Understanding salt transport in the hyporheic zone of ephemeral streams

Abstract

This research was driven by portentous water management complications that transpired due to the accumulation of large volumes of saline water within the Bowen Basin coal mines during recent years. The ongoing realities of either low or no flow hydrographs within semi-arid regions, together with tightened discharge criteria, have resulted in restricted discharge opportunities; these restrictions render it impossible to meet mines’ discharge demands. Considering the potentially deleterious impacts of saline discharge on biota and water quality for downstream water users, concerns regarding the saline discharge criteria in ephemeral streams have been raised. Hyporheic exchange, which is the interaction between stream water and subsurface water, might affect the outcome of the discharged saline water. However, fundamental knowledge is currently missing in regards to salt transport in the hyporheic zone within ephemeral streams. More specifically, research about the potential consequences of salt exchange to flow dynamics and the impacts of the drying and rewetting of the hyporheic zones in ephemeral streams on salt movement has not yet been conducted. Consequently, the aim of this research is to investigate hyporheic exchange in the salt transport process within ephemeral streams, and further, to identify the major factors that control salt transport. Streambed sediments were collected for the salt storage quantification from Cherwell Creek, which is an ephemeral stream within the Bowen Basin. Spatial and temporal variations of streambed salt storage were investigated, and factors impacting the distribution of salt storage were explored. Vertically, peak storages were found in the top twenty centimetres of the streambed, and below that depth, the salt storage variation was negligible. The spatial variations in the surface sediment salt storage were found to be related to the streambed surface elevation. The temporal variation of salt storage suggested that the duration of evaporation strongly impacted the amount of streambed salt storage. Low flows tend to dominate areas within Cherwell Creek. Under low flow conditions, stream water flows along the bedform meanders. Past research has shown that reach-scale meanders can induce hyporheic exchange, but the processes, which are driven by bedform meanders, are largely unknown. This research hypothesised that hyporheic exchange can also be induced by bedform meanders. Field measurements of hydraulic head and salinity distributions were conducted in a bedform meander within Cherwell Creek. Minimal exchange was observed between surface and subsurface water. The hydraulic head distributions within the meander suggested that subsurface water flowed from the centre to the sides of the meander. This finding revealed a fundamental difference between processes driven by bedform scale meanders and reach scale meanders. A bedform meander is submerged by surface water during the peak flow period, and during the recession period, the water level drops rapidly, and accordingly, the drainage process dominates the flow regime. In contrast, the land surface of a reach scale meander is always above the water table. Therefore, the hydrological processes in the bedform reach scale meanders are different during the recession period. The initial solute flush has been widely reported and hypothesised as being due to connections between previous solute storage zones and the stream. This study hypothesised that the salt stored in the streambed contributed to the initial salt flush in Cherwell Creek. MODHMS was employed to simulate the first flow event in Cherwell Creek during 2012. The three-dimensional flow processes were explored, and the rate of the stored salt in the streambed during rewetting was estimated. Water volume and salt mass balances were examined to identify the controlling processes of flow and salinity variations. The simulation results suggest that the infiltration process dominates the flow regime at the beginning of the flow event. After the streambed became fully saturated, lateral flow slowly redistributed the subsurface water that was observed during the recession period. Salt storage in the streambed decreased to half that of its initial magnitude by the end of the simulation due to the redistribution process to the stream banks. According to the simulation results, streambed salt storage had minimal impact on the surface water salinity. Findings in this research contribute to a better understanding of hydrology and salt transport in ephemeral streams. The functions of the processes that took place at the reach scale during the rewetting period are represented. The streambed salt storage has a non-significant impact on stream salinity variations during the rewetting period. The differences between the processes driven by reach scale and bedform scale meanders during the recession period are highlighted in this study.