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Aquifer Biothermoremediation using Heat Pumps : sound theoretical basis and results on thermal, geochemical and biological impacts on aquifers

Abstract : Geothermal heat pumps have been operated on aquifer over the past thirty years. Currently, these systems constitute a major sector of renewable energy development. However, this development is accompanied by several difficulties at scientific, technical and administrative levels, to describe short and long term interactions with aquifers, at both the local and regional scale, especially close to urbanized or industrialized areas. As a typical example, the long-term use of groundwater heat pumps for air conditioning of homes or buildings can induce significant increases in temperature of the aquifer, if, for given groundwater flow direction and rate, there is an imbalance between the demands for cooling and heating. Therefore, the aquifer (groundwater and sediments) can be affected. In fact, it is expected that a wide variety of geochemical reactions and microbiological changes with variable intensity should occur which depends on the sediment characteristics and hydrogeological background. The presence of organic pollutants in the aquifer can amplify these phenomena. The predictable results may progressively be: (i) precipitation, which reduces the porosity of the aquifer and/or the well productivity, (ii) an inappropriate temperature for the use of groundwater heat pumps for air conditioning, but also and especially (iii) pollutant bioremediation, which has a beneficial impact on the aquifer. Under certain conditions, this bio-thermo-remediation can be considered as an attractive option for some contaminated aquifers, taking into account the fact that, administratively in European countries, any action is subject to authorization. Notably in the framework of the collaborative project BIOTHERMEX (French Region Centre, convention n° 200800034163), exploratory studies were focused on three objectives: i) Improving the understanding of the concept of bio-thermo-remediation of aquifers by using effects induced by geothermal heat pumps. ii) examining realistic configurations, on a laboratory scale, pilot-scale and in-situ, in order to identify the most determining factors for possible (undesirable or helpful) aquifer impacts, e.g. temperature increase, bacterial development, organic pollutants degradation iii) evaluating if those configurations can present interests for bio-thermo-remediation of polluted aquifers by using heat pumps. A literature survey led to the consolidation of the basic principles of bio-thermo-remediation of most organic pollutants: increase in volatilization, solubility, degradation kinetics and mobility of the residual pure phases by reduction of their viscosity. The different heat pump systems and processes are discussed in the article, to identify what is suitable for bio-thermo-remediation. To reproduce the underground thermal conditions encountered in groundwater heat pumps systems (temperature range of water production from 10 to 25 °C, temperature range of water injection between 20 and 35 °C), batch-experiments, at laboratory and pilot scale have been carried out to investigate the impacts of the different parameters on the geochemical equilibrium (solubility, mobility, precipitation, dissolution, volatilization,) on the aquifer inorganic content and organic pollutants as well as on microbial populations activities. Two different sediments were used. Depending on the nature of the sediments and the water composition, the impacts are more significant: (i) thermally, with a low water flow rate, (ii) geochemically, with the high bicarbonate and Ca/Mg contents in water and (iii) biologically, with a organic contaminants contents in water. Some results from in-situ measurements (especially with an aquifer polluted by chlorinated solvents) are used to corroborate the conclusions. Even if the most part of the work is still in progress, the obtained results are validated by biogeochemical modelling. The combination of a low-enthalpy geothermal system using aquifer heat pumps (water pumping and re-injection) and remediation approaches appears to be an appropriate method to treat aquifers polluted by organic contaminants without use of any other way than the thermal energy.
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Soumis le : vendredi 18 novembre 2011 - 17:17:00
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Frédéric Garnier, Hervé Lesueur, Mikael Motelica-Heino, Ioannis Ignatiadis. Aquifer Biothermoremediation using Heat Pumps : sound theoretical basis and results on thermal, geochemical and biological impacts on aquifers. 5th European Bioremediation conference, Jul 2011, Chania, Crête, Greece. 6 p. ⟨hal-00593563⟩

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