Summary - Nitrate contamination in groundwater report

Key findings

1. Nitrate contamination is widespread

Monitoring and modelling show that elevated nitrate concentrations occur across multiple Freshwater Management Units (FMUs) and Groundwater Management Zones (GMZs).

• 66% of assessed GMZs have mean nitrate-nitrogen concentrations above 3.5 mg/L\
• Mean concentrations for some GMZs exceed 8.5 mg/L, and one zone exceeds the drinking water Maximum Acceptable Value (MAV) of 11.3 mg/L
• Elevated concentrations are not confined to one catchment or FMU, they are regionally distributed.

2. Long-term trends show deterioration at many sites

Trend assessments of long-term monitoring wells show that:

• 60% of assessed sites show likely or very likely degrading trends over 10 years
• 56% show degradation over 15 years
• 71% show degradation over 20 years

This indicates that nitrate increases are persistent and reflect long-term land-use pressures rather than short-term fluctuations.

3. Several identifiable “hotspot” areas exist

Spatial analysis highlights areas where nitrate concentrations are consistently elevated. These include:

• Balfour / Waimea Plains
• Wendonside Terrace
• Five Rivers / Castlerock
• Edendale
• Knapdale
• Mabel Bush / Woodlands
• Central Plains / Waimatuku
• Otahu Flat

These areas often share common characteristics:

• Shallow, unconfined alluvial aquifers
• Well-drained or bypass-prone soils
• Oxic groundwater conditions (limited natural attenuation)
• Long histories of intensive land use

4. Agriculture is the primary driver at regional scale

The spatial patterns, monitoring data, and established nitrogen cycling science indicate that diffuse nitrogen losses from agricultural land use are the primary source of nitrate contamination at the regional scale.

Key contributing mechanisms include:

• Livestock urine deposition
• Fertiliser application
• Effluent irrigation
• Winter forage crop grazing

Dairy systems typically generate higher nitrogen losses to water than most other land uses due to higher stocking rates and nitrogen inputs. The expansion and intensification of dairy farming in Southland since the 1990s coincides with widespread increases in nitrate concentrations.

Point sources (e.g. septic tanks, infrastructure failures) can cause localised contamination, but the regional scale and persistence of elevated nitrate concentrations is consistent with diffuse inputs across the landscape.

5. Drinking water risk is location-specific

Groundwater vulnerability was assessed at a screening level using spatial modelling and supply location data.

Findings show:

• 49% of domestic drinking-water supply wells are located in areas modelled to have nitrate concentrations between 3.5–8.5 mg/L at 10 m depth
• 3% are located in areas predicted to exceed 8.5 mg/L
• 2% are located in areas predicted to exceed the drinking water MAV (11.3 mg/L)

This represents a screening-level assessment of vulnerability. It does not mean that these supplies experience these concentrations or exceed drinking water standards.

Registered public supplies are routinely monitored and managed to meet national drinking water standards.

6. Lag times are important

Groundwater responds over long timeframes.

• In many parts of Southland, nitrate can travel from soil to shallow groundwater in less than two years
• In other locations, travel times may be longer.

This means current groundwater quality reflects both historical and more recent land-use practices.