The use of groundwater as a carrier of thermal energy is an important source of sustainable heating and cooling. However, the effects of thermal use on geochemical and biological aquifer characteristics are poorly understood. Here, we have assessed the impacts of heat discharge on an uncontaminated, shallow aquifer by monitoring the hydrogeochemical, bacterial and faunal parameters at an active thermal discharge facility. The observed variability between wells was considerable. Yet, no significant temperature impacts on bacterial or faunal abundance and on bacterial productivity were observed. Also, we did not observe an improved survival or growth of coliforms with temperature. In contrast, the diversity of bacterial terminal restriction fragment (T-RF) length polymorphism fingerprints and faunal populations was either positively or negatively affected by temperature, respectively, and the abundance of selected T-RFs was clearly temperature dependent. Canonical correspondence analysis indicated that both the impact of temperature and of surface water from a nearby river, were important drivers of aquifer biotic variability. These results demonstrate that aquifer thermal energy discharge can affect aquifer bacteria and fauna, while at the same time controlling only a minor part of the total seasonal and spatial variability and therefore posing no likely threat to ecosystem functioning and drinking water protection in uncontaminated, shallow aquifers.

[1] Many hydrological systems exhibit complex subsurface flow and storage behavior. Runoff observations often only provide insufficient information for unique process identification. Quantitative modeling of water and solute fluxes presents a potentially more powerful avenue to explore whether hypotheses about system functioning can be rejected or conditionally accepted. In this study we developed and tested four hydrological model structures, based on different hypotheses about subsurface flow and storage behavior, to identify the functioning of a large Mediterranean karst system. Using eight different system signatures, i.e., indicators of particular hydrodynamic and hydrochemical characteristics of the karst system, we applied a novel model evaluation strategy to identify the best conceptual model representation of the karst system within our set of possible system representations. Our approach to test model realism consists of three stages: (1) evaluation of model performance with respect to system signatures using automatic calibration, (2) evaluation of parameter identifiability using Sobol's sensitivity analysis, and (3) evaluation of model plausibility by combining the results of stages (1) and (2). These evaluation stages eliminated three out of four model structures and lead to a unique hypothesis about the functioning of the studied karst system. We used the estimated parameter values to further quantify subsurface processes. The chosen model is able to simultaneously provide high performances for eight system signatures with realistic parameter values. Our approach demonstrates the benefits of interpreting different tracers in a hydrologically meaningful way during model evaluation and identification.Citation: Hartmann, A., T. Wagener, A. Rimmer, J. Lange, H. Brielmann, and M. Weiler (2013), Testing the realism of model structures to identify karst system processes using water quality and quantity signatures, Water Resour. Res., 49,[3345][3346][3347][3348][3349][3350][3351][3352][3353][3354][3355][3356][3357][3358]

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