IN-SITU QUANTIFICATION OF LESS-MOBILE POROSITY IN SAND-AND-GRAVEL GLACIAL KETTLE POND SEDIMENTS AT SNAKE POND, CAPE COD, MASSACHUSETTS

As surface water enters the subsurface from a lake or stream, it commonly loses oxygen to biogeochemical reactions. It is hypothesized that pockets of less-mobile porosity embedded within shallow, more mobile oxygenated zones locally increase residence time, creating anoxic microzones where denitrification can occur. Salt tracers and geophysical measurements have been paired in laboratory experiments to determine the presence of less-mobile microzones in various sediments, but such experiments have not previously been implemented in-situ at the surface-water/groundwater interface. An apparatus was designed and constructed with the capability of inducing a constant-rate tracer-labeled downwelling zone in a stream or lake bed. As injection fluid conductivity is altered, collocated bulk and fluid conductivity measurements are collected to track exchange of water throughout the dual-domain (mobile and less-mobile porosity) sediment matrix. A field test was conducted at Snake Pond, Cape Cod, Massachusetts, in July 2016, during which several focused tracer experiments were conducted to evaluate paired mobile and less-mobile porosity zones. Results indicate that less mobile pore space is substantial in the shallow subsurface of the pond, even in highly connected sediments, caused by the hydraulic effect of embedded cobbles and gravel within the sandy matrix. Graphical and numerical modeling techniques are used to quantify the less-mobile domain and evaluate the average residence time within the context of denitrification processes. This new focused injection technique, paired with fine-scale geophysical measurements, will lead to a better understanding of nitrate transport and transformation within interface sediments.