AN AUTOMATED HYDRAULIC POTENTIOMETER FOR MEASURING GROUNDWATER-SURFACE WATER INTERACTIONS

Diffuse groundwater discharge from bottom sediments into surface waters (e.g., baseflow, submarine groundwater discharge, etc.) is an important conveyer of water and dissolved materials to the surface in terrestrial and coastal environments. Due to its diffuse nature and low specific discharge, groundwater-surface water exchange is challenging to quantify and monitor. Existing direct methods for measuring groundwater-surface water exchange (e.g., seepage meters, piezomanometers, benthic flux chambers, etc.) are prohibitive for long-term, high-frequency studies because they are labor intensive, slow to respond, expensive, and/or susceptible to interference and error. Here we introduce a modified piezomanometer design we call a “hydraulic potentiometer” that is capable of high-frequency (i.e., sub-second) measurement of the small hydraulic head differences typical between bottom sediments and surface waters. Building upon existing Arduino-based designs used for cave pressure transducers and the light-oil piezomanometer, our design consists of a pair of waterproofed, gel-filled electronic pressure sensors (i.e., MS5803-01BAs) connected to an Arduino microcontroller. One of the pressure sensors is left open to the surface water, and the other pressure sensor is plumbed to a temporary, polyethylene tube piezometer driven into the subsurface. Following calibration, the difference in pressure between the two sensors is recorded and used to calculate a freshwater hydraulic head difference and used to estimate specific discharge or submarine groundwater flux with Darcy’s Law. Initial deployment of the hydraulic potentiometer in an estuarine, tidal river produced sub-minute hydraulic head difference time-series that reveal subtle (i.e., mm-level) and overlapping groundwater-surface water exchange processes including meteoric groundwater discharge, tidal pumping, wave pumping, and atmospheric groundwater forcing. Notable advantages of the hydraulic potentiometer include rapid response to short-lived groundwater-surface water exchange processes, high-frequency automated measurement, resolution of subtle (sub-mm) hydraulic head differences, potential for expansion, ability for remote real-time monitoring, and low cost.