Arsenic is a common trace element in groundwater and its fate and transport are controlled by combination of (i) natural processes, including redox conditions, salinity and pH, (ii) sedimentary and geochemical environment, and (iii) anthropogenic influences such as groundwater extraction, managed aquifer recharge (MAR), Aquifer Thermal Energy Storage (ATES), and pollution. We investigated the relative influence of these processes by presenting 10 cases from The Netherlands. Our review showed that the primary controlling factor for arsenic mobility in natural coastal dune systems is the redox state of groundwater, with concentrations between 2 and 10 μg/L. Strongly reduced greensands (containing glauconite and, more importantly, associated minerals) exhibited elevated As concentrations, with concentrations up to 40 μg/L. Groundwater systems modified by MAR or those that are influenced by nitrate pollution (NP) showed elevated As concentrations (20–110 μg/L), as a result of either pyrite oxidation (MAR, NP) or reductive dissolution of iron(hydr)oxides (MAR). Increasing temperature at ATES systems may cause mobilization of As at temperatures beyond 25 °C. The highest As concentrations were observed at sites where muddy sediments were recently deposited in surface water bodies (200–820 μg/L), for example in dammed Rhine River tributaries and sand pit lakes south of the city of Amsterdam. The reduction of arsenate to arsenite and competitive desorption during intrusion of polluted water also form important As mobilizing processes. The data for the Netherlands show that high CH₄ and NH₄ concentrations may form a risk indicator of elevated As levels in some fresh groundwater systems.

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Groundwater monitoring and pumping wells set in anoxic aquifers require attention to keep the groundwater free of dissolved oxygen (DO). In properly constructed monitoring or pumping wells, two processes can however still introduce oxygen to anoxic groundwater: (1) permeation of oxygen through polymer materials such as silicone, PVC, HDPE or Teflon, and (2) thermally driven convection, which can occur in all types of piezometers or wells, regardless of construction material, when the water table or pressure head is close (

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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.@en

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.@en

Aquifers used for the production of drinking water are increasingly being used for the generation of shallow geothermal energy. This causes temperature perturbations far beyond the natural variations in aquifers and the effects of these temperature variations on groundwater quality, in particular trace elements, have not been investigated. Here, we report the results of column experiments to assess the impacts of temperature variations (5°C, 11°C, 25°C and 60°C) on groundwater quality in anoxic reactive unconsolidated sandy sediments derived from an aquifer system widely used for drinking water production in the Netherlands. Our results showed that at 5 °C no effects on water quality were observed compared to the reference of 11°C (in situ temperature). At 25°C, As concentrations were significantly increased and at 60 °C, significant increases were observed pH and DOC, P, K, Si, As, Mo, V, B, and F concentrations. These elements should therefore be considered for water quality monitoring programs of shallow geothermal energy projects. No consistent temperature effects were observed on Na, Ca, Mg, Sr, Fe, Mn, Al, Ba, Co, Cu, Ni, Pb, Zn, Eu, Ho, Sb, Sc, Yb, Ga, La, and Th concentrations, all of which were present in the sediment. The temperature-induced chemical effects were probably caused by (incongruent) dissolution of silicate minerals (K and Si), desorption from, and potentially reductive dissolution of, iron oxides (As, B, Mo, V, and possibly P and DOC), and mineralisation of sedimentary organic matter (DOC and P).@en

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 SO4 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 °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 °C are limited; most interestingly was an increase in arsenic concentration. At 60 °C, carbonate precipitation, mobilisation of dissolved oxygen concentration, K and Li, and desorption of trace metals like As can occur.@en