Columbia Formation sediments in the Middletown-Odessa area, Delaware, are composed of boulders, gravels, sands, silts and clays. Subsurface geology was interpreted on the basis of well-log data. The sediments were deposited in channels, flood plains, cut-off meanders, and levees. Twenty-two electrical resistivity soundings and two traverses were made in an attempt to determine the usefulness of the resistivity method in predicting Columbia thickness. Resistivity techniques appear to be useful in determining gross lithologies, but accurate depth solutions are not always possible. The Columbia Formation is the water-table aquifer in the Middletown-Odessa area and in most other parts of the State. Saturated thickness and water levels were determined in the study area by temporary installation of piezometers. The water-table was found to average about 30 feet below land surface in the central part of the study area. The transmissivity of the main paleochannel east of Middletown is about 40,000 gpd/ft. High iron content (1-2 mg/l) and low pH are common, and may present treatment problems. Extrapolation of Columbia aquifer coefficients from one area to another is not reliable and may produce erroneous results. (WRSIC abstract).