Impacts of borehole heat exchangers (BHEs) on groundwater quality: the role of heat-carrier fluid and borehole grouting

This study investigated the role of two technical features of borehole heat exchangers (BHEs) in causing impacts on groundwater quality: the use of propylene glycol (PG) and other additives in heat-carrier fluids and the BHE grouting. The PG is the most diffused antifreeze compound used in heat-carr...

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Bibliographic Details
Published in:Environmental earth sciences 2018-03, Vol.77 (5), p.1-11, Article 175
Main Authors: Bucci, Arianna, Prevot, Alessandra Bianco, Buoso, Sandro, De Luca, Domenico Antonio, Lasagna, Manuela, Malandrino, Mery, Maurino, Valter
Format: Article
Language:English
Subjects:
Additives
Alluvial aquifers
Alluvial plains
Antifreezes
Aquifers
Biogeosciences
Boreholes
Computational fluid dynamics
Computer simulation
Defects
Earth and Environmental Science
Earth Sciences
Environmental Science and Engineering
Fluctuations
Fluids
Flux
Geochemistry
Geology
Groundwater
Groundwater quality
Grout
Grouting
Heat
Heat exchangers
Heavy metals
Hydraulic conductivity
Hydraulics
Hydrology/Water Resources
Iron
Leaching
Manganese
Mathematical models
Metabolism
Metals
Microcosms
Mineral nutrients
Nickel
Original Article
Oxidoreductions
Propylene
Propylene glycol
Redox potential
Soil
Soil water
Terrestrial Pollution
Voids
Water quality
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Summary:This study investigated the role of two technical features of borehole heat exchangers (BHEs) in causing impacts on groundwater quality: the use of propylene glycol (PG) and other additives in heat-carrier fluids and the BHE grouting. The PG is the most diffused antifreeze compound used in heat-carrier fluids circulating into the ground loops. Laboratory-scale experiments were performed to simulate the release of PG into soil–water microcosms. PG itself is not toxic, but it is a potential organic nutrient able to stimulate the micro-organisms’ metabolism, thus resulting in O 2 consumption and subsequent decrease in redox potential of the system. Indeed, a leaching of the main heavy metals was observed: Fe, Mn and Ni ions reached 10 2 –10 4  μg l −1 concentrations after 60 days. Secondly, the role of BHE grouting in changing the hydraulic conditions of aquifers was investigated because defects and voids in the grout may increase the hydraulic conductivity along the boreholes, leading to inter-aquifer flux. A field-scale experiment was performed by grouting a 7-m-long pilot BHE, and many defects and void spaces were detected along the BHE. A simplified analytical model was thus applied to detect the magnitude of the inter-aquifer flux. Basing the calculations on typical hydraulic parameters of the alluvial aquifer of Piedmont Po Plain (NW Italy), the downward flux is up to 2.65 cubic metres per day in the worst scenarios with high head difference and thin separation layer between two superposed aquifers.
ISSN:1866-6280
1866-6299
DOI:10.1007/s12665-018-7375-9