What are the Potential Sources of Groundwater Contamination?

Groundwater can become contaminated from natural sources or numerous types of human activities. Waste from residential, commercial, industrial and agricultural activities can seriously affect groundwater quality. These contaminants may reach groundwater from activities on the land surface, such as industrial waste storage or spills; from sources below the land surface but above the water table, such as septic systems; from structures beneath the water table, such as wells, or from contaminated recharge from the aquifers.

The following is a list of common sources of contamination threatening water quality at municipal wells:

Natural Resources

Naturally occurring substances in rocks like iron, manganese, chlorides, fluorides, sulfates or radionuclides are dissolved in groundwater. Organic matter can move in groundwater as particles. Some of these substances in higher concentrations may pose a threat to human health, while others may result in poor taste, odour or colour. Some of these substances can often be removed from groundwater by common treatment methods.

Septic Systems

Homes that rely on septic systems dispose of their human waste into the native soils. Septic systems that are improperly sited, designed, constructed, or maintained can contaminate groundwater with bacteria, viruses, nitrates, detergents, oils, and chemicals. Commercially available septic system cleaners may contain synthetic organic chemicals, which in turn, may contaminate drinking water wells.

Disposal of Hazardous Materials

Hazardous waste such as paints, cleaners, batteries, motor oil, antifreeze, stain, weed killer, insecticides etc., cannot be disposed of into on-site septic systems. Many substances used in industrial processes cannot be disposed of in drains at workplaces because they outlet to the environment and, therefore, can contaminate groundwater. Domestic hazardous waste should always be disposed of through municipal licensed depots. For more information contact the Waste Management Facility closest to you or contact us at wastesites@region.durham.on.ca.

Chemical Storage and Spills

Underground and aboveground storage tanks are used for storage of chemicals, such as heating oil. If these tanks develop a crack, which often occurs in old and corroded tanks, chemicals seep out and migrate through soil and eventually contaminate groundwater. Abandoned underground tanks pose a threat to groundwater because their locations are not always known. Tracks and trains transporting chemicals pose another potential threat if accidentally the contents of the train are spilled.


Solid waste is disposed of in landfill sites. During the decomposition process, chemicals are released into the environment in a form of leachate. Untreated leachate can severely impact the quality of groundwater. Moreover, industrial and hazardous chemicals, that should be disposed of at designated facilities, sometimes make their way into waste landfills, which are not designed for disposal of these materials. Once chemicals leach into the ground, they can contaminate groundwater. It is very important not to dispose of hazardous waste with domestic waste.

Sewage Lagoons

Municipalities and private industries sometimes treat liquid waste in sewage lagoons to reduce contaminant levels by biological activity prior to release to the environment. This is a more economical solution in rural areas where populations, and resulting waste flows, are typically lower and land is relatively inexpensive so the sewage lagoon can be constructed with enough volume to store the liquid a sufficient length of time to provide adequate treatment. Sewage lagoons are constructed with a sealed bottom to prevent seepage into the ground. However, any time that liquid waste is stored for an extended length of time, there is a risk that some seepage may occur. Therefore, this form of land use is not permitted within an area designated as a wellhead protection area.

Sewers and Other Pipelines

Sanitary sewers sometimes break resulting in seepage of raw sewage. The raw sewage consists of organic matter, nitrogen, inorganic salts, heavy metals, bacteria and viruses. Some industries use pipelines to carry chemicals and oils. If the pipe is made of corrosive material or has weak spots, chemicals may seep and contaminate soil and groundwater.

Pesticide and Fertilizer Use

Fertilizers and pesticides are used for crop production. In addition, homeowners and industries such as golf course, utilities, and municipalities also use these chemicals. Some of these pesticides and fertilizers are highly toxic. If these contaminants enter groundwater they can remain there for many years polluting municipal water supplies. Animal waste on farm feedlots may also percolate into the ground. In addition, common fertilizers contain nitrogen, phosphorous and potassium. If fertilizers are applied too often, or too much, the chemicals contained in fertilizers can percolate into soils and contaminate groundwater. For more information contact the Durham Health Department, Environmental help line at 1-800-841-2729 ext. 3232 and/or Durham Region Works Department, Mr. Glen Pleasance, Water Efficiency Coordinator at waterefficientdurham@region.durham.on.ca.

Improperly Constructed Wells

Improperly constructed or poorly maintained wells can result in groundwater contamination from seepage of contaminated surface water or contaminated shallow groundwater. Improperly abandoned wells act as a conduit through which contaminants can reach an aquifer. Some people use abandoned wells to dispose of wastes such as motor oil. These wells may contaminate aquifers, which supply water to municipal wells. In addition, abandoned exploratory well, test wells, and geotechnical boreholes, if not properly protected and covered, are potential conduits for contamination.

Active wells can also be poorly constructed resulting in groundwater contamination. Poor casings, inadequate covers, or lack of concrete pads, may allow contaminated surface water into the well. Older wells are more prone to construction problems as they were constructed prior to established standards and regulations guiding well drillers in their construction. Other types of wells that may be sources of contamination are "dry wells" and sumps that collect storm water runoff and spilled liquids, drainage wells, which are used in wet areas to remove water and transport it into deeper soils and poorly constructed irrigation wells. For more information, please contact the Ministry of the Environment (MOE) at www.ene.gov.on.ca/en/water/index.php

Roadway De-Icing

Salt and winter sand/salt mix is commonly applied to Regional Roads for winter maintenance to reduce the hazards of winter driving. As the salt dissolves and forms a brine solution the majority of the runoff is contained in ditches or storm sewers. The runoff then discharges through storm water management ponds (where available) and into the creeks and rivers within the Region. However, precipitation can move some of the brine solution into the soils and into the groundwater. Elevated sodium and chloride levels are often recorded close to the roadside ditches. Sometimes higher sodium and chloride levels in the groundwater can contaminate wells constructed in shallow aquifers.

The Region of Durham has a Council adopted "Salt Management Plan" to ensure effective salt management practices through the use of new technologies, guidelines and procedures, such as pre-wetting and the use of treated salts. These practices should continue to minimize the winter driving hazards and also minimize the amount of salt entering the environment.