COUPLED TEMPERATURE-PRESSURE PROBES FOR MONITORING PORE WATER FLUXES IN COASTAL SEDIMENT

In inland settings, hydrologists have built on seminal research from Stallman (1965) and others to demonstrate that pore water fluxes can be quantified by time series of multi-depth pore water temperatures. Heat is a naturally occurring pore water tracer that is relatively inexpensive to collect and simple to analyze. In particular, the damping and lagging of sinusoidal, diel thermal signals in sediment underlying surface water bodies can be interpreted to reveal the direction and magnitude of groundwater-surface water exchanges. Also, the curvature of sediment temperature-depth profiles can be used to quantify vertical groundwater fluxes.

While heat-as-a-tracer techniques have been widely applied to inland environments, pore water exchanges in coastal settings are difficult to quantify with heat due to the highly dynamic nature of tidal environments. These complications arise because the groundwater flux itself may be periodic due to tidal oscillations. This flux periodicity may induce signal interference in the thermal diel signals. The frequency of the pore water flux oscillations does not coincide with the frequency of diel temperature oscillations. To address the challenging physics of pore water exchange in coastal settings, we have developed a novel, multi-level temperature-pressure probe for quantifying porewater fluxes in coastal sediment. The pressure readings reveal the hydraulic gradient as well as the period and magnitude of the tidal fluctuations, which are important controls on groundwater flux. Having hydraulic data available helps limit equifinality issues when interpreting the thermal data to quantify the fluid flux. Also, when the fluid flux is inferred from the thermal data, the hydraulic conductivity can then be estimated from the pressure data by rearranging Darcy’s Law. Challenges in the design and implantation of this instrument will be highlighted, and field data and interpretation will be presented for early testing in mega-tidal settings in the Canadian Maritimes.