High-frequency hydrogeochemical monitoring has revolutionized our understanding of surface water in the past decade by providing a detailed record of hydrologic system response to cyclical forcings (e.g. days, seasons) and instantaneous perturbations (storms, anthropogenic contaminations or mitigations), but the high-frequency dynamics of groundwater chemistry remain enigmatic because of practical and technological hurdles such as high pressure in deep aquifers or the inaccessibility of field sites. Prior attempts to collect high-frequency groundwater chemistry measurements have utilized surface-based sensor suites that rely on pumps for sample delivery to the sensors. However, this continuous pumping approach perturbs groundwater systems in unknown ways, and the machinery can introduce a variety of contaminants including organic lubricants, which frustrate geochemical studies. Furthermore, these invasive and mechanically complex systems have not been proven at depths greater than 100 m and cannot be deployed at inaccessible field sites. The Well Geochemistry Logger (WGL) is a small data-logging sensor suite enclosed in a pressure-rated aluminum capsule. The WGL has two operational modes: depth profiling and continuous measurement at static depth. In its current stage of development, the WGL can measure electrical conductivity, temperature, and depth (CTD) of well fluids as deep as 400 m; future iterations will include pH/ORP sensors. Upon recovery, the WGL can connect to a laptop to transfer data. The WGL was successfully field-tested in January 2023 at the Samail Ophiolite, Oman, where it was used to profile the electrical conductivity of extremely basic well waters to depths exceeding 250 m. The WGL recorded conductivities over 6000 µS/cm and helped identify two distinct fluid layers based on their conductivities. These real-time observations informed sampling strategies and validated other measurements obtained from fluid samples. The WGL is made from affordable, commercially available components and based on Arduino, an open-source electronics framework. We propose the WGL as a solution to challenges that currently impede the study of groundwater chemistry dynamics.
Graduate student Geology, CU Boulder