Nitrate nitrogen (NO3-N) response domains following a large-scale rainfall event on the Hekeao Hinds Plains of Mid-Canterbury, New Zealand

• High frequency sampling of groundwater enables characterisation of NO 3 -N after rainfall. • NO 3 -N responses after rainfall are spatially & temporally heterogeneous. • NO 3 -N concentration was affected by soil type and profile available water. • NO 3 -N mobilisation was influenced river & fault proximity, and transmissivity. Nitrate-nitrogen is a highly mobile contaminant in the alluvial gravel aquifers that compose the Hekeao Hinds Plain in mid-Canterbury, New Zealand. The area is an intensively farmed agricultural catchment where nitrate-nitrogen is delivered to the aquifer through land surface recharge. We analysed water samples from 54 bores following an extreme rainfall event where > 500 mm of rain fell over a three-day period. Samples collected from the bores at weekly, then monthly resolutions, showed different temporal and spatial responses to the rain event, which are partially explained by factors, such as distance to main rivers, and soil profile available water. A scheme of six different responses were classified on their temporal variations and principle discriminant analysis identified variables influencing nitrate-nitrogen response types, and underscores that differences in aquifer properties may explain nitrate-nitrogen responses to significant rainfall events. The specific mechanisms for spiked and parabolic responses in groundwater nitrate concentrations were attributed to piston flow along preferential flow pathways and the mobilization of stored nitrate with ground saturation. The effects of elevated nitrate following significant rain at some locations may persist for months after the rain event. Prolonged elevation in nitrate may occur in some bores following extreme rainfall and have implications for interpreting nitrate trends in longer-term monitoring programmes, and that in bores with high variance in concentrations these changes may be in part attributable to specific local-scale processes that affect nitrate mobilisation and transport through an aquifer.