How do groundwater wells get contaminated?

Water is a natural solvent, and because of this, all groundwater generally has dissolved minerals and other constituents in it. These added minerals are what can give it a particular taste, odor, or — in some cases — smell. This is why some people feel their well water tastes better than treated “city water.” However, water’s ability to dissolve material also means that water can sometimes “pick up” contaminants from the surface and transport them down into your groundwater system (Figure 1).

Figure 1. Adapted from http://ecommons.library.cornell.edu/bitstream/1813/3928/2/Groundwater%20-%20Protection%20%28Fact%20Sheet%29.pdf

The most important factors that influence groundwater quality near your well include vulnerability to surface influences, existing natural groundwater quality, and proper well construction. In most cases, if a well has a water quality problem, it is related to one or more of these three issues. The problem with groundwater contamination is that we have learned the hard way that it is much cheaper to protect groundwater in the first place than it is to try to remove the contaminant from the aquifer. Understanding your own well provides you with the awareness needed to identify possible threats.

Contaminants that can get into groundwater are of two types: naturally occurring and anthropogenic. Both natural and anthropogenic contaminants can be a concern to well owners. Anthropogenic simply means the result of human activities.

Natural and anthropogenic contaminants fall into two categories: nuisance or have possible health risks. We care about both. Nuisance contaminants like iron can cause staining, and sediment can cause taste problems or possibly pump wear, for example. Those that that pose a health risk speak for themselves and we all need to be concerned about ensuring our well water is free of any unwanted constituent.

Natural groundwater quality is what it is. If you are in an area with an elevated natural contaminant, like iron or arsenic, there isn’t a lot that can be done except to treat the water. The contaminant is part of the make-up of the geologic material and will likely be found commonly in the aquifer near your current well. Aside from treatment, the only other option may be to drill a well into a different aquifer, if that is possible at your location.

Anthropogenic contaminants are the result of human activities. Both the construction of your well and the proximity of potential contaminants at the surface impact the likelihood of contamination. Finding a contaminant may mean that a number of wells are deemed unusable, and generally a regulatory agency will get involved to try and trace the source of the contamination. Contamination can occur local to the well or at a distance, traveling through the aquifer.

Surface contamination issues are more of a concern for shallower wells and for near surface geology that is less protective of the underlying aquifers. Soil is a natural filter, so the longer the flow path from the surface to where your water is being taken out of an aquifer, the better. The surface infiltration of contaminants is most influenced by having a source of contamination near your well, or having a well with sandy soils or fractured rock at the surface that allows more water to infiltrate.

Finally, extreme weather events such as floods and hurricanes can also introduce natural and man-made contaminants into your well. This can occur even if you do not otherwise have vulnerability to contamination because wells are not designed to be submerged. If you suspect your well as been overtopped by stormwater, you should have the water tested. Treatment measures such as disinfection may be required.

The Private Well Class is a collaboration between the Rural Community Assistance Partnership and the University of Illinois through the Illinois State Water Survey and the Illinois Water Resource Center, and funded by the United States Environmental Protection Agency. The views expressed here are solely from the class authors and not endorsed or reviewed by USEPA.