Ground water in the Mohall area, Bottineau and Renville Counties, North Dakota

Abstract

The Mohall area includes about 120 square miles in Bottineau and Renville Counties in northwestern North Dakota. Mohall, whose 1950 population was 1,073, is the only town in the area. The area is part of the Drift Prairie section of the Central Lowland physiographic province. It is characterized by the gently undulating ground moraine plain which slopes regionally to the northeast. It is drained by several southeast-trending intermittent streams which run almost at right angles to the regional slope. The geologic formations in the Mohall area may be conveniently grouped into three units: the alluvium or alluvial deposits, which are found in the valleys of the intermittent streams, the till and associated glaciofluvial deposits, and the bedrock formations. Ground water of reasonably good quality is obtained in the area only from the alluvial deposits. Because of the limited areal extent of these deposits, only a small number of farms obtain water supplies from them. However, the municipal water supply of the town of Mohall is obtained from these deposits in Spring Coulee northeast of the town, and much water for rural domestic use is hauled from the Becker well southwest of the town, in Little Deep Creek. The principal source of recharge to the alluvial deposits is seepage during the spring runoff period. Natural discharge occurs by underflow down the stream valleys, by evaporation from open water and marshy areas and transpiration by plants. The coefficient of transmissibility of the alluvium, n the basis of short pumping tests on Mohan wells 3 and 4, is indicated to be about 6,000 gallons a day per foot at well 3 and about 20,000 gallons a day per foot at well 4. The specific yield was computed as 0.25 from the pumping test on well 3. No gravel of importance was en countered in the two cross sections that were drilled across Spring Coulee near the municipal wells, and the greatest thickness of satu rated alluvial deposits found was only about 10 feet. A Saturated thickness of 20 feet of sand and gravel was penetrated in test drilling along Cut Bank Creek. This is the greatest thickness of saturated alluvial deposits penetrated anywhere in the area. Ground water in storage in these alluvial deposits has been estimated to be about 150 million gallons per mile. These deposits are favorably situated to receive recharge, as Cut Bank Creek drains a rather large area and contains long stretches of open water perennially, Therefore, it is believed that the alluvial deposits of Cut Bank Creek offer the best promise for the development of moderately large perennial ground-water supplies for the present and probable future needs of the town of Mohall. Test drilling in West Cut Bank Creek and in Little Deep Creek did not reveal alluvial deposits of such character and saturated thickness as to be considered favorable for the development of moderate to large water supplies, though some of the material encountered should yield adequate quantities for farm supplies. The till with its associated glaciofluvial deposits is the surface formation in the area except where covered by the alluvial deposits in the stream valleys. In the Mohall areas, this formation is not an important aquifer and fewer than half a dozen wells are known to obtain water from it. The glaciofluvial deposits penetrated by the test holes are not considered adequate sources for permanent municipal or industrial supplies because they are likely to have small-areal extent and the overlying, relatively impermeable till makes seasonal recharge to them practically impossible. Test drilling in the Souris River Valley about 12 miles west of Mohall penetrated as mutt as 58 feet of fluvial sediments, but practi-cally all the material is clay and silt. No important aquifers were found there. At least 50 percent of all the farm water supplies in the area are obtained from wells in the bedrock formations, which probably consist of the Fox Hills sandstone, the Cannonball formation, and the Ludlow and Tongue Rtter members of the Fort Union formation. Underlying these formations is approximately 2,600 feet of Cretaceous shale, which'is not water bearing. Water from the "Dakota" sandstone (including the possible equivalent of the Fuson shale and Lakota sandstone) may be obtained at depths of about 3,200 to 3,300 feet. The "Dakota sandstone" probably would yield water in sufficient quantity for municipal and many industrial purposes in this areas but the water is likely to be too highly mineralized for most domestic uses. Jurassic formations underlie the Cretaceous formations in the area. They do not constitute aquifers of importance, and, any water found in them is likely to be too highly mineralized for most purposes. In the Mohall area, by far the most suitable water for general purposes is obtained from the shallow alluvial deposits in the stream valleys. Of the seven samples of this water water analyzed, the highest concentration of dissolved solids was 1,242 parts per million and the lowest, 317 parts. The iron content was higher than desirable in two samples but satisfactory in all the others. Total hardness ranged from 196 to 570, which is higher than desirable. Nitrate was present in all samples analyzed and was excessively high in two samples. Water samples for analysis were not obtained from the till and associated glaciofluvial deposits but the water from these aquifers is likely to be more highly mineralized than the water from the alluvial. deposits. The water from the upper part of the bedrock is highly mineralized, but its mineral content varies considerably. The chloride concentration of the samples analyzed ranges from 608 to 3,740 parts per million and the bicarbonate concentration ranges from 160 to 860 parts per million. The specific conductance ranges from 3,190 to 11,120 micromhos and may represent total mineralization on the order of 1,500 to more than 6,000 parts per million in the water with high chloride and bicarbonate content.