Hydrologic factors affecting the discharge of ground water in the Red River of the North Basin, northeastern North Dakota
Ground-water discharge from bedrock aquifers contributes significant amounts of salinity to the Red River of the North. The area of major discharge is represented on the land surface by saline soils, wetlands, and flowing wells. As tributaries flow through these areas, the dissolved-solids concentrations in the streams increase. This salinity is then transported to the Red River of the North and northward to Canada. The bedrock units that supply the saline water are mainly the Dakota Group. Overlying the bedrock are sediments from numerous glacial advances and lacustrine sediments from glacial Lake Agassiz. These units form confining layers over the bedrock aquifers and limit the rate of ground-water discharge from bedrock sources. The three mechanisms responsible for salinity reaching the land surface are advection, diffusion, and flowing wells. Hydraulic conductivity of the surficial units were measured. The major confining unit in the area is the Brenna Formation, which has a hydraulic conductivity of about 5.0 x 10-5 ft/d, would transmit water through its entire thickness in about 3,500 years under the present hydraulic gradient. Diffusion may contribute to salinity at the land surface, but has a minor role compared to advection. Estimates of flowing well discharge and distribution show that these wells may have an effect on surface-water quality during low-flow periods. A pressurized slug test was designed for use in low-permeable sediments. Results indicate that this test provides an approach for estimating hydraulic conductivities in tight formations over short time periods. A ground-water flow model was used to investigate hydrogeologic factors affecting groundwater discharge. The thickness of the bedrock aquifers and confining units influences ground-water discharge. Saline soils relate to both the occurrence of the lnyan Kara aquifer and the thickness of surficial sediments. The effect of ground-water discharge on streams was examined using field measurements of streamflow and dissolved-solids load, and historic surface-water data in northeastern North Dakota. The effects of ground-water discharge became more significant during periods of low flow. Major sources of salinity to the Red River of the North are the Turtle, Forest, and Park Rivers.
Groundwater--Red River of the North; Groundwater--North Dakota; Water quality--Red River of the North; Water quality--North Dakota; Geology--North Dakota, Geology
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