Groundwater is water that comes from the ground. It's water that's travelled through the soil to underground 'aquifers'. Aquifers are porous geological formations, often gravel and sand which act like a sponge, soaking up and storing water.
Groundwater is the main source of water in the middle of the Poverty Bay Flats away from the Waipaoa River. The aquifers supplying groundwater were formed from gravel deposits left behind by the Waipaoa River over many thousands of years.
Groundwater in the Gisborne region is used for the irrigation of crops, water for stock and to provide vital backup drinking water when other sources are unavailable. Groundwater is particularly important as the rivers and streams of the region all typically have low flows in summer.
Almost all the known groundwater aquifers in the region are located on the Poverty Bay Flats. There are over 1,400 bores on the flats drawing water for irrigation and domestic use.
Map of Poverty Bay Aquifers (225kb)
There's also shallow unconfined sand aquifers at Tolaga Bay, Tokomaru Bay, Ruatoria, Tikitiki and Te Araroa.
Aquifers in Poverty Bay
A gravel aquifer that can be encountered at depths of 45-80m depending on location and ranges in thickness from 5-15m.
- Aquifer is the most utilised in this region and is under some stress with water takes ranging from 100-6000 m3/day.
- Seasonal water level fluctuations tend to be within 5-10m, therefore submersible pumps are recommended as they are not limit by lift capacity.
- Surface pumps are not recommended as a reliable efficient extraction system during the peak of summer as generally they only have a lift capacity of 4-6m.
- Water quality is variable.
Information about the Managed Aquifer Recharge Trial
A narrow gravel aquifer that can be encountered at depths of 90-140 metres in the subject area.
- This aquifer is presently under little stress however a large water permit which has had little use to date (5760m3/day) is likely to put the system under stress if exercised in the future.
- Past pump tests indicate that this aquifer experiences large drawdown when pumped, therefore surface pumps are not recommended as a reliable efficient extraction system.
Te Hapara Sands aquifer
A shallow sand aquifer that can be encountered just below the topsoil and can be up to 20 metres thick.
- Aquifer is under little stress and has water takes ranging from 45-1850 m3/day.
- Seasonal water level fluctuations tend to be within 0.5-1 metres, therefore surface pumps are usually sufficient to extract water.
- Water quality is variable.
Waipaoa gravels aquifer
Gravels laid down within the meandering river channel cut into fluvial, swamp, lagoon and estuarine sediments.
- Aquifers located at depths of 15-30 metres
- Recharge likely to be from the Waipaoa River.
- Aquifer appears to vary in water level by 1-2 metres between winter and summer.
Shallow fluvial deposits are also found at depths of 0-20 metres at a number of locations around the Flats and can be up to 10 metres thick.
Te Karaka gravels aquifer
Gravels aquifer that can be encountered at depths of 10-30m depending on location.
- Seasonal water level fluctuations tend to be within 1-3m.
- Water quality is variable and further information is available by contacting our water resources team.
Permit requirements to install a bore
You need to apply for a bore permit to install any type of water bore. This is to ensure we have a record of all bore locations.
A bore permit does not mean that a water permit will be issued to take water from the bore.
Up to 10 cubic metres of water can be taken for domestic and stock water needs from an underground bore. For more than 10 cubic metres - a water permit is required.
For a bore permit - see resource consent application forms
There's no fee for domestic or stock monitoring bores.
Non domestic bore permits are charged at actual cost - see fees and charges
A bore or well can be drilled into an aquifer and secured with a casing pipe which brings the water to the surface.
The cost of installing a groundwater bore depends on the depth and how the water is to be pumped. Pump testing of the bores is sometimes required to find out if it's having a negative effect on neighbouring properties. A pump test also provides council with valuable information that can determine the storage capacity and how quickly the aquifer can recover from pumping for an individual bore.
For bores which are to be used all summer, a submersible pump is recommended, as water levels can drop below the lift capacity of most surface pumps. A screen should be fitted at the bottom to avoid the pipe or pump from clogging.
The groundwater travels through the soil until it reaches a depth where the sediments are filled with water - the 'saturated zone'. This process 'recharges' the groundwater supply. The top of the saturated zone is called the water table in an unconfined aquifer and may be very close to the ground surface or many metres down.
Groundwater can also be recharged when water from rivers or streams travels down through the riverbed to replenish the aquifer below.
In Gisborne, water entering through the groundwater system beneath the Poverty Bay Flats flow towards the coast through the sediments. Although - with the exception of the Te Hapara sand aquifer - the aquifers have no direct connection with the sea.
Potential groundwater contaminants
Groundwater is not protected from contaminants. Soil can act as a filter, although it's not able to filter out all impurities.
Contamination of groundwater can happen gradually and go unnoticed for some time. Most contaminants come from the ground surface, shallow watertable aquifers with thin soil cover are easily contaminated.
Contamination of groundwater can come from:
- animal effluent
- fertilisers and pesticides
- leakage from pipelines and underground storage tanks
- salt water intrusion.
Because the flow of groundwater is slow with little turbulence, contaminants are not easily diluted or dispersed. Pollution can therefore stay a long time in groundwater and can be very difficult or impossible to clean up.
Naturally occurring contaminants include minerals which dissolve out of the surrounding rock such as iron, calcium, magnesium and manganese. These are generally only a nuisance, at worst - clogging irrigation nozzles, rather than being a risk to human health or the environment.