Preface: Hydrogeology of shallow thermal systems

Abstract

Hydrogeologyplaysanimportantroleinthetransferofheatin geological systems. Large-scale variations in thepermeabilitydistribution(includingthepresenceoffracturezones and faults) can significantly impact the coupled flowof heat and fluid (Smith and Chapman 1983). At theregional scale, groundwater flow can alter the heatdistribution, depressing or enhancing thermal gradients inspace and time, and carry heat over significant distances(Smith and Chapman 1983; Woodbury and Smith 1985;Molina-Giraldoetal.2011).Atsmallerscales,groundwaterflow can present challenges to the proper design of geo-exchange systems and can limit the success of aquiferthermal energy storage (ATES) and borehole thermalenergy storage (BTES) systems (e.g. Sauty et al. 1982a, b).This special issue of Hydrogeology Journal presents acollection of articles that aim to advance understanding ofcoupledhydrogeologicalandthermalprocessesoverarangeof spatial and temporal scales. The articles are broadlyclassified into three topics including springs (thermal andmineralized), basins (sedimentary and structure controlled),andgeo-exchangesystems.Theissuefocusesonnovelfield-and modeling-based studies that have been carried out tostudytheseshallowthermalsystems,wherehydrogeologicalprocesses play an active role in the redistribution of heat.The decision to focus on shallow thermal systems asopposed to deep thermal systems was driven largely by ajoint interest in highlighting low-to-moderate temperature(enthalpy) geothermal systems. This is not to say that high-temperature systems and/or deep systems are not consid-ered herein, but rather that the emphasis is on lower-temperature systems that are often strongly influenced bygroundwater flow in such a way as to alter the thermalregime, and that tend to exist at relatively shallow depth.Hebig et al. (2012) reviewed some of the early work thatled to definitions of deep groundwater, both from theperspective of flow and geochemical characteristics. Mostof the definitions of deep groundwater point to the absenceof meteoric water (low tritium and a lack of bicarbonate)and slow circulation rates, but emphasize that the term deepgroundwater is not bound to a defined depth below surface.The authors noted that such deep-lying aquifers havebecome important resources for the production of geother-mal energy and alternative potable water sources, or forrepositories (nuclear waste and CO