The Anthropocene in the Deep Subsurface

Abstract:

Deep groundwater systems are weakly connected to the rest of the hydrologic cycle. Fluxes of fluids and solutes are typically low under background conditions due to the presence of lower permeabilities and effects of negative buoyancy. Groundwaters at depths exceeding several 100 m often have ages exceeding millions of years old along with geochemical and isotopic compositions that do not resemble meteoric waters. During the past century, these deep groundwater systems have been dramatically altered in many areas by anthropogenic activities in the subsurface. Large volumes of fluid have been both produced and injected from many sedimentary basins, mainly by the energy industry. While in many cases the overall effect on basin-scale fluid budgets is near zero, this has resulted in local rearrangement fluid pressures, groundwater velocities and fluid chemistry. Groundwater flow in many areas is driven primarily by production and injection rates rather than regional flow systems. Installation, stimulation and abandonment of wells has created high permeability pathways locally, which could further impact groundwater flow and solute transport. Our understanding of these systems typically comes from pressures measured during the installation of oil and gas wells along with isolated geochemical and isotopic measurements taken at various points in time. Where appropriate screening approaches are taken, this can provide insight into background conditions, but does little to inform us about how these systems are changing. New approaches are required to understand the behaviour of deep groundwater systems in the Anthropocene.