Hydrologic Setting and Geochemical Characterization of Free-Phase Hydrocarbons in the Alluvial Aquifer at Mandan, North Dakota, November 2000

摘要

Free-phase hydrocarbons are presented in the alluvial aquifer at Mandan, North Dakota. A large contaminant body of the hydrocarbons (light nonaqueous phase liquid (LNAPL)) floats on the water table about 20 feet below below land surface. The main LNAPL body is about 6 feet thick, and the areal extent is about 657,000 square feet. A study was conducted to describe the hydrologic setting and characterize the geochemical composition of the free-phase hydrocarbons in the alluvial aquifer. Most of the study area is underlain by alluvium of the Heart River Valley that ranges in thickness from about 25 to 109 feet. The alluvium can be divided into three stratigraphic units-silty clay, silty sand, and sand-and is underlain by shales and sandstones. Regional ground-water flow in the Heart River aquifer generally may be from west-northwest to east-southeast and is influenced by hydraulic connections to the river. Hydraulic connections also are probable between the aquifer and the Missouri River. Ground-water flow across the north boundary of the aquifer is minimal because of adjacent shales and sandstones of relatively low permeability. Recharge occurs from infiltration of precipitation and is spatially variable depending on the thickness of overlying clays and silts. Although the general water-table gradient may be from west-northwest to east-southeast, the flow directions can vary depending on the river stage and recharge events. Any movement of the LNAPL is influenced by the gradients created by changes in water-level altitudes.