Arthur Marble Aquifer and Te Waikoropupu Springs, New Zealand: flow contributions and nitrate sources

Abstract

This paper investigates sources of early signs of nitrate pollution in a karst aquifer and major karst spring. Te Waikoropupu Springs are the largest springs in New Zealand and amongst the largest in the Southern Hemisphere with a mean outflow of 13.4 m3/s. They discharge the clearest water ever recorded from a karst spring, but early warning of water quality deterioration is signalled by increasing nitrate concentrations. The springs discharge water from a partly artesian aquifer of 2.85 km3 volume in karstified Ordovician marble with a transmission time averaging 7.9 years. Reservoir porosity is unusually high for a crystalline carbonate rock, estimated as 6%. Water emerging at Main Spring (9.89 m3/s) and adjacent Fish Creek Springs (3.48 m3/s) is a mix of recharge from four main sources. The paper investigates by means of mass balance modelling the proportions of water coming from each source, the origins of nitrate contamination and the contribution of nitrification. Water losses into the bed of the upper Takaka River, about 17 km from the Springs, provide 13% of the mean flow at Main Spring, 34% of the flow at Fish Creek Springs, and 83% of the volume discharged at submarine springs. Infiltration from rain falling over farmed land in the middle valley is estimated to supply 10% of total outflow at Main Spring and Fish Creek Springs but to contribute 82% of the NO3-N output discharged at the springs. Nitrification accounts for 16% of the nitrate discharged. A 34% reduction of leachate concentration in the middle valley would be required to bring NO3-N values at Main Spring back to levels recorded in the 1970s.