The storage of thermal energy, either heat or chill, in aquifers or impoundments creates local perturbations of the indigenous microflora and the environmental properties. Within an international working group of the International Energy Agency (IEA Annex VI*) possible environmental impacts of ATES**-systems were recognized and investigated. Investigations of storage systems on natural sites, man-made aquifers and large-scale models of impounded aquifers showed changes in microbial populations, but until now no adverse microbiological processes associated with ATES-systems could be documented (Winters, 1992). However, examinations with a model system indicate an increased risk of environmental impact. Therefore, the operation of ATES-systems should be accompanied by chemical and biological investigations.