Delineating E. coli occurrence and transport in the sandy beach groundwater system by radon-222

Fecal pollution poses a global threat to environmental safety and ecosystem, but the mechanism of microbial transport and occurrence in the beach groundwater system is still poorly explored. Here, we leveraged one-year field data of Escherichia coli (E. coli) and radon-222 (²²²Rn) and found that E. coli occurrence and transport in the sandy beach groundwater system can be delineated by ²²²Rn. The underlying mechanism behind this phenomenon is due to similar half-lives of ²²²Rn and E. coli in the sandy beach groundwater system. Thus, the unique relationship between ²²²Rn and E. coli can provide additional critical context to the microbial water quality assessments and ecosystem resilience. Also, the beach aquifer in this study is found to be a vital compartment for E. coli removal. The net E. coli removal/production capacity is identified to be highly impacted by submarine groundwater discharge. Finally, a conceptual model is constructed for a better understanding of the occurrences and characteristics of E. coli and ²²²Rn at multiple spatial scales. These findings are constructive to mitigate the hazardous influences of microbe on water quality, especially in recreational sandy beaches and mariculture zones.