Is thermal use of groundwater a pollution?

Thermal use of the shallow subsurface and its aquifers (< 400 m) is steadily increasing. Currently, more than 2800 aquifer thermal energy storage (ATES) systems are operating worldwide alongside more than 1.2 million ground source heat pump (GSHP) systems in Europe alone. These rising numbers of shallow geothermal energy (SGE) systems will put additional pressure on typically vulnerable groundwater systems. Hitherto, suitable criteria to control the thermal use of groundwater in national and international legislations are often still at a preliminary state or even non-existing. While the European Union (EU) Water Framework Directive (WFD) defined the release of heat into the groundwater as pollution in the year 2000, the cooling of groundwater for heating purposes is not explicitly mentioned yet. In contrast, some national legislations have stricter guidelines. For example, in Germany, detrimental changes in physical, chemical and biological characteristics have to be avoided. In the Swiss water ordinance, it is even recommended that the groundwater biocenosis should be kept in natural state. However, exact definitions of 'detrimental changes' and 'natural state' are still missing. Hence, the current study provides an overview on natural and affected thermal groundwater conditions and international and national legislations of the thermal use of groundwater. Also, it presents recent studies on groundwater ecosystems and proposes a sustainable policy framework for the thermal use of groundwater. In addition to geothermal heat sources, other anthropogenic heat sources such as climate change, underground car parks, heated basements, district heating systems, land fills, wastewater treatment plants and mining are considered, although no legislation on these anthropogenic heat sources and their impact on groundwater is currently in place. Finally, we intend to answer the above question and provide recommendations for the further discussions on the joint use of shallow groundwater systems for drinking water production and thermal use.

Chloromethane formation and degradation in the fern phyllosphere.

Chloromethane (CH3Cl) is the most abundant halogenated trace gas in the atmosphere. It plays an important role in natural stratospheric ozone destruction. Current estimates of the global CH3Cl budget are approximate. The strength of the CH3Cl global sink by microbial degradation in soils and plants is under discussion. Some plants, particularly ferns, have been identified as substantial emitters of CH3Cl. Their ability to degrade CH3Cl remains uncertain. In this study, we investigated the potential of leaves from 3 abundant ferns (Osmunda regalis, Cyathea cooperi, Dryopteris filix-mas) to produce and degrade CH3Cl by measuring their production and consumption rates and their stable carbon and hydrogen isotope signatures. Investigated ferns are able to degrade CH3Cl at rates from 2.1 to 17 and 0.3 to 0.9µggdw-1day-1 for C. cooperi and D. filix-mas respectively, depending on CH3Cl supplementation and temperature. The stable carbon isotope enrichment factor of remaining CH3Cl was -39±13‰, whereas negligible isotope fractionation was observed for hydrogen (-8±19‰). In contrast, O. regalis did not consume CH3Cl, but produced it at rates ranging from 0.6 to 128µggdw-1day-1, with stable isotope values of -97±8‰ for carbon and -202±10‰ for hydrogen, respectively. Even though the 3 ferns showed clearly different formation and consumption patterns, their leaf-associated bacterial diversity was not notably different. Moreover, we did not detect genes associated with the only known chloromethane utilization pathway "cmu" in the microbial phyllosphere of the investigated ferns. Our study suggests that still unknown CH3Cl biodegradation processes on plants play an important role in global cycling of atmospheric CH3Cl.