This report provides a summary of groundwater quality state and trends in New Zealand based on data collected from 973 sites over the period 1995 to 2008. The dataset includes sites in State of the Environment (SOE) monitoring programmes operated by regional councils and the National Groundwater Monitoring Programme operated by GNS Science. This report updates a previous report on groundwater quality at the same sites and based on data collected from 1995 to 2006 (Daughney and Wall, 2007).
This report focuses on ambient groundwater quality. Some of the monitoring sites considered in this report are used to supply single dwellings or small communities with water supply, but many other monitoring sites considered in this report have non-potable uses (e.g. irrigation, stock drinking water). Drinking water guidelines are used in this report to provide context for assessment of ambient groundwater quality, but for focused assessment of drinking water quality in New Zealand, readers are directed to the Annual Review of Drinking Water Quality reports produced by the Ministry of Health (e.g. Ministry of Health, 2009).
Median values and trend magnitudes for key groundwater quality indicators reported here are very similar to those reported previously (Daughney and Wall, 2007). Nationally, ambient groundwater quality in New Zealand is similar to other countries such as Finland, Canada and the Netherlands. New Zealand has two main groundwater quality issues:
- Contamination with nitrate and/or microbial pathogens (of presumably human or agricultural origin) occurs in many regions, particularly for shallow wells in unconfined aquifers. Nationally, median concentrations of nitrate and Escherichia coli exceed their respective health-related standards for human consumption at 5% and 23% of the monitoring sites considered in this report, respectively.
- Naturally elevated concentrations of ammonia, iron and/or manganese are found in many regions, especially for deeper wells in confined aquifers. Nationally, 4%, 21% and 27% of the sites considered in this report have median concentrations of ammonia, iron and manganese above their respective aesthetic guidelines for human consumption, and 10% of sites have median manganese concentration above the health-related standard (there are no health-related standards for ammonia or iron).
Groundwater quality is either constant over time or changing slowly (parameter values change less than 2-5% per year) at about three quarters of the sites considered in this report, probably due to the natural process of water-rock interaction. Changes in groundwater quality over time are more rapid at the remaining sites, with patterns that suggest human influence. With respect to nitrate, significant time trends are detectable at roughly one third of the monitoring sites considered in this report, and of these, roughly twice as many sites show increasing nitrate concentration over time compared to sites that show decreasing nitrate concentration over time. In general however, this report shows that attempts to identify and interpret time trends in groundwater quality are complicated by year-by-year changes in the structure of the various groundwater monitoring programmes operated by regional councils.
This report has revealed certain significant relationships between groundwater quality and well depth and/or aquifer characteristics. In contrast, this report has not revealed any systematic or significant 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 (e.g. Daughney and Wall, 2007) 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.
The main recommendation from this report is that similar studies should be conducted at a regular interval in the future, in order to identify changes in the status of groundwater quality in New Zealand. Future studies will require national and regional commitment to regular (quarterly) monitoring of key groundwater quality indicators (nitrate, Escherichia coli, ammonia, iron, manganese, electrical conductivity) via standardised sampling and analytical methods, on an on-going basis, and at a consistent network of monitoring sites, all having adequate well-head protection.
The main sources of bias in the aggregated regional and national statistics are, in order of influence:
- changes in sampling procedure, such as collection of field-filtered instead of unfiltered samples;
- addition or removal of a large proportion of sites from an SOE network; and
- changes in analytical procedure, such as replacement of total coliform counts with Escherichia coli counts.
In order to elucidate the drivers of groundwater quality, there is also a need to determine the age and origin of the groundwater that is actually being sampled at each monitoring site, to permit meaningful comparison to current and past land use.