The Corrective Action Investigation Plan (CAIP) for Corrective Action Unit 274, Septic Systems, has been developed in accordance with the Federal Facility Agreement and Consent Order (1996) that was agreed to by the State of Nevada, the U.S. Department of Energy, and the U.S. Department of Defense. The purpose of the investigation is to ensure that adequate data are collected to provide sufficient and reliable information to identify, evaluate, and select technically viable corrective actions. Corrective Action Unit (CAU) 274 is located in Areas 3, 6, 9, and 20 of the Nevada Test Site, which is 65 miles northwest of Las Vegas, Nevada. Corrective Action Unit 274 is comprised of the five Corrective Action Sites (CASs): 03-02-01, WX-6 ETS Building Septic System; 06-02-01, Cesspool; 09-01-01, Spill Site; 09-05-01, Leaching Pit; and 20-05-01, Septic System. These sites are being investigated because existing information on the nature and extent of potential contamination is insufficient to evaluate and recommend corrective action alternatives. A corrective action investigation (CAI) will be conducted and the information obtained during the investigation will be used to evaluate corrective action alternatives prior to selecting the appropriate corrective action for each CAS. The results of the field investigation will support a defensible evaluation of viable corrective action alternatives that will be presented in the Corrective Action Decision Document.