[en] The Missouri Department of Natural Resources, Division of Geology and Land Survey (MDNR-DGLS) conducted investigations of the upper aquifer in the vicinity of the abandoned Weldon Spring Chemical Plant in southwest St. Charles County, Missouri. The study began October 1, 1987, and concluded September 30, 1990. The study area included most of the original Weldon Springs Ordinance Works property plus a smaller area to the northeast between U.S. Highway 40-61 and Dardenne Creek. The investigations included a literature review, classification of stream segments as gaining (effluent) or losing (influent), location of springs, water tracing tests, testing of the spontaneous potential geophysical technique, continuous gaging of selected streams and continuous water-level monitoring in selected monitoring wells. The objective of the investigation was to better define the relationships between precipitation, surface runoff, groundwater recharge and shallow groundwater discharge within the study area, thereby assisting the Department of Energy in designing an appropriate groundwater monitoring plan for the Weldon Spring Site Remedial Action Project. A review of literature revealed that a number of reports had been written pertaining to shallow groundwater hydrology near the Weldon Spring site; five by the U.S. Geological Survey and several short ones by the MDNR-DGLS. These reports noted various aspects about the hydrology of the area including the locations of some springs and losing streams, the presence of some subsurface hydrologic connections and some piezometric data. Using these data as a base, eleven surface water drainages around the site were examined during various climatic conditions to locate springs and identify losing stream stretches. Seventy-five springs were located and losing stream stretches were identified in nine of the eleven surface-water drainages