Maine is a largely rural state where nearly half of the population uses drinking water from private wells. Arsenic (As) is present in some Maine groundwater, has been linked to cancer, and a lack of testing and treatment may expose people with private wells to elevated As levels. Disinfection byproducts (DBPs) include known and suspected carcinogens that form when chlorine or chloramines are added to water. People served by public water systems may be exposed to elevated levels of regulated DBPs such as trihalomethanes and haloacetic acids associated with chlorine and/or unregulated nitrogenous DBPs, or N-DBPs, such as nitrite and N-nitrosodimethylamine associated with chloramines. Contrary to initial expectations, there were no significant associations between median town As in well water and bladder, lung, kidney, or skin cancer incidence. Furthermore, bladder, melanoma, and other skin cancer incidence rates were negatively correlated with the percent of the town population using private wells. Analysis of cancer incidence associated with chlorine and chloramine disinfection showed elevated melanoma, and other skin cancer with chloramine use and elevated bladder and non-melanoma skin cancer with chlorine use compared to the no disinfectant case. We recommend more research on the links between disinfectant use and cancer.

  • Maine towns with more private wells had fewer bladder, melanoma, and other skin cancers.

  • Bladder and non-melanoma skin cancer were higher with chlorination; melanoma and other skin cancers were higher with chloramination.

  • Bladder, kidney, melanoma, and other skin cancers were higher with surface water sources.

  • Arsenic did not correlate with cancer incidence.

  • No disinfection being limited to small towns may confound the results.

Maine is a largely rural state in which a larger than average percentage of the population self-supplies drinking water: roughly half compared to 13% of the population nationwide (Dieter et al. 2018). It is also known that naturally occurring arsenic (As) contaminates the groundwater in some parts of the state (e.g., Yang et al. 2014). According to the US Centers for Disease Control and Prevention (CDC), more rural areas are more likely to have poorer drinking water quality, whereas more urban areas are more likely to suffer from worse air quality (Strosnider et al. 2017). This background prompted the research reported here on the connections between water supply and the incidence of several cancers across the state.

As is a confirmed carcinogen linked to lung and bladder cancers, a suspected carcinogen for liver and prostate cancer and some skin cancers, and has been associated with diabetes, neurological effects, cardiovascular disorders, reproductive health effects, childhood brain development issues, and a variety of other diseases and conditions (Smith et al. 1992; Hong et al. 2014; Wasserman et al. 2014; D'Ippoliti et al. 2015; Bedaiwi et al. 2022). Low-level As exposure has been associated with the formation of two non-melanoma forms of skin cancer: basal cell carcinoma and squamous cell carcinoma (Huang et al. 2019). Epidemiological studies have also indicated an association with more malignant (melanoma) forms (Haque et al. 2003; Beane Freeman et al. 2004).

Several studies have established dose-dependent relationships between long-term As exposure through drinking water intake and lung and bladder cancer incidence, even at relatively low As levels (Chiou et al. 1995; Saint-Jacques et al. 2014). Studies specific to the New England states, including Maine, USA, have identified a positive correlation between bladder cancer mortality and the use of private water supplies in New England (Ayotte et al. 2006; Koutros et al. 2018). A cross-sectional study of school-aged children and families in Maine by Wasserman et al. (2014) demonstrated a strong association between As levels in drinking water and reduced childhood and maternal intelligence quotient (IQ). Another study in nearby New Hampshire showed a relationship between As levels in drinking water and negative birth outcomes (Gilbert-Diamond et al. 2016).

Elevated As levels are, unfortunately, a common occurrence in Maine drinking water and are strongly correlated with the geologic setting including the chemistry of bedrock and overlying unconsolidated deposits (Robinson & Ayotte 2006). As levels of 0.5 to more than 5,000 μg/L, some of the highest concentrations in the United States, have been identified in groundwater samples across the state. The As levels in groundwater appear to be particularly elevated in some large regions including the southern coastal region, central Kennebec County, and around the town of Ellsworth in Hancock County. However, there are also several smaller areas with noted elevated As levels and isolated wells or small clusters of wells in all parts of Maine. A study by the United States Geological Survey (USGS) examined As concentrations in water samples analyzed at the Maine Health & Environmental Testing Laboratory (HETL) between 2005 and 2009, including samples from 11,111 private wells representing 531 Maine towns. Data from the 1990 census, which included a survey of household water use, was used in the USGS study to estimate the percentage of private vs. public water use per town. Their study found that 18.4% of private wells had As concentrations exceeding 10 μg/L and 4% exceeded 50 μg/L (Nielsen et al. 2010). The USGS dataset was used in our study, as discussed in later sections.

Public water systems add chemical disinfectants to the water supply to control microbial infection risk. This practice has saved countless lives since its inception and contributed to the increased life expectancy over the 20th century (Angelakis et al. 2021). The most common chemical additives are chlorine compounds such as sodium hypochlorite. Chemical disinfection has the potential to react with organic matter and other source water constituents to form various disinfection byproducts (DBPs). Chlorination has been shown to generate DBPs such as trihalomethanes (THMs) and haloacetic acids (HAAs), which can have carcinogenic health impacts (Richardson & Plewa 2020). Regulated public water systems classified as ‘community water systems and non-transient non-community systems, including those serving fewer than 10,000 people that add a disinfectant to the drinking water during any part of the treatment process’ (US EPA 2024) are required to comply with the US Environmental Protection Agency (EPA) Stage 1 and Stage 2 disinfectants and DBPs rules to reduce exposure to DBPs, including total THMs and HAAs, chlorite, and bromate.

Some public water systems choose to apply monochloramine, which is created by the addition of ammonia to chlorinated water, to reduce the formation of the four EPA-regulated DBPs mentioned above. This practice is also referred to as using chloramines or ‘chloramination’. The use of chloramines has been found to be especially effective in controlling Legionella in building piping systems, such as in hospital settings (Marchesi et al. 2020). However, one of the challenges with chloramination is that during the time water resides in the distribution system (e.g., underground piping and storage reservoirs), chloramines may decay, releasing free ammonia into the water. Ammonia can subsequently be oxidized by nitrifying microorganisms to nitrite and nitrate. Incomplete nitrification can lead to the accumulation of toxic levels of nitrite (Liu et al. 2020), which is associated with several types of cancers. The use of chloramines has also been linked to the formation of N-nitrosodimethylamine (NDMA), a currently unregulated DBP and probable human carcinogen (Choi & Valentine 2002), dihalogenated DBPs, iodo-THMs, and several other classes of nitrogen-containing organic compounds collectively referred to as nitrogenous DBPs or N-DBPs (Richardson & Plewa 2020). While less is known about the toxicity and formation of N-DBPs, there is an apparent gap between the toxicity measured in chloramine-treated water and the concentration of regulated DBPs in the water (Li & Mitch 2018).

Unlike public water systems that must maintain some level of disinfection to manage microbial infection risk in distribution piping and storage tanks or systems that draw water from surface water bodies, most private wells in Maine are not equipped with disinfection control and are therefore free from DBPs and N-DBPs.

The study described in this paper uses existing datasets including As concentrations in private well water tabulated on a per-town basis from a USGS study (Nielsen et al. 2010), and case records from the Maine Cancer Registry (MCR) to examine potential associations between lung and bronchus, urinary and bladder, kidney and renal pelvis, skin melanoma, and other non-epithelial skin cancer occurrences and As concentrations in Maine. We also compared the reported frequencies of these cancer diagnoses with the type of disinfection chemicals used (chlorine, chloramine, or none), the use of ozone or ultraviolet light (UV) treatment prior to secondary disinfection, and source water type (surface vs. groundwater) using data from the Maine Center for Disease Control and Prevention's Drinking Water Program (DWP).

Several studies, including those referred to earlier in this article, have been performed to evaluate associations between private well As use and health outcomes for individuals and groups (Chiou et al. 1995; Saint-Jacques et al. 2014; Wasserman et al. 2014; Gilbert-Diamond et al. 2016). However, to our knowledge, there has been limited examination of disease occurrence in the general population in Maine over a significant time period to determine if an association can be established between town-based As concentrations in private well water and health outcomes. Furthermore, there is very little information available about the influence of disinfection practices on cancer rates (Weisman et al. 2022), despite the known carcinogenicity of several DBPs and N-DBPs.

The results of this study provide an example of using population-based health and environmental data to infer relative risk to people using drinking water from private wells as compared to public water systems that apply chemical disinfection methods. This study also provides evidence to support further study on methods to reduce DBPs in public water and possible regulatory policies for currently unregulated N-DBPs associated with the use of chloramines.

This study involved the analysis of several existing datasets containing no personally identifiable information (i.e., secondary data). We performed descriptive and inferential statistical analysis on information from a dataset provided upon request by the MCR. The MCR dataset contains case records of diagnoses from 1995 (when the registry began collecting population-based records) up to 2020 (MCR 2023). Data elements maintained by the MCR follow standards from the North American Association of Central Cancer Registries (NAACCR). This dataset included 61,303 tumor records for lung (including lung and bronchus), bladder (including urinary and bladder), and kidney (including renal pelvis and liver) cancer types, along with melanoma of the skin and other non-epithelial skin cancers, listed by date and town. In all, 519 towns and unorganized territories were included in the MCR dataset.

To explore possible associations between DBPs (DBPs and N-DBPs) and cancer incidence in Maine, we analyzed data obtained from the DWP in conjunction with the MCR dataset. The DWP dataset included information for community public water systems, which serve water to at least 25 people for at least 60 days per year. Approximately 370 public water systems in Maine are classified as community water systems, serving populations ranging from 25 customers to about 142,000 customers (Maine DWP 2023). These population estimates are typically based on the number of customer accounts or service connections, assuming 2.5 persons per household connection, rather than a census-type population count.

Data provided by the DWP included the name of the town(s) served by each community water system, As concentration, and the type of disinfectant used, typically reported as chlorine or chloramine, and water source type (76 surface water and 443 groundwater). Of the 370 community water systems included in this study, 167 were disinfected with chlorine, while 44 systems used chloramine. Some community water systems reported the use of other disinfection techniques such as ultraviolet disinfection or ozone oxidation (30), usually in combination with chemical disinfection. Several smaller systems (i.e., having a small distribution system and limited storage) reported using no disinfection method. Towns in the MCR dataset without community water systems were classified as using groundwater without disinfection.

Groundwater As concentrations (median of private well water samples), and percent private well usage data were obtained according to the information presented in Nielsen et al. (2010). For the analysis of As, town exposure concentrations were calculated as (% private well usage)/100 * town median private well As concentration + (100 – % private well usage)/100 * community water supply As concentration. Median age and household income data were obtained by town from ArcGIS (Esri 2023). Cancer incidence (MCR 2023) rates per 100,000 population per year for each town were calculated as the total number of diagnoses over the 1995–2020 time span in the town, times 100,000, divided by 26 years, and divided by town population. The compiled data are provided as supplementary information Table 1.

All plotting and statistical analyses were done in R. Disease incidence for each town was compared with groundwater As concentrations using nonparametric one-way analysis of variance (ANOVA), also referred to as the Kruskal–Wallis test. Using logistical and linear regression analysis, we also compared cancer incidence with median As concentration and the percentage of private well usage in each town. Data for cancer incidence was plotted using categorization based on disinfection type, water source type (surface or groundwater), and additional treatment (ozone or UV) use. Inferential statistics, including linear regression, were utilized to determine the significance of any association and/or correlation between the rate of occurrence for each cancer type, per town, and the different disinfection methods.

According to the MCR database, between 1995 and 2020, there were 26,979 reported cases of lung cancer; 9,775 cases of bladder cancer; 5,471 kidney cancers; 13,153 skin melanomas; and 622 other non-epithelial skin cancers as summarized in Table 1. This table also shows incidence rates in cases per 100,000 per year, for the entire population and the median value per town. Incidence rates were not age-adjusted. During the reporting period, lung and bronchus cancers were reported most frequently in the general population (76.5 cases/100,000/year) and had the highest median incidence rate per town (76.2 cases/100,000/year) as well as the highest annual incidence rate for an individual town (962 cases/100,000/year).

Table 1

Summary of MCR cancer data

Cancer type
Lung and bronchusUrinary and bladderKidney and pelvisMelanoma of the skinOther non-epithelial skin
Total reported case countsa 26,979 9,775 5,471 13,153 622 
Statewide incidence rate (cases/100,000/year) 76.5 27.7 15.5 37.3 1.8 
Median incidence rate per town 76.2 26.4 14.7 31.2 <0.01 
range 0–962 0–186 0–128 0–365 0–24.3 
Cancer type
Lung and bronchusUrinary and bladderKidney and pelvisMelanoma of the skinOther non-epithelial skin
Total reported case countsa 26,979 9,775 5,471 13,153 622 
Statewide incidence rate (cases/100,000/year) 76.5 27.7 15.5 37.3 1.8 
Median incidence rate per town 76.2 26.4 14.7 31.2 <0.01 
range 0–962 0–186 0–128 0–365 0–24.3 

aData from Maine Cancer Registry, 1995–2020.

In our comparison of cancer incidence against As exposure concentration through drinking water, concentrations in each town were categorized as follows, with the results shown in Figure 1:
  • 1. Trace levels below typical reporting limits (0–0.5 μg/L; n = 71).

  • 2. As levels up to the EPA's practical quantitation limit (0.5–3 μg/L; n = 174).

  • 3. As levels up to the EPA's proposed Maximum Contaminant Level (MCL) and standards adopted by New Jersey and New Hampshire (3–5 μg/L; n = 41).

  • 4. As levels up to the current EPA MCL (5–10 μg/L; n = 28).

  • 5. As concentration >10 μg/L, (n = 15).

Figure 1

Annual cancer incidence/100,000 population and % self-supply vs. As exposure concentration (μg/L) NS means the relationship is not significant.

Figure 1

Annual cancer incidence/100,000 population and % self-supply vs. As exposure concentration (μg/L) NS means the relationship is not significant.

Close modal

Nonparametric ANOVA (Kruskal–Wallis test) yielded significant associations between As levels and kidney cancer (p = 0.001) and other skin cancers (p = 0.000) with higher As exposure concentrations. Surprisingly, the correlations were inverse (Figure 1). The percent of the population that self-supplied drinking water was positively correlated with As concentration, indicating that this population was indeed exposed to more As in their water.

Multivariate linear modeling including disinfectant types, As exposure levels, percent of the population on community water supply, median age and income confirmed the negative correlations between kidney (p = 0.014) and other skin (p = 0.044) cancers and As. This analysis also showed a significant positive correlation between other skin cancer incidence and both chlorination (p < 0.001) and chloramination (p = 0.035).

These results indicate that higher As exposure concentration for a given town in Maine is not a good predictor of cancer incidence, at least for the forms of cancer included in our study. These findings were contrary to our expectations based on previous studies indicating that unmitigated As in private wells may present significant exposure risk for lung and bladder cancers, even at low levels. Kidney and other skin cancers were the two lower incidence cancers in our study and the results may have been more skewed in the smaller population towns with higher % self-supply, however, this was not confirmed through our linear modeling.

Simple linear regression between cancer incidence and percentage of private well usage (Figure 2) showed significant negative associations with bladder cancer (p = 0.007), skin melanoma (p = .01) and other skin cancers (p < 0.000). These results indicate that people using private wells may be at less risk for some of the cancers considered in this study.
Figure 2

Annual cancer incidence/100,000 population and town population vs. percentage of private well use (n0–20% = 48; n20–40% = 41; n40–60% = 48, n60–80% = 47, –80-100% = 332; NS means a relationship with private well use is not significant).

Figure 2

Annual cancer incidence/100,000 population and town population vs. percentage of private well use (n0–20% = 48; n20–40% = 41; n40–60% = 48, n60–80% = 47, –80-100% = 332; NS means a relationship with private well use is not significant).

Close modal

The inverse correlation observed between the incidence of bladder and skin cancers with the percentage of private well use may indicate that any risk associated with the use of private wells is outweighed by other risk factors for these cancers in Maine. This outcome may be confounded by the negative correlation between the percentage of private well usage and the town population, however. Towns with smaller populations are more likely to have no reports of low-incidence cancers over the reporting period even if the risk to individuals is actually higher, skewing the mean lower relative to that observed in the larger population centers.

Nonparametric (Kruskal–Wallis) testing of cancer incidence against the type of disinfection chemical used in public water systems (mean data shown in Table 2) indicated that the occurrence of all cancers varied with the disinfectant type (p < 0.000–0.002). Pairwise comparisons (with the Bonferroni correction and unequal variance) showed that bladder cancer was significantly higher in towns that used chlorine (31.9/100,000/year) disinfection than those with no disinfection (25.8/100,000/year; p = 0.015), representing an increase of 6.1 cases/100,000/year. Towns using chloramine had rates that were not significantly different from either chlorine or no disinfection. The incidence of skin melanoma was higher in towns that used chloramination than chlorine (p = 0.009; risk increased by 11.6/100,000/year) or no disinfection (p = 0.0005; 15.5/100,000/year increase), towns using chlorine or no disinfection had similar rates. Other skin cancers were higher in towns using chlorine (p < 0.000; 1.2/100,000/year increase) or chloramine (p = 0.001; 1.3/100,000/year increase) than without disinfection. No other pairwise comparisons showed significant differences (Table 2 and Figure 3).
Table 2

Comparison of mean cancer incidence with disinfection type, additional oxidant use, and source type

 Cancer type 
Incidence rate (cases/100,000/year) Lung and bronchus Urinary and bladder Kidney and pelvis Melanoma of the skin Other non-epithelial skin 
 Disinfection type 
No disinfection (n = 308) 81.1 25.8a 14.3 32.2c 1.1e 
Chlorine (n = 167) 88.0 31.9b 17.0 36.1c 2.3f 
Chloramine (n = 44) 75.2 30.1ab 16.7 47.7d 2.4f 
 Use of additional oxidant 
No other oxidant (n = 489) 83.4 28.1 15.3 34.0a 1.59a 
UV and/or ozone (n = 30) 73.1 28.2 16.3 48.0a 1.65a 
 Source water type 
Ground water (n = 443) 83.0 27.5a 15.1a 32.7a 1.5a 
Surface water (n = 76) 81.7 31.6a 17.0a 46.6a 2.1a 
 Cancer type 
Incidence rate (cases/100,000/year) Lung and bronchus Urinary and bladder Kidney and pelvis Melanoma of the skin Other non-epithelial skin 
 Disinfection type 
No disinfection (n = 308) 81.1 25.8a 14.3 32.2c 1.1e 
Chlorine (n = 167) 88.0 31.9b 17.0 36.1c 2.3f 
Chloramine (n = 44) 75.2 30.1ab 16.7 47.7d 2.4f 
 Use of additional oxidant 
No other oxidant (n = 489) 83.4 28.1 15.3 34.0a 1.59a 
UV and/or ozone (n = 30) 73.1 28.2 16.3 48.0a 1.65a 
 Source water type 
Ground water (n = 443) 83.0 27.5a 15.1a 32.7a 1.5a 
Surface water (n = 76) 81.7 31.6a 17.0a 46.6a 2.1a 

Note. n = number of samples. Letters (a–f) in disinfection type indicate statistically significant differences (p = 0.05). Pairs with the same letter are not significantly different.

aSignificant difference in cancer incidence rate at p = 0.05.

Figure 3

Annual cancer incidence/100,000 and town population vs. disinfectant (means of samples with the same letter are not significantly different).

Figure 3

Annual cancer incidence/100,000 and town population vs. disinfectant (means of samples with the same letter are not significantly different).

Close modal

Table 2 also shows significantly elevated (p < 0.05) risk of bladder (4.1/100,000/year), kidney (1.9/100,000/year), melanoma (13.9/100,000/year), and other skin cancers (0.6/100,000/year) when surface water was the source in comparison to groundwater. The use of additional oxidation steps (UV or ozone) prior to chlorine or chloramine use also corresponded with significantly higher melanoma (14/100,000/year) and other skin cancer (0.06/100,000/year) rates (Table 2/100,000/year). These relationships are not independent of disinfectant type, however: almost all the systems that chloraminated used a surface water source (40/44 systems that used chloramine started with surface water), and of the water treatment systems that used UV or ozone treatment prior to final disinfection (total of 30), 23 used chloramine as the secondary disinfectant, thus the effects of these factors cannot be separated using our dataset. That stated, if these cancer rates are influenced by the presence of DBPs in drinking water, the presence of organic matter, which is typically higher in surface water, and the use of pretreatment would both be expected to increase the rates of DBP formation on exposure to chlorine or chloramine (Richardson & Plewa 2020).

Because the cancer incidence rates in our study are not age-adjusted, the relationship between median age in the town (data from ArcGIS) and incidence rates was also checked. Contrary to expectation, if age were an important factor contributing to elevated cancer rates, town median age was negatively correlated with bladder, kidney, melanoma, and other skin cancers (p < 0.001), but not lung cancer (p = 0.312). Similarly, a check on the relationships between cancer incidence and median household income (data from ArcGIS) showed a significant negative relationship with lung cancer (more cancer in areas with lower median household income), but positive for bladder, kidney, melanoma, and other skin cancers (p < 0.001). Thus, low income and advanced age are not factors contributing to the elevated bladder, melanoma, and other skin cancer incidence rates observed in the towns with chlorinated and chloraminated water.

Comparison of cancer incidence with the type of disinfection chemical used in public water systems in Maine provides evidence that the use of these chemicals may be associated with increased cancer incidence for urinary/bladder, melanoma, and other skin cancers, and possibly kidney cancer. It is noted that these results can only be used to make general inferences about exposure risk on a community level. Further studies would be needed to identify causal relationships at the individual case or cohort level. However, our results are supported by other studies demonstrating that chlorine DBPs are associated with bladder cancer (Weisman et al. 2022), and the use of chloramines, while potentially reducing the formation of the four regulated DBPs, can result in the formation of unregulated nitrogenous DBPs, or N-DBPs, at significantly higher concentrations (Richardson & Plewa 2020). Furthermore, epidemiological studies have indicated that many N-DBPs have greater toxigenic potential as compared to chlorine-associated DBPs (Liew et al. 2016).

In this work, the elevated risk of bladder, kidney, melanoma, and other skin cancers in towns using surface water sources may also support the conclusion that exposure to DBPs contributes to these higher cancer rates. Surface water sources are likely to have higher dissolved organic matter than groundwater sources. Dissolved organic matter reacts with chlorine or chloramine to produce DBPs. Pretreatment of water containing dissolved organic matter with ozone or UV prior to chlorination or chloramination can also increase the production of DBPs (Liew et al. 2016), presumably by generating more reactive forms of dissolved organic matter. Thus, these systems might be expected to generate higher levels of DBPs. In this work, melanoma and other skin cancers were significantly elevated in towns that used additional disinfectants. Taken together, our results support the possibility that DBPs are contributing to these elevated cancer rates. It should be reiterated, however, that there is a lot of overlap between the towns that use surface water sources, pretreatment with ozone or UV and chloramine disinfection. It should also be noted that the high quality of the source waters for several of the larger water districts enables them to use the water without filtration, which could also enable other contaminants to enter the distribution system that could contribute to the observed health outcomes. More research using data from many more communities would be needed to develop more conclusive evidence of the link between disinfectant use and cancer rates.

Our findings indicate that it may be appropriate to consider regulation under the Safe Drinking Water Act to control levels of chloramine-associated N-DBPs in public drinking water, similar to the current regulatory framework for chlorine-associated DBPs, especially given the increasing levels of dissolved organic matter in many surface waters in recent decades (e.g., see SanClements et al. 2012) that could increase DBP production during treatment. The US EPA listed N-nitrosamines and other N-DBPs in the Unregulated Contaminant Monitoring Rule 2 between 2008 and 2010, to be considered for regulation. The World Health Organization (WHO), Health Canada, and the Australian National Health and Medical Research Council have established guideline values for NDMA in drinking water (Marchesi et al. 2020). However, there are currently no regulatory standards in place in the US for any of the N-DBPs known to be associated with chloramine use.

Limitations of our study include the effect of confounding factors that may influence health outcomes in the general population. For example, urinary/bladder, kidney/liver, and skin cancers have numerous genetic, behavioral, and environmental risk factors. The elevated cancer risk being concentrated in higher population towns could provide additional risk factors that were not addressed in this research, such as longer retention times in the drinking water distribution system, air pollution, and other environmental exposures as well as lifestyle-related factors. Furthermore, there is potential for disproportionality in the data such as the observation that although the number of systems using chloramines (n = 44, 8.5%) was relatively small, the population served by these systems represents a large percentage (28.4%) of the total population. Medium-sized towns, which were more likely to chlorinate, could also have oversized distribution systems with long retention times to ensure adequate capacity for fire suppression; thus, many questions remain about causal relationships. Finally, the relatively smaller populations of the towns without disinfection could skew incidence rates for these towns lower, especially for the lower incidence cancers such as non-melanoma skin cancer.

Our analysis indicated that private well water use and median town private well water As concentrations were not associated with higher cancer incidence at the town level in Maine. To date, both the WHO and US EPA have maintained a regulatory level of 10 μg/L for As despite advances in laboratory quantification limits and treatment technology and the growing body of risk assessment data, indicating that a lower limit (3–5 μg/L) is warranted (Frisbie & Mitchell 2022). Some states, including New Jersey and New Hampshire, have adopted a lower drinking water standard of 5 μg/L. In Maine, there are no regulatory requirements for homeowners using private wells to test for As or treat for elevated As levels. While our analysis did not show an increase in cancer incidence with private well water usage or median As concentration in well water, the variability in As concentrations in areas with known geological origin As (Nielsen et al. 2010), small population sizes of the relevant towns compared to the incidence rates of these cancers, and efforts to reach out to homeowners and connect homes in known As hotspots to public water supplies may all contribute to this lack of a relationship. Exposure to elevated As levels in private wells may be addressed through the installation of treatment at a point-of-use (POU) location (e.g., a single faucet used for drinking and cooking) or in a point-of-entry (POE) treatment system designed to treat all water entering the household from the private well. Removing untreated water from the diet can result in an 8–32% reduction in urinary As for young children and a 14–59% reduction for adults (Smith et al. 2016). Other strategies to mitigate elevated As levels may include using an alternative drinking water source, such as purchased bottled water (Smith et al. 2016).

The finding that the use of chlorine and chloramine disinfection in public water systems may be associated with increased risk for certain cancers, especially urinary/bladder, and skin cancers, provides strong justification for further examination of the health risks posed by the use of chlorine-based disinfectants and the formation of DBPs and chloramine-associated N-DBPs. Although some N-DBPs have been considered for federal regulation, there are currently no requirements under the Safe Drinking Water Act (SDWA) to monitor or mitigate elevated levels of these compounds. DBPs and N-DBPs are most likely to form at significant levels when water contains organic matter and remains in distribution piping and storage reservoirs for long periods of time, for example where there are ‘dead-end’ lines, oversized piping and/or tanks, and within large internal plumbing systems. Delivery of safe water has social justice implications (Greenberg 2016) and this can also be applied to the management of DBPs. Often, the most significantly elevated DBP levels occur in portions of a water distribution system with limited water flow and circulation, where the water can sit in pipes or tanks for long periods of time before it is used. In many cases, this problem is most pronounced in disadvantaged or underserved communities or parts of a community where inadequate capital is available to pay for proper management of water age or to invest in upgrades to treatment and distribution systems to improve water circulation and reduce DBP and/or N-DBP formation.

The concentration of DBPs and N-DBPs can be managed through the removal of the organic matter prior to disinfection. The amount of organic matter in northeastern US waters is increasing (see SanClements et al. 2012), and can vary significantly with seasonal changes and disruptive climatic events and therefore presents a constant challenge for water system operators to manage (Leonard et al. 2022). While acknowledging the overlap in systems using surface water, additional oxidants, and chloramination in our dataset, the elevated cancer risk we observed with surface water sources, which are usually higher in organic matter than groundwater, supports the idea that increased cancer incidence could be due to exposure to DBPs. The elevated skin cancer risk when using UV or ozone pretreatment could further point to an increase in the rate of DBP formation as has been reported after chlorination or chloramination of pretreated water (e.g., see Richardson & Plewa 2020). Since pretreatment tends to reduce the overall concentration of organic matter in the water, the effect could be due to the production of more reactive organics available for DBP formation, so the means by which organics are removed prior to disinfection will need to be carefully considered if this is used as a strategy to reduce DBP formation. More research is needed on this subject. DBP and N-DBP formation can also be lowered by reducing contact time and/or the concentration of disinfectant added to the water. Contact time can be managed by using strategic flushing to reduce the length of time that water is held in pipes and storage tanks to avoid stagnation in underused areas or where the system is oversized, and joining dead ends in the distribution system to create more loops.

The authors of this article recommend additional research to further examine the relationship between cancer incidence and DBPs in public water systems in Maine and elsewhere and continued exploration of new methods to mitigate DBPs and associated health risks, including alternatives to chemical disinfection and better management practices to ensure fresh, safe drinking water that is free from harmful compounds.

The authors wish to thank Carolyn Bancroft and Katherine Boris from the Maine DHHS/CDC/Maine Cancer Registry for their assistance in providing data from the Registry to support this research. We would also like to thank Amilyn Stillings from the Maine DHHS/CDC/Drinking Water Program for providing data relating to treatment techniques at regulated public water systems in Maine.

This research was conducted without funding.

All relevant data are included in the paper or its Supplementary Information.

The authors declare there is no conflict.

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