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Factsheet: Calculating groundwater quality state

What do we mean by 'state'?

On LAWA, we use the term 'state' to give a general description of groundwater quality at a point in time. For a given indicator, the ‘state’ is an estimate of the concentration or value that you might expect to find in the groundwater if you were to take a sample from a well today.

How is state calculated for our groundwater quality sites?

The state value presented for nitrate nitrogen, dissolved reactive phosphorus, chloride and electrical conductivity for each groundwater quality monitoring well on LAWA are based on the median of the monitoring results from the last five years (2015-2019).  All of the results from a given well for a given indicator were sorted from highest to lowest value, and the median was taken as the middle value in the rank, or the average of the two middle values if there were an even number of results.  For E. coli, the results are represented by whether E. coli was detected between 2015 and 2019.  Here state is represented as 'not detected' (E. coli were below detection limits in all samples) or 'detected' (E. coli were detected in at least one sample). 

In order to provide the most complete picture of the data we have available, LAWA presents state from whatever data is available, whether the well has been sampled many times or only once. If only one sample was collected during the five-year period, then the value from that one sample will be presented as the median (or detected/not detected for E. coli).

Because of this, caution should be used in interpreting state for wells that have few data points, and/or if samples have not been collected during each season (winter, spring, summer and autumn) over the five-year period.  You can see the monitoring history for a well site by selecting the indicator you are interested in.

How is state presented for groundwater quality sites?

State values are colour-coded on LAWA for each indicator. In general, lower values are represented by a lighter colour, and higher values are represented by darker colours:

Chloride

The thresholds were based on the Guideline Value (GV) of 250 mg/L set by the Ministry of Health in the Drinking Water Standards for New Zealand (DWSNZ, 2005, revised 2018). Chloride concentrations greater than the GV can affect the taste of the water. There are three categories on LAWA:

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

Dissolved Reactive Phosphorus

There are no environmental or health thresholds for DRP concentrations in groundwater, so the thresholds on LAWA are simply based on the distribution of the data. Roughly one third of the wells presented have median concentrations in each of the following three categories:

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

E. coli

On LAWA, E. coli results in groundwater are compared to New Zealand drinking-water standards, because groundwater is commonly used as a 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. If it is detected, it indicates the presence of faecal material in the water and the risk that other harmful pathogens may also be present. In a broad sense, that risk increases with higher E. coli counts, but even an E. coli count of 1 in a 100 mL sample indicates a risk, and it exceeds the New Zealand drinking-water standard.

LAWA groups the state values as one of the following:

  • Detected: E. coli was detected (1 or more E.coli in a 100 mL sample) in one or more samples from the well during 2015-2019
  • Not detected: E. coli was not detected (<1 E. coli in a 100 mL sample) in any sample from the well during 2015-2019. 

Electrical conductivity

There are no environmental or health thresholds for electrical conductivity in groundwater, but electrical conductivity is closely related to the concentration of total dissolved solids, which does have a GV of 1000 mg/L. Concentrations above this can affect the taste of the water. LAWA separates electrical conductivity values into three ranges:

  • 0 to 500 μS/cm: low values, corresponding to dissolved solids concentrations that are probably too low to affect the taste of the water
  • 500 to 1000 μS/cm: corresponds to dissolved solids concentrations that may affect the taste of the water
  • > 1000 μS/cm: corresponds to higher salinity that could be associated with seawater contamination

Nitrate-nitrogen

LAWA groups nitrate-nitrogen concentrations based on the Maximum Acceptable Value (MAV) in the DWSNZ (11.3 mg/L), half the MAV (5.65 mg/L), and a lower threshold of 0 to 1 mg/L which divides the wells with median concentrations less than 5.65 mg/L into two roughly equal groups. This creates four concentration ranges:

  • 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: The Ministry for the Environment used a nitrate nitrogen concentration of 3 mg/L in its Environment Aotearoa 2019 report to represent the upper end of the range of expected concentrations for natural conditions.  The threshold of 1 mg/L was used on LAWA, rather than the 3 mg/L threshold used in Environment Aotearoa 2019, simply because it provides a better split of the wells with concentrations less than half the MAV. Of those wells, roughly half have state values (median concentrations) less than 1 mg/L, and half have concentrations between 1 and 5.65 mg/L.

 

What does the groundwater quality state tell us?

When plotted together on a map, the state results give us a general picture of the state of groundwater quality at a regional or national scale. They help us to identify areas where groundwater quality is good or degraded, and they help to identify areas where intervention may be needed to help improve groundwater quality.

The results from an individual well are less useful on their own. They do give a picture of the groundwater quality at one point within the aquifer – the zone immediately outside the screen of the monitoring well – but it is very difficult to determine the factors influencing the groundwater at that site-specific scale. This is why regional council data are much more valuable when considered at an aquifer or regional scale.

Note also that the monitoring data reported on LAWA tends to be biased toward wells with higher contaminant concentrations. Regional councils tend to focus on wells with known contamination or high risk of contamination, rather than spending their limited resources monitoring water of good quality and low risk.