Riverbed depth-specific microplastics distribution and potential use as process marker.

Riverbed sediments have been identified as temporary and long-term accumulation sites for microplastic particles (MPs), but the relocation and retention mechanisms in riverbeds still need to be better understood. In this study, we investigated the depth-specific occurrence and distribution (abundance, type, and size) of MPs in river sediments down to a depth of 100 cm, which had not been previously investigated in riverbeds. In four sediment freeze cores taken for the Main River (Germany), MPs (≥ 100 µm) were detected using two complementary analytical approaches (spectroscopy and thermoanalytical) over the entire depth with an average of 21.7 ± 21.4 MP/kg or 31.5 ± 28.0 mg/kg. Three vertical trends for MP abundance could be derived, fairly constant in top layers (0-|30 cm), a decrease in middle layers (30-60 cm), and a strong increase in deep layers (60-100 cm). The dominant polymer types were polyethylene (PE), polypropylene (PP), and polystyrene (PS). Polyethylene terephthalate (PET) and PP were also found in deep layers, albeit with the youngest age of earliest possible occurrence (EPO age of 1973 and 1954). The fraction of smaller-sized MPs (100-500 µm) increased with depth in shallow layers, but the largest MPs (> 1 mm) were detected in deep layers. Based on these findings, we elucidate the relationship between the depth-specific MP distribution and the prevailing processes of MP retention and sediment dynamics in the riverbed. We propose some implications and offer an initial conceptual approach, suggesting the use of microplastics as a potential environmental process tracer for driving riverbed sediment dynamics.

Seasonal dynamics modifies fate of oxygen, nitrate, and organic micropollutants during bank filtration - temperature-dependent reactive transport modeling of field data.

Bank filtration is considered to improve water quality through microbially mediated degradation of pollutants and is suitable for waterworks to increase their production. In particular, aquifer temperatures and oxygen supply have a great impact on many microbial processes. To investigate the temporal and spatial behavior of selected organic micropollutants during bank filtration in dependence of relevant biogeochemical conditions, we have set up a 2D reactive transport model using MODFLOW and PHT3D under the user interface ORTI3D. The considered 160-m-long transect ranges from the surface water to a groundwater extraction well of the adjacent waterworks. For this purpose, water levels, temperatures, and chemical parameters were regularly measured in the surface water and groundwater observation wells over one and a half years. To simulate the effect of seasonal temperature variations on microbial mediated degradation, we applied an empirical temperature factor, which yields a strong reduction of the degradation rate at groundwater temperatures below 11 °C. Except for acesulfame, the considered organic micropollutants are substantially degraded along their subsurface flow paths with maximum degradation rates in the range of 10-6 mol L-1 s-1. Preferential biodegradation of phenazone, diclofenac, and valsartan was found under oxic conditions, whereas carbamazepine and sulfamethoxazole were degraded under anoxic conditions. This study highlights the influence of seasonal variations in oxygen supply and temperature on the fate of organic micropollutants in surface water infiltrating into an aquifer.

Temperature-dependent redox zonation, nitrate removal and attenuation of organic micropollutants during bank filtration.

River bank filtration (RBF) is considered to efficiently remove nitrate and trace organic micropollutants (OMP) from polluted surface waters. This is essential for maintaining good groundwater quality and providing high quality drinking water. Predicting the fate of OMP during RBF is difficult as the biogeochemical factors controlling the removal efficiency are not fully understood. To determine in-situ removal efficiency and degradation rates of nitrate and OMP indicator substances we conducted a field study in a RBF system during a period of one and a half years incorporating temporally and spatially varying redox conditions and temperature changes typically occurring in temperate climates. RBF was analyzed by means of mixing ratios between infiltrated river water and groundwater as well as average residence times of surface water towards the individual groundwater observation wells. These results were used to calculate temperature dependent first order degradation rates of redox sensitive species and several OMP. Five out of ten investigated OMP were completely removed along RBF pathways. We demonstrate that degradation rates of several OMP during bank filtration were controlled by redox conditions and temperature whereby temperature itself also had a significant influence on the extent of the most reactive oxic zone. The seasonal variations in temperature alone could explain a considerable percentage of the variance in dissolved oxygen (34%), nitrate (81%) as well as the OMPs diclofenac (44%) and sulfamethoxazole (76%). Estimated in-situ degradation rates roughly varied within one order of magnitude for temperature changes between 5 °C and 20 °C. This study highlights that temporal variability in temperature and redox zonation is a significant factor for migration and degradation of nitrate and several OMPs.