Quantifying groundwater–surface water interactions in braided rivers has considerable uncertainties, due to the dynamic nature of the river system and the heterogeneity of the sediments. The ability to quantify recharge rates to groundwater from these complex systems is of vital importance for freshwater management. Environmental tracers such as radon have proven to be useful tools to aid in characterising groundwater–surface water interactions in other settings. This research investigates the spatial variability of radon equilibrium concentrations in the braid plain sediments of the upper Waikirikiri Selwyn River, Canterbury, New Zealand. This is important because previous studies have assumed the radon concentration within the sediments is evenly distributed.

Sediments were collected during the drilling of monitoring wells in the braid plain of the Waikirikiri Selwyn River. These monitoring wells were drilled through two geological formations, the Springston Formation (0–6 m) and the Burnham Formation (6-16 m). Grain size analysis was carried out on 32 sediment samples. Following this, 318 radon equilibrium experiments were carried out on samples from across the study site. Grain size fractions from one of the installed piezometers, Selwyn 5, were also used in the radon experiments.

Analysis of sediment cores revealed a heterogeneous structure. The grain size analysis presented relatively consistent distribution curves showing a large range in grain size fractions. Hydraulic conductivity values were estimated from the grain sizes using a number of empirical formula and were found to vary vertically and spatially depending on the percentage of core recovery and formula used. The values ranged from ~ 80– >1,000 m/d. The calculated porosity ranged from 0.18 to 0.3 and bulk density values ranged from 1400 kg/m3 to 1700 kg/m3 and were similar to studies within the same region.

Radon equilibrium concentrations of water samples were tested after the water had been in a sealed jar with the sediments for 30 days, until secular equilibrium was reached. It was evident from the 318 samples analysed that radon concentration varied with differing grain size. Specifically, the <63 μm fraction contained significantly more radon than the other grain size fractions sampled. When comparing the spatial variability with the Springston and Burnham formations it appears that the radon concentration increases with depth within both these formations.

This study has provided baseline data within the research site on the radon equilibrium concentrations both spatially and within differing grain size fractions. The results suggest that radon concentrations in groundwater depend on the volume of silts and clays in the subsurface. Therefore, using radon as a tracer for groundwater–surface water interactions may be more complex than first thought.