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How The Greymouth Bore System Works

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The three existing bores are about 10m deep into the gravels at the Coal Creek site. We are very fortunate to have clean deep gravels in the Grey Valley because it provides a natural filter bed that holds back fine sediment and other impurities before it is distributed through the network. Systems in NZ and overseas often require the construction of filtration systems that are large, complex and expensive, both to construct and to maintain.

This system of filtration through gravels is a real advantage when the river is running dirty, in flood.

There is a huge amount of water in the gravels in the Grey Valley. That is why the river keeps running for days after it has stopped raining, the groundwater constantly flows through these gravels, it’s like a giant sponge that is slowly draining.

The groundwater level is higher when the gravels are re-charged and you’ll see higher river flows because the ground drains into the river at a higher rate.

As the water level in the ground drops the river also feeds water back through the gravels and that is a state that we are in currently.

Water in the ground exerts what is known as hydrostatic pressure, the deeper the column of water in the ground the more pressure there is.

Pumps are positioned at the bottom of the bores. If they are turned off the water level in the bores and in the surrounding gravels would be at the same height.

When the pumps are on, the water drawn from the wells is assisted by the hydrostatic pressure, which causes water to flow towards a negative pressure zone created by the pumping.

So more water can be extracted when the groundwater level and associated hydrostatic pressure is high and conversely when water levels in the ground are low and less hydrostatic pressure is exerted, less water can be extracted.

When water is pumped up from the bores it enters a treatment plant where it is treated by ultraviolet light, Lime is added to balance PH levels, which protects the network and home plumbing from corrosion, and chlorine is added. It then enters a balance tank which is like a large reservoir, where the chlorine evens out and acts on any bacteria which may be present in the water.

It’s useful to know that one of the main reasons that chlorine is added to the water supply is that it’s difficult to ensure that bacteria cannot contaminate the water supply within the network once it has left the treatment plant so the chlorine acts on any bacteria that may be present in the distribution network, keeping people safe.

Water is pumped from the balance tanks under high pressure to a series of pipes and reservoirs which feed water to the community.

Why can’t water be pumped directly from the Grey River to the network?

While the river looks quite clean, it actually contains a lot of impurities like algae, E.coli and nitrogen and the volume of water needed to supply the town would result in considerable contamination of the network. UV treatment consists of glass tubes with UV lights and the tubes would quickly build up material and become ineffective. A boil water notice could be issued but the water would still be quite dirty, clog things like shower heads, toilet cisterns pressure reducing valves on hot water systems, cause ceramic taps to drip and it would have an unpleasant smell and taste. When reverting back to the clean supply Council would have to completely decontaminate the entire plant, shutting it down and cleaning out the large balance tank, UV treatment, valves and other parts at great cost.

As a last resort the Council could consider pumping water from the river into the network but the technical problems would be significant and would also result in unhappy property owners who would have problems with their own plumbing systems.

Why doesn’t the Council just dig a trench from the river to the bores?

The gravel bed, that acts as a filter, is very sensitive to disturbance and there is a potential that excavating too near the bores could permanently damage the filtration properties of the gravel, and if we excavate a trench to the bores, next time it floods water might not be filtered and would contain high levels of turbidity, potentially running brown when you turn on your taps.

Council staff are seeking specialist geotechnical engineering advice to see if there is a safe limit for excavation that may provide higher flows to the bores without damaging the filter effect.

Why doesn’t the Council drill the bores deeper?

A Kaiata mudstone (Parpa) layer exists in the ground at the bottom of the existing bores. The exiting bores are currently at the most optimum depth and drilling deeper would not provide any advantages.

The Council has a project currently in progress, constructing a 4th bore. This will improve the operational capability of the existing plant.

Councils obligations as a water supply authority

Water supplies are registered with Taumata Arowai, as the national regulator for public water supplies. Drinking water supply authorities are required to meet the requirements of the Water Services Act 2021 and associated regulations. Over recent years the regulations related to water supplies have become more stringent and continue to do so. It is complex maintaining compliance, and rule changes that occurred at the end of last year mean that the testing Council needs to do now to prove compliance has tripled. When we make changes to the supply we must inform the regulator and show them how our supply continues to comply. It would be very difficult to show the regulator that we were compliant if we did something like pumping straight from the river into the network.

Would the three waters reform prevent the type of water shortage we are currently experiencing?

When a water supply network is designed and constructed, engineers calculate the required demand, including long term calculations, to understand the volumes of water that a network needs to produce for the community. Investigations are undertaken onto the environmental conditions to understand and determine the most suitable method of extracting the required volume from the environment. A design is produced, and the system is constructed and commissioned.

Councils use the same pool of engineers from the private sector for this work that the new Entity D water authority will likely use.

Runanga and the Sids Road bores

Council has started to prepare the Sids Road plant for potential use to service the Rapahoe, Dunollie and Runanga areas. There are several factors that will determine whether or not the bores can be used and it’s not a quick process of just turning on the bores. The ground contains impurities that may mean that the water would not meet current drinking water standards, so the bores are being flushed to clear impurities. Because of the ground structure we may end up clearing those impurities, but levels could increase as more are drawn towards the bores from further out. The plant has never been fully commissioned and so how successful this is at this point in time is not fully understood but is worth the effort as that additional capacity may take pressure off the Coal Creek supply and we really don’t know how long the dry period of weather may persist. Chlorination equipment is being installed and contractors are continuing to work on commissioning, including PH management and filtration. This is a complex process and there are numerous factors which could affect our ability to produce compliant drinking water.

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