Our groundwater resource

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Groundwater quality

The hydrogeological setting of an area determines how vulnerable groundwater aquifers are to contamination. For example, shallow aquifers lying under thin permeable soils, especially sandy soils, are at most risk from contamination.

Aquifers of the Waikato region are highly variable and include:

basalt aquifers (Pukekohe)
coastal sand aquifers (Coromandel Peninsula)
sand and gravel alluvial aquifers (Hamilton Basin and Hauraki Plains)
ignimbrite and pumice aquifers in the Central North Island (Tokoroa and Taupō ).

The chemical character of groundwater is determined by:

rainfall
type of soil
aquifer geology
land use.

In some aquifers the natural water quality is unsuitable for some uses because of its chemical characteristics. For example, geothermal groundwater is usually not suitable for drinking.

Contamination of groundwater can occur when substances make their way down through the soil into the underlying aquifers. This generally happens slowly and may not be noticed for some time, but once groundwater is polluted it is very difficult to clean up. That’s why it’s so important to protect and monitor our valuable groundwater resource.

Contaminants in our groundwater come from either point (from specific locations) or non-point sources (from wider areas).

Point sources include:	Non-point sources include:
Septic tanks, leaking effluent treatment ponds and landfills	Pesticide and fertiliser applications
Leaking underground fuel tanks and pipelines	Agricultural land use
Mines and waste tailings	Application of wastes to land
Chemical storage areas and timber treatment sites	Saltwater intrusion.
Waste disposal sites, such as offal holes.

In many areas groundwater quality is declining due to:

an increase in the amount of waste water discharged onto land – to about 460,000 m³/day
increased use of nitrogen fertiliser
a doubling of stocking rates over the last forty years - animal waste from intensive farming contaminates groundwater with nitrate.

The nutrients present in groundwater affect the plants and animals that live in the rivers and streams it flows into.

Groundwater use

Rainfall naturally replaces water pumped from aquifers. The amount of water in our aquifers will sustain our needs as long as we don’t take too much.

Recent years have seen large increases in the amount of surface and groundwater used in our region. When too much groundwater is taken:

the level of groundwater left in aquifers lowers (lowering of the water table)
there may not be enough water for everyone to use, resulting in competition for water
less groundwater flows into streams - during extended dry periods most of the water present in a stream may be from groundwater
and may subside
in coastal areas salt water may come into freshwater as the water table drops.

Our groundwater resources themselves are not generally under pressure. Our climate provides enough rainfall to replace (recharge) the amount of groundwater being taken at present. Average rainfall in our region is 1,250 mm, varying from:

1,100 mm in the Hauraki Plains.
3,500 mm in the Coromandel Ranges.
5,000 mm at Mt Ruapehu.

Where possible, Waikato Regional Council encourages the use of groundwater rather than surface water. Although, in some catchments groundwater is connected to surface water and groundwater allocation may be limited by surface water availability.

Waikato Regional Council issues around 400 consents each year to drill groundwater wells. Taking smaller amounts of groundwater (less than 15 m³ per day) does not require resource consent. Together, taking these smaller amounts of groundwater add up to a similar amount of water use requiring consent to take (around 305,000 m³ per day).

Groundwater around the region

Most of the Coromandel Peninsula is made up of volcanic rocks (andesite and rhyolite), which generally have few fractures and little space to hold water.
Some Coromandel settlements, including Whangamata, Hahei and Onemana, take water from these fractured rock aquifers. Other settlements, such as Pauanui, Cooks Beach, Matarangi, Whiritoa and Whangapoua rely on small, coastal sand aquifers for their community water supply. So far these aquifers have provided enough water to meet demand. However, groundwater demand in these areas is very high over the summer months and is increasing. Increased demand for groundwater increases the risk of aquifers drawing in salt water (salt water intrusion).
Groundwater quality in the Coromandel Peninsula is generally high. However, localised microbial and nitrate contamination has been found in areas reliant on septic tanks.
Groundwater supplies up to 85 percent of the water in Hamilton Basin streams. Only three percent of the total water available in the Hamilton Basin is being used. Of this, about 75 percent is from groundwater.
Groundwater quality in the Hamilton Basin is highly variable. High nitrate levels are common in shallow, unconfined aquifers. High levels of iron are common in deeper aquifers located in peat sediments.
The Hamilton Basin is filled with alluvial sediments (mainly tephra from volcanic eruptions) distributed over time by the ancestral movements of the Waikato River. The main aquifers in the basin are made up of gravel and sand found amongst layers of silt and clay.
Aquifers in the Hauraki Plains are located in areas of alluvial sediments. These aquifers are similar to those found in the Hamilton Basin, being made up of gravel and sand found amongst layers of silt and clay.
Aquifers are recharged by rain falling on the southern Hauraki Plains. In the lower lying northern plains the emerging groundwater feeds streams and swamps by upwelling (artesian) flow.
Little use is made of groundwater in the Hauraki Plains compared with the total water available each year. As a result, long-term monitoring does not show any evidence of declining water levels.
The map below shows that groundwater quality in the Hauraki Plains is very variable. Nitrate and dissolved oxygen levels in groundwater are higher in the southern parts of the Plains than in the north. Nitrate levels also decrease with depth below the ground surface.
Iron and ammonium levels have the opposite pattern, being higher in northern parts of the Plains. This reflects the higher peat content in the soil in northern areas. High iron levels make groundwater unsuitable for drinking.
Concentrations of nitrogen and ammonia at sampled sites in the Hauraki Plains
In both Pukekawa and Pukekohe, the shallow basalt aquifers make up most of the base flow in streams, especially during periods of drought. Summer demand for water is often greater than the flow in streams so water use has to be managed carefully.
Groundwater quality in the basalt aquifers is naturally very high. But shallow aquifers are becoming increasingly degraded from intensive land use.
Pukekohe
Pukekohe’s water resources are in high demand, particularly for watering commercial vegetable gardens during summer. During dry periods the water available in local streams is fully allocated, and sometimes over allocated. If too much water is taken from streams, aquatic plants and animals will suffer.
Groundwater makes up most of the base flow in streams of the Pukekohe area. In order to protect them, careful management is needed to balance the use of both shallow groundwater and stream water.
The Pukekohe area is divided by the administrative boundary of the Auckland and Waikato Regional councils. Both authorities share the management of these water resources. Waikato Regional Council manages water resources in the Pukekohe area by:
- restricting the use of groundwater from shallow aquifers. Shallow groundwater is only available for stock and domestic use.
- declining new applications to take water from streams, except where water is taken during times of higher flow and stored for later use
- applying ‘no take’ days and water use restrictions when stream levels are extremely low
- regularly monitoring groundwater levels to make sure not too much water is being taken.
Much of the land in the Pukekohe area is cultivated for commercial vegetable growing. Groundwater underlying cultivated land is particularly vulnerable to contamination due to:
- use of agrichemicals (fertilisers and pesticides) to enhance crop production
- low organic content of many cultivated soils
- low levels of soil microbes needed to break down harmful chemicals such as pesticides
- low water holding ability of cultivated soil (water does not get ‘filtered’ before reaching aquifers).
Pukekawa / Onewhero
Market gardening is the main land use in the Pukekawa/Onewhero area. During the summer water resources are heavily relied upon to keep crops well watered. Groundwater from shallow aquifers supplies a third of the area’s water supply, and also sustains the local streams. Intensive land use is degrading the water quality in the shallow aquifers.
Tokoroa’s groundwater is sourced from fractured volcanic rock (ignimbrite) aquifers. Two of these, the Waiotapu and Whakamaru aquifers, provide most of the water required for Tokoroa’s industry, communities and agriculture.
The Waiotapu aquifer is one of the most productive in our region. More than 40,000 m³ of groundwater is taken from it daily. The Kinleith Pulp and Paper Mill uses most of this water, but it also provides the Tokoroa town water supply.
Total groundwater use is about 20 to 40 percent of total flow through the aquifer, but there is no evidence of over use.
There is a strong interaction between groundwater and surface waters around Tokoroa. Over 90 percent of runoff in the Pokaiwhenua catchment comes from groundwater outflow.
The quality of Tokoroa’s groundwater is naturally very high. The diagram below shows the large plume of contamination that extends northward from the Kinleith Pulp and Paper Mill, but drinking water guidelines are not exceeded at any neighbouring water supply wells.
Hydrogeology
Local geology influences both the flow and quality of water entering the lake. Groundwater in the Taupō area is taken mainly from ignimbrite aquifers and less commonly from alluvial aquifers.
Rainfall easily soaks through ash soils down to the aquifers, carrying with it nutrients such as nitrate. Groundwater is recharged from rainfall and drains toward the lake, which acts as the local sink. Most aquifers in the area produce only small groundwater flows. However, coarse pumice airfall deposits and alluvial sediments can produce larger flows.
Effects on Lake Taupō
Lake Taupō currently has excellent water quality, but long-term water clarity has fallen and is threatened by increasing amounts of nitrogen entering the lake.
Nitrogen enters the lake predominantly from groundwater, directly or indirectly via streams.
Dating of groundwater in Lake Taupō catchment shows that higher nitrate concentrations are often found in younger groundwater. It also indicates where nitrate concentrations are likely to increase.
Nitrate concentrations are expected to continue to increase for some time unless land use changes.
Geothermal extraction
Most of the groundwater taken in Taupo is from geothermal areas, and is used for power generation. About 242,000 m³ per day was taken for power generation in 2002, compared with 547,000 m³ taken per day across the whole region.
Geothermal extraction for electricity production at Wairakei and Ohaaki/Broadlands is causing:
- reduced pressure and temperatures in geothermal fields, leading to ground subsidence
- changes in heat flow
- loss of some geothermal features such as geysers
- changes in the plants and animals that live there.
There is also evidence of cold water flowing into the geothermal systems, especially at Wairakei where most geothermal wastewater has not been put back into the system.

Our groundwater resource

Groundwater quality

Groundwater use

Groundwater around the region

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