The Merksplas Formation consists of a gray medium to coarse grained sand with glauconite and wood fragments. The

sands contain shell fragments in the lower part and occasionally gravel. The Brasschaat Formation is a dominantly sandy complex with a grain size distribution ranging from very fine to medium grained sand. Beside typical minerals such as micas and glauconite, the unit also contains vegetation remains, peat and wood fragments. The Merksplas and Brasschaat Formations are partly lateral facies ( Gullentops et al., Everolimus 2001). The Formations of Berchem, Diest, Kattendijk, Mol, Merksplas and Brasschaat together form the Neogene Aquifer. The natural groundwater compostion of this aquifer is characterized by low levels of chloride (<25 mg/l). The composition of the groundwater is further determined by the oxidation of organic matter creating a strong vertical variation in groundwater quality. Pyrite oxidation occurs in the shallow groundwater introducing high amounts of sulfate (to 100 mg/l) and iron (>50 mg/l). Deeper in the aquifer these concentrations decrease due to sulfate reduction ( Coetsiers et al., 2014). For several ATES systems (A, E, F, G) (Supplementary data – Figs. S1, S5–S7), the samples from the cold and warm well(s) were taken only once a year in the same season. Z-VAD-FMK molecular weight Therefore the effect of temperature on the groundwater quality could not be determined for these systems,

as the extracted water always originates from the same well. When sampling during winter, water extracted from both the warm and cold well originates from the warm bubble, when sampling during summer, the sampled water from both wells originates from the cold bubble. For other ATES systems however, water was sampled once or twice a year in different seasons (B, C, D) (Supplementary data – Figs. S2–S4), whereby water originating from both the cold and warm bubble was displayed in the time series. Comparing the quality of the water extracted from the cold well during summer Pembrolizumab manufacturer (cold bubble) with the quality

of the water extracted from the warm well during winter (warm bubble) shows no larger differences than between the samples from the same season over time. Fig. 3 shows a chart summarizing the data of the ATES systems and the ambient values compared with the Flemish drinking water standard. The chart shows upward or downward trends for some of the considered species for several of the investigated ATES systems. The measured values however stay well within the drinking water standard for calcium, sodium, magnesium, sulfate and chloride. For the pH, manganese, iron and ammonium the analyses for several ATES systems show values outside the drinking water standard. This is especially the case for iron and ammonium where for all ATES systems, except respectively one (C) and two systems (C and E), values above the drinking water standard are reported.