Geoscientific input to business case for “Lønstrup spa”: Overview of relevant water quality regulations and the chemical composition of Gassum Formation brine

Research output: Book/ReportReport (publicly available)


The purpose of this report was two-fold: 1) to overview existing national and international
regulation and guidelines with water quality requirements for spas using thermal water; 2) to
provide preliminary evaluation on the potential use of Gassum Formation brine for the future
spa in Lønstrup. Thermal water from deep wells has not been used yet in Denmark for
recreational purposes, so in this report we outline some of the potential issues.

The key findings from this assessment are:

1. The primary Danish water quality requirements relevant to spas are those for public
bathing (BEK nr 918 of 27/06/2016), which concern the water clarity, pH, chlorine
levels, and microbiological safety of swimming pools. The Danish drinking water
quality requirements should also be met (BEK nr 2361 of 26/11/2021), which could
be a problem for highly mineralized waters, such as the Gassum Formation brine.
Thermal water quality requirements for bathing purposes are not explicitly formulated
in the existing Danish laws, so further guidance should be sought by the Danish EPA
(Miljøstyrelsen), the Danish Health Authority (Sundhedsstyrelsen), and the local

2. There is no unified EU legislation on the use of thermal waters for recreational
purposes (bathing), and both the EU Drinking Water Directive and the EU Directive
on the Exploitation and Marketing of Natural Mineral Waters do not apply for mineral
waters used at source for curative purposes in thermal or hydromineral

3. The common theme in all reviewed scientific papers, national legislation, and the
WHO guidelines is that the specific challenges in operating public pools with thermal
water, are related to the high temperature and the unique chemical composition (rich
in specific ions, high mineralization, and/or organic matter), which may favour
microbial growth and the formation of unintended disinfection by-products (esp.
brominated or iodinated compounds). Another specific issue to consider is potential
radioactivity of the thermal water from deep wells. Risk assessment should be carried
out to assess the source-water quality and how the selected spa design,
management, and operation of the facilities would affect both the bathing water and
indoor air quality in the spa.

4. The characterisation of Gassum Formation, which is the most probable candidate for
the Lønstrup spa, is uncertain because of the limited seismic data and deep wells in
North Jutland. Based on regional models, it is estimated that the depth to the top of
the formation is 1400-1600 m (± 15%) with temperature of the brine ~50°C (± 20%).
The dominating lithology is sandstone, and the hydraulic conductivity is expected to
be relatively good. There is a need to acquire more seismic data and to drill a deep
well in the area to reducing the hydrogeological and the associated economic risks.
Original languageEnglish
Number of pages41
Publication statusPublished - 21 Mar 2022

Publication series

SeriesDanmarks og Grønlands Geologiske Undersøgelse Rapport

Programme Area

  • Programme Area 2: Water Resources


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