Obliteration of Thermal Springs by Lateral Flow of Groundwater: Implications for Resource Assessments

Abstract

Analysis of geothermal characteristics of hydrogeologic provinces of Brazil has identified the existence of an association between the geographic distributions of thermal springs and areas of occurrences of lateral flows of groundwater. Occurrences of thermal springs are found to be rare or altogether absent in regions inferred to have lateral flows of groundwater. This trend is remarkably evident in the sedimentary basins of the Amazon region, in the central parts of the Parnaiba basin and in the west-central parts of the Paraná basin. Results of model simulations indicate that groundwater flows with velocities in excess of few cm/year, along flat-lying aquifers, are potentially capable of masking the occurrences of thermal anomalies. Also, down-flows through distributed recharge zones are found to lead to the development of large regions of relatively low temperatures, capable of suppressing surface manifestations of deep geothermal resources. The observational data sets of temperature gradients and Peclet numbers have been employed outlining advection - convection domains of subsurface strata. The results indicate that advective down flows are capable of opposing thermal buoyancy forces and obliterating upward movements of thermal fluids. Vertical and lateral flows of cold groundwater can also lead to development of large zones of relatively low temperatures, capable of suppressing surface manifestations of deep geothermal resources. Though hydrogeological and geochemical data for the thermal spring systems of Brazil are limited it appears that an understanding of the perturbing effects of lateral flow systems in the subsurface is important in assessment of geothermal resources.