Nitrate in groundwater

What is nitrate and why does it matter?

Nitrate (NO₃⁻) is a naturally occurring form of nitrogen that is highly soluble and readily moves through soil into groundwater. At low concentrations, nitrate is part of natural nutrient cycling. However, elevated nitrate concentrations in groundwater can pose risks to human and ecosystem health indicates broader impacts from land use.

The New Zealand Drinking Water Standards set a maximum acceptable value (MAV) of 11.3 mg/L nitrate-nitrogen (NO₃-N) in our drinking water to protect against methemoglobinemia (blue baby syndrome) in infants. Ongoing research has also examined possible associations between long-term nitrate exposure and other health outcomes. Health risk thresholds and compliance responsibilities are determined nationally, with Taumata Arowai responsible for regulating drinking water suppliers.

What causes nitrate in groundwater?

Low concentrations of nitrate occur naturally in groundwater as part of soil nitrogen cycling. National assessments of reference (minimally impacted) groundwater conditions indicate that nitrate-nitrogen concentrations in oxic groundwater have an estimated 80th percentile of approximately 1.65 mg/L, while anoxic groundwater systems have much lower reference concentrations of around 0.04 mg/L. These values reflect natural inputs from soil organic matter mineralisation and atmospheric deposition.

In comparison, many Southland aquifers commonly exceed 3.5 mg/L, and in some areas exceed the drinking water standard of 11.3 mg/L, levels that cannot be explained by natural processes alone. Regional and national scientific evidence shows that elevated nitrate concentrations are primarily associated with anthropogenic nitrogen inputs, particularly from pastoral agriculture and related land-use intensification. Livestock urine deposition, fertiliser use, and nitrogen mineralisation in intensively managed soils contribute to nitrogen losses that can leach into groundwater, especially in shallow, well-drained, unconfined aquifers.

While septic tanks and other point sources can contribute locally, evidence indicates that widespread elevated nitrate concentrations are predominantly driven by diffuse land-use pressures.

What does recent monitoring show?

Environment Southland’s 2026 regional nitrate assessment synthesises long term groundwater monitoring data. Key findings include:

• Elevated nitrate concentrations are widespread across multiple groundwater management zones.
• Many sites show long-term deteriorating trends.
• Shallow, oxic, unconfined alluvial aquifers are most vulnerable.
• Concentrations frequently exceed 3.5 mg/L and in some locations exceed the drinking water standard of 11.3 mg/L.
• Screening analysis indicates that many domestic and public groundwater supplies are located within areas mapped as highly vulnerable to nitrate contamination, representing potential exposure risk and increased reliance on monitoring and treatment.

Groundwater systems respond slowly to land-use change. Improvements or reductions in nitrogen losses at the land surface may take years to decades to be reflected in groundwater quality.

What is Environment Southland doing?

Environment Southland:

• Monitors groundwater nitrate concentrations across the region.
• Assesses long-term trends and identifies vulnerable aquifers.
• Works with communities and land users to reduce nitrogen losses.
• Provides science and evidence to inform regional planning and freshwater management.

More detailed information is available in the Nitrogen Contamination in Southland Groundwater (2026) technical report.

Practical actions to reduce nitrogen loss risk this winter

• Reduce nitrogen surplus by avoiding nitrogen fertiliser applications during winter and other low-growth periods, and by regularly reviewing feed plans, fertiliser plans, and nutrient budgets so nitrogen inputs are matched to demand.
• Keep stock off saturated and vulnerable soils during high-risk periods, particularly on free-draining and pugging-prone areas. Use stand-off pads, feed pads, barns, deferred grazing, or lower-risk paddocks where possible.
• Improve effluent management before winter pressure builds by lowering effluent pond levels early, ensuring adequate storage, applying effluent over larger areas, using low application rates, and avoiding application when soils are saturated or heavy rain is forecast.
• Maintain ground cover where possible and capture residual nitrogen by planting catch crops as soon as practical after winter forage grazing, and by planning spring re-sowing early so bare soil is not left exposed for longer than necessary.
• Use lower-risk feed and forage options where practical, including lower-protein feeds, mixed pastures, plantain or other alternative pasture species, and forage systems that reduce urinary nitrogen loading. Some of these changes are more relevant to next season’s planning than to crops already in the ground.
• Use weather and soil moisture information to make day-to-day decisions, especially before grazing, fertiliser application, irrigation, or effluent spreading. For regional environmental data, visit envdata.govt.nz.

Private water schemes

Some properties have their water supplied by a privately owned community water scheme. If your water is supplied by a privately owned scheme, you should contact the scheme's owner directly if you want information. That provider must have a drinking water safety plan or acceptable solution.

Taumata Arowai maintains the public register of drinking water supplies which you can search and find the drinking water supplies in your local area. Some drinking water supplies may not yet appear on the register because of the phased introduction of this requirement for smaller suppliers.

Keeping your private bores and wells safe

In many cases, problems with the reliability of bores and wells can be avoided by regular maintenance. Regular maintenance may also reduce pumping and equipment replacement costs.

Water can be contaminated by bacteria, viruses, nitrate, metals and other chemicals. Some contaminants occur naturally, while others come from human activities. If you have your own well or surface source, the responsibility to keep your drinking water safe rests with you.

Check out the factsheet for actions you can take to keep your water supply safe and test your water supply for contaminants.