With all of the interest regarding PFAS over the past year, it is worth discussing other substances that are present in our drinking water that can potentially impact on human health. Some of these substances are detected in every sample tested and across hundreds of water supplies and are associated with water treatment. One such group of chemicals are called Trihalomethanes (THM's).
THM's are created when chlorine which is used as a disinfectant, reacts with organic molecules (mainly humic and fulvic acids) in the source water and create other chemicals, known as Disinfection Byproducts (DBP's). Hundreds of DBP's have been discovered since 1974. (Some of these DBP's can be viewed in another blog post here).
THM's are definitely in Melbourne's drinking water and are present almost whenever chlorine is used. THM's consist of 4 chemicals: Bromoform, Chloroform, Dibromochloromethane and Bromodichloromethane. To work out Total THM's all four chemicals are combined and are calculated as a whole.
According to the 2011 Australian Drinking Water Guideline's (ADWG's) "...The International Agency for Research on Cancer has concluded that chloroform and bromodichloromethane are possibly carcinogenic to humans (Group 2B, inadequate evidence in humans but sufficient evidence in animals); and that bromoform and dibromochloromethane are not classifiable as to their carcinogenicity to humans (Group 3, inadequate evidence in humans and limited evidence in animals)(IARC 1991)...)
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Average THM levels detected throughout Melbourne over 2023-2024. The highest average levels of 80μg/L were detected in two locations, Belgrave and Monbulk, in the Dandenongs. Generally speaking, THM's are detected at higher levels at the end of distribution networks. Blue Pins represent 20μg/L to 40μg/L, Pink pins represent detections of 41μg/L to 80μg/L.
The ADWG's provide a guideline level of THMs of 250μg/L (parts per billion). However, the National Health and Medical Research Council, who oversee the ADWG's have a committee who have been looking at potentially reducing these guidelines for a decade or more. The NHMRC and the committee has so far resisted calls to reduce THM guideline levels in Australia.
By contrast the United States have a drinking water guideline for THM's at 80μg/L or 32% of the total of Australian guideline. If THM's are detected at above 80μg/L, water customers in the United States have to be provided with their water bill, an explanation about what THM's are and why the guideline level has been exceeded. No such information is available to consumers in Australia and it would be fair to say that barely anyone in Australia has even heard of Trihalomethanes. Europe have a THM guideline of 100μg/L.
The Environmental Working Group in the United States goes even further than the US EPA and recommends a level of 0.15μg/L for Trihalomethanes, or 1,667 times lower than the Australian guideline!!! If the EWG group guideline was accepted every drinking water sample in Melbourne would be well in breach of the EWG guideline.
THM's have been linked to increase risk in illness, particularly bladder and colorectal cancer in men. THM's are also found in steam from boiling water and showers and can be inhaled and absorbed through the skin. A study from Europe a couple of years ago, suggests that 5% of all bladder cancers can be attributed to exposure of THM's in drinking water. In 2022, 723 Victorians were diagnosed with bladder cancer with three quarters of these cases being men. The biggest risk factors for bladder cancer however are smoking and exposure to certain chemicals in the workplace. Pinpointing an exact cause of cancer however is almost impossible.
It also appears that there may be people more susceptible to being impacted by THM's than others. This is generally the case with health guidelines, which may represent the largest cohort of the population, but may not so accurately represent people with particular health issues such as chemical sensitivity or genetic susceptibility: "... Lymphocyte Specific Protein 1 is responsible for recruiting leukocytes to inflamed sites, leading the researchers to hypothesize that exposure to THMs for people with the GG genotype may exacerbate bladder inflammation, which may in turn increase risk of bladder cancer. More mechanistic research into these interactions is needed..." Source
Key health issues in relation to high THM levels include cancer of the bladder, colon, rectum, liver and kidneys. Reproductive effects such as low birth weights, birth defects and miscarriages have also been suggested, as have increased risk of anaemia in patients using water for dialysis if THM levels exceed 8μg/L.
This is an image for Victoria, highlighting average THM levels in 2023/24. The yellow pins represent THM levels >80μg/L (the US guideline). The red pins indicate breaches to the Australian Drinking Water Guidelines (250μg/L) for a portion of 2023/24. At these locations average levels also exceeded the US guideline. Generally speaking THM levels have been reducing across Victoria over the last decade as water authorities are under pressure to lower THM levels. THM levels are also higher in many rural areas. The highest THM level in Victoria was recorded at Balmoral in 2007/08 at 970μg/L.
THM levels can be reduced by removing precursors in the source water. This can include oxidisation by ozone or chlorine dioxide. Better filtration at water treatment plants can also reduce THM formation. THM's can also be reduced by better aeration and the use of Granular Activated Carbon which can "absorb" THM's. Better filtration methods at water treatment plants can also be employed. Acidity of water (pH) is also a factor in THM creation. Alternative disinfectants can also be used, most notably chloramines, where Ammonia is added into the disinfection process. Chloramination and treatments such as ozone and chlorine dioxide also come with their own issues.
A bladder cancer map for Victoria with the highest rates appearing west of Bendigo and south of the Grampians.
Image Source Renewall Water: Trihalomethanes are compounds where three or four of the hydrogen atoms in methane have been replaced with halogen atoms. Halogens are very reactive and are very toxic.
Resolving THM and other disinfection byproduct issues in drinking water is not always an easy task and problematic as it could lessen the effectiveness of disinfection, increasing the risk of potentially dangerous microbiological organisms with increased likelihood of waterborne disease. Waterborne diseases such as cholera, diarrhoea, dysentery, hepatitis A, typhoid and polio were largely eliminated in the 20th century through application of water disinfectants, such as chlorine. Waterborne diseases can potentially have devastating impacts and if present in Melbourne's drinking water supply could impact hundreds, if not thousands of people. It is a delicate balancing act.
According to the Australian Water Association in 2021: "The ADWG requires that the treatment of pathogens is not compromised due to chronic issues, such as THMs. This, in some instances, has legitimised some water service providers to not implement control measures for disinfection by-products, which often only occur seasonally and can be expensive to manage.
However, with a growing body of research, Queensland Health has suggested that 250µg/L could be high enough to cause health concerns for communities, and as such are taking tougher action on exceedances of THM limits.
Water service providers need to ensure that risk assessments adequately assess this risk, appropriate monitoring is undertaken, and measures are taken to mitigate the risk. These may include reducing water age in the reticulation, avoiding chlorinating unfiltered water, enhancing coagulation to remove precursors, managing chlorine levels, cleaning mains and reservoirs in the reticulation or switching to a chloramine residual in the distribution.
Increasing pressure will be applied to those water service providers operating schemes with seasonal exceedances. This should be an area for continual improvement, it is almost inevitable that guideline limits will be reduced at some point, the limit in the UK is 100µg/L and in the US, 80µg/L."
Average detections of THM's across Victoria are actually higher in areas serviced by Westernport Water. The lowest THM levels are in North East Victoria.
Chlorine disinfection of water supplies is not going to disappear in the foreseeable future. If you're interested in reducing THM's in your drinking water have a think about buying a water filter that has Granular Activated Carbon filter or a reverse osmosis system. If you are concerned about exposure to THM's from shower water and steam investigate shower filters.
For more information contact: [email protected]
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The ten highest average THM levels in Victoria in 2023-2024 are listed in the table below.
| Location | Water Authority | Maximum THM Level 2023-2024 μg/L | Average THM level 2023-2024 μg/L | Approximate Population |
| Quambatook | Grampians Wimmera Mallee Water | 0.22 | 0.183 | 229 |
| Natimuk | Grampians Wimmera Mallee Water | 0.21 | 0.16 | 514 |
| Mount Macedon | Greater Western Water | 0.24 | 0.16 | 1275 |
| Coleraine | Wannon Water | 0.18 | 0.153 | 1834 |
| Clunes | Central Highlands Water | 0.2 | 0.153 | 1844 |
| Pyalong | Goulburn Valley Water | 0.231 | 0.151 | 772 |
| Riddells Creek | Greater Western Water | 0.22 | 0.15 | 3673 |
| Fish Creek | South Gippsland Water | 0.22 | 0.149 | 827 |
| Tarnagulla | Coliban Water | 0.37 | 0.1438 | 153 |
| Dunolly | Coliban Water | 0.35 | 0.1421 |
893 |