Study of connectivity of open framework gravel facies in the Canterbury Plains aquifer using smoke as a tracer

Open framework gravels (OFGs) are an inherent textural component of alluvial gravel outwash deposited by braided river systems. Being exceptionally permeable, they play an important part in facilitating the transmission of water and contaminants through alluvial gravel aquifers. Understanding how connected OFG facies are is helpful in making informed predictions about groundwater flow and contaminant transport through such aquifer systems. This work examined a section of the Rakaia fan, Canterbury, New Zealand. A 3×3 grid of large diameter auger holes was drilled in close proximity to a sea cliff, which provided very good three-dimensional exposure of the fan architecture. A novel smoke tracing experiment and water tracing field tests were conducted to measure the dynamic connectivity of the OFG facies. Smoke proved to be an effective tracer for measuring the interconnectedness of OFGs over set distances of 5 m. The water tracing tests confirmed that OFGs are connected across much longer distances – in excess of 18 m. Results from both tests revealed how rapid, and non-uniform, aqueous transport can be through alluvial outwash materials. The connectivity information will be used to improve realizations of the heterogeneity of the Canterbury Plains aquifer and inform future hydrogeological modelling.