Date of Award
Master of Science (MS)
The West Brush Creek Development Area, a part of the Beulah Mine, occupies a rolling upland approximately 7 kilometres southwest of Beulah in west-central North Dakota. This study describes the geology, hydrology, and hydrogeochemistry of an 8 km2 area which is scheduled for strip mine development, and addresses the potential impacts of mining on the local hydrologic and hydrogeochemical regime.
Four previously recognized stratigraphic intervals in the Paleocene Sentinel Butte Formation were described and correlated. They are, from oldest to youngest, the Spaer, Beulah-Zap, Schoolhouse and Twin Buttes intervals, each named for the lignite bed which caps the interval. The pore-water chemistry of the elastic sediments within these intervals was defined using saturated-paste extracts; most samples had a mean SAR in the 12 to 20 range. Texturally, these sediments are composed of approximately equal amounts of fine to very-fine sand, silt, and clay.
All four lignite beds are saturated in parts of the study area; because of its limited occurrence, however, the Twin Buttes Bed was not instrumented. The Schoolhouse Bed forms a perched aquifer throughout the area, and often has a few metres of confined potentiometric head. Hydrologically, the Beulah-Zap Bed occurs as both a confined and a water table aquifer; near the center of the study area this bed is dry. The Spaer Bed is confined where instrumented and is probably below the local water table. Single-well response tests showed geometric mean hydraulic conductivities of 2.2 x 10-6, 5.2 x 10-8, and 1.6 x 10-7 m·s-1 for the Schoolhouse, Beulah-Zap and Spaer Beds, respectively.
The McDonald-Harbaugh Modular Three-Dimensional Finite-Difference Groundwater Flow Model was used to predict the inflow of groundwater to the mine pits, as well as potential short and long-term drawdowns of the lignite aquifers. The model predicted that both pit-water inflows and drawdowns will be negligible.
Three chemically distinct types of groundwater occur within the study area. The most common is the sodium-bicarbonate, sulfate type which is characteristic of the deeper Spaer and Beulah-Zap Beds. The shallower Schoolhouse Bed generally has a calcium, magnesium, sodium-sulfate type water which is believed to reflect a less evolved phase of the sodium-bicarbonate, sulfate type. At one sampling location the Beulah-Zap Bed had a sodium-carbonate type water with an unusually high pH of 13. The hydrolysis of silicates associated with nearby "scoria" is believed to produce the high pH which controls the groundwater chemistry at this site.
Groundwater recharge within the area to be mined is minimal and will probably remain so in the post-mining setting. The mechanism of spoils saturation, therefore, will be lateral inflow from adjacent undisturbed lignite beds south of the mine pits. ln situ hydrogeochemical reactions between the spoils water and soluble salts displaced from the oxidized parts of the pre-mining landscape to the pit bottom, will result in groundwater which is approximately 2 to 3 times more mineralized than before mining, primarily due to increases in the sodium, calcium, and sulfate concentrations. The spoils water quality should begin to improve following an initial flushing of one pore-water volume, a process which should take a few hundreds of years.
Winbourn, Gary D., "Geology, hydrology and hydrogeochemistry of the West Brush Creek Development Area, Beulah Mine, Mercer County, North Dakota" (1986). Theses and Dissertations. 327.