The National Groundwater Monitoring Network (NGMP) : structure, implementation and preliminary results

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Rosen, M.R. 1997 The National Groundwater Monitoring Network (NGMP) : structure, implementation and preliminary results. Lower Hutt: Institute of Geological & Nuclear Sciences. Institute of Geological & Nuclear Sciences science report 97/26 47 p.

Abstract: The New Zealand National Groundwater Monitoring Programme (NGMP) aims to assess the long-term trends in the water quality of New Zealand's groundwater aquifers. It is considered to be a ''nationally significant database'' by the New Zealand Foundation for Research Science and Technology. The programme is a cooperative effort between the Institute of Geological & Nuclear Sciences and New Zealand's Regional Councils. It started as a pilot programme in 1990 in four regions of New Zealand (Tasman District, Waikato, Manawatu-Wanganui, and Bay of Plenty regions until 1994 when more regions were added. Currently the NGMP receives representative samples from 12 regions, 9 on the North Island and 3 on the South Island, 70 samples are analysed quarterly for major cations and anions, nutrients, and minor elements such as iron and manganese. The ultimate size of the programme is expected to be 100 quarterly samples from 14 regions. All of the major aquifers in New Zealand are sedimentary. However, detailed geochemical information on the aquifers is limited. The aquifers in Canterbury, Hawke' s Bay, Wellington, and Gisborne are composed of gravels and sands, mostly derived from greywacke boulders brought down into valleys by rivers and alluvial fans. Many of the sedimentary aquifers in the Tasman District have been derived from granitic material. Other important aquifers included in the NGMP are composed of fractured basalts (Waikato), pumice (Bay of Plenty, and limestones (Tasman, although these aquifers are minor in comparison to the alluvial gravel aquifers Forty-two percent of the aquifers are unconfined and shallow, 18 percent are semi confined, but 40 percent are confined and deep. The average thickness of the aquifers is 19 m, and the average depth to the top of the aquifers is 38 m. However, 38 percent of the wells have water levels less than 10 m below the surface. Trends in groundwater quality over 5 years are varied. Important trends show: 1) virtually no change in water quality with time, 2) some detrimental effects of land-uses on water quality, 3) the effects of seasonal fertiliser application of water quality, and 4) in some cases an improvement in water quality over time. Preliminary analysis of the data collected indicates that 70 percent of the samples collected have anion abundances of HCO3>Cl>SO4>NO3-N on an equivalent basis. Cation abundances are more variable The ranking Ca>Na>Mg>K>NH4-N is the most abundant (34 percent, and is found in most of the regions studied, although there are 4 other rankings greater than 5 percent. Seventeen percent of the wells sampled in the NGMP had average NO3-N concentrations greater than 3.0 g/m3. Six percent had average concentrations greater than the New Zealand Drinking water maximum acceptable value (MAV) of 11.3 NO3-N g/m3. A total of 15 wells in the NGMP have NH4-N concentrations greater than 0.3 g/m3, and five of t hose wells are in the Taranaki region an d five are from Gisborne. Seven of the 15 wells have NH4-N concentrations greater than 1.0 g/m3. Almost all of the wells with high NH4-N concentrations are located in confined aquifers. (auth)