Spatial-temporal distribution and fluxes of microplastics in Jiulong River basin

Microplastics (MPs) pollution has emerged as a critical global environmental issue, for which rivers serve as key conduits for the transport of MPs from land to ocean. There remained a lack of clarity regarding the characteristics and transport patterns of MPs pollution in diverse environmental media and seasons within a river basin. The Jiulong River basin in Fujian, SE China was studied as case, whose spatial-temporal distribution, pollution characteristics, and transport dynamics of MPs in both surface water and groundwater were analyzed. Additionally, the features of MPs in different land use types during the dry season were examined. Results show that in wet season, the abundance of MPs in river water varied from 840 to 12 300 p/m3, with an average of 3 920 p/m3; in groundwater, the range was 740-1 820 p/m3, averaging 1 107 p/m3; and from 68 to 951 p/kg, with an average value of 265 p/kg in the soil (dry weight). In dry season, levels of MPs in river water ranged from 580 to 8 880 p/m3, with an average of 4 034 p/m3; groundwater exhibited a range of 860-14 980 p/m3, averaging 3 453 p/m3. Fibers consistently dominated the morphology of MPs, of which polyethylene terephthalate (PET) was identified as the predominant composition in each environmental medium. Our findings underscore the spatial and temporal variability in distribution and contamination characteristics of MPs across different media. Notably, MPs abundance in river water and groundwater exhibited a seasonal pattern, being higher in levels in dry season than in wet season. The downstream transport of MPs was facilitated by surface runoff, in which MPs in groundwater and surface water could complement to each other. Furthermore, population density and human activities were identified as influential factors on the distribution and abundance of MPs. This study revealed the order of magnitude of difference in annual estimates of MPs entering the sea among different models. Additionally, the interception volume by mangrove forest in the Jiulong River estuary was estimated approximately 1.51x1010 MPs particles per year. Ecological interception shall be considered when estimating the fluxes of MPs into the sea.