Abstract: A non-destructive and non-invasive method of detecting underground tank leaks using three phase testing. A plurality of boreholes are drilled around the tank with the boreholes preferably extending below the tank. In the first phase of testing gas vapor analyzer is used to determine which borehole contains volatile organic material. In the second phase, a neutron back scatter gauge is lowered into those boreholes indicating presence of volatile organic material, and readings are taken at intervals along the length of the borehole. All of the readings are correlated. A sharp increase in reading, over a base reading, detects a probable "leaking" organic material beyond that present in the ambient soil. The third phase of the testing is to identify the probably "leaking" organic material and confirm if it is the same as that contained in the underground tank.