Bedrock units at the Weldon Spring ordnance works in St. Charles County, Missouri, dip to the northeast at about 60 feet per mile, as measured by the top of the Chouteau Group. The top of the bedrock forms a generally east-west trending ridge through the Weldon Spring training area and the Weldon Spring chemical plant. This surface contains a large, broad bedrock low centered about the unnamed tributary to Dardenne Creek that contains Burgermeister spring. The low has been interpreted to be a paleodrainage that existed before deposition of glacial drift. This feature consists of smaller, more elongate paleovalleys at and west of the chemical plant where more dense drillhole data provide better definition.
The uppermost bedrock unit throughout most of the ordnance works is the BurlingtonKeokuk Limestone of Mississippian age. It is subdivided based on weathering characteristics into a lower, unweathered unit; an upper, weathered unit; and a strongly weathered subunit of the weathered unit. The unweathered unit is a light to medium gray, coarse to less commonly fine crystalline, thin to massive bedded, fossiliferous, cherty limestone. The unweathered unit can be silty or argillaceous, or can locally be dolostone or siltstone. The weathered unit is characterized by an increase in mostly horizontal fractures and partings, increased porosity, vugs, voids, breccia, and discoloration by iron oxides. A strongly weathered subunit of the weathered unit is identified in some monitoring wells where these features are particularly abundant or intense.
The overburden units are, in ascending order: residuum, basal till, glacial till, including a glacial outwash subunit, the Ferrelview Formation, loess, alluvium, and fill. Some of the thickest overburden occurs in the northern part of the training area and north of the training area and may be caused by a larger thickness of glacial drift. The paleodrainage centered about the unnamed tributary to Dardenne Creek that contains Burgermeister spring appears to have been partially filled by glacial drift, and a surface-water divide now exists southeast of the tributary.
The upper, more permeable part of the shallow aquifer consists of the residuum, basal till, glacial outwash (where there is no glacial till below it), and the weathered unit of the Burlington-Keokuk Limestone. The lower, less permeable part of the shallow aquifer consists of the unweathered unit of the Burlington-Keokuk Limestone and the Fern Glen Formation. Generally, the upper part of the shallow aquifer thins to the north, reflecting the thin to absent weathered unit north of the training area and chemical plant. A glacial drift confining unit consists of parts of the glacial till and the Ferrelview Formation. Ground water as recharge and discharge probably moves in fractures through this unit. It confines ground water where the potentiometric surface of the shallow aquifer is above its base. There are stream reaches where the streams have cut through the glacial drift confining unit to expose the underlying shallow aquifer.
A potentiometric surface map of the shallow aquifer shows a large ground-water mound in the south-central part of the training area. This mound is part of a generally east-west trending ground-water ridge through the training area and the chemical plant that defines a ground-water divide. Precipitation that percolates downward through fractures in the glacial drift confining unit recharges the shallow aquifer. Where the glacial drift confining unit is not present, precipitation can be expected to recharge the shallow aquifer more readily. There is the potential for groundwater flow in permeable overburden units where the potentiometric surface is above the top of bedrock. Generally, the residuum and locally other overburden units of the shallow aquifer potentially become more important as mediums of ground-water flow north and downgradient of the ground-water ridge. This is probably limited where clay-rich zones in the residuum confine ground water below in the bedrock. Because the thickness of the weathered unit generally decreases to the north, it generally becomes a less important medium of ground-water flow downgradient to the north. Also to the north, the potentiometric surface of the shallow aquifer is above the base of the glacial drift confining unit over a large area, indicating that the aquifer is confined. Upward ground-water gradients measured in monitoring well pairs, Burgermeister and other springs, the gaining unnamed tributary to Dardenne Creek upstream of Burgermeister spring, and Dardenne Creek indicate ground-water discharge in the northern part of the ordnance works.
|Title||Geohydrology of the Weldon Spring ordnance works, St. Charles County, Missouri|
|Authors||Douglas N. Mugel|
|Publication Subtype||USGS Numbered Series|
|Series Title||Water-Resources Investigations Report|
|Record Source||USGS Publications Warehouse|