Migration of tire rubber crumbs through variably saturated laboratory-scale sand columns

Tire wear rubber particles (TWRP) fall under the umbrella of microplastics and are responsible for synthetic particulate pollution in the urban environment. Urban stormwater runoff carries these particles towards receiving water bodies (e.g., aquifers, rivers, or sea). In particular, studies on their migration behaviour to the subsurface through stormwater infrastructures are still elusive.The current study investigated the vertical migration of end-of-life truck and track tire rubber particles (TPs) in synthetic stormwater (SSW) with 5 mg C/l dissolved organic carbon through laboratory-based infiltration sand columns. Thereto, 200 mg of the particles, with a size range of 63 – 180 µm, were placed inside the column at a depth of 3 cm. Subsequently, 6 litres of SSW were flushed intermittently through the columns in varying wetting, drying, and saturation cycles simulating heavy precipitation patterns. Effluents were collected after each cycle and retained particles within the column were extracted at specific depth intervals to test for Zn concentration (as ZnO is a commonly used additive agent in tire manufacturing) as a proxy for tire particles. We found that 45 – 95% of the truck and track TPs were retained in the seeded depth of 0 – 3 cm varying with different scenarios. Significant migration occurred in the first depth interval layer (3 – 8 cm) of the columns, whereas the other layers received fewer and nearly uniform amounts of TPs. The truck particles showed 24% more penetration for wetting and drying cycles, whereas upon reduction of gap time between two subsequent wetting cycles track TPs penetrated 18% more. Furthermore, longer saturation (24-hour contact time with SSW between cycles) consistently released 2 – 6 times more Zn than shorter duration times. Our results indicated that large rubber particles in the size ranges we studied remain in the topmost part of the soil. In case of moist or wet soils, these particles will act as a source of pollution, which will finally leach into groundwater, thereby polluting aquifers.