Tracing submarine groundwater discharge in a semi-enclosed bay using pulsed ionization chamber radon detector

Natural occurring radon (²²²Rn, t_1/2 = 3.8 d) is a powerful tracer for investigating submarine groundwater discharge (SGD) and other hydrological and oceanographic processes. Continuous ²²²Rn measurement approach is essential but remains inadequate to provide high spatiotemporal resolution field dataset, especially for low-activity surface seawaters. Here, we present a labor-less technique for continuous ²²²Rn observations using the pulsed ionization chamber detector (PIC-1000). The measurement efficiency of PIC-1000 is ∼7 and ∼3.5 times that of the commercial RAD7 and the existing PIC-200 detectors, respectively. High efficiency and low background make the PIC-1000 device more suitable for low and variable radon activity measuring environments. Based on over 18-day continuous field measurements and a radon mass balance model, we estimated the groundwater flux at 12.6 ± 8.9 cm d^-1 in a semi-enclosed bay during wet season of 2023. We further identified a positive correlation between groundwater flow and tidal amplitude within individual tidal cycles, highlighting the dynamic response of groundwater-seawater interactions to both diurnal flooding-ebbing and biweekly spring-neap tidal forcing.