Time-series measurements of stream and sediment temperature for understanding river-groundwater interactions: Border Rivers and Lower Richmond catchments, Australia

Monitoring of temperature in the stream sediment (0.25-1.2 m depth) as well as the stream itself was used to investigate groundwater-surface water interactions in two different Australian catchments (Border Rivers and Lower Richmond). When interpreted with hydrographic and hydraulic conductivity data, the temperature monitoring provided useful insights into the spatial and temporal variability of stream-aquifer connectivity. At one site, sediment temperatures fluctuated with the diurnal temperature variation of the stream, reflecting river leakage (losing conditions). No diurnal signal was detected in the sediment temperatures at other sites, which is a typical indicator of gaining conditions. However, with water-level measurements indicating negative gradients and the stream sediments dominated by clay at these sites, this lack of sediment temperature variability is interpreted to reflect very low rates of downward seepage. At one site, a transition from gaining to losing conditions was observed through time. In the field trials, operational issues such as timing the monitoring to coincide with reasonable diurnal variations of stream temperature, the requirement of understanding the shallow stratigraphy of the stream bed and separating out localised effects (such as weirs) were highlighted. The trials also highlighted that interpretation of the temperature data can be ambiguous when viewed in isolation. Results indicated that time-series measurements of sediment and stream temperatures can be a useful screening tool for identifying gaining and losing reaches and for identifying temporal variations in seepage flux. It is suggested that temperature loggers can be readily and cheaply incorporated into existing hydrographic networks to provide a greater understanding of stream-aquifer connectivity. It is also recommended that existing routine temperature logging (such as with pressure transducers) be upgraded to sufficient accuracy for seepage studies. Temperature monitoring would be particularly useful in estimating seepage from Australian ephemeral streams. These data also have relevance to the investigation and management of aquatic ecosystems, notably in the hyporheic zone.