Editors’ message: The housing crisis from underground—damage to a historic town by geothermal drillings through anhydrite, Staufen, Germany
Message des éditeurs: La crise immobilière par le sous-sol—une ville historique endommagée par des forages géothermiques au travers d’anhydrites, Staufen, Allemagne
Editorial: Die Immobilienkrise aus dem Untergrund—Beschädigung einer historischen Altstadt aufgrund von Geothermiebohrungen durch Anhydrit, Staufen, Deutschland
Mensaje de los editores: La crisis inmobiliaria del subsuelo—Daños en la ciudad histórica de Staufen (Alemania) causados por sondeos geotermales en anhidrita
编辑的讯息 : 来自地下的住房危机——地热钻井穿透膏盐层给德国斯道芬古城带来破坏
Mensagem dos Editores: A crise alojada no mundo subterrâneo—danos causados a uma cidade histórica por furos geotérmicos em anidrite, Staufen, Alemanha
Geothermal energy can be used for heating, electric power generation and other applications (e.g. Seyboth et al. 2008). The confidence of the public in this technology is currently compromised by several bad examples where inappropriate implementation of geothermal installations has caused avoidable damage. A dramatic but instructive case is presented here. This article is a plea both for the reasonable use of geothermal energy and for more geologic and hydrogeologic competence and education in geothermics.
Staufen is located in the Upper Rhine Graben, within the tectonic transition zone between the central graben and the Black Forest. The graben is filled with several kilometres of sediments; the Black Forest mainly consists of crystalline rocks. In the transition or foothill zone (German: Vorbergzone), Mesozoic sedimentary rocks are present below the surface or crop out as ridges and hills, reflecting the fault pattern and tilting of the strata. In this region, the Upper Triassic (Keuper), which is also present below Staufen, includes gypsum and anhydrite layers and lenses of substantial thickness, which is common knowledge and documented in publications and geologic maps. Even a standard German-language textbook entitled Geologie von Mitteleuropa (Geology of Central Europe) mentions noteworthy anhydrite occurrences in this region (Walter 1992).
Anhydrite transforms into gypsum when it comes in contact with water, resulting in a volume increase of up to 61% that can generate pressures of 5–10 MN/m2. Geotechnical engineering books, including traditional German textbooks, discuss this problem and recommend avoiding any contact between anhydrite and groundwater in all types of construction activities (Prinz 1991). There are numerous well-studied examples, internationally and in southwest Germany, where swelling of anhydrite caused severe geotechnical problems during construction of roads, tunnels or buildings. In the USA alone, an oft-cited study by Jones and Holtz (1973) calculated that shrinking or swelling materials inflicted at least $2.3 billion USD annually in damage to houses, buildings and roads.
Despite the strict correlation between the time and place of the drillings and the time and place of uplift and damage, an expert report (H. Schad, MPA Stuttgart, 19 September 2008, prepared for the Regional Court of Freiburg) states that natural geologic movements at faults may also have caused this phenomenon, by changing the previous flow system so that groundwater suddenly entered the anhydrite; according to this report, the probability of natural causes versus the drillings is 1:2. This estimate is asserted without justification. The report also states that the drillings were accomplished in accordance with “state of the art” drilling techniques, and that no additional requirements would have been necessary.
One can only speculate how such statements found their way into an expert report, but they are indefensible. Firstly, natural geologic movements, strong enough to alter the groundwater flow system abruptly, would have associated seismic activity, i.e. a significant earthquake in late 2007, centred in Staufen. However, there was no such earthquake. Secondly, assuming that such phenomena are possible without seismic activity (an assumption without precedent), the probability that they would naturally occur precisely at this time and at this place would still not be 1:2 but, based on rough assumptions, less than “one-in-a-million”. Thirdly, it is undoubtedly no comfort to the homeowners to learn that the drillings represent the state of the art; and, if this is the state of the art for geothermal drilling, clearly something must be wrong with it.
Although geothermal energy is a safe and renewable energy with a great potential in general, it also has its technical and economic limitations and associated risks (albeit less than other forms of energy production, including other renewable energies). These limitations and risks should be investigated, evaluated, discussed and communicated forthrightly.
Geologic and hydrogeologic expertise is indispensable in the process of planning, authorising and implementing large geothermal installations. This expertise should include essential competency in mineralogy, stratigraphy and structural geology as well as an understanding of groundwater occurrence and flow, and groundwater–rock interactions.
More and better training and education are required.