Typically, groundwater is naturally clean and safe for consumption. Because the overlying soil acts as a filter, groundwater is usually free of disease-causing microorganisms. However, contamination may occur following improper installation of well casings or caps, after a break in the casing or as a result of contaminated surface water entering the well. Contamination can also occur if wells are drilled in fractured bedrock without an adequate layer of protective soil and with less than the recommended minimum casing length.
In order to prevent illness, wells should be properly maintained and the water regularly tested for the presence of microbial contaminants. Well water should also be tested occasionally for possible inorganic and organic chemical contaminants.
Testing Well Water for Microbiological Contamination
New wells should be disinfected by the well driller at the time of construction to eliminate any microbiological contamination that may have occurred during drilling. This should be done before collecting a sample for microbiological testing. Existing wells should be tested two or three times a year. The best time to sample your well water is when the probability of contamination is greatest. This is likely to be in early spring just after the thaw, after an extended dry spell, following heavy rains or after lengthy periods of non-use.
Depending on the province, bacteriological testing of well water is done either by the provincial health laboratory in your area or by a certified private laboratory. They will supply you with a clean, sterile sample bottle and the necessary instructions. Samples collected in any other container will not yield meaningful results and will not be accepted by the laboratory. In all instances, samples should be refrigerated immediately and transported to the laboratory within 24 hours.
If you have experienced gastrointestinal illness and suspect that it might be associated with your well water, consult your physician and local health unit.
Interpreting the Results of Water Well Testing
The microbiological quality of your water is determined by looking for the presence of bacteria indicative of faecal (sewage) contamination — namely, total coliforms and Escherichia coli. Total coliforms occur naturally in soil and in the gut of humans and animals. Thus, their presence in water may indicate faecal contamination. E. coli are present only in the gut of humans and animals. Their presence therefore indicates definite faecal (sewage) pollution.
The presence of total coliform bacteria in well water is a result of surface water infiltration or seepage from a septic system. According to Health Canada’s Guidelines for Canadian Drinking Water Quality (Sixth Edition, 1996), drinking water should not contain more than 10 total coliform bacteria per 100 mL of water. Any water containing more than this amount should be resampled. If the repeat sample contains more than 10 total coliform bacteria per 100 mL, corrective action should be taken immediately.
Water containing fewer than 10 total coliform bacteria per 100 mL is considered marginally safe to drink. Nevertheless, the water should be resampled. If fewer than 10 total coliform bacteria per 100 mL are detected, the cause of contamination should be determined if possible and corrective action taken as appropriate.
E. coli appear in water samples recently contaminated by faecal matter; thus, they indicate the possible presence of disease-causing bacteria, viruses or protozoa. Water containing E. coli is not safe to drink. Corrective action should be taken immediately.
The maximum acceptable concentration of E. coli is “0” per 100 mL of water.