Date of Award
Master of Science (MS)
An area of saline/brackish groundwater south of Oakes, North Dakota has been reported by the North Dakota State Water Commission. The concentrated water (over 12,000 mg/L of total dissolved solids) is in direct hydraulic connection with relatively fresh water and shows an abrupt change in concentration and water chemistry between these two end members. The saline/brackish water is beneath a shallow trough or depression which has an approximate area of 6 square miles (15.5 square kilometres). The trough is within the northern part of the Glacial Lake Dakota plain. Sediments in the lake plain, which is an unconfined aquifer, are sands that grade downward to gravels. Within the trough sands are overlain by Holocene lacustrine silts and clays. A discontinuous unit of lacustrine silt lies beneath the upper sand and gravel aquifer. Below both these units is glacial till. Locally, a second sand and gravel aquifer underlies the till. The second aquifer is under confined conditions and is underlain by a second till unit.
In May 1983 observation wells were installed to determine the horizontal extent of the saline/brackish water. In order to determine the vertical extent of that water, and to determine groundwater movement, two detailed traverses of nested piezometers were installed.
Interpretation of hydraulic head data for the two traverses shows a general northerly flow along the N-S traverse and lateral flow along an E-W traverse, toward a central sink. Here, water is assumed to move along the axis of the trough toward the James River. During major recharge events, reversals of hydraulic gradients occur, indicating outward flow from the center of the depression and downward flow into the lower sand and gravel aquifer. Alternating recharge and stable flow results in relatively stagnant water.
The major constituents of the saline groundwater are Na+ and SO42-; lesser amounts of HCo3-, Ca2+, Mg2+; and trace amounts of Fe, Mn, B, F, SiO2, and Cl-. Due to the large percentage of carbonates in the aquifer and tills, CaCO3 and (Ca,Mg)CO3 dissolved in the presence of acidified water yield Ca2+, Mg2+, and HCO3- ions. Gypsum (CaSO4 .2H2O) yields Ca2+ and SO42- ions. Pyrite oxidation yields Fe and SO42- ions. The tills and surficial clays contain montmorillonite, presumably sodium montmorillonite, which in the presence of Ca2+, exchanges Na+ for Ca2+. If the system is saturated with gypsum, loss of Ca2+ would create an undersaturated condition causing further dissolution of gypsum. Continued Ca2+ exchange for Na+ leads to a dominance of Na+ and SO42- ions.
Evaporation is suggested as the concentrating mechanism within the center of the depression. Mineral precipitation occurs at the water table in surficial clays just below land surface. Major recharge events dissolve the precipitated minerals, move the saline water down to the water table, and-then by miscible displacement, transport the denser, saline water to the base of the aquifer. Repeated mineral precipitation and dissolution cycles have led to the concentrated saline/brackish water.
Williams, David L., "The geochemical evolution of saline groundwater within a fresh water aquifer south of Oakes, North Dakota" (1984). Theses and Dissertations. 325.