Submarine groundwater discharge: An overlooked but quantifiable pathway for microplastics inputs to the coastal ocean

Marine microplastics (MPs) pollution has become a serious concern, in which submarine groundwater discharge (SGD) is an overlooked yet potentially significant pathway for land-based MPs entering the ocean. This study examined the occurrence, correlated factors, and SGD-derived flux of MPs in the groundwater of a representative coastal city in China. MPs were widely detected in groundwater at relatively low levels, with an average abundance of 2.4 ± 1.0 n/L, and exhibited distinct physicochemical characteristics. Fibers represented the most abundant morphological types, while transparent and black particles were the predominant colors. Over half of the MPs were less than 500 μm in size. MPs occurrence was primarily governed by hydrological season (dry > wet), functional zone (estuarine > transitional > urban), and well type (open > covered), reflecting the combined effects of dilution, pollution sources, land use, and input pathways. Based on a radium isotope (²²⁶Ra) mass-balance model, the total SGD flux was estimated at 4.0 × 10⁷ m³/d, of which the freshwater component constituted 3.3 × 10⁶ m³/d. We conservatively estimated that fresh SGD carried (2.6 ± 0.4) × 10¹² microplastic particles (2.6 ± 0.5 tons) into coastal waters annually, accounting for approximately 11.3% ± 6.0% of the total riverine MPs flux into Xiamen Bay. These findings confirm that SGD constitutes a non-negligible pathway for land-based microplastics, highlighting the necessity of incorporating groundwater transport into coastal plastic pollution assessment and management strategies.