Spatiotemporal variabilities of major, minor and trace elements in the German part of the river Rhine

Recently developed multi-element methods for filtered and unfiltered river water enable the analysis of 67 elements in a single analytical run, facilitating the assessment of multi-element fingerprints in monitoring programs. To elucidate the occurrence and possible pathways of emerging contaminants, it is essential to have baseline data for as many elements as possible to detect anomalies at specific environmental events. However, the variability of element concentrations in river water on the temporal and spatial scale is only poorly understood, causing considerable uncertainty for river water monitoring. Therefore, we conducted comprehensive sampling campaigns to assess the spatiotemporal variabilities of element concentrations in the river Rhine (Germany) at different discharge levels. Both, the long-term temporal (one-year data) and the spatial variability data revealed a distinct behavior of two element groups in relation to the discharge: elements showing a dilution effect (e.g.: B, Mg, S, K, Ca, Br, Sr, Mo, and U) or a co-rising effect (e.g.: Al, Si, P, Ti, V, Mn, Fe, Ni, Cu, Ga, As, Rb, Y, Cs, Ba, La, Ce, Pr, Nd, Sm, Gd, Pb, and Th) with higher discharge in the unfiltered river water. However, certain elements such as K, Ba or Gd displayed a variable behavior throughout the dataset, underlining the importance of collecting enough baseline data from various locations and conditions to detect anomalies. The analysis of unfiltered water samples in comparison with the filtered fraction allowed us to detect opposite behavior between the dissolved and particulate fractions for Co, Ni, As, Rb, Cs, and Gd. Cross-profile measurements were conducted to investigate spatial variabilities, revealing that lateral spatial gradients in element concentrations (up to a factor of 4) are more pronounced than depth gradients, likely caused by insufficient mixing of river influents or point-sources. Thus, fixed monitoring stations or single-point sampling for long-term data acquisition might not be able to capture the whole picture regarding element behavior in rivers.