Date of Award
Doctor of Philosophy (PhD)
The Little Missouri valley in western North Dakota is deeply incised into slightly permeable Mesozoic and Cenozoic sediments. An evaluation of the groundwater flow in part of this valley was 'based on the present groundwater flow-system models. Abrupt changes in the hydrochemical facies have been explained relative to a continuous flow system.
The Little Missouri valley ls the discharge area of a regional groundwater flow system; it affects the potential distribution of the flow system for a minimum of 1.000 feet below the valley floor. The discharge area is approximately the same width as the valley floor and has a ratio of the vertical to longitudinal flow component (in terms of' gradient) of at least 100.
The groundwater divides in the center of the study area are asymmetrically spaced relative to the discharge area and underlie the surface-water divides. The recharge area to the west of the valley has a higher potential and steeper gradient than the eastern recharge area as a result of its higher elevation and overall lower per meability.
Lithology strongly controls the groundwater chemistry. The highest concentrations of dissolved solids are associated with some of the shortest groundwater flow paths; this water usually contains abundant sulfate.
Sodium is the dominant cation (comprising 90 to 99 percent of the total cations) in the discharge area and. below a depth of about 200 feet in the recharge areas. This sodium facies is the result of the exchange of sodium for calcium by cation exchange minerals in the sediment. Groundwater rich in calcium and magnesium occurs where much of the calcium and magnesium exchange capacity of the sediment has probably been exhausted .
Repeating anion facies occur in a vertical section of the flow system. Bicarbonate facies, occurring at depths of several hundred feet are attributable to a sequence of reactions initiated by the reduction of sulfate. The total change of the groundwater chemistry as a result of this reduction is (1) a decrease in the absolute amount of sulfate, (2) an increase in the amount of bicarbonate, (3) an indirect increase in sodium, and (4) an increase in the total dissolved solids.
The highly-permeable valley-fill sediment receives seepage contributions related to three separate sources. The probable order of relative importance of these contributions is (1) seepage from the regional groundwater flow system, (2) seepage through the beds of ephemeral streams during runoff, and (3) seepage through the river banks at high stream stage. Much of the groundwater in the valley-fill sediment probably leaves the basin as underflow.
The cations in the groundwater of the valley-fill sediment are composed of as much as 50 percent calcium and magnesium. This high percentage may be explained as a reversal of the cation-exchange equilibrium when calcium rich clays, that were eroded from the recharge areas, are brought into contact with the sodium-rich groundwater of the regional discharge area.
Hamilton, Thomas M., "Groundwater flow in part of the Little Missouri River Basin, North Dakota" (1970). Theses and Dissertations. 121.