Effect of microplastics on arsenic transport in shallow groundwater of coastal transition zones.

Coastal transition zones (CTZs) are dynamic zones where seawater intrusion and freshwater discharge interact. Arsenic (As) is a common contaminant in these zones, however its co-transport with increasing abundance microplastics (MPs) remains unclear. In this study, saturated column experiments combined with Hydrus-1D modeling were used to investigate the co-transport of As with polystyrene (PS), polyethylene (PE), and polylactic acid (PLA). Increasing salinity inhibited As transport in the absence of MPs. Under the same salinity, MPs generally increased As breakthrough, following the order PS > PE > PLA. With salinity increased, PS maintained a slight promoting effect on As breakthrough, whereas PE and PLA exhibited reduced breakthrough under high-salinity conditions. Co-transport was most pronounced at an As:MP ratio of 1:2, although PLA showed a non-linear response and lower As mobility at 1:1. Simulated seawater intrusion indicated that ionic-strength fluctuations produced distinct release peaks. MPs further enhanced non-equilibrium As release during flushing. Metagenomic sequencing of samples from the 21-day experiment indicated that MPs altered the microbial communities and increased the relative abundance of several As-related genes (such as AS3MT and arsC), particularly in the PS treatment. These findings clarify the effects of MPs on As transport and support risk assessment for coastal groundwater systems.