In 1992, the U.S. Army Corps of Engineers and the U.S. Army Pueblo Depot Activity requested that the U.S. Geological Survey study the hydrogeology of the Pueblo Depot Activity, a 36-square-mile facility that has been operated by the U.S. Army since 1942. The purpose of the study was to provide an updated hydrogcological framework to facilitate the investigation of specific sites on the facility that might require remediation. This report describes the hydrogeology of the alluvial aquifers beneath the facility and the distribution of specific conductance of ground water in those aquifers.

The Pueblo Depot Activity is underlain by two alluvial aquifers: (1) The terrace alluvial aquifer, which is a southernmost, downgradicnt part of an erosional remnant of an extensive terrace deposit; and (2) the Chico Creek alluvial aquifer, a smaller alluvial system along Chico Creek. These aquifers primarily consist of sand separated by clay layers and are underlain by the almost impermeable Pierre Shale of Upper Cretaceous age.

The bedrock surface, which has an average slope of 28 feet per mile to the south-southeast, is relatively regular beneath the northern two-thirds of the terrace deposits at the Pueblo Depot Activity, but forms an irregular surface of troughs, hills, and ridges in the southwestern part of the terrace alluvium. Saturated thickness of the terrace aquifer ranges from 0 to about 45 feet.

The bedrock surface beneath the Chico Creek aquifer slopes about 31 feet per mile to the south. Saturated thickness of the Chico Creek alluvium ranges from 0 to about 30 ft, but generally is less than 15 ft. Total thickness of the Chico Creek alluvium in the saturated area ranges from 16 to 41 ft.

Water in the terrace alluvial aquifer generally flows southward, except in the southwestern part where directions of flow are complex. Measured hydraulic conductivity ranges from 0.4 to 400 feet per day (median 26 feet per day). Estimates for vertically averaged ground-water-flow velocity range from 0.02 to 3 feet per day (median 0.9 foot per day).

Water in the Chico Creek alluvial aquifer generally flows southward to the Arkansas River alluvium. Measured hydraulic conductivity ranges from 14 to 310 feet per day (median 42 feet per day). Estimates for vertically averaged ground water-flow velocity range from 0.5 to 4 feet per day (median 0.7 foot per day).

Specific conductance of ground water in the terrace alluvial aquifer generally is less than 800 microsiemens per centimeter; the smallest values were observed in the north-central part of the Pueblo Depot Activity. In the southwestern part of the terrace alluvial aquifer, values varied in an irregular pattern, and values as large as 3,300 microsiemens per centimeter were measured locally. Water in the terrace alluvial aquifer was dominated by the sodium cation and usually by the bicarbonate anion, and sulfate usually was present in substantial (and locally predominant) concentrations.

Measured specific conductance of water in the Chico Creek alluvial aquifer ranged from 683 to 1,460 microsiemens per centimeter. This water was dominated by the sodium cation and by the bicarbonate and sulfate anions; sulfate was more predominant to the south.