A key indicator of groundwater contamination typically arising from land use intensification and/or on-site wastewater disposal systems. Nitrate-nitrogen in groundwater can affect its quality for drinking-water supply. See the LAWA nitrate factsheet for more information. Switch tabs below to look at different nitrate-nitrogen measures.
Map usage: Drag and scroll on the map to move and zoom in on areas of interest, click on a site circle to see more information and subset the table below to that specific site, and use the selection box on the map ( ) to subset table data to sites in that area.
Table usage: All calculations use nitrate-nitrogen (mg/L). Click on the +/- button at the end of a row to see the raw sample data and highlight that site on the map above. Click on table headers to sort the data by that column – holding the shift key allows sorting over multiple columns.
| Concentration rating | No. Sites |
|---|---|
| Elevated (>5.6 to ≤11.3 mg/L) | 5 |
| Intermediate (>2.4 to ≤5.6 mg/L) | 15 |
| Low (≤2.4 mg/L) | 52 |
| Total sites | 72 |
Groundwater nitrate-nitrogen concentrations are evaluated below in terms of likely human influence, as excess concentrations can have negative health effects.
Sites with annual median concentrations above the Drinking Water Standards for New Zealand Maximum Acceptable Value (MAV) of 11.3 mg/L nitrate-nitrogen are flagged as ‘Exceeds DWSNZ’, and concentrations above half of this MAV are highlighted as ‘Elevated’.
| Connectivity | ANZECC rating | No. Sites |
|---|---|---|
| A (direct) | Exceeds DGV | 11 |
| A (direct) | Within DGV | 3 |
| B/C + unknown | Exceeds DGV | 37 |
| B/C + unknown | Within DGV | 21 |
| Total sites | Exceeds DGV | 48 |
| Total sites | Within DGV | 24 |
Groundwater discharges from aquifers into a number of surface water bodies throughout the region and there is the potential that groundwater high in nitrate-nitrogen could contribute to the decline of surface water quality.
The 2000 Australia New Zealand Guidelines for Fresh and Marine Water Quality (ANZECC) define default guideline values (DGVs) for the 80th percentile of nitrate-nitrogen based on second-level River Environment Classification (REC) class reference conditions. Annual median groundwater nitrate-nitrogen concentrations are evaluated below against these DGVs.
Shallow groundwater within Connectivity Category A (direct) poses a greater risk from groundwater nitrate-nitrogen to ecosystem health in the surface water system than groundwater within other Connectivity categories. See groundwater connectivity for more information on surface water connectivity categories.
| Connectivity | Attribute band | No. Sites |
|---|---|---|
| A (direct) | A/B (≤2.4 mg/L) | 11 |
| A (direct) | B/C (>2.4 mg/L) | 3 |
| B/C + unknown | A/B (≤2.4 mg/L) | 41 |
| B/C + unknown | B/C (>2.4 mg/L) | 17 |
| Total sites | A/B (≤2.4 mg/L) | 52 |
| Total sites | B/C (>2.4 mg/L) | 20 |
The National Policy Statement on Freshwater Management (NPS-FM) classes river water nitrate-nitrogen concentrations into four attribute bands based on nitrate toxicity effects on aquatic life. The NPS-FM sets a National Bottom Line for nitrate toxicity of 2.4 mg/L nitrate-nitrogen. Annual median nitrate-nitrogen concentrations above this threshold (Bands C and D) are deemed a high risk to aquatic life. Median groundwater nitrate-nitrogen concentrations are evaluated below against these bands.
Shallow groundwater within Connectivity Category A (direct) poses a greater risk from groundwater nitrate-nitrogen to aquatic life in the surface water system than groundwater within other Connectivity categories. See groundwater connectivity for more information on surface water connectivity categories.
Ten and fifteen year trends in nitrate-nitrogen concentrations are presented on the LAWA website. See the LAWA trend factsheet for details on how the trends are calculated.