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Mineral Precipitations and Hydrochemical Evolution of Brines in Graduation Towers


The purpose of this study is to examine hydrochemical changes of saline groundwater and precipitation of mineral phases during evaporative graduation processes. Therefore, concentrated brines and mineral precipitates were sampled at four graduation towers with differing evaporation approaches. Solid phase compositions were qualitatively and quantitatively analysed. Hydrochemistry changes of dissolved ion concentrations during successive evaporation was assessed by comparing natural with concentrated brines. PHREEQC was used to simulate sequence and quantity of mineral precipitates. Finally, a prognosis for expected precipitations of a newly built graduation tower was made using PHREEQC modelling. The identified mineral phases included calcite, aragonite, gypsum and halite in varying proportions. Concentrations of all investigated ions (except bicarbonate) increase, eventually leading to supersaturation and sequential precipitation of evaporite minerals. Modelled saturation indices show that calcite is the first and halite is one of the last precipitating phases at all sites. Further calculated precipitates include the carbonates dolomite, siderite and strontianite; manganese oxide and hematite; and the sulphates baryte, celestine and gypsum which precipitate depending on local hydrogeochemistry and graduation conditions. Calculated precipitation quantities reach a maximum of 48.6 g/L of applied natural brine at an evaporation grade of 90%. After 25 years, the total expected mass of precipitates at the new graduation tower is about 34.4 t for a graduation process up to a salt content of 19%, and about 356 t for a permanent evaporation grade of 90%.


• Except for bicarbonate, concentrations of all ions increase to different degrees.

• Mineral precipitations are composed of calcite, aragonite, gypsum and halite.

• Modelled precipitation sequence: carbonates, sulphates, chlorides.

• Modelled precipitation amounts: max. 48.6 g/L for natural brine at 90% evaporation.

• 25 y precipitation forecast for new graduation tower: max. 356 t at 90% evaporation.

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Data Availability

on request.

Code Availability

PHREEQC, publicly available code for hydrogeochemical calculations:


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The authors thank the contact persons of the graduation towers: Mr. Vleugels (Rheine), Mr. Meck (Bad Sassendorf), Mr. Köchling and Mr. Kemper (Bad Westernkotten) as well as Mr. Wilker (Bad Rothenfelde) and the Kurverwaltung Bad Rothenfelde GmbH for enabling the sampling as well as for providing data for the natural brines. Our thanks also go to Ilka Hinzer (RUB) and Nikolaus Richard (RUB) for carrying out hydrochemical analyses and to Hartmut Mammen (RUB) for carrying out powder X-ray diffraction measurements.

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Conceptualization: Andre Banning, Laura-Jane Ostwald; Methodology: Andre Banning, Laura-Jane Ostwald; Formal analysis and investigation: Laura-Jane Ostwald; Writing—original draft preparation: Laura-Jane Ostwald; Writing—review and editing: Andre Banning; Resources: Andre Banning; Supervision: Andre Banning.

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Correspondence to Andre Banning.

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Ostwald, LJ., Banning, A. Mineral Precipitations and Hydrochemical Evolution of Brines in Graduation Towers. Environ. Process. 8, 729–746 (2021).

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  • Graduation tower
  • Saline groundwater
  • Concentrated brine
  • Münsterland Cretaceous Basin
  • Germany
  • Evaporite mineralogy