Coupling automated radon and carbon dioxide measurements in coastal waters

Groundwater discharge could be a major, but as yet poorly constrained, source of carbon dioxide to lakes, wetlands, rivers, estuaries, and coastal waters. We demonstrate how coupled radon (²²²Rn, a natural groundwater tracer) and pCO₂ measurements in water can be easily performed using commercially available gas analysers. Portable, automated radon and pCO₂ gas analysers were connected in series and a closed air loop was established with gas equilibration devices (GED). We experimentally assessed the advantages and disadvantages of six GED. Response times shorter than 30 min for ²²²Rn and 5 min for pCO₂ were achieved. Field trials revealed significant positive correlations between ²²²Rn and pCO₂ in estuarine waterways and in a mangrove tidal creek, implying that submarine groundwater discharge was a source of CO₂ to surface water. The described system can provide high resolution, high precision concentrations of both radon and pCO₂ with nearly no additional effort compared to measuring only one of these gases. Coupling automated ²²²Rn and pCO₂ measurements can provide new insights into how groundwater seepage contributes to aquatic carbon budgets.