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6 Summary and Conclusions

This study has shown that data from New Zealand’s various SOE groundwater quality monitoring programmes can be meaningfully compiled at a national scale. Based on the current status of groundwater quality in New Zealand (section 2), the major national-scale issues are:

  • contamination with NO3-N and/or microbial pathogens, especially for shallow wells in unconfined aquifers

  • naturally elevated salinity, Fe, Mn, NH4-N and/or As, especially for deeper wells in confined aquifers.

These issues vary in importance at different monitoring sites and in different regions, but at the national scale the current status of groundwater quality can be summarised as follows.

  • The health-related guideline value for microbial pathogens is exceeded at 20% of the monitoring sites for which indicator data were available. This issue is probably more related to poor well-head protection than to the vulnerability of any particular type of aquifer.

  • Median NO3-N concentration exceeds the health-related MAV for drinking-water and the TV for ecosystem protection at 4.9% and 10.3% of monitoring sites, respectively.

  • At 39% of monitoring sites the groundwater currently shows some level of human influence in the form of above-background concentration of NO3-N, often with elevated K, Cl and/or SO4 (this is especially common in Waikato and Southland). The median concentration of NO3-N exceeds the threshold for “probable” human impact (> 1.6 mg/L) at almost all sites in this category.

  • At 30% of monitoring sites the groundwater currently shows little or no evidence of human influence, but due to high levels of oxygen in the aquifer there is a risk that any introduced NO3-N or SO4 will persist and accumulate.

  • At 31% of monitoring sites the groundwater is oxygen-poor and is not likely to accumulate significant NO3-N; however, the groundwater can accumulate high concentrations of Fe, Mn, As and/or NH4-N due to natural processes (especially in parts of Gisborne, Auckland and Manawatu-Wanganui). Aesthetic drinking-water guidelines are exceeded for Fe and Mn at almost all sites in this category. Health-related guideline values for As and Mn are exceeded at about a quarter to a third of sites in this category (10% and 15% of sites overall, respectively). Many groundwaters in this category also have high salinity and can exceed aesthetic water-quality guidelines for Cl, Na, SO4 or TDS.

Trend assessments based on data for the period from 1995 to 2006 (section 3) reveal that about two-thirds of the monitoring sites considered in this study show slow changes in groundwater quality that probably indicate natural water-rock interaction. The remaining third of the monitoring sites show more rapid changes in groundwater quality, with patterns of change that appear to reflect human influence.

  • At 6% of monitoring sites, rapid decreases in parameters such as Na, Ca, HCO3 and Cl in deep aquifers may suggest dilution (most common in Northland, Gisborne and the Bay of Plenty), possibly due to pumping of coastal aquifers.

  • At 12% of monitoring sites increases in parameters such as NO3-N, K, Cl and/or SO4, usually in shallow unconfined aquifers and often in Waikato or Southland, may indicate increasing human or agricultural impact in the recharge area.

  • At 10% of monitoring sites decreases in parameters such as NO3-N, K, Cl and/or SO4 may indicate decreasing anthropogenic influence in the recharge area.

Findings from this study support the implication that human activities on the land have led to the degradation of groundwater quality, particularly in shallow, unconfined aquifers (ie, those most vulnerable to pollution from the land surface). However, proving relationships between the particular land use types/intensities surrounding monitoring bores and the magnitude or pattern of impacts of these land uses on groundwater quality has not been possible. This is in fact a common result that has been observed in several previous studies in New Zealand and overseas. There are some important information gaps identified in Section 2 that should be addressed in order to better understand the drivers of groundwater quality and underpin the development of sound resource management strategies:

  • Information related to well construction, aquifer characteristics and surrounding land should be available for all monitoring sites, and, if possible, consistent metadata standards should be applied nationally.

  • The age and origin of the groundwater monitored should be determined.

  • Routine groundwater quality monitoring programmes should include a balanced number of shallow and deep sites to ensure local, regional and national-scale representativeness, along with a certain number of sites in pristine areas to provide valuable baseline data.

  • Baseline data should be used to determine what threshold should be used to identify groundwater quality issues and trends that are important from a management perspective.

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