Little Muddy Aquifer Computer Model for Water Permit No. 5757

Abstract

Water Permit No. 5757 is for a proposed well which would discharge water from the Little Muddy Aquifer in the Little Muddy Management Area (LMMA) in Williams County, North Dakota. The extraction of water in this well could adversely affect the water levels in the aquifer and the water levels in the previously installed wells. To predict the affect that this well will have on the aquifer, a MODFLOW computer model of the Little Muddy Aquifer was constructed. The purpose of this report is to explain how the model was constructed and what conclusions were drawn from the model output. • The model represented a land area of just under 90 square miles and was made up of approximately 2700 cells. The input hydraulic conductivity and recharge data were split into zones to best represent the natural aquifer texture of the sediments. Recharge was added to the model where other aquifers discharged into the Little Muddy Aquifer. Surface water was represented either as river cells or general head boundaries. • The model output data calibrated closely in the southern part of the aquifer when all the wells were not pumping. The model output data were not as close when all the wells were pumping at maximum rate, but the water budget outflows were within 1.5% of the inflows. • The model was not sensitive to small changes in hydraulic conductivity. • The installation of Water Permit No. 5757 will not negatively affect Water Permit Nos. 4036 and 4036A, but it will contribute a drawdown of 3-4 ft in Water Permit No. 5004. However, this drawdown will occur in an area of the aquifer where the saturated thickness is approximately 50 ft thick and is not expected to influence the quantity of groundwater available for Water Permit No. 5004