LAWA Groundwater Quality National Picture Summary 2022

Publish date: 22 March 2022

Groundwater is a vital resource for New Zealand. It is widely used as a source of drinking water, whether by individual households with their own wells, or by communities and cities including Christchurch and Wellington. Groundwater is also used for irrigation and industry.  It feeds many streams and lakes. Understanding the quality of groundwater helps us to manage land uses that affect it and the uses that depend on it.

Pathogens, nitrate and other contaminants can affect the suitability of groundwater for various uses. Groundwater contaminants, especially nitrate and phosphorus, can also affect water quality in spring-fed streams and lakes. Over-use of groundwater in coastal areas can draw seawater into aquifers, rendering the groundwater unsuitable for drinking or irrigation.

LAWA shows groundwater quality information from wells located throughout New Zealand. Data and information are available from more than 900 wells.  Regional and unitary councils monitor groundwater for a range of water quality indicators, five of which are displayed on LAWA: chloride, electrical conductivity, dissolved reactive phosphorus, E. coli and nitrate-nitrogen.  State and trend analyses for these indicators have been updated using data up to the end of December 2020.

Data collected by regional councils and unitary authorities shows that while most groundwater is of very good quality, contamination from E. coli and nitrate does occur. All groundwater is potentially vulnerable to pathogen contamination (indicated by E. coli), so people should have their well water tested regularly. Nitrate concentrations exceed drinking-water standards in some wells, and long-term trends are variable, with roughly equal numbers of wells displaying improving and degrading trends. Seawater intrusion is not widespread, but continued monitoring is important so that we know if the situation changes.

This national picture presents an aggregated view of the state of long-term monitored wells and how the groundwater quality has changed over the past ten years.

LAWA evaluates the five groundwater quality indicators against various thresholds, depending on the indicator. The state for any given year is a median value based on available data over the previous five years, except for E. coli where we report on whether E. coli has been detected at all over the previous five years.  For more detail on how state values are calculated for groundwater quality, see this factsheet.

Nitrate-nitrogen

LAWA groups nitrate-nitrogen concentrations based on the Maximum Acceptable Value (MAV) in the Drinking-water Standards for New Zealand (11.3 mg/L) and half the MAV (5.65 mg/L). For more information, refer to the LAWA factsheet or to this page on the Ministry of Health website.

To help illustrate where lower concentrations occur, LAWA also defines a threshold of 1 mg/L based on the distribution of the data; of the wells with median concentrations less than 5.65 mg/L, roughly half have state values (median concentrations) less than 1 mg/L, and half have concentrations between 1 and 5.65 mg/L.

Groundwater from about two thirds of the wells presented on LAWA has a five-year median concentration greater than 1 mg/L. For the most part, this reflects contamination from human activities. Concentrations are higher than half the MAV in around 25% of the wells presented, and they exceed the MAV in about 6% of monitored wells. There has been little change in these proportions over the past ten years.

The higher nitrate concentrations are generally found in areas of intensive agriculture, either grazing (for example Canterbury, Southland and Waikato) or vegetable farming (such as Pukekohe in Auckland and the Horowhenua in Manawatu-Whanganui). A few regions, such as Bay of Plenty, Gisborne and Marlborough, appear to have relatively low concentrations.

Nitrate-nitrogen concentrations have been fairly stable at the national level over the past ten years (Figure 1), though at a regional level, there is a mix of areas with improving and degrading trends. For more detailed information, you can browse LAWA to look at state and trends at individual wells.

LAWA National Groundwater Quality State Change Over Time (2011-2020)

Nitrate-Nitrogen

Figure 1. Changes in the state of nitrate-nitrogen concentrations from 2011-2020 in groundwater from the 623 wells where there were enough data to determine a result each year. The location of these monitoring sites is shown on the map.

Dissolved reactive phosphorus (DRP)

There are no environmental or health thresholds for DRP concentrations in groundwater, so the results shown on LAWA are based on the distribution of the data. Approximately one third of the sites shown on LAWA fall into each of the three categories shown on the map and in Figure 2.

DRP can leach into groundwater from farm land, but for the most part, DRP concentrations in groundwater are related to aquifer material. Groundwater in volcanic rocks or limestone aquifers tends to have higher DRP concentrations because the aquifer material contains more phosphorus. This explains many of the higher DRP concentrations in the volcanic areas of Northland, Taranaki and Bay of Plenty, or in limestone aquifers in Hawke’s Bay and northern Canterbury.

In contrast, alluvial gravels derived from greywacke contain very little phosphorus. Therefore, groundwater beneath the Canterbury Plains and other alluvial gravel plains, such as in Tasman, Marlborough and West Coast, has relatively low DRP concentrations.

At a national scale, it appears that the number of wells in the lowest concentration category has decreased somewhat, while the number of wells in the middle category has increased (Figure 2). The reason for this is not yet clear. The number of wells in the highest concentration category has remained fairly stable.

LAWA National Groundwater Quality State Change Over Time (2011-2020)

Dissolved Reactive Phosphorus (DRP)

Figure 2. Changes in the state of dissolved reactive phosphorus (DRP) concentrations from 2011-2020 in groundwater from the 363 wells where there were enough data to determine a result each year. The location of these monitoring sites is shown on the map.

E. coli

LAWA treats E. coli results differently to the results for the other four indicators. When calculating a value for state, LAWA considers whether E. coli was detected in any of the samples collected over the past five years. If so, the state for E. coli is shown as “detected”.

This is because 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 E. coli 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.

Roughly half the wells presented on LAWA have had at least one E. coli detection in the past five years. This shows that groundwater supplies are widely vulnerable to faecal contamination. The most common sources of this contamination include grazing animals, farm effluent disposal and on-site wastewater systems.

In many cases, wells are contaminated by water at the land surface flowing directly into the well. This risk can be minimised by making sure the well head is secure. The area immediately surrounding a well should be fenced off to prevent stock access and kept clear of any potential contaminants. Well casing should be elevated above ground and above stormwater and flood levels. The top of the well should be securely sealed or capped, and any hoses or cables should also be securely sealed.

 There is little pattern to where E. coli has been detected, except that contamination is most common where the water table is shallow. Even then, detections have been recorded in wells where the water table is more than 50 metres below the ground surface.

Figure 3 shows how E. coli detections have changed over the past ten years. There are 469 sites where we can calculate a state value for every year between 2011 and 2020. In 2011, E. coli had been detected in 45% of those wells over the previous five years. By 2020, that percentage had increased to 55%. The reason for this trend is not clear, but it may reflect increased E. coli contamination of groundwater, possibly related to more intensive animal grazing and growing numbers of onsite wastewater disposal systems.

As a final note, please keep in mind that the wells presented on LAWA are not all necessarily used for drinking-water supply, and the results do not represent drinking water quality in New Zealand. The results represent the source water, the raw water before any treatment, which could potentially be used for water supply.

LAWA National Groundwater Quality State Change Over Time (2011-2020)

E. coli

Figure 3. Changes in the state of E. coli detections from 2011-2020 in groundwater from the 469 wells where there were enough data to determine a result each year. The location of these monitoring sites is shown on the map. Note that unlike the other indicators shown on the page, which use median values, the state of E. coli in a given year is determined only by whether E. coli was detected in any sample over the preceding five-year time period.

Chloride

LAWA compares chloride state to thresholds 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 water.

For the vast majority of wells, chloride concentrations in groundwater are far below the taste threshold, and this doesn’t change much over time (Figure 4).

The highest chloride concentrations are found in groundwater in coastal areas. In some cases, a very high concentration may indicate seawater intrusion, where overuse of a well can draw in seawater from offshore. Aquifer sediments deposited in marine environments may have a high salt content, which can lead to naturally elevated chloride concentrations in groundwater even many kilometres from the coast. Older groundwater and groundwater in volcanic rock aquifers may also have somewhat elevated chloride concentrations, though these concentrations are still likely to be well below taste thresholds.

LAWA National Groundwater Quality State Change Over Time (2011-2020)

Chloride

Figure 4. Changes in the state of chloride concentrations from 2011-2020 in groundwater from the 670 wells where there were enough data to determine a result each year. The location of these monitoring sites is shown on the map.

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.

As with chloride, most groundwater in New Zealand does not have high enough salt content to affect its taste, and electrical conductivity values are well within the low value range on LAWA (Figure 5). Electrical conductivity can also reflect increased water hardness, which is caused by increased concentrations of calcium and magnesium derived from limestone. Old groundwater that has had a long time to dissolve minerals from the aquifer material can also have higher electrical conductivity.

LAWA National Groundwater Quality State Change Over Time (2011-2020)

Electrical Conductivity

Figure 5. Changes in the state of electrical conductivity from 2011-2020 in groundwater from the 689 wells where there were enough data to determine a result each year. The location of these monitoring sites is shown on the map.

This National Picture Summary provides an aggregated overview of the state of groundwater quality across five indicators using data from regional councils and unitary authorities. Council groundwater quality monitoring programmes are designed to capture environmental state and trend on a regional scale.

Looking at data from individual wells provides further information about state and trend and additional details such as well construction. Information about monitored wells can be accessed either by clicking on a site dot on the map to the left of the LAWA main screen (desktop and tablet users), or by navigating via the region buttons above, then the groundwater zone, and well site of interest. The agency responsible for monitoring an individual site may be able to provide further contextual information on the results shown here.