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.

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.

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 is a key issue as it affects 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.

What you can do

Tips for reducing nitrate in groundwater

  • Think about the specific risks on your farm. Do you have well drained soils?  Is there excess nitrogen in your farm system?
  • Avoid irrigating farm dairy effluent during wetter months (June/July) – most nitrate enters groundwater during this time.
  • Irrigate with farm dairy effluent during summer and periods of grass growth when there is less risk of leaching
  • Only apply N fertiliser during time of grass growth, this gives you best bang for buck and reduces leaching.
  • Develop a nutrient budget/nutrient management plan and wintering plan to minimise nitrate leaching, remember, lost nitrogen is money down the drain

Tips for protecting our wells

New bores
Landowners are required to obtain resource consent from us before drilling or altering a new or existing bore or well. Find out about the resource consent process.

Ensure new bores are located as far as practical from potential sources of contamination, such as septic tanks, offal holes and effluent disposal areas.

Maintaining your bore or well
Problems with reliability of bores and wells can be reduced/avoided by regular maintenance. Some key things you can do include:

  • Regularly flushing out the pressure tank to remove sediment and ensure sufficient air is retained.
  • Using an experienced contractor to flush the bore.
  • Controlling slime growths in the bore and reticulation system (simple treatment options are available).

Protecting your groundwater
Bacteria contamination is the main problem with water quality in groundwater in Southland.

Taking 10 minutes to check the condition of your bore or well may help prevent you or your family getting sick. It will also help to protect the quality of groundwater in your area.

Tips for checking your bore or well

  • Check the casing extends far enough above ground to prevent stormwater runoff entering the bore or well. If possible, place a sloping concrete pad around the casing.
  • Check the top of the bore or well is securely sealed. Tip: Silage tape is excellent for sealing around pipes and cables to make your wellhead secure.
  • Check all pipes and fittings for leaks.
  • Check the oil or grease is not leaking from pumps.

Keep stock and chemicals away

  • Keep chemicals, fertilizers, rubbish etc away from your well or bore (avoid spraying herbicides around your bore or well to control weeds).
  • Fence off the bore or well to prevent stock access.

Tips to protect your health

  • Have a sample of your well or bore tested once a year for faecal coliform bacteria. (Use one of the laboratories listed in the Yellow Pages). *
  • Ensure water filters are regularly maintained.

*Contact an Environmental Health Officer at your local council or Public Health South for advice, if sample results indicate contamination of your water supply.

Aquifer Pump Tests

Aquifer pump tests are a reliable, effective way of quantifying groundwater flow and determining aquifer properties at a particular location. A resource consent is required before undertaking an aquifer pump test if you are going to take more than 20m3 per day. You can apply for a consent at the same time you apply for your drilling permit if you wish. See our aquifer pump test application form for more information.

You can check out our monitoring information here, and you can find out more about Southland's groundwater management zones at the Land, Air, Water Aotearoa website.

Page reviewed: 05 Dec 2016 3:26pm