Geothermal phenomena in the context of gravity-driven basinal flow of groundwater

Abstract

Gravity-driven groundwater flow systems function in topographic basins as subsurface conveyor belts. They pick up and move fluids, gases, solutes, colloids, particulate matter and heat from loading sites in recharge areas and/or on their way to the discharge areas and can deliver them "en route" or in discharge regions. Gravitational flow systems of various horizontal and vertical extents are organized into hierarchically nested complex patterns controlled by the configuration of the water table's relief and modified by the rock framework's heterogeneities of permeability. The systems are ubiquitous and act simultaneously on broad ranges of the spatial and temporal scales of measurement. Their universal geologic agency is manifest by numerous different, even disparate, natural processes and phenomena. Several of these are associated with geothermal heat flow. The understanding of geothermal phenomena in the context of basinal flow systems requires, therefore, an intimate familiarity with the overarching "Theory of regional groundwater flow" which, in turn, comprises two component theories: "The hydraulics of basin-scale groundwater flow systems" and "The geologic agency of basin-scale groundwater flow-systems". The paper's outline is based on this conceptual structure. The paper presents examples for geothermal effects of groundwater flow by means of the first theoretical models and some case studies of thermal springs and wells, and petroleum accumulations. The final section reflects the author's conviction that geothermal studies cannot be complete without consideration and understanding of the area's groundwater flow regime.