What we know
The cause of the gastric disease was Campylobacter. Campylobacteriosis is the most common form of water and food borne illness in NZ. And poultry is the most common source of this disease in NZ. However, in this case, sheep droppings were almost certainly the source. The question is how did bacteria get from sheep get into the Havelock North water supply?
There is no suggestion yet that the contamination entered the distribution network after safe water had been drawn from the well/s so that restricts the inquiry to how dirty water got to the bottom of the well/s in the first place.
We also know that the outbreak occurred just after a heavy rainfall/flooding event. There is not much in the documentation provided so far to indicate whether that was particularly relevant but we shall see.
The theoretical ideal
The various possible threats to human health from contaminated water are supposed to be prevented either through only supplying safe groundwater or treating water thoroughly before it is consumed by humans. For a better explanation of the science go here. In Havelock North only safe groundwater was supposed to go out in the pipes. The diagram below shows how this works - not only in Havelock North but in many other water supplies around the country:
OK it's a cow but we are talking sheep here. So groundwater that started its life way up country is isolated from surface contaminants by the impervious layer of solid clay or whatever. By the time the water gets to the bottom of the well it is no threat to human health. On the surface, contaminants in animal faeces may get soaked into the ground by rain but they float along the surface layer and don't get into drinking water.
Scenario A - the well was faulty
The first scenario that will get tested is whether the well that draws drinking water up from the depths to be distributed was poorly constructed.
Drilling a hole through the impervious layer obviously creates a weakness. When it comes to court this week Hawkes Bay Regional Council will allege that Hastings District Council failed to seal the top or the sides of their well correctly. This would allow contaminated water to flow from the surface down the sides of the well and then get drawn back back up through the well into the water supply.
If proved this is pretty much the slam dunk. The techniques for creating what is called a sanitary seal around the well are well known and are contained in the New Zealand Drilling Standard. Proof will be difficult because HDC have at least two independent engineer's reports stating the well-heads are safe. HBRC will also have to explain why they signed off on the safety of the well-heads year after year then changed their minds after a major incident. As will the Drinking Water Assessor working for the Ministry of Health.
But a poorly constructed well would definitely make HDC the sole culprit.
Scenario B - the impervious layer was broken
Hastings District Council appear to be lining up to allege that Hawkes Bay Regional Council consented earthworks that damaged the impervious layer and allowed contaminated water to enter the "secure" aquifer upstream from the water intake.
Apparently it is possible that earthworks (even relatively shallow ones) can disturb the impervious layer enough that it is no longer impervious.
From a purely legal point of view this would put HBRC firmly in the frame. Remember that since 2008 they have been responsible for delivering safe-ish groundwater to HDC. If they gave consent to someone to carry out major earthworks that compromised a public water supply then they are almost entirely responsible for the gastro outbreak.
Even if this is proved HDC and MoH will still need to explain why they did not include the risk of a failure in the impervious layer in their Water Safety Plan. It is a risk and should have been documented even if they did not take any action to prevent or mitigate the risk.
Scenario C - the Hand of God
Of course it is possible that the contaminated water simply found a previously unknown pathway through the impervious layer. In fact this was one of the first suggestions made by a scientist with knowledge of the local hydrogeology. His theory was that prolonged drought had allowed fissures to open in the ground providing that pathway.
It is identical to Scenario B above except that there is no human intervention.
Again HDC and MoH will still need to explain why they did not include the risk of a failure in the impervious layer in their Water Safety Plan. It is a risk and should have been documented even if they did not take any action to prevent or mitigate the risk.