By Magali Moreau, Chris Daughney and Zara Rawlinson
To date, more than 200 aquifers have been mapped across the country. These aquifers vary widely in their volumes, depths, host-rock lithologies, related geological structures, water circulation pathways and water age. Our knowledge of the individual characteristics of these aquifers grows as we gather more data through our active monitoring networks.
Active monitoring of groundwater is undertaken by the National Groundwater Monitoring Programme (NGMP) and, at the regional scale, by the State of the Environment (SOE) networks. Initiated in 1990, the NGMP achieved national coverage in 1998 (currently 108 sites) and is a collaboration between GNS Science and 15 regional authorities. Groundwater samples are collected quarterly at NGMP sites and tested for more than 25 parameters using a consistent sampling protocol and suite of analytical procedures. At the regional scale, SOE networks are operated by each regional authority, with groundwater samples collected at varying intervals (from monthly to annually). Nation-wide, the total number of monitored SOE sites exceeds 1,000. Monitored groundwater indicators vary between each region based on individually set regional monitoring objectives. The denser SOE network is accurately represented by the NGMP, which provides a perspective on groundwater quality throughout New Zealand.
Key groundwater chemical indicators used to assess the state and trends are:
- Nitrogen species (nitrate-nitrogen and ammoniacal-nitrogen) for environmental and health reasons (blood circulation disorder). Both species need to be monitored because under low oxygen conditions, nitrate-nitrogen is converted to ammoniacal-nitrogen by natural processes. Groundwater is unsuitable for human consumption when its nitrate-nitrogen concentration exceed the Drinking Water Standard for New Zealand (DWSNZ) maximum admissible value (MAV) of 11.3 mg/L.
- E. coli, indicative of faecal contamination. Groundwater used for drinking-water purposes should not contain any E. coli.
- Iron and manganese, indicators of the oxidation state of groundwaters (high concentrations of these ions occur in anoxic conditions). The aesthetic guideline values for iron and manganese concentration are 0.2 mg/L and 0.04 mg/L, respectively (unpleasant taste and for manganese, laundry staining). Chronic health problems can also appear if groundwater containing manganese concentration above 0.4 mg/L is ingested. Some elements such as arsenic can also become mobile in groundwater under anoxic conditions.
- Electrical conductivity is an indicator of total dissolved solids content. Infiltrated rain water conductivity will naturally increase as groundwater flows through the aquifer material, due to water-rock interaction. Electrical conductivity is a good indicator of saltwater intrusion in coastal aquifers.
Based on data from the NGMP, three categories of groundwater quality have been identified in New Zealand:
- “Natural fresh” groundwater (32% of NGMP sites, shown on the map in white) has little to no evidence of human impact and is chemically similar to clean river water. NGMP sites that have this type of groundwater are found throughout New Zealand with high proportions in central Otago and in the West Coast.
- “Natural evolved” groundwater (26% of NGMP sites, shown on the map in green) has chemistry that is indicative of natural water-rock interaction and may contain elevated concentrations of iron, manganese and/or arsenic. Most of the NGMP sites with this character are found in the North Island, for example Gisborne.
- “Impacted” groundwater (42% of NGMP sites, shown on the map in red) has nitrate concentration above natural levels due to some human or agricultural influences, and neither iron nor manganese are present. NGMP sites in this category are found across the country, but especially in regions of more intensive agriculture, such as Southland and Waikato. Only 11% of all NGMP sites have nitrate concentration above the guideline for protection of aquatic ecosystems, and only 5% of NGMP sites have nitrate concentration above the DWSNZ MAV.
Magali Moreau is a groundwater geochemist and manager of the National Groundwater Monitoring Programme, Dr. Chris Daughney is the Director of the National Isotope Centre, and
Zara Rawlinson is a geophysicist at GNS Science.
Ministry of Health. (2005) Drinking-Water Standards for New Zealand 2005. Ministry of Health, Wellington, New Zealand.
Daughney, C. J.; Randall, M. (2009) National groundwater quality indicator update: state and trends 1995-2008. GNS Science Consultancy Report 2009/145. 62p.
Daughney, C.J.; Reeves, R.R. (2005) Definition of hydrochemical facies in the New Zealand National Groundwater Monitoring Programme. Journal of Hydrology, New Zealand, 44(2): 105-130.
Rosen, M. R.; White, P.A. (2001) Groundwaters of New Zealand. New Zealand Hydrological Society Publication. 498 p. ISBN 0-473-07816-3