Journal of Hydrology (NZ) 50 (2): 313-338 2011 © New Zealand Hydrological Society (2011) Nitrate sources and residence times of groundwater in the Waimea Plains, Nelson Michael K. Stewart1, Glenn Stevens2, Joseph T. Thomas2, Rob van der Raaij3, Vanessa Trompetter3 1 Aquifer Dynamics & GNS Science, PO Box 30368, Lower Hutt, New Zealand. Corresponding author:
[email protected] 2 Tasman District Council, Private Bag 4, Richmond, New Zealand 3 GNS Science, PO Box 30368, Lower Hutt, New Zealand Abstract the various wells. The timing of the derived Nitrate concentrations exceeding Ministry of nitrate input history shows that both the Health potable limits (11.3 mg/L nitrate-N) diffuse sources and the point source were have been a problem for Waimea Plains present from the 1940s, which is anecdotally groundwater for a number of years. This work the time from which there were increased uses nitrogen isotopes to identify the input nitrate sources on the plains. The large sources of the nitrate. The results in relation piggery was closed in the mid-1980s. to nitrate contours have revealed two kinds Unfortunately, major sources of nitrate of nitrate contamination in Waimea Plains (including the piggery) were located on groundwater – diffuse contamination in the the main groundwater recharge zone of the eastern plains area (in the vicinity and south plains in the past, leading to contamination of Hope) attributed to the combined effects of the Upper and Lower Confined Aquifers. of the use of inorganic fertilisers and manures The contamination travelled gradually for market gardening and other land uses, northwards, affecting wells on the scale of and point source contamination attributed to decades.