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Al Introduction 1 Investigative background 1 Purpose and scope 3 Approach 3 Physiographic and hydrologic setting 4 Acknowledgments 5 Geologic framework 5 Rock properties 5 Structural properties 9 Hydrogeologic framework 12 Hydrology 12 Hydraulic characteristics 17 Initial storage and flow concepts 17 Mathematical model 20 Representation of the aquifer 21 Hydrologic data 23 Hydrogeologic data 23 Simulation of flow 26 General description of simulations 26 Selected simulation 27 Refinement of storage and flow concepts 31 Delineation of storage units 31 Delineation of flow units 36 Identification of possible flow across external boundaries 36 Description of geologic structures that restrict and convey ground water 38 Topics for future investigation 42 Summary and conclusions 44 Selected references 45 Glossary 48 IV Contents

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The relation between water quality and water movement within the ground‐water reservoir may be better understood if studies of flow systems are used in conjunction with hydrochemical methods. Within small watersheds, local, intermediate, and regional flow systems may develop, depending upon the shape, the relief, and the thickness of the ground‐water reservoir. Lateral and vertical variations of water quality in the ground‐water reservoir reflect the difference in length of flow paths within a system. The relative ion concentration of the major ions in solution in ground waters change as water moves from recharge areas to discharge areas. These changes are used to interpret the direction and rate of ground‐water movement. Ground‐water types mapped in the Two Rivers watershed of northwestern Minnesota indicate that calcium bicarbonate type water occurs near the source of recharge and progressively changes to the sulfate type as water moves downgradient toward the discharge area. Local recharge areas are indicated by the greater penetration of calcium bicarbonate waters into the ground‐water reservoir. The calcrum bicarbonate type water of surface streams during low flow indicates that water moves into the stream from the upper part of the ground‐water reservoir. Chloride type water moves from the Paleozoic rocks and mixes with water in the glacial drift probably at places where permeable zones in the Paleozoic rocks are in contact with the overlying drift.