5.1 Overview

A complete view of domestic well water quality is not available because most well owners do not test their wells regularly and reporting test results is voluntary in most jurisdictions. Furthermore, when domestic well owners test their water, they often analyze only for bacteria or a limited suite of parameters (such as bacteria, nitrate, and arsenic), rather than a comprehensive group of pathogens and chemicals that may be present in well water. Although water quality information for domestic wells is limited compared to public wells, regional and local surveys have been completed that provide some insight. The results from these surveys tell us that a high proportion of domestic wells do not meet drinking water quality guidelines.

Domestic well water quality data from a selection of surveys are provided in Table 2. The data indicate that it is common for more than 40 percent of domestic wells to exceed one or more health-based water quality guideline.

Table 2 – Domestic well water quality results from selected surveys.

1. Bacteria = Total Coliform Bacteria.
2. Total exceedances = Total percentage of wells that exceeded at least one health-based water quality guideline. This may include other parameters tested but not shown in this table.
3. Water quality guidelines vary between jurisdictions and are periodically revised. The guidelines presented here are the ones used by each of the referenced surveys. The guidelines used to determine exceedances in these surveys were as follows: Arsenic = 10 µg/L; Bacteria = 10/100 mL (Canadian surveys) and zero detections (USA surveys); Lead = 15 µg/L (Virginia) and 5 µg/L (New Jersey); Manganese = 300 µg/L (North Carolina) and 50 µg/L (New Jersey); Nitrate = 10 mg/L (Nitrate-N); Fluoride = 1.5 mg/L (India surveys) and 2 mg/L (USA surveys).

The high exceedance rates of health-based water quality guidelines in domestic wells and the reliance on domestic wells by hundreds of millions of people worldwide indicate that these wells can have a significant impact on public health. It is important to note, however, that not all domestic wells are used as drinking water supplies because some well owners use bottled water or another source for drinking water and rely on their water well for all other water needs. Therefore, the presence of contaminants in a well does not necessarily mean the well owner is exposed to these contaminants via drinking water (although other exposure routes such as dermal contact and inhalation may be possible depending on the type of contaminant present). For example, census data from Australia indicate that 5.6 percent of the population rely on domestic wells for their water supply but only 0.5 percent of the population use domestic wells for their drinking water (Australian Bureau of Statistics, 2013).

In terms of global health impacts, the most significant contaminants in domestic wells are microbial contaminants, arsenic, and fluoride. These three contaminants have been identified as the highest priority for global monitoring in drinking water (WHO and UNICEF, 2017). Microbial contaminants are a concern in all areas of the world, whereas arsenic and fluoride problems are greater in some areas because of the local geology.

Other contaminants and water quality problems commonly found in domestic wells include those that can cause health problems (e.g., lead, manganese, nitrate, and radionuclides such as uranium, radium, and radon), and those that cause aesthetic concerns (e.g., iron, chloride, hardness, sulphate, odor, color, turbidity). Some common water quality parameters have other adverse effects. For example, turbidity can interfere with treatment systems used for disinfection, which is why filtration is recommended upstream of disinfection systems. Although pH itself is usually not a health concern, groundwater with low pH can cause corrosion of pipes and plumbing fixtures, which can release lead and copper to the water.

Contaminants found in domestic wells can be naturally occurring or anthropogenic. Naturally occurring water quality problems are usually associated with geologic sources, including soils, sediments, and bedrock. Examples of contaminants that are primarily associated with geologic sources include arsenic, fluoride, hardness, iron, manganese, sulphate, and radionuclides (e.g., uranium, radium, radon). The degree and extent of these naturally occurring contaminants in groundwater varies from area to area because their occurrence is controlled by the local geology, groundwater flow path, and groundwater resident time. The most commonly detected anthropogenic contaminants in domestic wells are those associated with septic systems (e.g., microbial contaminants, nitrate, salt), commercial/industrial activities (e.g., chlorinated solvents, petroleum hydrocarbons) and agricultural activity (e.g., microbial contaminants, nitrate, pesticides).

Some contaminants originate from both natural and anthropogenic sources. For example, salt can come from geologic formations that contain salt or from sea spray near the coastline. Salt can also come from road de-icing or dust control operations, from seawater intrusion caused by over-pumping in coastal aquifers, and from the discharge of water softener treatment systems. Another example is microbial contaminants, which can come from natural wildlife activity or from human sources and activities, such as septic systems, domestic pets, and manure spreading.