ferric Sulphate) that are used in some water treatment processes, are carried over from the treatment stages and can be found in drinking water.

How are iron exceedances dealt with?
All exceedances for iron are notified to the HSE and Irish Water consults with them to seek their advice in relation to the protection of public health. Iron exceedances are also notified to the EPA where there may be a risk to public health, or where the exceedance indicates a persistent issue. Iron exceedances are fully investigated to determine the cause, with suitable corrective actions being identified and implemented. Irish Water is working to reduce iron exceedances through regular mains flushing and control of coagulant dosing processes (where iron-based coagulants are used in water treatment processes).

What is lead?
Lead is a metallic element and is present in trace quantities in most surface and ground waters (e.g. rivers or natural wells). It is a chemical parameter for drinking water.

What is a lead exceedance?
A drinking water exceedance is a result from a drinking water sample which is above the acceptable limit.
The limit for lead in drinking water is 10µg/l, therefore any result greater than this is seen as a lead exceedance.

What causes a lead exceedance?
The main cause of lead in drinking water is due to lead ‘leaching’ from lead pipes and plumbing fittings into the water.

How are lead exceedances dealt with?
Irish Water reports all detections of lead in drinking water to the HSE and EPA. Exceedances for lead are usually site specific, i.e. a lead exceedance detected at one location within a water supply does not mean that the entire water supply will be affected. All information regarding a lead exceedance is provided to the HSE, who are the experts in public health and a decision is made following a consultation if a Water Restriction is required. For more information on the possible health implications of a lead exceedance in drinking water, please visit www.hse.ie/water

Further information on lead in drinking water is available here.

What is nitrate?
Nitrate comes from sources in the environment such as waste discharges, animal slurries and artificial fertilisers. It is a chemical parameter for drinking water.

What is a nitrate exceedance?
A drinking water exceedance is a result from a drinking water sample which is above the acceptable limit.
The limit for nitrate in drinking water is 50mg/l, therefore any result greater than this is seen as a nitrate exceedance.

What causes a nitrate exceedance?
Nitrate concentrations can increase in surface water and ground water and cause exceedances in drinking water as a result of agricultural activities in an area, e.g. fertiliser use on fields or from sewage contamination.

How are nitrate exceedances dealt with?
All exceedances for nitrate are notified to the HSE and Irish Water consults with them to seek their advice in relation to the protection of public health. Nitrate exceedances are also notified to the EPA where there may be a risk to public health, or where the exceedance indicates a persistent issue. Nitrate exceedances are fully investigated to determine the cause and suitable corrective actions (e.g. putting an exclusion zone around a water source, re-drilling a water source at a different location or installing a nitrate removal system) are identified and implemented where required.

For more information on the possible health implications of a nitrate exceedance in drinking water, please visit www.hse.ie/water

What are total coliforms?
Total coliforms are bacteria that exist naturally in the environment. They are an indicator parameter for drinking water which means that they may indicate the presence of other parameters in a water sample.

What is a total coliforms exceedance?
A drinking water exceedance is a result from a drinking water sample which is above the acceptable limit. Any detection of total coliforms in a drinking water sample is seen as an exceedance.

What causes a total coliforms exceedance?
Exceedances for total coliforms can occur following a localised contamination incident or where water is not adequately chlorinated in the disinfection treatment process.

How are total coliforms exceedances dealt with?
All exceedances for total coliforms are assessed by Irish Water and notified to the HSE where necessary to seek their advice in relation to the protection of public health. Total coliform exceedances are also notified to the EPA where there is a risk to public health, or where the exceedance indicates a persistent issue. All total coliform exceedances are investigated to determine the cause, with corrective actions (e.g. checking of chlorination levels and flushing of mains) being identified and implemented where required.

What are trihalomethanes?
Trihalomethanes or THMs are a side-effect or by-product of the water disinfection process. THMs are a chemical parameter for drinking water and are typically formed by the reaction of chlorine (used to disinfect the water) with natural organic matter e.g. algae, twigs or leaves etc. that may be present in the water.

What is a THM exceedance?
A drinking water exceedance is a result from a drinking water sample which is above the acceptable limit.
The limit for THMs in drinking water is 100µg/l, therefore any result greater than this is seen as a THM exceedance.

What causes a THM exceedance?
Exceedances for THMs can occur in large networks, where the chlorine in the water has a longer period of time to react with any organic material that may be present. This process can occur faster during warmer weather and/or where higher chlorine levels are present in the water.

How are THM exceedances dealt with?
Irish Water reports all exceedances of THMs to the HSE and EPA. All information regarding a THM exceedance is provided to the HSE and their advice is requested in relation to protecting public health. All exceedances for THMs are fully investigated to determine the cause and to ensure that suitable corrective actions are taken.

Irish Water is currently in the process of developing a national THM Plan.

With the THM Plan, Irish Water is committed to improving water quality through investment in water services infrastructure and targeting Water Supply Zones with THM exceedances. Controlling the concentration of chlorine in the water is one of the key measures in controlling THM formation. Irish Water is upgrading dosing controls to optimise chlorine concentrations and therefore reduce the risk of THM formation across the 856 water treatment plants throughout the country. Additional measures being taken by Irish Water to achieve THM compliance include process optimisation to increase the efficiency of organic material removal, the reorganisation of networks and also regular flushing of storage reservoirs and pipelines to reduce the formation of THMs in the system.

For more information on the possible health implications of a THM exceedance in drinking water, please visit www.hse.ie/water

What is turbidity?
Turbidity is a measure of the "cloudiness" of the water. The cloudiness of water is typically caused by tiny particles that are suspended in the water. Turbidity is measured in Nephelometric Turbidity Units (NTU) and water below 4NTU will be visibly clear to the human eye. Turbidity is an indicator parameter for drinking water.

What is a turbidity exceedance?
The limit for turbidity in drinking water leaving a surface water treatment plant is 1NTU (Nephelometric Turbidity Units). However, for drinking water samples tested at the point of compliance (which is typically the kitchen tap), the limit is aesthetic. A general guideline limit of 4NTU for aesthetic purposes is recommended.

What causes a turbidity exceedance?
Exceedances for turbidity can be due to disruption of sediment in cast iron mains or can also indicate that the water treatment process may not be operating effectively.

How are turbidity exceedances dealt with?
All exceedances for turbidity are notified to the HSE and Irish Water consults with them to seek their advice in relation to the protection of public health. Turbidity exceedances are also notified to the EPA where there may be a risk to public health, or where the exceedance indicates a persistent issue. Turbidity exceedances are fully investigated to determine the cause and suitable corrective actions (e.g. regular flushing of mains) are taken.

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