Environmental Science and Engineering Seminar

Over 97% of groundwater is sub-modern, poorly constrained beyond radiocarbon dating, and weakly connected to the rest of the hydrological cycle. Million- to billion-year old groundwater exists at kilometers depth in the Earth's crust, deepening our perspective on the timescales and extent of the terrestrial water cycle. But where is the bottom of the terrestrial water cycle and how has it evolved over geologic time in response to land surface changes (e.g., burial, exhumation, glaciation)? Tackling these questions is important for defining the lower boundary of watershed to earth system models, constraining subsurface geochemical fluxes, and understanding the deep biosphere. In addition, circulation of meteoric water impacts groundwater and mineral resources and subsurface storage of anthropogenic waste (e.g., CO2, spent nuclear fuel) and alternative energy (e.g., H2). To begin to address these questions, this talk explores the volume, quality, and residence time of kilometers-deep groundwater; existing subsurface competition for these waters; drivers and depth of active meteoric water circulation over geologic time; and connection of deep groundwaters with the near-surface water cycle. We find that there is more water stored as groundwater on the continents (up to 10 km depth) than in ice sheets and glaciers, although the vast majority is saline; in some locations deepening water supply wells are approaching limits of freshwater and presence of existing oil and gas activities; active meteoric circulation extends hundreds of meters to several kilometers dependent on geologic setting; less than 0.1% of groundwater beyond 500 m contributes to river discharge; and deep groundwater flow systems have been activated within the last several million years in response to recent landscape changes (e.g., incision of the Colorado Plateau and Pleistocene glaciation of northern latitudes).