Grandpa used pesticides on my property 50 years ago. Should I be concerned about contamination of my well?

Posted on

About Peter Doris

Peter is an Environmental Specialist for the Ontario Ministry of Agriculture, Food and Agribusiness (OMAFA) in east central Ontario. He provides information and recommendations to farmers, agri-businesses, municipalities and other individuals and groups on issues related to the Nutrient Management Act, “normal farm practices” and the Farming and Food Production Protection Act and interacts regularly with the public and other ministries and agencies about agricultural nutrient use, water quality, soils, drainage and other agricultural environmental issues.

See all posts by Peter Doris.

Occasionally, I receive a question or concern about impacts from the previous use of pesticides on a property. These concerns could be categorized as historical or legacy concerns from the use of pesticides over 40 or 50 years ago. This article provides some context for these past concerns and some options for follow up for the current property owner.

There are several good sources of information that can help a farmer minimize the risks associated with current use of pesticides. While the current use of pesticides is not the focus of this article, a list of some of these resources can be found in the appendix for further reading. These are specific to Ontario, Canada, but your region should have similar resources.

How common is pesticide contamination of well water?

In an extensive project that collected water samples from 1,290 private wells throughout Ontario in the 1990s, University of Guelph and Waterloo University researchers found that most of the problems with well water were related to bacteria; approximately 1/3rd of well water did not meet the standard for coliforms and E. coli followed by nitrates (14% exceeded guidelines). It important to state that these problems are NOT related to previous or current pesticide management! Of the 1,290 wells sampled, water samples from six of the wells did show pesticide residue levels above the interim maximum level established at the time of publication for this study. The text below is copied from the published scientific article that discusses the findings for bacteria, nitrates, pesticides, and petroleum derivatives in the water well sample for this study.

“About 40% of the nearly 1,300 wells tested contained one or more of the target contaminants above the maximum acceptable concentration (Table 4). Bacteria were the most widespread form of contamination with about 34% of wells having more than the maximum number of coliform bacteria (faecal coliforms, or E. coli, or total coliforms) permissible in drinking water. Some 14% of the wells contained NO -N concentrations above the 10 mg/L limit and about 7% of the wells were contaminated both with bacteria and nitrate. Six wells contained pesticide residues above the interim maximum acceptable concentration (IMAC). One contained alachlor, one contained metolachlor, and the remainder contained more than 5 µg/L of atrazine, or the total concentration of atrazine plus deethylatrazine exceeded 5µg/L . Records -1 showed that a spill caused the one well to be contaminated with metolachlor. None of the wells tested contained detectable petroleum derivatives.”

-Goss, M.J, Barry, D.A.J. and Rudolph, D.L.  Contamination in Ontario Farmstead Domestic Wells and its Association with Agriculture 1. Results from Drinking Water Wells. 1998.

While this study is almost 30 years old, there are a couple of key takeaway messages here that are still valid today.  For all Ontarians that rely on private wells for their drinking water, samples should be taken for free testing through your local Health Unit for bacterial contamination.  This should be done regularly, at least once or twice per year.  Based on the study results, other contaminants are less likely to be found in your drinking water but water samples can be tested for these other potential contaminates including pesticides for a fee; see section below on “where can I send my water samples to test for pesticides”.

What are some other sources of information about previous pesticide use on a property?

The best source of information is likely the person who was responsible for previous pesticide management on the property; typically, this is the farmer. The farmer may be able to recall the type of pesticide products used or at least give an indication of previous cropping practices and livestock husbandry which can provide some indication to the type of products that may have been used. In addition, the farmer may be able to provide an indication where/how products were stored and mixed and application practices. But a word of caution here: some farmers might be hesitant to answer questions “out of the blue” about the past management practices with pesticides because of liability concerns. 

In some cases, the Ministry of Environment, Conservation and Parks (MECP) may be helpful with this type of inquiry. MECP oversees the Pesticides Act in Ontario and as such they might be aware if there are any local legacy or historical issues with pesticides spills or pesticide contamination of groundwater in your area. Some lawyers involved in real estate transactions will query the local District MECP office to see if there have been any environmental orders filed related to a specific property prior to finalizing a sale. 

What are some additional considerations when thinking about the historical use of pesticides on a property?

Based on the information from the 1990 water well study mentioned above, pesticide contamination appears to be a rare occurrence; 6 samples out 1,290 indicated some presence of pesticide residual in the samples. If there were problems with previous management, it may relate:

  1. Improper storage of pesticides
  2. Poor techniques with the mixing or handling of pesticides
  3. Disposal of empty pesticide containers, or
  4. Equipment malfunction such as improperly closed valve on a sprayer

Concerns related to storage or mixing and handing can help to focus in on a specific area(s) on the farm. For example, if it is known or there is a strong belief that the pesticides were stored in “the old shed” or that pesticides were routinely mixed in the sprayer while filling from the tap beside the barn and there is a well near this area, then testing of a water sample for pesticides may provide some peace of mind.

If we test our well water for pesticides, what pesticides do we test for?

Because there are many different chemical formulations for current and past pesticides, the lab will ask which class or type of chemicals that you want the water analyzed for. Asking for the lab to analyze for many different types of pesticides will increase the lab cost substantially. 

Having some background on the property can be helpful here. For example, if the concern is from the 1960s and 70s and the crop rotation on the property at time was hay-cereals-corn, then a couple of the commonly used pesticides at that time were atrazine (corn) and 2,4-D (cereals). 

While this background information may help to narrow down the focus to fewer pesticides and save money on lab analysis, it should be noted that there were multiple pesticide formulations used 50 years ago so if the testing is narrowed too much, you might miss the pesticide(s) that was actually used on the farm at that time. There is a trade-off here by narrowing down the number of pesticides and keeping lab costs lower versus an increased risk of not analyzing for right pesticide. 

Where can I send my water samples to test for pesticides?

There are several private labs that can analyze water samples for pesticide residues for a fee. Depending on the number of parameters (i.e. the number of pesticides) that you ask to be analyzed for will be a major factor in how much the analysis cost. 

The Ontario Groundwater Association, the association that represents well drillers in Ontario, has a link to a water testing program called “My Water Quality” and one of its water testing packages will test for approximately 20 different pesticide residues in water. The cost for this testing package is $998 per sample at the time of writing this article in the winter of 2025.

Some concluding thoughts

It is understandable that rural residents relying on wells for their drinking water may be concerned about the quality of the water coming out of their taps. Bacteria contamination is the most common problem found with private well water and fortunately, you can test your private well water for free through your local health unit. 

There is limited data on pesticide contamination of water well in Ontario and based on this limited information, it appears to be a rare problem, thankfully! However, if you are still concerned about the impacts of pesticide use and management in the past on your property, there are private laboratories that will test your water for a fee. For some people, paying the fee for this type of testing may allow them more peace-of-mind the next time that they turn on the tap.

Appendix

The following links will help Ontario farmers and landowners understand and manage the risk associated with current pesticide use and management on their properties. Again, for readers outside Ontario, there should be similar resources in your area.

Ontario.ca and search “Pesticide contamination of farm water sources

Learn how to avoid contaminating any well or surface water source by properly mixing, loading or applying pesticides and what to do if a spill should occur. This technical information is for Ontario producers.

Ontario.ca and search “Assessing the potential for ground water contamination on your farm

Learn about a risk assessment procedure to select best management practices to reduce groundwater contamination. This technical information is for Ontario producers.

Ontario Pesticide Education Program and Grower Pesticide Safety Course

The Ontario Pesticide Education Program supports Ontario farmers and pesticide vendors to achieve pesticide safety certification and training. Renewal is every 5 years.

Author

  • Peter is an Environmental Specialist for the Ontario Ministry of Agriculture, Food and Agribusiness (OMAFA) in east central Ontario. He provides information and recommendations to farmers, agri-businesses, municipalities and other individuals and groups on issues related to the Nutrient Management Act, “normal farm practices” and the Farming and Food Production Protection Act and interacts regularly with the public and other ministries and agencies about agricultural nutrient use, water quality, soils, drainage and other agricultural environmental issues.

    View all posts