In this plan, the background and pollution potential of underground environment in total 130 gas stations in middle-northern Taiwan had been investigated. Including “Gas station set-up data”, “UST(underground storage tank) basic data”, “piping basic data”, “unloading data”, “reparation data”, “monitoring equipment data”, “oil vapor recycling system data”, “car wash data”, “SVE(soil vapor extraction) equipment data”, “environment data” and so on, totally 10 kinds of data were collected to provide Taiwan EPA(Environmental Protection Administration) to do the follow-up works. In Phase I Investigation, we inspected 130 gas stations and 1,607 soil vapor monitoring wells by using LEL(Lower Explosion Level) detector, PID(Photoionization Detector) and checked the capabilities of those wells (including the cap could be opened or not, water accumulation and ventilation test, etc.). There were 28 soil vapor-monitoring wells failed. According to investigation results, 42 gas stations were screened to enter the next step (Phase II). In Phase II Investigation, we inspected 42 gas stations, sampled and analyzed 131 soil vapor samples from monitoring wells and 131 samples from direct push process. Base on the analyzing values of benzene, toluene, ethylbenzene, xylene and total petroleum hydrocarbons (TPH), 21 gas stations were sieved out to do soil and groundwater sampling investigation. (Phase III) According to the analyzing results of soil and groundwater samples, 12 gas stations have higher pollution potential among 21 gas stations. In order to confirm the pollution situation, Taiwan EPA assigned 12 gas stations to conduct the identification task. According to the identification results，8 gas stations were exceeding the “Soil Pollution Control Standards ” or the “Groundwater Pollution Control Standards”. Taiwan EPA have to do the follow-up control according to the “Soil and Groundwater pollution Remediation Rules”。

Gas Station, Soil, Groundwater, Soil Vapor Monitoring Well