Nitrate sources and residence times of groundwater in the Waimea Plains, Nelson

Abstract

Nitrate concentrations exceeding Ministry of Health potable limits (11.3 mg/L nitrate-N) have been a problem for Waimea Plains groundwater for a number of years. This work uses nitrogen isotopes to identify the input sources of the nitrate. The results in relation to nitrate contours have revealed two kinds of nitrate contamination in Waimea Plains groundwater - diffuse contamination in the eastern plains area (in the vicinity and south of Hope) attributed to the combined effects of the use of inorganic fertilisers and manures for market gardening and other land uses, and point source contamination attributed to a large piggery to the south of Hope. Once nitrate is introduced to a groundwater system it can take many years for it to be flushed out. Tritium measurements in wells are interpreted to give mean residence times, and the spread of residence times around the mean, for groundwater in different parts of the plains. Mean ages are youngest in the area south of Hope, where nitrate concentrations are highest, and increase to the south, west and north. The age distributions have been used to produce a nitrate input history for the Upper and Lower Confined Aquifers by simulating the nitrate measurements in the various wells. The timing of the derived nitrate input history shows that both the diffuse sources and the point source were present from the 1940s, which is anecdotally the time from which there were increased nitrate sources on the plains. The large piggery was closed in the mid-1980s. Unfortunately, major sources of nitrate (including the piggery) were located on the main groundwater recharge zone of the plains in the past, leading to contamination of the Upper and Lower Confined Aquifers. The contamination travelled gradually northwards, affecting wells on the scale of decades. Input of nitrate to the groundwater has been decreasing since about 1988 due to closure of the piggery. The resulting decrease in nitrate concentrations is now also gradually travelling northward. Groundwater to the south and west already had relatively low nitrate because of river and/or rainfall recharge with low nitrate concentrations. Improved monitoring and practices (e.g., best management practices and nutrient budgeting) need to be encouraged among market gardeners and other land users, taking special account of the location of the groundwater recharge areas around and south of Hope.