[A-HW18] Material transportation and cycling at the land-sea interface: from headwaters to the ocean

[AHW18-02] Importance of considering hydrogeological conditions to understand microplastics occurrence in the subsurface

★Invited Papers

*Juliana Gardenalli de Freitas¹, Tamiris Ferreira¹, Décio Semensatto¹, Reginaldo Bertolo², Ricardo Hirata² (1.Universidade Federal de São Paulo, 2.Universidade de São Paulo)

Keywords:microplastics, groundwater, soil, tropical

Microplastics have been detected in the environment, particularly in oceans, for decades. However, the concern about its impacts has increased significantly, as it has been found to be ubiquitous in other environmental compartments, including rivers, lakes, soil, groundwater, atmosphere and even within the human body. The sources of microplastics to groundwater are several, and mainly are non-point sources. In urban environments, the sources are mainly related to drainage and wastewater systems. Leakages from the wastewater network, infiltration from septic systems, and discharge from wastewater treatment plants constitute some of the major sources. The body of literature on microplastics in groundwater is increasing rapidly, with a significant part of the studies focusing on mapping their occurrence in different aquifer systems. However, most studies did not relate their occurrence to the hydrogeological conditions or the aquifer physical-chemical properties. On the other hand, some laboratory studies showed that microplastics transport and fate in the subsurface are strongly dependent on groundwater chemistry and physical, chemical and mineralogy properties of the medium. In addition, few studies have been conducted in tropical zones, where the environmental characteristics, such as soil type, are significantly different. In our research, we investigated the occurrence of microplastics in soil and groundwater in an urban area, in the city of Bauru, Brazil. The study site was near the Bauru river, in a region of groundwater-surface water interaction by subsurface flow and also by occasional floods. In the city of Bauru, groundwater is contaminated by nitrate, derived from sewage from leaks in the collection system and from previous septic systems. Considering the source of contamination, other contaminants are likely to be present, including microplastics. Soil samples were characterized for morphology, grain size distribution, and chemical properties. Groundwater was also characterized for physical-chemical properties. Microplastics were analyzed and detected in all soil and groundwater samples. In soil samples, microplastics were detected in highest concentrations at the shallower zones (up to 0.5 m depth). The concentrations decreased in the vadose zone below that layer, but increased again at the saturated zone. Therefore, there were two concentration peaks: the top layer and the saturated zone. Besides the concentration, other properties of microplastics are important to be considered, such as the shape, size, composition, and color. Fibers were dominant in all samples, but fragments were also detected in groundwater. Also, mainly colored fibers were detected in the groundwater. In the soil, colored fibers were predominant in the samples from the saturated zone, and transparent in the unsaturated zone. This indicates that the microplastics reaching the soil from the surface (brought by river flooding or surface runoff, for example), are mostly being retained in the shallower zone, possibly by straining (due to the higher clay content and lower pore size) and sorption, associated with higher concentrations of organic carbon and clay in this zone. The microplastics in the saturated zone are liked being transported from other regions upgradient. The saturated zone is a sandy aquifer, which might facilitate particles transport. Some sorption could occur in this zone to iron oxide minerals present, but the relatively high groundwater pH does not favor this process, resulting in higher microplastics mobility. Therefore, the microplastics occurrence in the site was shown to be determined by the porous medium physical and chemical properties and groundwater flow. It is crucial to have a good characterization of the subsurface properties to understand microplastics distribution and transport in the subsurface. The authors thank the São Paulo Research Foundation (FAPESP) for support through grants 2020/15434-0 and 2022/00652-7.