There is little known about the short-term dynamics of groundwater-surface water exchange in losing rivers. This is partly due to the paucity of chemical techniques that can autonomously collect high-frequency data in groundwater bores. Here we present two new instruments for continuous in situ 222Rn measurement in bores for quantifying the surface water infiltration rate into an underlying or adjacent aquifer. These instruments are based on 222Rn diffusion through silicone tube membranes, either wrapped around a pole (MonoRad) or strung between two hollow end pieces (OctoRad). They are combined with novel, robust, low-cost Geiger counter 222Rn detectors which are ideal for long-term autonomous measurement. The down-hole instruments have a quantitative response time of about a day during low flow, but this decreases to <12 h during high-flow events. The setup was able to trace river water bank infiltration during moderate to high river flow during two field experiments. Mass-balance calculations using the 222Rn data gave a maximum infiltration rate of 2 m d-1. These instruments offer the first easily constructible system for continuous 222Rn analysis in groundwater, and could be used to trace surface water infiltration in many environments including rivers, lakes, wetlands, and coastal settings.