The database includes 1035 datasets from 560 different measurement points in the entire Hessian territory and adjacent areas (Fig. 1). Of these, 890 datasets include concentrations of major elements and therefore TDS and electrical balance. The database covers a period of more than 200 years. An analysis of the Schwefelquelle (“Sulfur spring”, no. 520) in Flörsheim from 1810 is the oldest dataset (Thews, 1969). Figure 2 shows the data distribution with respect to depth, temperature, TDS and carbon dioxide content. The depth is classified into four groups (Fig. 2a): 24.6 % (n=259) of the datasets have a depth of 0 to 10 m and are springs, shafts or very shallow boreholes tapping the uppermost aquifer. They are in many cases related to fault zones and therefore releasing water of a deeper origin. Another 30.6 % (n=313) of the datasets belong to the second group (depth > 10 to 100 m), most of them being wells used for drinking-water supply. A limit of 100 m is defined here in accordance with the formation water database of the BGR. The third group (depth > 100 to 400 m) is with 37.3 % (n=386) the largest one. These are mainly wells to supply drinking water, thermal baths or spas, where the limit of 400 m follows the definition of shallow geothermal systems (e.g. PK Tiefe Geothermie, 2007; Sass et al., 2016). The fourth group (deeper than 400 m) consists of 6.9 % (n=71) of the datasets and includes mostly deep spa wells, research drillings or boreholes from hydrocarbon exploration. Borehole Pfungstadt I (no. 853), south-west of Darmstadt, is the well with the deepest hydrochemical analysis (2291 m). In 0.6 % (n=7) of the datasets no information on the depth is available.

In total, 12.7 % (n=131) of the datasets are thermal waters and 70.6 % (n=734) are below 20 ∘C (Fig. 2b). The remaining 16.7 % (n=170) do not have an entry for temperature. A dataset from Kochbrunnen (“Cooking well”) in Wiesbaden has a temperature of 67.3 ∘C and represents the hottest natural thermal water of Hesse. It has 12 entries from 1849 to 2011 (nos. 997–1008) and therefore has the largest number of analyses for one sampling location within the database.

The majority of the datasets, 53.8 % (n=557), fulfil the definition of mineral waters due to their TDS; another 11.3 % (n=117) have sufficient sodium and chloride concentration to be classified as brine in a balneological sense (Fig. 2c). Solebohrung III (“Brine wellbore”) in Bad Karlshafen (no. 59), in the northernmost part of Hesse north of Kassel, shows by far the highest TDS with 301 gL-1. This extreme high concentration is caused by its origin where the brine leaches halite rocks of the Zechstein. Note that 14.0 % (n=145) cannot be defined due to missing hydrochemical data.

In total, 36.5 % (n=378) of the datasets fulfil the definition of mineral waters due to their free carbon dioxide content, also called acidulous waters (Fig. 2d). Georg-Viktor-Quelle B (no. 345) in Bad Wildungen, located between Marburg and Kassel, shows the highest content of free carbon dioxide (< 21.5 mgL-1). Of the datasets, 18.8 % (n=194) have no entries for the carbon dioxide content.

The first step of the statistical evaluation was to match datasets with the individual hydrogeological units of Hesse (see Sect. 1.2) based on their location (i.e. coordinates). The second step was to prepare and condition the bulk database itself. The conditioning included the following steps:

Delete datasets with incomplete entries (e.g. missing hydrochemical data).

After the conditioning, the database comprised 544 individual datasets. For the statistical evaluation the parameters temperature, pH and TDS as well as major ions (Na+, K+, Ca2+, Mg2+, Cl-, HCO3- and SO42-) were considered.

In total, 219 datasets describe the Upper Rhine Graben and Mainz Basin (HYR-03) including Vogelsberg volcanites (HYR-03_VV; n=7). The pH values of water samples from this group range from 5.7 to 8.2. The average is 6.9 with a standard deviation of 0.6. At 18 locations, the groundwater pH is below 6. These originate from Silurian gneisses and phyllites, Devonian limestones and quartzites, and unconsolidated rocks of the Quaternary. Temperatures within HYR-03 are between 6.0 and 66.3 ∘C (average is 17.6; standard deviation is 13.8). Note that as many as 50 datasets do not have an entry for temperature. Twenty-six datasets have groundwater temperatures above 20.0 ∘C. All but one of these groundwaters show Na+ as the dominating cation, most often in combination with Cl- (pure Na–Cl water type at 19 locations; one location has a Ca–Na–SO4 water type). Within HYR-03, the highest groundwater temperatures above 40 ∘C mainly occur in Silurian gneisses and phyllites of Wiesbaden. By plotting groundwater temperature versus depth (Fig. 3), several distinct subgroups can be differentiated. The largest subgroup is characterized by mixed groundwater due to convection. Convection occurs when hydraulically permeable subsurface pathways, e.g. faults or fractures, allow deeper, connate groundwater to migrate upwards. This groundwater is relatively warm and highly mineralized. Typically, it mixes with colder groundwater of meteoric origin in the shallower subsurface (up to approximately 250 m b.s.l.) and is therefore diluted (e.g. Schäffer et al., 2018). The subgroup “normal” represents groundwater with temperatures roughly following the typical Hessian geothermal gradient (green). One subgroup shows increased groundwater temperatures at or close to the surface (hot, yellow), and the subgroup “very hot” shows groundwater temperatures ≥ 55 ∘C. Groundwater mineralizations (i.e. TDS) of HYR-03 range from 72 to as much as 122 661 mgL-1 (Table 3). The groundwater from location no. 673 (see above) has the maximum TDS with a pronounced Na–Cl water type. This is noteworthy as from this site no halite deposits are known that may explain the high TDS and Na–Cl domination. Because this value is approximately twice as high as the second highest (64 396 mgL-1), it is considered an outlier and hence excluded from further statistics. Groundwater with TDS concentrations above 10 000 mgL-1 (n=29) show a pronounced Na–Cl dominance. These high mineralizations mainly occur in unconsolidated rocks of the Neogene (n=10) as well as in Permian Zechstein (n=5) and Devonian limestones (n=4). TDS concentrations below 1000 mgL-1 are dominated by Ca2+ and HCO3-. These mainly occur in groundwater from Triassic Buntsandstein, Quaternary and Tertiary aquifers. No depth–TDS correlation is apparent.

Groundwater from the Vogelsberg volcanites (HYR-03_VV; n=7) are characterized by increased pH values between 8.0 and 9.9 and low mineralizations between 151 and 256 mgL-1 (Table 3). Chemically, Na+ and HCO3- are the major dissolved ions, and the groundwater of HYR-03_VV represents Na–HCO3–Cl water types (one Ca–Mg–HCO3).

The central German basin range (HYR-05) comprises 192 datasets. Groundwater temperatures are between 7.0 and 27.3 ∘C. Note that 21 datasets have no temperature entries. At five locations groundwater temperatures are above 20 ∘C. These originate from depths greater than 100 mb.s.l. and mainly occur in Permian formations. In Fig. 4 depth–temperature dependence for the groundwater of HYR-05 is displayed. While a large subgroup of groundwater is characterized by convection (purple) and another subgroup roughly follows the typical Hessian geothermal gradient (normal, green) similarly to groundwater of HYR-03, one new features is apparent. There is one subgroup with groundwater temperatures below the geothermal gradient (cold, blue).

The pH values of samples from HYR-05 are between 5.2 and 9.3 (average is 6.8; standard deviation is 0.7). Increased pH values above eight (n=4) are found in groundwater from Permian Rotliegend and Rhön volcanite. These datasets are dominated by Na+ and HCO3-. Lower pH values below 6 typically occur in Triassic sandstones and Permian rock series. Note that Schlossquelle in Bad Wildungen (no. 361) has a pH of 6.0 and a Mg–Ca–Na–HCO3 water type reflecting Mg2+ domination. A total number of 67 datasets do not have an entry for pH. Mineralization ranges from 65.7 to 300 983 mgL-1 (Table 3). The average is 10 084 mgL-1 (note the median of 1592 here) with a standard deviation of 27 758 mgL-1. Hence, HYR-05 has the highest TDS average of the five major hydrogeological regions of Hesse. Groundwater with TDS contents above 10 000 mgL-1 (n=40) are all characterized by Na–Cl water types. At TDS values above 50 000 mgL-1 (n=9) groundwater mainly occur in Zechstein formations. Lower mineralization below 1000 mgL-1 (n=76) relates to groundwater from sandstone of the Buntsandstein. These groundwaters are characterized by Ca–Mg–HCO3 water types. A correlation of TDS with depth is not evident for HYR-05, whereas in northern and eastern Hesse, deep waters from the Buntsandstein are usually mixed by ascending leachate waters of the Zechstein, especially next to the salt slope.

The South German Scarplands in south-eastern Hesse (HYR-06) comprise 15 groundwater samples mainly from Buntsandstein and Zechstein formations. Temperatures are between 10.0 and 20.5 ∘C (four datasets do not have temperature values). Only one groundwater has a temperature above 20.0 ∘C; the remainder are between 10.0 and 14.0 ∘C. pH values range from 6.0 to 7.3, with seven samples having no pH entry. Mineralization ranges from 34.8 to 27 176.0 mgL-1 (Table 3). The average TDS value is 5524 mgL-1, and the standard deviation is 8621 mgL-1. The lowest mineralization occurs in groundwater from formations of the Middle Buntsandstein. These typically have a Ca–HCO3(–Cl) water type. TDS values above 10 000 mgL-1 are Na–Cl dominated; the Martinusquelle in Bad Orb additionally shows a significant SO42- influence of 2100 mgL-1. Here it is noteworthy that in Bad König three samples show a Ca–K–HCO3–Cl water type. These are the only datasets of the entire database with K+ as the co-dominating cation. Among the thermal waters of Hesse, Bad König is the only location where thermal water from granites is extracted. These granites – presumably mica (i.e. muscovite) – may explain the high content of dissolved K+.

The Rhenish Massif in western and north-western Hesse (HYR-08) comprises 113 datasets in total. The depth interval is between 0 and 853 mb.s.l. Groundwater temperatures within HYR-08 range from 6.2 to 49.1 ∘C; the average is 13.9 ∘C with a standard deviation of 7.2 (eight cases of no data). By far the highest groundwater temperature of 49.1 ∘C is from Schützenhofquelle (no. 1019) in Wiesbaden (Fig. 5, subgroup very hot). Increased temperatures between 20.0 and 34.3 ∘C are mainly found both in Silurian–Ordovician phyllites and gneisses (n=9) and in Devonian rocks (n=4). Temperatures below 10.0 ∘C are typical of Devonian schists (e.g. Hunsrück and Wissenbach schist). While the majority of groundwaters belong to the subgroup “convection”, one subgroup shows increased groundwater temperatures partially at or close to the surface (“hot”). Analogous to HYR-05, a subgroup with groundwater temperatures below the geothermal gradient (“cold”) is apparent in Fig. 5.

The pH values of HYR-08 are between 5.4 and 8.0; the average is 6.2. Note that 51 datasets do not have a pH entry. As many as 35 datasets show a pH value below 6. These mainly belong to Silurian–Ordovician gneisses and phyllites (n=18) and Devonian schists (n=11).

Groundwater mineralization within HYR-08 ranges from 53.0 to 26 471.9 mgL-1. The average TDS value is 4479.0 mgL-1 with a standard deviation of 5161.6 mgL-1 (Table 3). It is noteworthy here that location no. 509 has the lowest mineralization of 53.0 mgL-1 of which 16.8 mgL-1 is H2SiO3. The highest mineralization above 10 000 mgL-1 (n=16) mainly occurs in groundwater from Silurian metavolcanites (n=6) as well as Silurian–Ordovician phyllites (n=7). These groundwaters show a pronounced Na–Cl water type. The lowest mineralization below 1000 mgL-1 are Ca–Mg–HCO3 dominated. These low TDS are typical of groundwater of Devonian schists and Triassic–Devonian sandstones. Two datasets of HYR-08 show groundwater with SO42- as the main anion. These are from the Permian (Zechstein) lithologies and show Ca–SO4 and Ca–Mg–SO4 water types. No correlation with depth is apparent for TDS.

The crystalline Odenwald in south-eastern Hesse as part of the south-western German basement (HYR-10) comprises five datasets: one each from Rotliegend (arkose), Zechstein (?) and Paleogene (marl), as well as two from sandstones of the Buntsandstein (S3 and S3–S6). Groundwater temperatures range from 10.9 to 18.0 ∘C (Tmax⁡ from Rotliegend). pH values are between 6.7 and 7.3 (two datasets without entry). Compared with the other major hydrogeological regions of Hesse, mineralization of HYR-10 is rather low, ranging from 277 to 2970 mgL-1 (Table 3). It is noteworthy that all the five groundwaters have different water types.

The database of mineral, thermal and deep groundwaters of Hesse has been made available by Schäffer et al. (2020) in xlsx and csv file format at TUdatalib, 10.25534/tudatalib-340. The second updated version of the database can be accessed directly at https://tudatalib.ulb.tu-darmstadt.de/handle/tudatalib/2508.2 (Schäffer et al., 2021).

The hydrochemical composition of groundwaters and related fluid properties are of great importance for the planning and trouble-free operation of geothermal power plants or direct heat utilization as well as for understanding hydrochemical processes in aquifers. Here, we describe the first publicly available data compilation of thermal, mineral and deep groundwater of Hesse. We compiled and evaluated hydrochemical data in a database for all geological units that are assessed as suitable for deep geothermal use. Through extensive literature research, it was possible to compile a dataset that thoroughly covers the Hessian state territory, both spatially and chronologically. In the near future, a major improvement of data accessibility is expected after implementation of the Geologiedatengesetz (Geological Data Act) passed in 2020. It transforms private into public data and allows state authorities to publish data which have been rated as classified so far. The HLNUG has thousands of documents, reports, statements, etc., which could not be evaluated and used up to now due to confidentiality and property rights. Thus, an update and extension of the database is planned, and contributions of other people are welcome.

The database can serve as a basis or tool for other projects or scientific questions. It can be used to estimate reservoir temperatures with solution geothermometers. In the best case, geothermometers can be used to determine reservoir temperatures for individual aquifers of the entire Hessian territory. The database can be also used to re-evaluate the genesis and circulation of deep groundwaters. In addition, the isotope data can be used for calculating groundwater ages or evaluating residence times, which in turn are criteria for the search of a repository for radioactive waste (StandAG, 2017). Furthermore, the hydrochemical database will be integrated by the BGR into the formation water database covering the entire German territory.