What is groundwater?
Groundwater is the water stored beneath the earth’s surface. When it rains, some of the rainwater soaks into the soil and percolates down into the ground. It fills cracks in rocks and pore spaces between grains of sand and gravel, creating an underground body of water that we call an aquifer. There are vast quantities of water in aquifers beneath the ground surface, slowly flowing and seeping through all those cracks and pore spaces. Groundwater provides habitat for a diverse range of aquatic invertebrates known as stygofauna. For more info read our ‘LAWA Groundwater Basics’ factsheet.
What is groundwater used for in New Zealand?
Groundwater is a vital resource. It accounts for over three quarters of all freshwater in New Zealand and is used as a supply for drinking water and industry. Forty percent of New Zealanders rely on groundwater for their drinking water. This includes whole cities such as Wellington and Christchurch, through to smaller communities and households across Aotearoa. Groundwater is also important because it is a major contributor of water to lakes, rivers, and streams. During summer, when rainfall is low, much of the water flowing in waterways has arrived via groundwater.
How does groundwater become contaminated?
Groundwater quality can be affected by land use or discharges to land and water that seep into the ground. In rural areas, the main risks to groundwater quality come from pathogens, nutrients and pesticides. In urban areas, pathogens are also a risk, but industrial chemicals pose a greater risk than nutrients and pesticides. In many cases, wells are contaminated by water at the land surface flowing directly into the well. This risk can be minimised by making sure the well head is secure.
What are the possible effects of groundwater contamination?
Pathogens (bacteria and viruses) and elevated nitrate concentrations in water supplies can make people sick. Elevated nutrient concentrations in groundwater can impact the health of spring-fed streams and lakes. Excessive groundwater use in coastal areas can draw sea water into aquifers making the water unsuitable as a supply for drinking water or irrigation. The data available on LAWA indicates that groundwater contamination from pathogens and nutrients does occur, but that seawater intrusion is not widespread.
What is the overall state of New Zealand’s groundwater quality?
Groundwater monitoring tells us that while most groundwater is of very good quality, contamination from E. coli (the most commonly-used indicator for the presence of pathogens) and nitrate does occur. All groundwater is potentially vulnerable to pathogen contamination, and people should have their well water tested regularly. Nitrate concentrations exceeded the New Zealand Drinking Water Standards in around 6% of wells tested. Roughly a quarter of groundwater sites had concentrations higher than half the Maximum Acceptable Value (MAV). At a regional level, we find a mix of areas with improving and degrading trends. Generally, the wells with higher nitrate concentrations are found in areas of intensive animal grazing or horticultural production.
Seawater intrusion is thankfully not a widespread problem, but ongoing monitoring is important so that we know if the situation changes. Pressures such as future sea level rise, drought, and human use can contribute to salinization of groundwater.
What information is available on the LAWA Groundwater Quality Topic?
On the Groundwater Quality Topic, you’ll find long-term data for almost 1,000 groundwater sites, factsheets that explain the science behind water quality monitoring, and an interactive map that allows for regional level exploration of groundwater quality state and trend.
The topic shows monitoring results for five widely recognised indicators of groundwater quality:
- E. coli is an indicator that pathogens could be present that could make people sick.
- Nitrate and phosphorous are nutrients that can affect spring-fed streams. At higher concentrations, nitrate may also be harmful to human health.
- Chloride and electrical conductivity are most important as indicators of seawater intrusion in wells near the coast.
The Groundwater Quality Topic presented on LAWA shows historical data and is intended to be used for regional level state of the environment insights. Anyone using groundwater as a source of drinking water should make sure their well is sealed and have their water tested regularly.
What can New Zealanders do to help protect local groundwater quality?
Dispose of waste chemicals properly, maintain on-site wastewater (septic) systems, and manage farming activities including fertiliser use and any animal stock.
Who is responsible for monitoring groundwater quality?
Regional councils and unitary authorities have the important job of helping their communities manage groundwater quantity and quality. As part of this management, their staff regularly monitor local groundwater sites working alongside GNS and other independent laboratories to analyse the data.
In New Zealand, households not connected to the mains are responsible for monitoring their own bore water. Testing is the only way to detect some contaminants such as nitrate.
What does groundwater monitoring involve?
Groundwater is monitored by taking a representative sample of water from the aquifer. For regional and unitary council state of the environment monitoring, this generally requires the bore to purged of around 3 times its standing volume to ensure the sample is drawn from the aquifer, although there are other methods that allow sampling directly from the screen where water is drawn into the bore.
Samples are collected in the appropriate bottles, cooled and then sent to an independent, accredited laboratory for testing. Some samples are sent to the GNS Science Wairakei Analytical Laboratory for analysis as part of the long-term National Groundwater Monitoring Programme (NGMP). This programme is operated by GNS Science in collaboration with regional councils and unitary authorities.
The results are then processed by councils and made publicly available through the LAWA website. Cawthron Institute has generated the site level state and trend information you’ll find on the LAWA Groundwater Quality Topic. For more read the ‘Monitoring groundwater in New Zealand’ factsheet.
Why is it important to also manage groundwater quantity?
It’s important to manage groundwater quantity because when the level of groundwater in aquifers lowers, the flow on effects can impact drinking water availability, river and spring flows, some wetlands, land stability, and in coastal areas salt water may come into fresh water as the water table drops. The LAWA Water Quantity topic has more info about river flow, rain and groundwater levels.