Groundwater

Groundwater is water that has made its way down through the soil to underground areas called aquifers.

Aquifers are not like areas of surface water, such as lakes. Instead they are areas filled with sand, gravel or rocks which 'hold' water. Water sits in the spaces between the gravel and rocks.

• Find out more about the outcomes (objectives) set for our freshwater and estuaries. These are the goals that have been set to ensure that we can all have the water we want in the future.
• Read the report that outlines the latest assessment on environmental health
• Check out the map that shows sites closest to you, change the different measurements and see the current state and the outcome we’re striving for.

The issues

Groundwater provides an important source of drinking water for people and livestock in Southland. It's also used for irrigation and dairy shed wash down, and it can be the primary source of water in streams over summer (baseflow).

However, what we do on top of the land can affect the quality of the groundwater sitting below. Nitrogen contamination of groundwater in Southland is very common. It is a key issue as it affects the health of people, livestock and the broader environment, particularly when nitrogen contaminated water enters streams in summer causing problem algal and plant growth. Go to 'Estimated Total Oxidised Nitrogen Concentration' layer (tick the layer on the left panel) on our Consents map for more information.

Nitrogen is an important ingredient in fertilisers and is extensively used in agriculture to promote grass growth. Although nitrogen is great for grass growth, we can have too much of a good thing. Often the amount of nitrogen in the soil exceeds the amount that can be used by the plants and this excess nitrogen changes to nitrate (NO3) in the soil. Nitrate is negatively charged and is not 'attracted' to soil (doesn't stick to soil particles). Instead it is water soluble, so will move with the flow of water via leaching down through the soil to the groundwater below.

Nitrate contamination of groundwater can affect the health of people, livestock and the broader environment.

Nitrate transfer

Nitrate dissolves in water so moves with it through the soil. How much nitrate moves down through the soil depends on the soils ability to hold water and remove nitrate through natural chemical processes (denitrification).

Unsaturated (drier) soil has the capacity to soak up and 'hold' more water. Water (and dissolved nitrate) tends to sit in the soil, rather than drain downwards. Soil is 'saturated' when it can't hold any more water. When more water is added (via rainfall); it will generally drain downward carrying nitrate with it. Leaching to groundwater occurs more frequently in gravelly or coarse soils that have a low ability to hold water and no restrictions to deep drainage.

Aquifers in Southland

Aquifers in Southland tend to be shallow. In many places the water level is only 1.5 to 5.0 metres below the ground. Elsewhere in the country, aquifers are often much deeper - up to 100 metres. Another key feature is their shape - in Southland they tend to be long and thin. They can cover a large area underground but don't tend to be very deep.

Types of aquifer

Aquifers can be divided into two broad categories:

1. Confined aquifers - have a layer of rock or other sediment above them that separates them from the surface above. This layer acts as a barrier to water so that it can't easily move vertically into or out of the aquifer
2. Unconfined aquifers - are connected to the water table. They are fed by rainwater and by areas of surface water such as streams or rivers.

What we are doing

Groundwater management zones

Our Regional Water Plan establishes a framework for managing groundwater abstraction in Southland. In order to manage groundwater resources, unconfined aquifers have been subdivided into 26 groundwater management zones based on areas of similar hydrogeological characteristics. Confined aquifers are managed separately. They respond differently to abstraction and recharge so are therefore more difficult to quantify. Currently in Southland there are confined aquifers with significant levels of abstraction in the Mossburn/Lumsden area and at Wendonside.

Information sheets have been produced for each groundwater management zone to provide an overview of the hydrogeology.

View the groundwater zone information sheets on our Beacon map. Click on the zone you're after to download the corresponding factsheet.

Surface water/groundwater links

Traditionally, management of water resources has focused on surface water or ground water as if they were separate entities. However, we now have a better understanding of the linkages between them. Nearly all areas of surface water (such as streams, rivers, lakes, and wetlands) interact with groundwater from unconfined aquifers. Water quality and quantity of surface water can affect the water quality and quantity of groundwater - and vice versa.

Scientists are continually learning about the complexities and links between groundwater, surface water and what we do with our land.

Read our scientists' quick explanation of groundwater - surface water interaction.

Environment Southland scientists have been investigating how long it takes for nitrate to move through the soil to underlying aquifers. This is known as 'lag time', and depends on rainfall, the type of substrate (rock, gravel, sand, silt and clay) sitting below the soil and the depth to groundwater.

Southland has a mosaic of different soil types, as well as different types of rock and gravel sitting below. Factor in different average rainfall levels for different areas and a complex picture emerges. However there are some general patterns.

Scientists estimate that for about 80% of Southland it takes less than one year for nitrate to travel from the soil down to the groundwater. For 90% of Southland, it takes less than two years. The quickest draining areas tend to be found on 'young' substrates located in lowland areas. The slowest draining areas are found on 'old' substrates at higher elevations.

Read our factsheet on groundwater and nitrate movement.