5.2 Contaminants in Domestic Wells

Descriptions of the most common contaminants found in domestic wells that can affect human health are provided below. Further information about groundwater quality and human health can be found in other Groundwater Project books.

Microbial Contaminants

Microbial contaminants are among the most common type of contaminants found in domestic wells. This group of contaminants includes several types of pathogens, such as bacteria, viruses, and protozoa (including Giardia and Cryptosporidium). Pathogenic microorganisms can cause gastrointestinal illnesses and usually occur in groundwater supplies that have been contaminated by human or animal waste. Because it is difficult and expensive to test for many of these microorganisms, domestic wells are usually tested for indicator organisms such as total coliform bacteria and Escherichia coli (E. coli). The detection of an indicator organism, which are themselves not necessarily harmful to human health, suggests that microbial pathogens may be present in the well or that the well is vulnerable to contamination by pathogens.

Microbial contaminants in drinking water have been estimated to cause over a billion cases of gastrointestinal illnesses per year worldwide (Johnston et al., 2001). Although these cases are more often related to surface water sources, several studies have looked at the health impacts of these contaminants in domestic wells. In Canada, it has been estimated that there are 78,000 cases of illness each year due to the consumption of untreated drinking water from domestic wells containing microbial contaminants (Murphy et al., 2016). The microbial contaminants included in this estimate were Giardia, Cryptosporidium, Campylobacter, E. coli O157, and norovirus. Norovirus was estimated to account for about 71 percent of these illnesses (55,000 cases). In the United States, a North Carolina study found that between 2007 and 2013, 99 percent of emergency department visits (29,200 cases) for acute gastrointestinal illness caused by microbial contaminants in drinking water were associated with domestic wells (DeFelice et al., 2016). The estimated cost of the emergency room visits associated with domestic wells in the North Carolina study was 40 million US dollars.


Arsenic is considered to be the second most important contaminant in drinking water after microbial contaminants. It has been estimated that worldwide more than 140 million people drink groundwater with high levels of arsenic (VanDerwerker et al., 2018). In the United States, arsenic is estimated to affect more than two million domestic wells (Ayotte et al., 2017). Long-term exposure to arsenic in drinking water has been linked to many types of cancer (bladder, kidney, lung, skin) and non-cancer health effects (skin lesions, cardiovascular disease, diabetes, neurological effects). The World Health Organization has estimated that arsenic in groundwater at concentrations above 500 µg/L causes death in one in ten adults (van Halem et al., 2009). In Araihazar, Bangladesh, where drinking water is sourced from 6,000 individual wells, it was estimated that 21 percent of all deaths were attributed to arsenic above 10 μg/L in well water (Argos et al., 2010). It has also been estimated that at least 100,000 cases of skin lesions in Bangladesh have been caused by arsenic in well water (Smith et al., 2000).

As shown in Table 2, regional surveys indicate that it is common for domestic wells to exceed water quality guidelines for arsenic. Prior to the 1980s, arsenic was not commonly tested for in drinking water, which caused it to go undetected. As discussed in Box 2, in some cases it has been the observed health effects that first led to the discovery of widespread arsenic and other contaminants in well water.


Fluoride is considered beneficial to dental health at low concentrations, but at high concentrations (> 1.5 mg/L) in drinking water it can be harmful to teeth and bones. At very high concentrations (> 10 mg/L) skeletal fluorosis can be crippling (WHO, 2004). It has been estimated that 200 million people from 29 countries around the world are exposed to high levels of fluoride in groundwater (Samal et al., 2015). In India, dental surveys in schools have indicated that 62 million people have dental fluorosis caused by high fluoride levels in drinking water (Podgorski et al., 2018). National surveys of domestic water wells in India (Table 2) indicate that 14 percent of wells exceed the WHO fluoride drinking water guideline of 1.5 mg/L (WHO, 2004). A study in central Malawi sampled 39 domestic groundwater supplies and looked at the results of a survey of 6,804 households for indicators of dental fluorosis. The study found that 44 percent of the wells exceeded the drinking water guideline of 1.5 mg/L and 28 percent of the households had someone in the house with evidence of dental fluorosis (Addison et al., 2020).


Nitrate in drinking water is associated with several health effects, including methemoglobinemia (or “blue baby syndrome”) in infants, thyroid effects, and cancer. Nitrate occurs naturally, but concentrations above about 1 mg/L are usually associated with human activities (DeSimone et al., 2009). The most common sources of nitrate include agricultural activities (e.g., fertilizer and manure application) and wastewater disposal (e.g., septic systems). As shown in Table 2, nitrate is commonly found in more than 10 percent of domestic wells in agricultural areas. Nitrate is also commonly found in domestic wells in urban areas, often related to wastewater disposal. A national survey in the USA found that 7.1 percent of domestic wells in areas dominated by agriculture exceeded nitrate drinking water guidelines, compared to 3.1 percent of domestic wells in urban areas (DeSimone et al., 2009). Because nitrate is associated with some of the main sources of groundwater contamination, its presence in water wells is often used as an indicator of aquifer vulnerability and an indicator that other contaminants could be present.

The cost to mitigate nitrate in groundwater can be high. For example, in Wisconsin, USA, nitrate is reported to be the most widespread groundwater contaminant with an estimated 10 percent of domestic wells (i.e., 42,000 wells) exceeding the nitrate drinking water guideline. The cost to replace these wells with deeper wells that access groundwater with low nitrate levels is estimated to be 440 million US dollars (Wisconsin Groundwater Coordinating Council, 2020).


Manganese commonly occurs in domestic well water and has been associated with health problems. For example, a study in North Carolina, USA, looked at water quality results from 73,000 domestic wells and the health outcomes of 17,000 children (Langley et al., 2015). The study found an association between the manganese concentration in domestic well water and adverse neurodevelopment and hearing loss in children. Approximately 8 percent of the wells exceeded the North Carolina health advisory level for manganese (200 µg/L).


Lead is another contaminant commonly found in domestic wells that causes adverse health effects. Lead is a neurotoxin that affects the neurological development and behavior of children and causes high blood pressure and kidney problems in adults. The regional survey results shown in Table 2 reveal that as many as 20 percent of domestic wells have lead levels above water quality guidelines. The presence of lead in domestic wells is usually caused by the corrosion of pipes and plumbing fixtures in the home’s water system. Most regulated public water supplies are required to have corrosion control programs to prevent high lead levels, however, domestic well owners rarely treat their water for corrosion control. A study in North Carolina, USA, looked at blood-lead levels in 59,000 children, 7,700 of which used drinking water from domestic wells (Gibson et al., 2020). The study found that children in homes with domestic wells were 25 percent more likely to have increased blood-lead levels compared to children in homes served by a regulated public water supply.

Pesticides and Volatile Organic Compounds

Pesticides are often detected in domestic farm wells, but they are not commonly found at concentrations that exceed drinking water quality guidelines. The two surveys in Table 2 from Nova Scotia and Ontario, Canada, reported that 41 percent and 11 percent of the wells tested had detectable levels of pesticides, respectively. None of the wells in the Nova Scotia survey had pesticide concentrations above drinking water guidelines and 0.5 percent of the wells in the Ontario survey exceeded guidelines. A survey done in agricultural areas of Wisconsin, USA tested 105 domestic wells and found 88 percent had detectable levels of pesticides, although none were found at levels that exceeded enforcement standards (Wisconsin Department of Agriculture, 2019). It is important to note that many pesticides do not have established drinking water guidelines, which makes it difficult to judge the significance of their detection in water wells. For example, no guidelines were available for about half of the 28 pesticides detected in the Wisconsin survey.

Volatile Organic Compounds (VOCs) are common components and additives in many commercial, industrial, and household products. They can be found in petroleum products (such as gasoline, and diesel fuel), carpets, paints, varnishes, glues, spot removers, cleaners, and fumigants. Surveys have detected VOCs in domestic wells, but they are not often found at concentrations above drinking water guidelines. A national survey in the United States tested for 55 VOCs in 2,400 domestic wells and found that 14 percent of the wells had detectable levels of VOCs (i.e., greater than 0.2 ug/L) and less than 2 percent of the wells had levels of VOCs that exceeded drinking water guidelines (Zogorski et al., 2006). The most common VOCs that exceeded guidelines in the survey were the fumigant Dibromochloropropane and the solvents Perchloroethene and Trichloroethene.

Emerging Contaminants

Emerging contaminants are another group of contaminants of concern for domestic wells. These are contaminants whose risk to human health and occurrence and distribution in groundwater are not yet fully understood. They may come from industrial, agricultural, and sewage sources including septic systems. Examples include pharmaceuticals, personal-care products, and per- and polyfluoroalkyl substances (PFAS). PFAS are used for many purposes, such as non-stick coatings for cookware and firefighting foam. They are highly resistant to degradation in the environment, difficult to treat, and have been associated with health problems such as kidney and testicular cancer, bowel disease, high cholesterol, and thyroid disruption (Lee and Murphy, 2020). PFAS monitoring to date has focused on public water supplies and, therefore, less is known about their occurrence in domestic wells. However, PFAS have been detected in domestic wells in Ohio and West Virginia, USA near a manufacturing facility; and in domestic wells in Alabama, USA near agricultural fields where wastewater treatment plant biosolids were applied (Lee and Murphy, 2020). Domestic wells may be more vulnerable to emerging contaminants like PFAS than public water supplies because domestic well owners are less likely to test their wells for these contaminants.


Domestic Wells – Introduction and Overview Copyright © 2022 by John Drage. All Rights Reserved.