Using ²²²Rn to Determine Groundwater Velocities in a Braidplain Aquifer

Surface water–groundwater exchange processes are uncertain in braided river settings where aquifer geomorphology and hydraulic gradients are highly heterogeneous. This study determines groundwater velocities in a braidplain aquifer in Te Waipounamu, South Island of New Zealand, where the Waikirikiri Selwyn River is losing to groundwater. ²²²Rn activities and hydraulic heads were measured both in the river and in a dense network of 22 piezometers installed at the study site. ²²²Rn activities were used to calculate groundwater residence times which, together with flow paths determined from hydraulic head contour maps, were used to calculate mean groundwater velocities. ²²²Rn residence time calculations based on a one-dimensional piston flow groundwater model yield a median groundwater velocity of 46 m day⁻¹ at high river flow and 38 m day⁻¹ at low river flow. However, an alternate (exponential) model demonstrates that residence times would be longer than those estimated using the piston flow model. At ²²²Rn activities above 6 Bq L⁻¹, the exponential model yields residence times twice those obtained from the piston flow model, and groundwater velocities that are less than half those obtained with the piston flow model. This highlights the importance of characterising groundwater flow paths in braidplain aquifers to improve estimates of groundwater velocities and recharge from losing braided rivers.