Factsheet: Calculating groundwater quality state

What does the groundwater quality state tell us?

When viewed on a map, state results provide a general picture groundwater quality at a regional or national scale. They highlight areas where groundwater quality is good, degraded, or where interventions may be needed.

Results from an individual well are less informative on their own. They represent groundwater quality only at one point within the aquifer, in the zone immediately outside the screen of the monitoring well, and it is difficult to determine the factors influencing groundwater at that site-specific scale. This is why data becomes much more useful when considered across an aquifer or region.

It is also important to note that monitoring data reported on LAWA is often biased toward wells with higher contaminant concentrations, as councils prioritise wells with known or high contamination risk over those with low risk and good water quality.

 

How is state calculated for our groundwater quality sites?

LAWA evaluates state (current conditions) at monitored wells for common indicators of groundwater quality.

For nitrate nitrogen, dissolved reactive phosphorus, chloride and electrical conductivity, the state value for each monitoring well is based on the median of all monitoring results collected over the last five years (2020-2024). Monitoring data are ordered from highest to lowest, and the middle value is taken as the median (or the average of the two middle values if there is an even number of results).  

For E. coli, state is represented by whether it was detected in any sample collected between 2020 and 2024:

• 'Not detected' - E. coli was below detection limits in all samples.
• 'Detected' - E. coli was detected in at least one sample. 

 

In order to provide the most complete picture, LAWA presents using all available data, whether a well has been sampled many times or only once. If only one sample was collected in the five-year period, that value is shown as the median (or as detected/not detected for E. coli).

Because of this, caution should be used in interpreting state for wells with only a few data points, or where samples were not collected across all seasons (winter, spring, summer and autumn). You can check the monitoring history for each well by selecting the indicator of interest.

 

How is state presented for groundwater quality sites?

On LAWA, state values are colour-coded for each indicator. In general, lower values are shown with lighter colours and higher values with darker colours.

 

Chloride

Thresholds are based on the Aesthetic Value (AV) for drinking water of 250 mg/L (see Aesthetic Values for Drinking Water). Chloride concentrations above this level can affect the taste of the water. LAWA uses three categories:

• 0 to 125 mg/L (up to half the AV)
• > 125 to 250 mg/L (up to the AV)
• > 250 mg/L (greater than the AV)

More information about chloride is available in this factsheet.

 

Dissolved reactive phosphorus (DRP)

There are no environmental or health thresholds for DRP concentrations in groundwater, so LAWA uses categories based on the distribution of the data. Roughly one-third of wells fall into each of the following ranges:

• 0 to 0.010 mg/L
• > 0.010 to 0.025 mg/L
• > 0.025 mg/L

More information about DRP is available in this factsheet.

 

E. coli

E. coli results in groundwater are compared with the Drinking Water Standards for New Zealand, as groundwater is a common source of drinking water. The most important thing about E. coli in a drinking-water sample is not the numerical result, but whether it is detected at all. Any detection indicates the presence of faecal material and a risk that harmful pathogens may be present. In a broad sense, that risk increases with higher E. coli counts, but even a single E. coli count of 1 in a 100 mL sample indicates a risk, and exceeds the New Zealand drinking-water standard.

LAWA groups E. coli state values as:

• Detected: E. coli was found (1 or more E.coli in a 100 mL sample) in one or more samples from 2020-2024.
• Not detected: E. coli was not detected (<1 E. coli in a 100 mL sample) in all samples from 2020-2024. 

More information about E. coli is available in this factsheet.

 

Electrical conductivity

Electrical conductivity measures how well water conducts electricity, and is reported in units of microSiemens per centimetre (μS/cm). It is closely related to the total concentration of salts dissolved in the water, and it provides a useful, inexpensive indicator of groundwater quality changes. There are no environmental or health thresholds for electrical conductivity in groundwater, so LAWA uses four ranges:

• 0 to 150 μS/cm: low values, indicating minimal concentrations of salts leached from the land surface or the aquifer material.
• 150 to 250 μS/cm: moderate values, indicating the presence of salts leached from the land surface or the aquifer material.
• 250 to 500 μS/cm: higher values, indicating higher concentrations of dissolved salts.
• > 500 μS/cm: values indicating concentrations that may be high enough to affect the taste of the water.

More information about electrical conductivity is available in this factsheet.

 

Nitrate nitrogen

Nitrate nitrogen thresholds are based on the Maximum Acceptable Value (MAV) for drinking water (11.3 mg/L) and half the MAV (5.65 mg/L) - see the Drinking Water Standards for New Zealand. It also defines a lower threshold of 1 mg/L based on the distribution of the data. Of the wells with median concentrations less than 5.65 mg/L, around half have median concentrations less than 1 mg/L, and half have median concentrations between 1 and 5.65 mg/L.

The four categories are:

• 0 to 1 mg/L (low values)
• > 1 to 5.65 mg/L (values up to half the MAV)
• > 5.65 to 11.3 mg/L (values up to the MAV)
• > 11.3 mg/L (values greater than the MAV)

Note that the MAV of 11.3 mg/L for nitrate nitrogen in drinking water is equivalent to 50 mg/L of nitrate.

More information about nitrate nitrogen is available in this factsheet.