## 1 Introduction  
Heavy metals as both point and nonpoint source pollutants have been the focus of numerous research endeavors in the past decades. Previous studies identified the source of heavy metals in runoff from urban environments as atmospheric deposition, road dust, tire wear, and surface wash-off from impervious areas such as roads, rooftops, and parking lots, mainly during rainfall events [1, 2]. Mercury (Hg), a type of heavy metal, is a highly toxic element with no recognized safe exposure threshold, posing serious risks to human health even at low concentrations. Methylmercury (MeHg), in particular, is known to bioaccumulate and can cause neurological, reproductive, and developmental disorders, especially in children and pregnant women [3]. Chronic exposure through ingestion or inhalation, as observed in artisanal mining regions, has been linked to elevated mercury levels in blood and urine, indicating systemic toxicity and potential long-term health hazards [4, 5]. In line with the confirmed presence of such heavy metals in stormwater runoff, innovative and cost-effective treatment mechanisms have also been the focus of study in recent years, particularly the removal of heavy metals in stormwater runoff using nature-based solutions (NbSs). NbSs have been defined as innovative stormwater management strategies that leverage natural processes and landscape features to enhance hydrological function and environmental resilience. Compared to conventional grey infrastructure, NbSs provide multifunctional benefits such as pollutant removal, flood mitigation, and ecological restoration [6; 7]. More specifically, heavy metals have been effectively removed or stabilized in NbSs through mechanisms such as ion exchange, co-precipitation, and surface complexation, often enhanced by plant-microhe interactions and biochar amendments [8]. Despite the confirmed presence of such heavy metals in stormwater runoff, particular types such as lead, cadmium, and copper, have been numerously identified to be treated effectively in NbS systems [9]. Such technologies leveraged ecohydrological biotechnologies, including constructed wetlands, which utilize plant uptake and microbial interactions to immobilize or remove toxic metals from water matrices [10]. However, Hg has been determined to be a challenging heavy metal to treat in NbSs [11]. Additionally, Hg accumulation has been observed in both natural soils and those within NbS systems, such as constructed wetlands and urban mangroves. In natural settings like boreal peatlands and mangrove sediments, high organic matter content and anoxic conditions promote long-term retention and transformation of Hg species, including methylmercury [12]. Similarly, NbS soils accumulate mercury due to atmospheric deposition, industrial runoff, and wastewater inputs. The extent of Hg sequestration in such environments was found to be attributed to soil organic carbon, and microbial activity, leading to spatial variability across land uses and NbS designs [13; 14]. Moreover, in urban wetlands, mercury was recently discovered to build up in sediments and vegetation due to industrial and residential runoff, with downstream reductions linked to retention by wetland plants [15]. Despite growing interest in plant-soil systems within NbSs, the integration of such approaches across diverse land uses remains underexplored. The unique biogeochemical challenges in urban, agricultural, industrial, and undeveloped land uses present uncertainties in mercury mobility and treatment efficiency, affecting the scalability of NbS systems. Therefore, this review aimed to synthesize existing peer-reviewed literature to characterize the occurrence and sources of mercury in NbS soils, evaluate the removal efficiencies of various NbS types and plant species, and assess emerging strategies for improving mercury treatment.