Runoff chemistry as an indicator of runoff sources and routing in semi-arid, badland drainage basins

The temporal variations of runoff chemistry during a storm event reflect the dynamics of runoff and solute sources. This study concerns the use of runoff chemistry to investigate runoff generation and routing in semi-arid, badland basins drained by ephemeral streams. Discharge, sediment concentration, and electrical conductivity (EC) were measured at the outlets of a 20.2 hectare and a 7.9 hectare basin. Runoff samples were analysed for sodium, potassium, calcium, magnesium, aluminium, and sulphate. EC values ranged from 255 to 800 μS cm −1. During the initial phase of flow, EC values dropped sharply owing to the flushing effect. The EC reached a minimum around the time of peak flow, and slowly increased during the falling stage owing to the dissolution of suspended sediment and, during large storms, the influx of solute-rich water from tunnel systems and shale surfaces. Sodium was the dominant cation, and variations of sodium concentration were similar to those of EC. The concentrations of the remaining cations showed little variation throughout a storm event. Cation composition was controlled by the displacement of sodium from the exchange sites by divalent cations. Sulphate concentration was strongly correlated with EC, and showed the same pattern of variation throughout a runoff event. Nevertheless, for a similar EC, sulphate concentrations were proportionally lower in runoff from sandstone and pediment surfaces, and proportionally higher in tunnel flow, runoff from shales, and during initial flushing. The SO 4-EC relationship hence proved to be a sensitive indicator of the start of tunnel flow and runoff generation on the shales, even though each variable had little value when used separately.