Morgenstern U, Davidson P. 2022. Groundwater tracers for improved understanding of water and nitrate flow through Te Hoiere / Pelorus Catchment to inform decision making in restoration programme. Lower Hutt (NZ): GNS Science. 21 p. (GNS Science report; 2022/50). doi:10.21420/937F-H779
Abstract
Using 2013 baseline isotope and hydrochemistry data, together with water-quality monitoring data over the last 10 years, we provide an improved understanding of the Te Hoiere / Pelorus catchment behaviour in relation to transport of water and nitrate. Radon, water isotopes and gas tracers enable detailed understanding of river/stream water loss into the gravel underflow system, as well as discharge of such underflows and locally recharged groundwater into the river. In Holocene gravel deposits, nitrate-loaded water from nearby intensive farming usually does not enter streams, as the streams are likely to lose water into the gravel instead of gaining water from the surrounding area, with the result that water quality in streams can be relatively good, despite surrounding irrigated dairy farming. Nitrate from animal farming on the gravel-deposit river terraces travels mainly through groundwater systems to the river. Where tributary streams and river waters derived from more pristine catchments lose water into Holocene gravel deposits, nitrate in groundwater from nearby intensive farming can be diluted. Overall, groundwater recharged on high-intensity farming land in the Rai Valley may commonly have high nitrate concentrations, up to 6 mg/L nitrate-nitrogen. Delayed arrival (lag) of nitrate in the Te Hoiere / Pelorus Catchment is insignificant. Tritium-based mean transit times of the water overall are less than four years, and the majority of nitrate is flushed into the river very quickly via the winter rain pulses. Shallow monitoring wells are likely to capture groundwater from seasonally changing sources, for example, very young water from surrounding areas in winter versus older groundwater from the deeper groundwater flow from further up in the catchment. Active groundwater flow was observed in the Rai Valley through anoxic zones, with potential to remove significant fractions of nitrate through denitrification before the groundwater discharges into the river. (The authors)
