Groundwater (the water under the ground surface) is an important source of drinking water, irrigation water, and a major contributor to surface water flows. It can sometimes take decades, or longer, for water (and any contaminants it contains) to cycle from the earth’s surface and through the ground to aquifers, so the impact of what we are doing today may not be measurable in groundwater for many years. Similarly, the quality of groundwater we see today may be the legacy of activities from many years ago.
Nitrogen and phosphorus occur naturally in groundwater, but additional input from agricultural discharges and urban land use can increase concentrations. High concentrations of nitrogen and phosphorus in groundwater can affect the quality of groundwater-fed rivers and lakes if aquifers have low attenuation capacities (McDowell et al, 2015). High nitrate-nitrogen or phosphorus concentrations may also have a negative effect on groundwater ecosystems. However, very little research has been done in New Zealand to define the effect of these nutrients on groundwater ecosystems. Nitrate-nitrogen and E.coli pose a public health concern if concentrations are above drinking water standards in groundwater used for drinking water. See Land-based activities are putting pressure on water quality for the pressures on water quality.
We report on the results for groundwater quality against the drinking water standards (see table 4), focusing on the variables that could affect public health.
Table 4: Drinking water standards
| Drinking water standards | Status | Water quality variable | What it means |
|---|---|---|---|
| Microbial variables – drinking water quality standards maximum acceptable value | Regulatory, for drinking water supplies | E.coli | Maximum concentration of E.coli in water that, based on current knowledge, constitute no significant risk to the health of a person who consumes two litres of that water a day over their lifetime (usually taken as 70 years). |
| Inorganic variables – drinking water quality standards maximum acceptable value | Regulatory, for drinking water supplies | Nitrate-nitrogen | Maximum concentration of nitrate-nitrogen in water that, based on current knowledge, constitute no significant risk to the health of a person who consumes two litres of that water a day over their lifetime (usually taken as 70 years). |
Some monitored groundwater quality sites are not meeting drinking water standards
In this report, we compare results from monitored groundwater quality sites with the drinking water standards (Ministry of Health, 2008), because there are no other standards for groundwater (eg no standards are available for cultural values or ecosystem protection). We assess the results from each groundwater sampling occasion against the drinking water standards. The sites are those monitored by regional councils and GNS Science in their environmental monitoring networks, which are not set up to monitor drinking water supply. Therefore, these results are not representative of drinking water quality in New Zealand.
The Health (Drinking Water) Amendment Act 2007 requires drinking-water suppliers who serve more than 500 people to implement a water safety plan for their water supply. However, the drinking water standards apply to water intended for drinking by the public irrespective of the water’s source, treatment, or distribution system, whether it is from a public or private supply, or where it is used.
The assessment was limited to sites that were sampled on 12 dates or more in the period 2012–14. We determined how many times a site failed to meet the drinking water quality standard. For E.coli, 50 of 70 groundwater sites (71 percent) did not meet the drinking water standard at least once (see table 5; analysed by Ministry for the Environment and Stats NZ). For nitrate-nitrogen, 47 of 361 sites (13 percent) did not meet the drinking water quality standard at least once.
Table 5: Number of times a site did not meet the drinking water standard
| Number of times a site exceeded the drinking water standard | E.coli (number of sites) | Nitrate-nitrogen (number of sites) |
|---|---|---|
| 0 | 20 | 314 |
| 1–2 | 15 | 19 |
| 3–5 | 14 | 5 |
| 6–10 | 15 | 16 |
| Over 10 | 6 | 7 |
| Total | 70 | 361 |
For more detail see Environmental indicators Te taiao Aotearoa: Groundwater quality [Stats NZ].
We have insufficient data to determine groundwater trends at most monitored sites
Of monitored groundwater quality sites, trends in nitrate-nitrogen were worsening at 26 percent of sites (130 sites), improving at 19 percent of sites (93 sites), and were indeterminate at 55 percent of sites (268 sites) for the period 2005–14 (see figure 20) (analysed by Ministry for the Environment and Stats NZ). Over the same period, trends in ammoniacal nitrogen at monitored groundwater quality sites were indeterminate at 72 percent of sites (209 sites), improving at 20 percent of sites (57 sites), and worsening at 8 percent of sites (24 sites). For dissolved reactive phosphorus, trends were indeterminate for 81 percent of sites (159 sites) for the period 2005–14. Trends were improving at 11 percent of sites (22 sites) and worsening at 8 percent of sites (15 sites) over the same period (analysed by Ministry for the Environment and Stats NZ).
We could not determine E.coli trends because a high proportion of samples at each site were below the detection limit.
Figure 20
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This graph shows trends in concentrations of dissolved reactive phosphorus, ammoniacal nitrogen, and nitrate-nitrogen at monitored lake sites between 2005 and 2014. Visit the MfE data service for the full breakdown of the data.
Note: Over the 10-year period 2005–14, monitored groundwater sites were assessed for trends in nitrate-nitrogen (491 sites), ammoniacal nitrogen (290 sites), and dissolved reactive phosphorus (196 sites).
For more detail see Environmental indicators Te taiao Aotearoa: Groundwater quality [Stats NZ].
Groundwater monitoring shows there is little health risk from pesticides in groundwater
Pesticides can move through soils and enter groundwater. Of 153 sampled wells in the 2014 survey, no pesticides were detected at 83 percent of the wells. One or more types of pesticides were detected in groundwater from the remaining 17 percent of wells, but only in one well (in Waikato) did the concentration of a pesticide (dieldrin, an insecticide) exceed its maximum acceptable value defined in the drinking water standards (Humphries & Close, 2014).
We report only on pesticides in groundwater as we do not have enough information on pesticide concentrations in surface water.
For more detail see Environmental indicators Te taiao Aotearoa: Groundwater pesticides [Stats NZ].
Monitored groundwater quality
March 2021
© Ministry for the Environment