Aquifer – stream connectivity at various scales: Application of sediment – water interface temperature and vulnerability assessments of groundwater dependent streams

Streams with greater connectivity to an aquifer are potentially more sensitive to changes in groundwater levels and fluxes than streams with less connectivity. Aquifer-stream connectivity during the summer low flow period is of particular concern because this is a period of maximum relative groundwater contribution to stream flow volumes, which coincides with periods of peak water demands and critical aquatic habitat needs. Field and statistical methods were used to characterize aquifer - stream connectivity and evaluate factors influencing the groundwater flux to streams at different scales during the summer low flow period. The research focused on the use of sediment-water interface temperature in combination with a range of field methods, including manual stream discharge measurements, seepage meters, and in-stream piezometers, to characterize aquifer - stream connectivity in Fishtrap and Bertrand Creeks in the Lower Fraser Valley of southwest British Columbia. A combination of field measurements aided in reducing measurement uncertainties and improved estimation of the groundwater flux. A simplified heat budget demonstrated that, despite their similar climate and geographical setting, the groundwater flux during the summer periods was higher in Fishtrap Creek than in Bertrand Creek, due to its more permeable geological substrate. Independent component analysis (ICA) combined with cross-correlation was a novel approach to temperature signal separation. ICA directly linked the extracted signals to factors in the heat budget that influence sediment-water interface temperatures within a stream reach. Surface heating from solar radiation was the dominant factor influencing the interface temperature in most years, but there is evidence that thermal exchanges took place at the water-sediment interface, and the correlation with groundwater levels indicated these heat exchanges were associated with groundwater influx. Overall, the combined approaches were able to attribute temporal and spatial variability in streamflow and sediment-water interface temperatures to relative contributions of groundwater to streams.The understanding of aquifer-stream connectivity at different scales was applied in the development of a vulnerability framework for assessing stream vulnerability to changes in groundwater conditions. This framework can be used in support of decision making surrounding Sensitive Stream Designation in British Columbia and water allocation under the Water Sustainability Act.