Groundwater lag times in the water discharges from the Whanganui, Rangitikei and Manawatu catchments

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Morgenstern, U.; Matthews, A.; Begg, J.G.; Roygard, J.; Townsend, D.B.; Clark, M.; Martindale, H. 2014 Groundwater lag times in the water discharges from the Whanganui, Rangitikei and Manawatu catchments. Lower Hutt, N.Z.: GNS Science. GNS Science report 2014/35 34 p.

Abstract: Tritium dating of water at 29 river and stream sampling sites in the Horizons Region was undertaken during base flow conditions. The sites were located in the Whanganui, Rangitikei and Manawatu river catchments. To obtain information about the origin of the stream and river water and potential impacts by evaporation, the stable isotopes of the water molecule 2H and 18O were also measured. The tritium dating results show that the mean residence times (MRT) of the water in the rivers and streams range from 0 to 11 years. Tritium ages show consistent patterns, with MRT of 6 – 7 years in the Whanganui River, 3 – 3.5 years in the Rangitikei River; and in the Manawatu catchment 9 – 11 years in the large discharges from the Tertiary sediments east of the Ruahine and Tararua ranges, 2.5 – 4.5 years in the discharges from the Tertiary sediments west of the Ruahine Range, and very young water with MRT of 0 – 2 years in the discharges from the eastern Ruahine and Tararua Ranges. The MRTs give insight into the lag times between rainfall infiltrating to groundwater and the arrival of the groundwater, and therefore nutrients associated with different land use, at the river. This lag time needs to be considered in models for prediction of land use changes on river water quality over time. In addition to this first survey of groundwater lag time in Horizons’ rivers, we recommend to analyse more samples collected at the 2013 low-flow condition to obtain the lag-time distribution through the whole region, covering all geologic formations, and to identify areas and rates of groundwater discharge into the rivers, with relevant groundwater capture zones. (auth)