Groundwater quality

  • Image, Groundwater quality: nitrogen.

    Many factors influence the quality of our groundwater. Nitrogen, which occurs naturally in groundwater, can increase due to agricultural and urban land use. High concentrations of nitrate-nitrogen in groundwater can affect human health and the quality of surrounding rivers and lakes. Ammoniacal nitrogen is undesirable for aesthetic reasons (ie smell) if groundwater is used for drinking. Surplus phosphorus drains (leaches) into groundwater as dissolved reactive phosphorus. Too much nitrate-nitrogen, ammoniacal nitrogen, and phosphorus can lead to excessive plant and algae growth where groundwater flows into surface water. Escherichia coli (E.coli) in groundwater can indicate the presence of pathogens (disease-causing organisms) from animal or human faeces. The pathogens can cause illness for anyone who ingests them.

    We report on the states and trends of four groundwater quality indicators: nitrate-nitrogen, ammoniacal nitrogen, dissolved reactive phosphorus, and E.coli.

    We classified Groundwater quality as a case study.

    Key findings

    Fifty of 70 sites (71.4 percent) had E.coli concentrations that did not meet drinking water standards more than once, and 47 of 361 sites (13.0 percent) had nitrate-nitrogen concentrations that did not meet drinking water standards more than once, for the period 2012–14. 

    • Dissolved reactive phosphorus and ammoniacal nitrogen do not have drinking water standards for the protection of human health.

    Trends, for the period 2005–14: 

    • Nitrate-nitrogen concentrations at 26.5 percent of sites worsened (130 sites), 18.9 percent improved (93 sites), and 54.6 percent were indeterminate (268 sites)
    • Ammoniacal nitrogen concentrations at 19.7 percent of sites improved (57 sites), 8.3 percent worsened (24 sites), and 72.1 percent were indeterminate (209 sites)
    • Dissolved reactive phosphorus concentrations at 11.2 percent of sites improved (22 sites), 7.7 percent worsened (15 sites), and 81.1 percent were indeterminate (159 sites).

    Figure 1

    18 April 2019: The interactive map will be available again when it has been updated.

    Table 1 shows the number of times monitored sties exceeded drinking water standards.

    Table 1 

     Number of times a site exceeded the drinking water standards 2012–14 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

    Figure 2

    Note: Trends by site are assessed within a 10-year period; trend time periods vary by site within the same period. Over the 10-year period 2005–14, monitored groundwater sites were assessed for trends in dissolved reactive phosphorus (196 sites), ammoniacal nitrogen (290 sites), and nitrate-nitrogen (491 sites).

    Definition and methodology

    Nitrogen, phosphorus and E.coli concentrations in groundwater are measured from laboratory analysis of samples from well water.

    We report on the following indicators of groundwater quality: 

    • Nitrate is measured and reported as the elemental nitrogen equivalent, described as nitrate-nitrogen. High concentrations of nitrate-nitrogen can make groundwater used as drinking water unsafe (Ministry of Health, 2008). Nitrate-nitrogen can contribute to excessive plant and algae growth, leading to environmental degradation, if it enters surface water. 
    • Ammoniacal nitrogen can be toxic to aquatic life and can contribute to excessive plant and algae growth, leading to environmental degradation, if it enters surface water. 
    • Dissolved reactive phosphorus can contribute to excessive plant and algae growth, leading to environmental degradation, if it enters surface water. 
    • Escherichia coli (E.coli) is used as an indicator organism for the presence of faecal pathogens that can cause illness if consumed. To meet drinking water standards, E.coli should not be present in drinking water.

    GNS compiled data from their National Groundwater Management Programme and from regional councils’ state of the environment data. Sampling frequency varied from monthly, quarterly, to annually.

    Trends were assessed over 10 years from 2005 to 2014. Most E.coli values were censored (ie below detection), so we could not calculate trends for E.coli. To assess trends, we only included sites that had data points for a minimum of six of 10 years in the analysis. We used the Mann-Kendall test, with statistically significant trends having p values <0.05.

    We report on states as medians over the 10-year period for sites that were assessed for trends; however, we report only on sites with indeterminate trends for state, so that the influence of trends does not affect median concentrations. We also report on exceedances of the drinking water standards (Ministry of Health, 2008) for E.coli and nitrate-nitrogen for the period 2012-14. Only sites that had at least 12 samples over the three year period were included.

    Table 2 shows the number of groundwater sites measured for the period 2005–14.

    Table 2

    Number of groundwater sites measured for states and trends in levels of nitrate-nitrogen, ammoniacal nitrogen, dissolved reactive phosphorus, and E.coli, 2005–14

    State of groundwater quality

    Trends in groundwater quality

     No. sites measured




     Ammoniacal nitrogen



     Dissolved reactive phosphorus






    If you want detailed regional-level information, we recommend you review the relevant regional council’s environmental reports.

    This is because although our data are sourced from regional councils, we adjust some datasets to ensure our reports are nationally consistent. The adjustments may include omitting information produced by non-comparable methods. As a result, our evaluations may differ from those produced by regional councils.

    Data quality 

    Topic Classification   Relevance Accuracy 
     Freshwater quality, quantity and flows  Case study

     Image, Partial relevance.


     Image, Medium accuracy.



    See Data quality information for more detail.


    Ministry of Health (2008). Drinking-water standards for New Zealand 2005 (revised 2008). Available from

    Archived pages

    See Groundwater quality: nitrogen and Groundwater quality: phosphorus (archived April 2017).

    Updated 18 April 2019

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