ABSTRACT. We present an overview of the risks that underground thermal energy storage (UTES) can impose on the groundwater system, drinking water production, and the subsurface environment in general. We describe existing policy and licensing arrangements for UTES in the Netherlands, as well as the capability of the current and future Dutch policy and legal framework to minimize or mitigate risks from UTES on groundwater resources. A survey at the European Union member state level indicates that regulation and research on the potential impacts of UTES on groundwater resources and the subsurface environment often lag behind the technological development of and ever-growing demand for this renewable energy source. The lack of a clear and scientifically underpinned risk management strategy implies that potentially unwanted risks might be taken at vulnerable locations such as near well fields used for drinking water production, whereas at other sites, the application of UTES is avoided without proper reasons. This means that the sustainability of UTES as a form of renewable energy is currently not fully understood, and the technology may be compromising the natural resilience of the subsurface environment. We recognize three main issues that should be addressed to secure sustainable application of UTES: Scientific research is required to further elucidate the impacts of UTES on groundwater; cross-sectoral subsurface planning is required to minimize negative conflicts between UTES and other subsurface interests; and EU-wide guidelines and standards are required for quality assurance and control when installing UTES systems.

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Shallow geothermal systems are increasingly being used to store or harvest thermal energy for heating or cooling purposes. This technology causes temperature perturbations exceeding the natural variations in aquifers, which may impact groundwater quality. Here, we report the results of laboratory experiments on the effect of temperature variations (5-80 °C) on redox processes and associated microbial communities in anoxic unconsolidated subsurface sediments. Both hydrochemical and microbiological data showed that a temperature increase from 11 °C (in situ) to 25 °C caused a shift from iron-reducing to sulfate-reducing and methanogenic conditions. Bioenergetic calculations could explain this shift. A further temperature increase (>45 °C) resulted in the emergence of a thermophilic microbial community specialized in fermentation and sulfate reduction. Two distinct maxima in sulfate reduction rates, of similar orders of magnitude (5 × 10(-10) M s(-1)), were observed at 40 and 70 °C. Thermophilic sulfate reduction, however, had a higher activation energy (100-160 kJ mol(-1)) than mesophilic sulfate reduction (30-60 kJ mol(-1)), which might be due to a trade-off between enzyme stability and activity with thermostable enzymes being less efficient catalysts that require higher activation energies. These results reveal that while sulfate-reducing functionality can withstand a substantial temperature rise, other key biochemical processes appear more temperature sensitive.

Large scale landfarming experiments, using an extensive range of treatments, were conducted in the Niger-Delta, Nigeria to study the degradation of oil in contaminated soils. In this work the effect of nutrient addition, biosurfactant, Eisenia fetida (earthworm) enzyme extract, bulking and sorption agents and soil neutralization were tested. It was found that these treatments were successful in removing up to 53% of the total petroleum hydrocarbon in the soil within 16 weeks. A comparison between treatments demonstrated that most were no more effective than agricultural fertilizer addition alone. One strategy that did show better performance was a combination of nutrients, biochar and biosurfactant, which was found to remove 23% more Total Petroleum Hydrocarbons (TPH) than fertilizer alone. However, when performance normalized costs were considered, this treatment became less attractive as a remedial option. Based on this same analysis it was concluded that fertilizer only was the most cost effective treatment. As a consequence, it is recommended that fertilizer is used to enhance the landfarming of hydrocarbon contaminated soils in the Niger Delta. The attenuation rates of both bulk TPH and Total Petroleum Hydrocarbon Criteria Working Group (TPHCWG) fractions are also provided. These values represent one of the first large scale and scientifically tested datasets for treatment of contaminated soil in the Niger Delta region. An inverse correlation between attenuation rates and hydrocarbon molecular weight was observed with heavy fractions showing much slower degradation rates than lighter fractions. Despite this difference, the bioremediation process resulted in significant removal of all TPH compounds independent of carbon number.

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Aquifer thermal energy storage (ATES) uses groundwater to store energy for heating or cooling purposes in the built environment. This paper presents field and laboratory results aiming to elucidate the effects that ATES operation may have on chemical groundwater quality. Field data from an ATES site in the south of the Netherlands show that ATES results in chemical quality perturbations due to homogenisation of the initially present vertical water quality gradient. We tested this hypothesis by numerical modelling of groundwater flow and coupled SO 4 transport during extraction and injection of groundwater by the ATES system. The modelling results confirm that extracting groundwater from an aquifer with a natural quality stratification, mixing this water in the ATES system, and subsequent injection in the second ATES well can adequately describe the observation data. This mixing effect masks any potential temperature effects in typical low temperature ATES systems (<25 W C) which was the reason to complement the field investigations with laboratory experiments focusing on temperature effects. The laboratory experiments indicated that temperature effects until 25 W C are limited; most interestingly was an increase in arsenic concentration. At 60 W C, carbonate precipitation, mobilisation of dissolved oxygen concentration, K and Li, and desorption of trace metals like As can occur. Key words | aquifer thermal energy storage, environmental impacts, ground source heat pumps, groundwater quality the natural groundwater temperature, can result in higher efficiency of the ATES 77 Downloaded from https://iwaponline.com/jwcc/article-pdf/4/2/77/374857/77.pdf by guest wells. 78 M. Bonte et al. | Environmental impacts of ATES investigated by field and laboratory experiments Journal of Water and Climate Change | 04.2 | 2013 Downloaded from https://iwaponline.com/jwcc/article-pdf/4/2/77/374857/77.pdf by guestFigure 5 | Simulated sulphate concentrations (shading) during several periods over the 3 year simulation period. Two ATES wells (K9 and W10) are shown, top left panel shows location of the monitoring screen set in the annulus of W10. The observed and simulated SO 4 concentrations for this monitoring screen are shown in Figure 6. 85 M. Bonte et al. | Environmental impacts of ATES investigated by field and laboratory experiments Journal of Water and Climate Change | 04.2 | 2013 Downloaded from https://iwaponline.com/jwcc/article-pdf/4/2/77/374857/77.pdf by guest Winters, A. L.  Summary of research on microbiological processes.

We used data from an aquifer thermal energy storage (ATES) system located 570 m from a public water supply well field in the south of The Netherlands to investigate the relation between production of renewable energy with an ATES system and the production of drinking water. The data show that the groundwater circulation by the ATES system can impact chemical groundwater quality by introducing shallow groundwater with a different chemical composition at greater depth. However, the observed concentration changes are sufficiently small to keep groundwater suitable for drinking water production. Microbiological results showed that the ATES system introduced faecal bacteria in the groundwater and stimulated the growth of heterotrophic micro-organisms. At the studied site this forms no hygienic risk because of the long distance between the ATES wells and the public supply well field A further degradation of either chemical or microbiological groundwater quality however may necessitate additional water treatment which raises the energy requirements. The additional energy requirements for drinking water treatment may be up the same order of magnitude as the harvested energy by the ATES system.

Aquifer thermal energy storage (ATES) systems are increasingly being used to acclimatize buildings and are often constructed in aquifers used for drinking water supply. This raises the question of potential groundwater quality impact. Here, we use laboratory column experiments to develop and calibrate a reactive transport model (PHREEQC) simulating the thermally induced (5-60 °C) water quality changes in anoxic sandy sediments. Temperature-dependent surface complexation, cation-exchange, and kinetic dissolution of K-feldspar were included in the model. Optimization results combined with an extensive literature survey showed surface complexation of (oxy)anions (As, B, and PO4) is consistently exothermic, whereas surface complexation of cations (Ca and Mg) and cationic heavy metals (Cd, Pb, and Zn) is endothermic. The calibrated model was applied to simulate arsenic mobility in an ATES system using a simple yet powerful mirrored axi-symmetrical grid. Results showed that ATES mobilizes arsenic toward the fringe of the warm water bubble and the center of the cold water bubble. This transient redistribution of arsenic causes its aqueous concentrations in the cold and warm groundwater bubbles to become similar through multiple heating cycles, with a final concentration depending on the average injection temperature of the warm and cold ATES wells.