Electrical geophysical data collected in the shallow sediments of Snake Pond, Cape Cod, USA

Quantification of mobile/less-mobile porosity dynamics at the sediment/water interface is critical to predicting contaminant storage, release, and transformation processes. Zones in groundwater flow-through lakes where lake water recharges the aquifer can strongly control aquifer water quality. Less-mobile porosity has previously been characterized in aquifers using flow path scale (10's of m+) tracer injections which are analyzed using numerical models. Methodology was recently developed to couple geoelectric measurements (bulk electrical conductivity, EC), which are directly sensitive to less-mobile ionic tracer exchange processes, with pumped fluid EC tracer data over time. If the fluid EC concentration history is assumed to reflect the more mobile porosity exchange processes, these paired fluid and bulk EC measurements can be used to quantify less-mobile porosity exchange in discrete cm-scale packets of sediment at the interface between surface and groundwater. For this study, tracer experiments were conducted in multiple rate-controlled downward flow experiments over several days. Although the bed was composed predominantly of highly permeable sands and gravels, which is not an intuitive sediment texture for less-mobile porosity, embedded cobbles created areas of less-mobile flow zones proximal to large cobbles. These experimental findings are described in detail in the associated publication: Briggs, M.A., Day-Lewis, F.D., Dehkordy, F.M.P., Hampton, T., Zarnetske, J.P., Singha, K., Harvey, J.W. and Lane, J.W.(2018), Direct observations of hydrologic exchange occurring with less-mobile porosity and the development of anoxic microzones in sandy lakebed sediments, Water Resources Research, DOI:10.1029/2018WR022823.