CAPÍTULO III: PROBLEMA DE INVESTIGACIÓN
3.4 Análisis de impacto regulatorio
Use of renewable energies (solar, wind, hydropower, geothermal) is globally increasing. Thus, a lot of natural
systems they offer a significant potential (hydraulic, heat, wind, current, etc.) are progressively prepared or equipped for getting energy. This induces the realization of dams, galleries, deep boreholes, windmills, hydro- turbines, solar panels, etc.
As they offer a structured network and a concentrated water and heat flux, karst environments may be a priori
favorable to support the development of hydropower devices or geothermal probes. Nevertheless,
characteristics of hydropower plants or geothermal devices should be carefully designed as they could be
disturbed by karst hazards as other building initiatives. Conversely, impacts of these constructions on karst
environment must also be carefully addressed.
3.1.3.1. Geothermal energy in karst
3.1.3.1.1. Development of geothermal probes in karst areas
3.1.3.1.1.1 Statement
As well as for other settled terranes, karst environments are concerned by the development of geothermal
devices. Two major types of devices are distinguished: deep geothermal extraction (hydrothermal and
geothermal systems) with relevant yield and shallower geothermal heat pump (or probes) for “domestic”
uses. The profitability of both devices depends on the evolution of the temperature gradient with depth and on the groundwater flux potential - the higher the geothermic gradient is, the more water could be captured and the more the device is effective. In Switzerland, due to the growing interest for renewable energies, the implantation of geothermal devices notably increases these last decades (up to +15%/year in Switzerland in the 2000s, Colliard [2004]; Lund et al. [2004]).
Two types of geothermal vertical probes could be distinguished; the open-loop systems and the closed-loop
system (Lund et al. [2004]; OWRC [2012]). Open-loop systems extract the groundwater from a well, inject it
through a heat exchanger, and discharge the temperature-altered water back into the aquifer in a return well or to a surface stream or lake. In closed-loop systems, water (or heat transfer fluid) circulates through one or several vertical or horizontal loops installed below the ground or in the aquifer zone.
Both systems work on the exchange (heating or cooling) of joules with the groundwater. In order to ensure the efficiency of the installation, the groundwater should be at a sufficient temperature (and a relative interesting gradient with depth) and a rapid renewal in the capture zone of the installation – reflecting a significant permeability of the aquifer. Depth of boreholes usually ranges from 50 to 400 m.
3.1.3.1.1.2 Temperature gradients and heat efficiency in karst aquifers
3.1.3.1.1.2.1 Temperature gradients
Due to high permeabilities and to fast circulations, the karst groundwater does not warm up as much as the groundwater circulating through fissured or granular aquifers. Haenel [1971] observed that the measured temperature gradient in boreholes is 2 to 8 times lower in karst aquifers than those measured in other
geological environments of Switzerland (~0.03°C/m). Indeed, most of the ascending geothermic flux is washed
away by the groundwater flows (Mathey [1974] found that nearly 80% of the flux is captured by the groundwater flows for the Areuse karst aquifer). This implies to address specific considerations regarding implantation and functioning of geothermal intakes.
Various authors assessed the evolution of the temperature gradient in karst aquifers (Badino [2005]; Cartwright [1971]; Drogue [1985]; Jeannin [1990]; Lütscher and Jeannin [2004]; Mathey [1974], etc.) and noticed that the evolution of the temperature gradient according to the depth is not homogeneous – as for other environments. This is strongly affected by (i) the location (i.e. the depth) of the saturated zone, (ii) the presence/geometry of the conduit networks, and (iii) the hydrology of the flow-system.
The evolution of the temperature gradient could be distinguished in the vadose heterothermic and
homothermic zones and in the saturated zone. Based on a series of measurements, Lütscher and Jeannin
[2004] provide a conceptual model of the temperature evolution in different zones of karst aquifers. The authors demonstrated that in temperate climatic areas, the air circulation controls the distribution of the
temperature in the upper unsaturated zone of karst aquifers. Two zones are defined: the heterothermic zone
influenced by seasonal variations of the outside temperature and the homothermic zone where the gradient is
stable over the year. In this zone, more the karst media is ventilated – by conduits - more the temperature gradient is high (>0.5°C/100 m). At the reverse high water flows (in the epiphreatic zone) and/or low air circulation tends to reduce this gradient to trends lower than 0.5°C/100 m. Close to the saturated area the gradient falls to 0.3°C/100 m. In the vadose zone, the temperature increases due to (i) the gravitational energy converted in thermal energy (>2°C/km) along the gravitational axis (Badino [2005]; Jeannin [1990]) and (ii) the
input of the outside air heat according to the ventilation capacity. Regarding the evolution of the temperature
in the saturated zone of the aquifer, Lütscher and Jeannin [2004] found that the temperature gradient does
not significantly evolve in the zone of the phreatic circulation. Observations also reveal that below the karstified base level, the temperature gradient significantly increases due to the lack of circulation – but conversely the available amount of water is low as permeabilities significantly decrease. In this zone,
groundwater is in thermic equilibrium with the surrounding rocks and the gradient similarly evolves as for
other environments.
Thus, geothermal potential in karst is far from being interesting – at least from point of view of the
temperature gradient. Ascending heat flux through karst is low compared to other rock massifs. Considering
for instance a 200 m long probe at an elevation of 650 m in karst environment, the highest recorded temperature will range between 8 and 10°C if the probe penetrates the saturated zone (see Figure 10—2) instead of 11 or 12°C for a similar device in non-karst area (molassic basin, etc.)... However, as karst aquifers
are able to provide large fluxes of water through few concentrated conduits, it may present an interesting
heat renewed potential that other geological media cannot provide equally.
3.1.3.1.1.2.2 Role of the conduits
As mentioned by Jeannin [1990], the assessment of the groundwater heat potential could not be strictly
restricted to the aquifer properties but it should also consider the flow-system properties, especially the
circulation in the conduits. Indeed, conduits act as a key parameter for heat transport in karst (Covington et al. [2011]). Knowing the principles that govern geothermal processes in karst aquifers, properties that affect the evolution of the temperature gradient in karst and the heat distribution are of two kinds:
- The physical properties of the conduit network: density and distribution of the conduits, length and
depth of the groundwater circulation, size of the conduits, etc. These properties are fixed and are specific to a system;
- The hydrological regime: fluctuations of the systems hydrological regime over the year notably affect
the heat transfer within the system (Badino [2005]). As mentioned by Covington et al. [2011], over a relative long timescale, heat exchanges in fulfilled karst conduits are dominated by heat conduction through the surrounding rocks under turbulent flows. For a similar timescale, in open channel, radiative heat exchanges are of relative great importance.
Badino [2005] demonstrated that the density and the distribution of karst conduits impact the ascending propagation of the geothermal gradient. Isolated conduit has a restricted area of influence while numerous and
equally distributed conduits will act as an “impervious” screen for ascending heat flux leading to inefficient thermal device on the ground (cf. Figure 3—4). In this view, poorly karstified rocks (dolomites, marly limestone, etc.) or rocks that have been recently exposed to the karstification will have a restricted influence on the ascending propagation of the geothermal flux (case a.). On the opposite, well karstified rocks or aquifers exposed to several phases of karstification will present a higher density of conduits (i.e. higher equivalent
permeability, case b.). These are expected to drain the geothermal flux in the phreatic zone, towards the
spring. Such zones are usually characterized by a temperature deficit over the ground and may present a heat- excess at the springs draining the massif.
Figure 3—4. Considering that conduits are efficient for heat drainage, their density and distribution
within the massif and the groundwater flux impact the ascending geothermal heat flux. At some
locations, aquifers of type (a.) reveal favorable for the implantation of geothermal probes while karst
systems of type (b.) may offer more interesting conditions for springs heat exploitation as most of the
geothermal flux is absorbed by the flows and carried out at the spring.
In addition to the density/distribution of the conduits, the groundwater flux also governs the drainage of the geothermal heat. For a large flux, both groundwater and aquifer will present low temperatures. For small flux, the groundwater flowing through the conduits will absorb most of the geothermal flux and will reach the equilibrium temperature of the aquifer and a part of the geothermal flux will be released upward. Considering the variability of the groundwater flux in the year (up to a factor 1’000 between low- and high-flow conditions) the situation of an equilibrate flux draining the geothermal heat-flux is not stationary. Different karst systems may present one or several periods in the year (short or long) where groundwater flux is equilibrate. Once the groundwater flux increases, aquifer and groundwater temperatures should decrease.
Here-above demonstrations show that exploiting heat in karst aquifers is not a trivial task. Heat propagation is
mainly controlled by the groundwater flux, resulting in contrasted thermic zones. Designing geothermal
probes or tapping heat at springs with return expectations requires a good knowledge of the aquifer properties.
3.1.3.1.1.3 Problems related to geothermal probes in karst
Drilling geothermal probes in karst environments may provoke disturbances on groundwater (quantity and
quality) and on the environment. Three types of hydraulic risks do exist for the aquifer and may lead to negative consequences over the ground:
- Artesianism and depressurization of the phreatic zone in confined parts of the aquifer;
- By-pass from an upper aquifer to a lower one (i.e. from the lower one to the upper one depending on the hydraulic head);
- Forcing infiltration may create or enlarge preferential flow-paths and provoke disturbances on the
ground such as collapse or subsidence (see in Great-Britain, Cooper et al. [2011]; Figure 3—5). The
recent increase of shallow geothermal heat pumps (mainly open loops devices) in karst aquifer increases collapses in suburban regions (houses, railways, etc.).
Figure 3—5. Open-loop systems in karst aquifer
enhance sediments mobilization or dissolution
and may provoke land-subsidence (Cooper et al.
[2011]). These mechanisms frequently occur in
confined /semi-confined aquifers. This lead UK
authorities to develop a decision support tool
(GeoSure) which gives indication on areas to
karst collapses susceptibility and recommends
the type of devices to implant (open or close
loop).
In addition two types of contamination hazards do exist for the groundwater:
- Chemical contamination by release of thermic-fluids into the aquifer (mainly due to deficiencies of
closed-loop systems).
- Change of the groundwater temperature. Swiss regulations mention that decrease/increase in
groundwater temperature must not exceed 3°C at the radial distance of 100 m from the drill (Eugster [2009]). This regulation is well adapted in porous or fissured aquifers where the groundwater temperature may evolve radially from the geothermal probes but it seems less relevant in karst aquifers. Furthermore, karst groundwater is expected to present more natural fluctuations of temperature compared to porous or fissured aquifers (see Appendix 10.4.1) - even if large fluctuations of temperature are exceptions;
Various configurations of geothermal probes implantation in karst aquifer are presented in Figure 3—6 in
considering potential hazards for the groundwater, the expected thermic efficiency and the usual status of
authorization. From a hydrogeological point of view, risky configurations are assumed if: (i) probes may
intersect two or more superimposed groundwater bodies because of a potential groundwater by-pass, (ii) probes are located in the direct vicinity of a groundwater intake like supplied springs, wells, etc., because of a potential groundwater contamination (ii) probes may intersect an artesian aquifer, because of sealing problems. On the other hand, from the point of view of the heat efficiency, fully-flooded and deeper probes may reveal the most profitable. As depicted on the figure, these aspects (regulations, risks and profitability) may reveal contradictory when looking for implanting a probe.
Figure 3—6. Various scenarios of geothermal probes in karst; A: although the probe may reveal
efficient, this scenario may impact the groundwater quality as the probe connects both aquifers. In
theory, usual regulations strictly prohibit this scenario but implementations in practice are difficult as
they require a consistent knowledge on the structuration of the aquifer with the depth; B: although the
probe is located in a non karst area, it intersects two superimposed aquifer and may connect them.
This configuration may reveal efficient for heat production if the probe intersects the conduits but it
may also impact the groundwater. Usual regulations should prohibit this scenario – at least if the
geological background is sufficient to assess the extension of the karst aquifer below the non-karstic
formations… C: the probe is located in a non-karst area even though it intersects the underlying karst
aquifer. This scenario a priori does not present hazard for the karst groundwater – at least until the
karst aquifer is non artesian; D: the probe is drilled in karst area and it reaches the saturated part of
the karst aquifer. This configuration is not supposed to be problematic for the groundwater as long as
it is located far from the supplied spring. However, some cantons strictly prohibit the probes to
penetrate in the phreatic zone. In such conditions the efficiency of the probe is supposed to be
moderate; E: the probe is located in karst area but it does not penetrate the phreatic zone. Although
this scenario is not supposed to be problematic for the groundwater, the efficiency of the probe lying
exclusively in the unsaturated zone is supposed to be moderate and to spatially evolve; F: the probe is
drilled in karst area, close to the supplied karst spring. As a potential impact on the groundwater does
exist (chemical contamination, etc.), this scenario is usually prohibited, G: in this scenario, the probe
is drilled in karst area and it intersects two superimposed aquifer. As the lower confined aquifer is
here artesian, a real risk of artesian by-pass has to be considered. Even if the probe may reveal
“highly” efficient, it should be prohibited in such a situation.
3.1.3.1.1.4 Swiss cases
In Switzerland the Federal Office for Environment (FOEN) promotes geothermal energies according to the
environmental and water resources respect (Eugster [2009]). The FOEN fixes the frame and the main guidelines
for a suitable exploitation but cantons are in charge of regulating geothermal probes as the sub-soil is their
property. In general, geothermal probes are de-facto prohibited inside S1, S2 (federal rules) and sometimes S3 groundwater protection zones. They are also prohibited in zones of waste deposits or already contaminated.
Restrictions may also concern the density and the depth of the probes.
In practice, cantons meet difficulties when the project is planned in karst area as it may present more risks for grounds and groundwater: contamination by the fluids, aquifer by-pass, stability of the ground, etc. Some cantons prohibit geothermal probes in karst area and strictly apply this principle where karst is outcropping (ex: Bern). Other cantons require a hydrogeological study in order to evaluate the project feasibility and its impact on the groundwater resources (cantons Fribourg, Valais, etc.). In reality, for most of the cantons, it reveals
difficult to apply the regulation rules they have defined because they often don’t have consistent decision
supports and authorizations may sometimes be questionable.
These regulations may also reveal problematic because on the one hand geothermal probes may be very appropriate in some places, and on the other hand, zones where probes may encounter problems or may endanger the groundwater cannot be easily delineated without a sufficient knowledge of the karst aquifer functioning (Jeannin [2014], see Figure 3—6). Even for cantons they strictly prohibit probes in karst areas, the distinction between karst and non-karst areas remains difficult to fix because of covering formations or because of the complex organization of the geological formation that result from tectonic arrangement.
3.1.3.1.1.5 Conclusion
Implantation of geothermal probes (vertical heat pumps) in karst aquifer still remains complex. Implantation
criteria should be balanced as conflicts do exist between appropriate locations from the point of view of thermic efficiency and related impacts on groundwater. As regulations for implantation are under the cantons authority - although they may not be familiar with karst aquifers - many of them strictly prohibit probes in karst environment or they apply restrictive guidelines (as for instance to not penetrate the phreatic zone). But even for such cases, regulation rules reveal difficult to implement as they require a consistent knowledge of the geometry of the karst aquifers as well as concrete documents for supporting decisions. Then, cantons are
interested for a systematic documentation of the aquifers and of their properties in order to address
authorization permits in a consistent way. Before the Swisskarst project starts, there was no dedicated
approach to address such questions and cantons were disarmed to apply consistent regulations. Since the Swisskarst project was run, the KARSYS approach now reveals suitable to address these questions.
However, regarding heat efficiency, in-depth studies are still required to discuss the real thermic potential of the saturated and unsaturated zones of the karst aquifers. Such in-depth studies would be crucial in order to provide consistent guidelines for dimensioning geothermal probes in karst regions.
3.1.3.1.2. Springs heat exploitation
As indicated above, undertaking geothermal probes in karst environment may reveal hazardous from the point
of view of the thermic efficiency and regulations may also reveal discouraging. As alternative solutions, karst
springs may offer an interesting heat potential which is now still under-exploited (expecting for thermal
springs, Goldscheider et al. [2010b]). Depending on the hydraulic properties of the karst aquifers and on the groundwater flux, heat exploitation at springs may reveal interesting. As springs are located at the downstream part of the flow system - they should offer the maximum heat potential (as already demonstrated by Drogue
[1985]). In coupling heat exchanger in the water circulation at the main discharge point major springs could
supply in heating numerous buildings with moderate cost.
Investigations in karst regions of the Vaud canton, i.e. Jura and Prealps (ISSKA and e-dric [2010]) focused on the assessment of springs heat potential by combining springs temperature and annual fluxes. Results show that springs heat potential may reach 273 GWh/yr in the Jura part and about 330 GWh/yr in the Prealps. On the whole canton, the annual heat potential offered by springs exceeds 600 GWh. This is far from being negligible, especially when considering that springs are often close to housings or to industrial zones. However, this resource is actually not optimally exploited although economic costs or global risks are weak. Indeed, such installations are usually cheaper and more suitable than expensive and unsuccessful boreholes. Few devices are already in use and most of them are installed in thermal karst springs.
Thermal springs in karst environments (i.e. springs that exceed 5°C above the mean annual air temperature as
defined by White [1957]) are evidences of geothermal anomalies. These result from particular tectonic contexts which make it possible for hot water to quickly flow along fault zones and to emerge at the ground surface. Thermal waters emerging from karst aquifers are far from being negligible as they constitute the most important thermal resources after those emerging in volcanic contexts (Goldscheider et al. [2010b]). These thermal waters often show a particular mineralization and are used for various purposes (thermal baths, specific industries or heating). A review about principles of thermal water circulation in carbonate aquifers is