This report presents the sampling and analysis plan for the hydrogeologic assessment to be conducted at McClellan AFB, California. Objectives of the assessment are characterization of hydrogeologic conditions and further definition of the extent of groundwater contamination beneath McClellan AFB. The primary uses of the data from the assessment will be to determine which areas may require groundwater operable unit remedial investigations and to evaluate groundwater remedial alternatives in future work. The hydrogeologic investigation is intended to: identify geologic and hydrologic characteristics of hydrogeologic units: determine continuity of waterbearing zones and aquitards; more clearly define horizontal and vertical determination. Data obtained from the assessment will provide a framework within which potential groundwater operable units may be determined and more effectively evaluated in the future. The scope of work includes: lithologic and geophysical logging of 19 pilot holes to depths of 360 feet; construction of as many as 51 monitoring wells in 19 well cluster locations; installation of 34 piezometric wells; sampling and analyses of all new monitoring wells; performance of 2 multiple-well and 40 single-well aquifer tests; and evaluation of the effects of local groundwater flow due to on-base water supply wells.