Seasonal influence of suspended particulate matter on metal and organic pollutant distribution under fluvial and tidal conditions in the Elbe River

The explanatory content of measurements of suspended particulate matter (SPM) for legacy contaminants in the aqueous phase was evaluated at the Elbe River and its estuary, focusing on cadmium, zinc, cooper, mercury, hexachlorobenzene (HCB), dichlorodiphenyltrichloroethane (DDT) and its metabolites DDD and DDE and polychlorinated biphenyl congener PCB 180 at two locations: nontidal (station Cumlosen) and tidal conditions at the fluvial estuary (station Seemannshöft). Although the affinity of heavy metals and persistent organic pollutants for fine suspended solids and organic matter is well documented, transferring singular observational knowledge to longer timescales, considering SPM-cycles, remains underexplored. This understanding is crucial for identifying key processes affecting the distribution of contaminants that heavily depend on varying SPM composition along the river. We performed a statistical analysis of long-term time series, assessing the explanatory content of SPM-related and other environmental parameters for different contaminant species using the autoregressive-moving-average model with exogenous input (ARIMAX). We obtained the variance of contaminant species explained by cross-correlated parameters, prewhitened through ARIMA-modeling, in both freshwater and tidal conditions. The results suggest that, in the fluvial estuary, the seasonal distribution of pollutants in the aqueous phase is controlled by the presence of very fine particles including aged organic matter and resuspended matter, whereas in the nontidal Elbe, algae-related organic matter influences the advective and diffusive transport of metals. These insights are crucial for enhancing contaminant transport models, highlighting the necessity to consider fine (likely colloidal) particle transport when forecasting metal concentrations and their distribution in solid-water phase systems. In addition, the analysis revealed effects on the distribution of contaminants following the prolonged drought recorded in the Elbe River after 2013, which are expected to become more frequent as a result of climate change.