Using hysteresis analysis to infer controls on sediment-associated and dissolved metals transport in a small humid temperate catchment

Understanding the metal dynamics in natural waters is critical to assessing the process and flow pathways that deliver these constituents to the stream network. Metal dynamics can be addressed by an analysis of concentration-discharge relationship across a large spectrum of hydrological conditions. However, metal-discharge responses have only been studied in detail for short periods or a few selected events. In this study, a detailed analysis of the hysteresis loops exhibited by metal concentration-discharge relationship was undertaken to assess the Al, Fe, Mn and Cu dynamics and factors controlling changes in metal concentrations in a small scale rural catchment. Metal (sediment-associated and dissolved) concentrations were measured at high temporal resolution during storm events in a small rural headwater catchment in Galicia (NW Spain). Hysteresis analysis indicated a contrasting concentration-discharge dynamic for sediment-associated and dissolved metals, highlighting a strong decoupling between the two due to their different sources or delivery pathways. For sediment-associated metals, clockwise hysteresis (56%) was the predominant pattern, highlighting the rapid sediment-associated metal mobilisation via surface runoff from sources located near the stream channel. In contrast, dissolved metal showed predominantly anticlockwise hysteresis (58% of storm events), emphasising the role of subsurface flow and soil leaching processes. The different flushing mechanisms between sediment-associated and dissolved metals imply that different catchment management strategies should be implemented for the control of metal export.