NOVELTY - Use of a triazole-free composition comprising water, 2-3C-alkylene glycol, 2-3C-polyalkylene glycol or glycerin, and a corrosion inhibitor, as a heat transfer fluid in near-surface geothermal plants, is claimed, where: the composition is biologically degradable according to the test method OECD 301 A; the composition has an anaerobic biodegradability of at least 75% according to the test method OECD 311; and the corrosion inhibitors according to OECD 301 A (for water-soluble corrosion inhibitors) or OECD 301 B (for poorly water-soluble corrosion inhibitors) are biologically degradable.    USE - The composition is useful as a heat transfer fluid in near-surface geothermal plants.    ADVANTAGE - The composition has improved biological degradability and anticorrosion properties, and does not freeze at temperatures below the freezing point.    DETAILED DESCRIPTION - Use of a triazole-free composition comprising water, 9.2-49.5 wt.% of at least one 2-3C-alkylene glycol, 2-3C-polyalkylene glycol or glycerin, and 0.1-4 wt.% of at least one corrosion inhibitor, as a heat transfer fluid in near-surface geothermal plants, is claimed, where: the composition is biologically degradable (readily degradable) according to the test method OECD 301 A; the composition has an anaerobic biodegradability of at least 75% according to the test method OECD 311; and the corrosion inhibitors according to OECD 301 A (for water-soluble corrosion inhibitors) or OECD 301 B (for poorly water-soluble corrosion inhibitors) are biologically degradable (readily degradable). INDEPENDENT CLAIMS are included for:    (1) the triazole-free heat transfer fluid comprising the above composition; and    (2) a triazole-free heat transfer concentrate for producing the heat transfer fluid, comprising 92.4-98.9 wt.% of at least one 2-3C-alkylene glycol, 2-3C-polyalkylene glycol or glycerin, and 1.1-7.6 wt.% of at least one corrosion inhibitor, where the concentrate is biologically degradable (readily degradable) according to the test method OECD 301 A, the concentrate has an anaerobic biodegradability of at least 75% according to the test method OECD 311, and the corrosion inhibitors according to OECD 301 A (for water-soluble corrosion inhibitors) or OECD 301 B (for poorly water-soluble corrosion inhibitors) are biologically degradable (readily degradable).