The national and regional assessments of groundwater quality made in this report (based on data collected from 1995 to 2008) are in good agreement with the findings of Daughney and Wall (2007) (based on data collected from 1995 to 2006).
At the national scale, groundwater quality in New Zealand is similar to other countries such as Finland, Canada and the Netherlands (Frapporti et al., 1994; Lahermo et al., 1999; Broers and van der Grift, 2004; Lesage, 2005; Griffioen et al., 2005). New Zealand has two major but mutually exclusive national-scale groundwater quality issues:
- Contamination with nitrate and/or microbial pathogens (of presumably human or agricultural origin) occurs in all regions, but is especially common in Waikato, Southland and Canterbury, and particularly for oxygen-rich groundwater extracted from shallow wells in unconfined aquifers. Nationally, the median NO3-N concentration exceeds the health-related MAV for drinking water and the TV for ecosystem protection at 4.8% and 13.2% of monitoring sites, respectively. The health-related MAV for E. coli is exceeded at 23.1% of the monitoring sites considered in this report, but may be more an indication of poor well-head protection than the vulnerability of any particular type of aquifer.
- Naturally elevated concentrations of NH4-N, Fe and/or Mn are found in many regions, especially Manawatu-Wanganui, Hawke’s Bay and Bay of Plenty, and particularly for oxygen-poor groundwater extracted from deeper wells in confined aquifers. Nationally, 3.8%, 21.3% and 26.9% of the sites considered in this report have median concentrations of NH4-N, Fe and Mn above their respective aesthetic GVs for human consumption, and 9.9% of sites have median Mn concentration above the health-related MAV (there is no MAV for NH4-N or Fe). Many groundwaters with elevated NH4-N, Fe and/or Mn also have high electrical conductivity and hence might exceed aesthetic water quality guidelines for Cl, Na, SO4 or TDS.
At about two thirds of the monitoring sites considered in this report, groundwater quality was found to be either constant over time or changing slowly (parameter values change less than 2-5% per year), probably due to the natural processes of water-rock interaction. The remaining one third of the monitoring sites show more rapid changes in groundwater quality, with patterns of change that appear to reflect human influence. However, attempts to identify and interpret time trends in groundwater quality are complicated by year-by-year changes in the structure of the various regional SOE programmes. The greatest uncertainties in the regional aggregated statistics are introduced by changes in sampling methodology, because these tend to be applied to all SOE sites at once. For example, the high proportion of sites and regions with decreasing trends in Fe and/or Mn might be caused by improvements in sampling methods between 1995 and 2008: several regional councils started to field-filter samples for Fe and Mn analysis some time around 2004 (e.g. Wellington), whereas in the past an unfiltered sample might have been collected for this purpose. A secondary source of bias in the aggregated statistics is caused by addition or removal of sites to a region’s SOE network. Generally, the addition or removal of just one site out of an entire SOE network has relatively little influence on the aggregated statistics, but in certain cases, there are changes made to a substantial proportion monitoring sites. For example, the apparent year-by-year decrease in NO3-N concentration in the Tasman region is an artefact of the expansion of the SOE network from 10 to 16 sites in 2002. Changes in analytical procedure can also influence the aggregated statistics, as observed for the shift from monitoring total coliform counts to monitoring of E. coli.
The factors that control groundwater quality are often difficult to identify. There were observable relationships between groundwater quality and well depth and aquifer characteristics, but no detectable relationships between groundwater quality (state or trends) and land use or land cover around the monitoring sites. This is in fact a common result that has been observed in several previous studies in New Zealand and overseas: it is hard to identify and understand relationships between groundwater quality and land use unless the age and source of the groundwater being monitored are accurately known.