BACKGROUND: The efficiency of in situ bioremediation observed in the field is often lower than that found in the laboratory, which can be due to the contact efficiency of air, substrates and bacteria achieved in the field being lower than that obtained in the laboratory. In this study, in situ pilot tests of air injection were conducted to evaluate the contact efficiency of both gaseous (toluene) and misty (tritiated water) substrates in the subsurface environment. RESULTS: Based on the observed distributions of air saturation, toluene and tritiated water in the pilot, the contact efficiency of the gaseous substrate was significantly better than that of the misty substrate with a median mist size of 10 gm. CONCLUSION: The contact efficiency of the gaseous substrate is very close to that of air. The sweep volume of the misty substrate through in situ sparging is only present near the injection point and therefore the contact efficiency is very limited. (C) 2009 Society of Chemical Industry