Bores and soft water
I would like to comment on recent articles about the city's water supply.
The unexpected soft water found in a bore at Aramoho gave those residents such delight, compared to the hard water, that relatives went there to do their washing. The bore was put down to provide more security of supply, as the Kai Iwi sources came into the city via pipelines vulnerable to landslips.
A report showed an average of $250 per year per household could be saved if the city supply was soft water. This was based on a comparison of supermarket sales of soap, shampoo and cleaning products from Whanganui and New Plymouth, with its soft water. It also considered liming up of hot water cylinders and kettles.
A referendum came down firmly on the side of softening the supply and this, along with the security-of-supply issue, led to Michael Laws asking engineers to seek soft water sources.
Six pilot holes were drilled into the limestone aquifer, their siting based on the water quality sample of the previous bores. Water quality data was also available from two supply bores put down by the MOW in Western Line and Blueskin Rd, three production bores at Kai Iwi, the production bore in Aramoho and the Whanganui East test bore.
The limestone Nukumaruan aquifer structure was well mapped, and considerable rainfall information where the recharge occurs over the area of the exposed outcrop was available.
All this information enabled an experienced consulting firm to model the aquifer, showing the direction of the underground flow and enabling flow quantities to be calculated for the entire aquifer and between flowlines.
About half of the aquifer flow is used by the city in summer. All but about 5 per cent of the remainder flows in a broad, curved path from the Kai Iwi bore sites into the Whanganui River, where it cuts deeply through the aquifer north of the city. As the water passes through the aquifer a chemical action takes place exchanging part of the hard calcium ions with softer sodium ions. The Aramoho bore is sited within this broad flowpath. The flowlines and this chemical exchange explain why the Abelard and Heloise bores produce soft water.
Unfortunately a small earthquake appears to have collapsed the screens at the Heloise bore, but the Abelard bore at the reservoir site has different screens and was unaffected.
Two new bores producing soft water could be built in this area to soften the entire city's supply. Although the bores need big pumps to draw the water up from much deeper than the Kai Iwi bores, the additional energy is largely offset by not having the pumping costs of the long pipelines from Kai Iwi.
Rather than 13,000 households spending $2000 to get soft water, about $5 million of that $26 million would give soft water, and more importantly allow replacement of the old bores at Kai Iwi and a much more secure supply for the city.
COLIN HOVEY
Former council project engineer, Whanganui
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