Understanding nitrate in Gore’s groundwater

Understanding nitrate

Nitrate is a naturally occurring form of nitrogen that dissolves easily in water and can move through soil into groundwater and at low levels, nitrate is part of normal nutrient cycling. However, higher concentrations of nitrate can pose risks to human health and the environment.

National drinking water standards set a maximum acceptable value of 11.3 mg/L nitrate‑nitrogen (NO₃‑N). While responsibility for drinking water compliance sits with water suppliers, understanding nitrate concentrations in groundwater helps inform monitoring, planning and land management decisions.

What the science investigation is looking at

The science-led investigation is focused on working with the other agencies involved in this issue to reduce uncertainty, better understand the risk, and support long‑term water management for the Gore community.

The staged approach focuses on identifying broad nitrogen source types, understanding groundwater flow and age, and how the aquifer at Coopers Wells is replenished. Land use mapping and statistical modelling will also provide insights into the impact of land use change on groundwater nitrate levels.

Results from the early stages of the investigation will be shared with the community once completed. The insights gained will inform further work, which could take up to three years and cost more than $100,000 if approved.

Further work would involve:

• Additional geophysical and hydrogeological investigations using data to identify key underground features - such as aquifer thickness, confining layers, high-conductivity zones and buried faults - while additional monitoring bores would improve understanding of how groundwater moves and interacts with surface water.
• Develop a groundwater forecast model which would provide valuable insights into system response to change and future trends.

The investigation will not identify individual farms, properties or specific activities as the single nitrate source of contamination in Coopers Wells. In complex pastoral systems, different nitrogen inputs behave similarly once they reach groundwater. The work is about understanding overall patterns, timing and risk.

Understanding the long-term data

Environment Southland’s science and monitoring programmes are built on decades of data, monitoring and investigations, and the recently released report, ‘Nitrogen Contamination in Southland Groundwater’, helps clarify the complexity of nitrate levels in Southland’s groundwater.

The report was commissioned as part of the initial response to the nitrate spike in Gore’s drinking water to bring together decades of published research, monitoring data, and regional modelling, and to present it in an accessible way, making it easier for communities and decision-makers to understand and respond.

While the report doesn’t present new data, it brings together a wide range of information to give an integrated view of groundwater quality, nitrate sources, and health risks.

Murihiku Southland’s geology and shallow water table mean there is a high proportion of vulnerable aquifers. Elevated nitrate concentrations are widespread across multiple groundwater management zones, and many sites show long-term deteriorating trends.

Assessment of long-term monitoring wells show that 71% of sites show degrading trends over the 20-year period (2004-2024). This indicates that nitrate increases are persistent and reflect long-term land-use pressures rather than short-term fluctuations.

Screening analysis indicates that many domestic and public groundwater supplies are located within areas mapped as highly vulnerable to nitrate contamination, posing potential exposure risks and increasing reliance on monitoring and treatment.

The spatial patterns, trend data, and known nitrogen cycling processes indicate that diffuse nitrogen losses from intensive land use are the primary driver at regional scale.

The full report can be found on our website, www.es.govt.nz/nitrate

Why we're investigating

Environment Southland has not identified any specific cause for the nitrate spike in Gore’s drinking water supply.

Understanding how nitrate behaves in Gore’s groundwater is essential for protecting drinking water now and into the future. The results will help improve monitoring, inform future planning decisions, and support discussions with landowners and communities about long‑term water management.