Mobilisation and transport dynamics of potential toxic elements during high flow events in a small river catchment

Export of potential toxic elements and other particle bound pollutants from catchments is highly dependent on the flow regime. The main driver is the higher mobilisation and transport capacity for suspended particulate matter (SPM) during high flow events.  But are there further dynamics in the concentrations which are not purely driven by the SPM transport?

To answer this question, we investigated the dynamics of the concentrations of potential toxic elements, several other elements and total suspended solids during high flow events by automated sampling and subsequent analysis of dissolved and total concentrations by ICP-MS after microwave assisted acid digestion. At 3 river monitoring sites 3 high flow events were sampled with 3-6 samples per event and site, covering different parts of the flow and turbidity peaks, which were recorded by online-measurements. To complement the river monitoring with data about potential sediment sources, landuse-stratified soil sampling in the catchment and river bed sediment sampling were conducted.

Our case study was the Wulka river in eastern Austria with a catchment area of 384 km² and two if its tributaries, one with a very high share of treated waste water and the other with no permanent input of waste water. With a mean annual precipitation of 695 mm and a mean elevation of 256 m a.s.l. the river can be classified as a low land river. The landuse is dominated by agriculture including significant share of viniculture.

A first explorative principal component analysis showed, that several elements are strongly related with each other and the suspended sediment concentration. As this was expected, we used the SPM concentration to normalize the elemental concentrations and therefore taking out the variability caused by the suspended solids dynamics for further analysis. The remaining variability will be investigated regarding temporal and spatial patterns and correlation with the sediment and soil concentrations which can give indications about the emission pathways and sources.

To characterize the sampled high flow events, a hysteresis index was calculated from the discharge and turbidity signal which revealed different types of hysteresis, some clockwise hysteresis, several complex hysteresis patterns with different directions of the hysteresis during different times of the event and one small event with anticlockwise hysteresis. Different types of hysteresis can give indications about the distance of the sediment source to the observation location, further contributing to the exploration of SPM sources.