Coastal saltmarshes as nature-based solutions for pesticide mitigation through groundwater-surface water interactions

Coastal saltmarshes are increasingly recognized for their role in mitigating agricultural pollution, yet their function in regulating pesticide transport via groundwater-surface water interactions remains poorly understood. Here, we quantify pesticide fluxes across the groundwater-surface water interface in a Yellow Sea saltmarsh using ²²²Rn as a tracer of submarine groundwater discharge (SGD) and analyze 20 commonly used pesticides in porewater and surface water. We find that 80% of pesticides are retained within the saltmarsh, with a mean removal rate of 170±41 μg m^-2 day^-1. Only a few hydrophobic compounds (e.g., tricyclazole) are exported to coastal waters via SGD. Extrapolation to China's total saltmarsh area suggests an annual removal of ∼185 tons, equivalent to 29% of riverine pesticide inputs. Historical wetland loss has reduced this natural filtration capacity, but restoration could enhance removal by 78%. Our results demonstrate that saltmarshes act as net sinks for most pesticides, challenging the paradigm of SGD as a contaminant source. These ecosystems provide a critical nature-based solution for coastal pesticide pollution control, emphasizing the need to integrate wetland conservation into watershed management policies.