Biodegradability and groundwater pollutant potential of organic anti-freeze liquids used in borehole heat exchangers

Abstract

Ground source heat pump systems are increasingly being used to exploit the energy content of shallow geothermal resources for space heating and cooling. In this study we evaluate the potential for groundwater contamination of the different organic anti-freeze compounds (ethylene glycol, propylene glycol and betaine) used in these pumps, based on a literature review of their biodegradability and the results of our own laboratory experiments on aquifer material. Ethylene and propylene glycol were found to be readily biodegradable under both oxic and anoxic conditions, without formation of toxic or persistent intermediates. Long-term groundwater contamination by the glycols is therefore not expected. Betaine is also expected to be readily biodegradable in oxic and anoxic groundwater. The potential formation of trimethylamine, an intermediate of anaerobic betaine degradation, is, however, regarded as critical due to its unpleasant odor even at very low concentrations. Additionally, betaine has the potential to complex metal ions and thus may mobilize toxic metals in groundwater. We therefore recommend that betaine not be used in borehole heat exchanger fluids. In addition to organic anti-freeze compounds such as glycols, borehole heat exchanger fluids also contain additives such as corrosion inhibitors or biocides. We demonstrate that potentially toxic additives in these fluids inhibit biodegradation of the organic anti-freeze compounds. In order to ensure environmental compatibility of borehole heat exchanger fluids, further research should be conducted on the impact of additives on subsurface microbiological activity and on groundwater quality.