Trihalomethanes (THMs) are formed in drinking-water primarily as a result of chlorination of organic matter present naturally in raw water supplies. The rate and degree of THM formation increase as a function of the chlorine and humic acid concentration, temperature, pH and bromide ion concentration.
THM compounds are undesirable in drinking water for two reasons. Firstly, the actual compounds themselves may pose a hazard to the health of the consumer if present in excessive amounts, as chloroform is a suspected carcinogen. Secondly, the presence of the THM group may be an indicator of the possible presence of other organic by-products of chlorination in trace amounts.
THMs are easily absorbed by the body when the water comes in contact with the skin, they can be ingested, if present in drinking water. THMs can also be inhaled, if present in in the air whilst showering or bathing.
According to the Groundwater Association, studies have suggested a small increase in the risk of bladder and colorectal cancers. Other investigations have found that chlorination by-products may be linked to heart, lung, kidney, liver, and central nervous system damage. Officially, THMs are classified as ‘possibly carcinogenic’ to humans.
There is a straightforward relationship between the degree of colour in the water prior to chlorination and the quantities of THMs present following chlorination. If colour is present at the point of chlorination, THMs are likely to be formed.
Agriculture, forestry, and peat extraction are all contributory factors
Sites on the (current) EPA Drinking Water Remedial Action List for THM non compliance
County, Name of Water Supply
Clare, West Clare RWS (New WTP)
Dun Laoghaire, Roundwood
Kerry, Caragh Lake
Kerry, Kilgarvan 046A
Kilkenny, Kilkenny City (Radestown) WS
Longford, Longford Central
Meath, Navan – Mid Meath Kilcarn PWS
Offaly, Clara/Ferbane RWSS
Tipperary, Nenagh Regional
Wicklow, Aughrim / Annacurra
Wicklow, Enniskerry Public Supply
Wicklow, Wicklow Regional Public Supply
Wicklow, Greystones/Windgates/ Templecarraig
Wicklow, Newtown Newcastle Kilcoole
FIE Complaint to EU
Infringement was first brought to the attention of the European Commission in 2011, when Friends of the Irish Environment made a complaint out of concern about reported levels of contamination by the Environmental Protection Agency
HSE and EPA Joint Position Statement on Trihalomethanes in Drinking Water
At the end of July (2018) there were 47 public supplies affected by THM contamination, with a population of 307,000 affected
EPA Prosecutes Irish Water, Drimoleague Public Water Supply, Co Cork
On 8th January 2020 the Environmental Protection Agency prosecuted Irish Water (Drimoleague Public Water Supply, Co Cork) at Dublin Metropolitan District Court. Irish Water pleaded guilty to:
- Failing to comply with a Direction given by the Agency, dated 5th June 2015. The direction required that Irish Water submit a final report to the Agency by or before 31st December 2018 containing monitoring results verifying that the trihalomethanes parametric value as specified in the European Union (Drinking) Water Regulations 2014 (as amended) had been complied with.
On hearing details of the offence Judge Halpin convicted Irish Water and imposed a fine of €1,000
Drinking water: Commission refers Ireland to the Court of Justice of the European Union over unsafe drinking water
The Commission has decided to refer Ireland to the Court of Justice of the European Union for failure to comply with the requirements of the Drinking Water Directive (Directive 98/83/EC).
The Directive requires Member States to ensure that water intended for human consumption is wholesome and clean. It requires that drinking water is free from micro-organisms and parasites, and from substances which could pose a potential danger to human health.
The European Green Deal sets for the EU a Zero Pollution ambition. Full implementation of the standards enshrined in EU legislation is important to effectively protect human health and safeguard the natural environment.
In Ireland, the level of the chemical substance trihalomethanes (THMs) in drinking water has long exceeded the parametric value established in the Drinking Water Directive in a number of water supply zones across the whole country.
Drinking water provided in 30 water supply zones in Ireland, serving a population of more than 200 000 citizens, continues to exceed the safe levels of THMs.
These chemicals are formed in drinking water due to the disinfection process. Exceeding the parametric value of trihalomethanes can entail potential risks to human health.
The Commission sent a letter of formal notice to Ireland in 2018, followed by a reasoned opinion in May 2020. The reasoned opinion concerned 44 water supply zones across the whole country. Since the reasoned opinion, 30 water supply zones remain in breach of the THM value.
Whilst the Commission welcomes the fact that Ireland has made progress in addressing elevated levels of THMs in the drinking water, today, more than three years after the opening of the infringement case, a number of water supply zones still do not comply with the requirements of the Drinking Water Directive. The Commission is therefore referring Ireland to the Court of Justice of the European Union.