6 Examples of How Pathogens May Get into a Water Supply
The following scenarios illustrate a range of ways in which pathogens can (and do) contaminate a community’s water supply.

  1. Damaged storage tank
    At a supply serving 50 people, groundwater is abstracted via a deep bore, chlorinated, and then stored in a wooden tank prior to use. Over time, some of the wooden planks that make up the roof of the tank have warped and twisted, allowing gaps to form. In addition, some of these planks rot. The gap formed allows birds and rats to get into the water, and they start nesting inside the reservoir. Even though the water is chlorinated and the water tested for free chlorine, the contamination is intermittent. Not all of the contamination involves microorganisms but rather organic matter that causes a chlorine demand – meaning that chlorine gets used up before it can control the micro-organisms in the water. The operator of the supply notices that there is very little free available chlorine in the distribution system. Then an E. coli test shows a positive result of 56 faecal coliforms per 100 mL. The problem is identified by the testing and fixed before anyone gets sick.
  2. High turbidity with a UV system
    In a small community supply serving 20 households and using a surface water source, a small river flows through lowland pasture with some dairying upstream. The water is pumped out of the river through the gravel (a form of pre-filtration) then disinfected via UV light immediately prior to use. During high flows, the degree of turbidity and contamination increases, causing poor UV light transmission, meaning that micro-organisms can get past without being inactivated. Users notice the change in water clarity but this cloudiness is a regular occurrence and consumers ‘get used to it’. Then some parents notice that children have diarrhoea after the water has been cloudy. Microbiological tests of the water when it is cloudy show high levels of E. coli. Tests of the children show that some of them have campylobacteriosis.
  3. Damaged well head
    A small school uses a groundwater supply for drinking-water. Clean, safe water is pumped from underground direct to use, without treatment, but the bore is in a paddock that also contains cattle. The well head is not secure due to a hole drilled in the concrete apron, allowing the otherwise safe water to be contaminated with runoff from the paddock. This results in intermittent contamination of the water, after heavy rain. This problem requires immediate action. Visual inspection would identify this particular problem but the school caretaker does not realise the risks of having an insecure well head. The water supply comes under suspicion when many of the children are sick with diarrhoea after a period of heavy winter rains.
  4. Chlorine pump failure
    A small community’s water is extracted from the ground via a pump, and chlorinated by a dosing pump dispensing liquid chlorine. The water is then stored in a well holding tank prior to use. As a community collaborative effort to operate the supply, different individuals are on a roster to check the free available chlorine levels and that the treatment plant is running, but some people forget or are not sure what to do. If the chlorine solution runs out or the pump fails to work, or even if chlorine is overdosed, there could be a problem. The chlorine dosing pump becomes blocked but still makes ‘pumping noises’. The chlorine levels drop. The water is from a shallow bore that contains contaminants from farm runoff. The water supply is contaminated with pathogenic micro-organisms until someone realises the chlorine pump is not working correctly and it is repaired.
  5. Spring water with no treatment
    Water is supplied via a spring that flows into a covered box, to exclude surface water interference. There is no filtration or disinfection. The springs are situated in lowland bush and there are livestock present in the area. Without the protection of barriers against micro-organisms getting into the water supply, any fault in this system would lead to illness. Without frequent and regular inspections, some problems could go unnoticed until it was too late. For example, if the lid of the collection box came off, it would allow surface contamination of the otherwise clean spring water.
  6. Roof water supplemented by tank water delivery
    The owner of a roof water supply has to purchase some drinking-water when the household supply runs low during a dry spell. A worker for a roading company uses the company’s water-tank truck to deliver some water during weekends and after hours. During the day, water from a small stream on a farm is used in the truck to wet down dirt on the roading site. After work, the worker fills the truck with water from the town supply without cleaning out its tank first. Some highly contaminated water from the stream is left in the truck, contaminating the load. The water delivered to the house is contaminated. The owner becomes ill with Giardia.