Assessment of nitrogen flow pathways and its potential attenuation in shallow groundwaters in the Lower Rangitikei catchment

Abstract

We assessed surface water and groundwater interactions, and nitrogen flow and its potential attenuation in shallow groundwater in the lower Rangitikei catchment. The study area covers about 850 km² between the townships of Bulls and Sanson in the east, Tangimoana in the south and Santoft in the north. A piezometric map, developed from measured depths to groundwater in about 100 wells in October 2014, suggests that groundwater flow, in particular the shallow groundwater (<30 m), is heavily influenced by the regional topography. Shallow groundwater flows mostly from elevated areas, such as Marton, in a south-westerly direction towards the Rangitikei River, whereas deeper groundwater (>30 m) showed relatively less topographic influence. Groundwater discharges to the river upstream and downstream of Bulls, while groundwater recharge or no interaction with the river is more likely to occur near the coast. The groundwater redox characterisation, based on sampling and analysis of 15 groundwater wells, suggests that in general the groundwater across the lower Rangitikei catchment is under anoxic/reduced conditions. The groundwater typically has low dissolved oxygen concentrations (<1 mg/L) and elevated levels of electron donors (particularly DOC and Fe²⁺) that are suitable for denitrification. We further measured NO₃⁻-N attenuation in shallow groundwater piezometers (3-6 m bgl) using the single-well push-pull tests. The push-pull tests showed N0₃⁻-N reduction at four of the five piezometers, with the rates of reduction varying from 0.04 mg N L⁻¹ hr⁻¹ to 1.57 mg N L⁻¹ hr⁻¹.