Horowhenua District - Artesian Water

Artesian Water

Much use is made of artesian water supplies. The northern part of the district, around Opiki, is situated above a series of aquifers. These are vertically separated from each other by layers of less permeable material, known as aquicludes. Within each aquifer, there may also be pockets of less permeable material, known as lenses. In descending order, the aquifers below the Opiki district are the Foxton, the Himatangi, and the Opiki Aquifers. Over its whole extent, which is much wider than the Opiki area, the Foxton Aquifer lies mostly between 30 m and 60 m below ground level. Around Opiki it is shallower, generally between 24 m and 35 m below ground.

Although not unique, the Foxton Aquifer in this area has the relatively uncommon quality of being under pressure so that when a bore is tapped into it, the water rises under its own pressure and will flow above ground; i.e. there is flowing artesian water. The head varies with ground level but can be as high as about 2.5 m above ground level. The level to which it will rise defines the potentiometric surface. For obvious reasons, the farmers and residential users who have bores tapping into the Foxton Aquifer regard this as a most useful attribute. In many cases it can avoid the necessity of pumping water from the bore and, for those even more fortunate, it can avoid the necessity of reticulating the water around farm properties by pumping systems. There are natural seasonal fluctuations in levels and pressures, but there is evidence that, even in times of extended dry periods, no property in the area loses positive pressure entirely, although some decline to a critical point. As an aside to that point, it is inevitable that the existing bores in the area will have reduced the potentiometric surface from its original level, and that future permitted bores will continue that trend. There must come a time when those properties which now become critical at those times will lose flowing artesian water.