Removal of Contaminants in Stormwater via Subsurface-Flow Wetlands: A Review with Focus on Nutrients, Heavy Metals, and PFAS

Subsurface-flow constructed wetlands (SSF CWs), which include horizontal subsurface–flow constructed wetlands (HSSF CWs) and vertical subsurface–flow constructed wetlands (VSSF CWs), have become increasingly popular for the treatment of various waste streams such as wastewater, landfill leachate, and stormwater runoff. These wetlands utilize a variety of natural physical, chemical, and microbial processes to effectively remove contaminants. This article aims to provide valuable insights into the application of subsurface-flow wetlands for the treatment of stormwater runoff by collecting and reviewing relevant articles. Within the realm of stormwater contaminants, this review primarily focuses on nutrients, heavy metal, and per- and polyfluoroalkyl substances (PFAS), which are commonly found in stormwater runoff worldwide. The removal percentages of these contaminants vary across different studies, depending on factors such as the characteristics of the contaminants, the type and scale of the subsurface-flow wetland, as well as the selection of plants and substrates used. This study provides valuable insights into the various processes that significantly contribute to the removal of contaminants, including adsorption, plant uptake, and biodegradation. The removal percentage of PFAS ranges from 1% to 99% in constructed wetlands. The wide range of PFAS removal comes from differences between PFAS category, plant and soil type, wetland configuration, and matrix effects. Additionally, this paper brings attention to the current gaps and limitations present in the research conducted within this field. It is imperative to conduct further studies utilizing real-world or synthetic stormwater runoff to deepen our comprehension in this domain. Furthermore, considering that each category of PFAS possesses unique properties, conducting individual studies on each group becomes crucial to attain a comprehensive understanding of the behavior and transformation of PFAS in CWs.