Water monitoring

Groundwater

About Groundwater

Groundwater is rainwater which has percolated under the influence of gravity into the ground and flows through spaces within the rock. Layers of rock which hold and yield water when accessed by bores are called aquifers, meaning water bearing. This rainfall replenishment is termed recharge.

Most of the region's groundwater is found in basalt, sandstone, sand and shell aquifers. The region also has two well-developed geothermal aquifers at Waiwera and Parakai For information on geothermal water in the Auckland region, take this link to ARC Waterfacts sheet Geothermal Water -a precious resource

Water bores are holes drilled into an aquifer which are cased with a lining to keep the hole open. The depth of the casing determines what level the water comes from when pumped. Pumps are normally positioned down the hole, just inside the bottom of the casing, but in some situations can sit on top.

There are two kinds of aquifer as illustrated in the diagram below, confined or unconfined. In confined aquifers, the water rises up inside the bore casing under artesian pressure, and can sometimes flow out of the bore without being pumped. Unconfined aquifers have a water level the same as in the surrounding aquifer, and so are also referred to as water table aquifers.

Common types of aquifer

Aquifers in the Auckland Region, though generally low yielding by national standards, provide an important source of water in rural areas, including essential reticulated water supplies for rural communities and irrigation water for crops. One quarter of the nations fresh vegetables are grown in the Pukekohe area, irrigated with groundwater from a volcanic rock aquifer. Some industry within the Auckland municipal water supply area also use their own groundwater bores as a cost effective alternative to reticulated water which has had to be treated to a potable standard.

For further information on groundwater and bores take this link to the ARC Waterfacts sheet Drilling a bore

What we measure

ARC automated recorder sites, such as the one in the picture, are constantly checking groundwater levels across the Auckland Region. Information is sent electronically to our Auckland office. The ARC monitor 12 automatic and 63 manual groundwater bores across the Auckland Region.

automated recorder

For the location of groundwater monitoring sites where water level data can be viewed and downloaded, go to Environmental Data online.

How is groundwater measured?

Groundwater level in monitoring bores is representative of the water level within the aquifer. Some reasons for groundwater level changes are:

• water users pumping water from the aquifer
• rainfall recharging the aquifer. The rate of recharge to the aquifer is dependent upon the geology through which the water percolates
• if the aquifer extends beneath the sea the tidal movement places pressure on the aquifer.

Automatic sites

The automatic sites record groundwater levels in the bores every 15 minutes using a water level sensor called a pressure transducer which is connected to a data logger. The pressure transducer is positioned a known distance down the bore, below the water level. The pressure of water above the pressure transducer is converted to a water level, which is stored in the data logger.

pressure transducer

The water level measured in monitoring bores is related to a common datum -the surveyed level of the top of the bore casing. A groundwater rating is used to relate the measured water level in the bore back to this datum. At automated sites, the rating is applied to every 15-minute measurement to obtain a time series of information. The level derived from the rating is referred to as "groundwater reduced level".

Manual sites

The manual groundwater readings are achieved by using a groundwater measuring probe. This probe is attached to a tape, which is lowered down the inside of the bore. Once the probe touches the water in the bore an electrical circuit is closed causing a beeping signal to occur. At this time the water level is read from the tape to obtain the depth from the top of the bore casing down to water level within the bore. This measured reading is subtracted from the surveyed datum level at the top of the bore casing to obtain "groundwater reduced level".

manual water measurement

This photo is of a manual water level measurement at a groundwater monitoring site.