Why you need to use protection
A little detective work reveals a common issue for anyone with bore-supplied drinking water.
In my early days as an enthusiastic new groundwater scientist, I was called to visit a lifestyle block in the Manawatu. I was met at the gate by a lovely man who was very concerned that his wife had become sick after drinking water from their private bore. He had two questions: who was responsible for contaminating his water supply, and what was I going to do about it?
I arrived at the farm armed with sample bottles, naively thinking this was going to be my Erin Brockovich moment and feeling every bit the ‘CSI’ scientist.
I surveyed the surroundings. The ‘bore’ wasn’t a bore but an open concrete well. A little gravel-bed creek next to the property was unfenced and the paddock around it was full of stock. “Sir, how deep is your... bore?” People often assume that if water comes out of the ground it is going to be safe to drink. Unfortunately, this isn’t always the case. The 2016 outbreak of campylobacter in the Havelock North water supply has drawn attention to the importance of ensuring water supplies are safe and secure. Discussion and debate around the potential cause of the outbreak also highlighted the range of ways in which bore water can become contaminated.
How we treat our land and waterways can impact the quality of groundwater, and this was certainly the case for the block owner I met that day. However, it is by no means the only way that water supplies can become contaminated.
Water on the move
Groundwater forms part of the larger flowing water system that is connected to our rivers, lakes and streams. Water moves through spaces between soil and rock, and is pumped to the surface to supply water for a range of uses.
Most stock water and domestic groundwater supplies in New Zealand are drawn from shallow sand and gravel aquifers that have formed in old river channels. Often this groundwater is ‘unconfined’, meaning that the aquifer has a direct path to the land surface. Water in shallow groundwater systems is often recharged directly from rainfall or from water lost from the base of streams and rivers. This means that shallow groundwater can be quickly replenished.
However, it also means there is an increased risk of contamination from nutrients and bacteria which can be readily transported by water moving through soils and waterways into the groundwater system.
In deeper aquifers, layers of sediment such as clay and mudstone create a barrier (known as a confining layer) that restricts the flow of groundwater between the aquifer and the surrounding rock. Water in deep, confined aquifers can be hundreds or even thousands of years old. Because of the time it takes for water to reach these aquifers, there is generally a low risk of contamination from bacteria or nutrients.
Water quality in confined aquifers can be very good, but can also be naturally mineralised and contain high levels of iron, manganese or arsenic. Soils and geology play an important role in the quality of groundwater and its suitability for different uses.