Missouri River Alluvial Groundwater Problem

Figure 1 – Screen shot of the model area near Glasgow, Missouri, United States. The Missouri River is highlighted as a green Zone feature in MAGNET.

Background

Groundwater reservoirs, like surface-water reservoirs and natural lakes, distribute water supplies over long periods of time.  Most groundwater reservoirs maintain a dual role in the water cycle.  During periods of high rainfall and streamflow, they store water, returning it to the streams as base flow during low-flow stages.  Alluvial aquifers play a particularly large part in this double role which emphasizes the strong interrelationship between surface water and groundwater.  The water flowing in any perennial stream may have infiltrated and been discharged to the stream a number of times.  Unconsolidated sediment adjacent to the stream is the most likely place for infiltration and discharge back to the stream.  Water can be drawn from neither groundwater nor surface water without affecting the other. 

This lesson uses information from the Missouri River alluvial valley on the lower reaches of the River and groundwater modeling to explore in more detail how water flowing in one of the world’s largest rivers interacts with its adjacent groundwater.  The study area is in the flood plain near Glasgow, Missouri that is typical of rivers that form at the distal end of continental ice sheets during continental glaciation and is an underfit stream.  Adjustments of discharge, sediment load from glacial debris, and base level during glacial and interglacial stages created a vertically fining-upward sequence of unconsolidated alluvium within a valley created by much larger volumes of water than in the present-day river.  It scoured a deep valley within sedimentary bedrock and filled the valley so that the modern river flows on top of previously deposited sediment (Figure 2).  The modern river only penetrates about 20 to 40 feet into alluvium that ranges from 80 to 120 feet in thickness. 

Water-level data were collected for 20 test wells in the flood plain and contoured for several river stages (Figures 2, 3, 4).  River stage rose by about 6 feet between the contoured data collected on April 26 and June 3, 1976.

Objective

You have been asked to determine if the rise in river stage explains most of the rise in groundwater level during these two dates? You will develop a 3D groundwater model to simulate the head distribution in the Missouri River Valley alluvium before and after the change in river stage between April 26 and June 4, 1976; and compare the simulated results to the head contours generated from the 20 wells.

Prepare a short report / memo that summarizes your key findings, supported by appendix including key graphics and descriptions of the model/analysis. Be sure to include an explanation of how your model compares to the head contours from the monitoring well data, and what improvements/further work might be needed.

Further Hints and Suggestions (MAGNET-related)

The suggested conceptual model is described as follows:

• Zoom to Glasgow Missouri and delineate a model along the stretch of the Missouri River south-southwest of Glasgow as shown in Figure 1, using the background map to demarcate the interface between the alluvium valley and the rocky uplands.
• The model should consist of three layers with the following aquifer elevations:
  • 1^st (top-most) layer:top follows the land surface as represented in the DEM from the MAGNET Data Center; bottom is a constant elevation of 577 ft.
  • 2^nd layer: top follows bottom of 1^st-layer (i.e., Aquifer top is left unchecked for this layer); bottom is a constant elevation of 535 ft.
  • 3^rd layer: top follows bottom of 2^nd layer; bottom follows bedrock top surface available from the MAGNET Data Center (select Data Center and Thickness as Bottom Elevation)
• Aquifer recharge (to the 1^st layer) is expected to vary between 12 in./yr. an 18 in./yr. in this area
• Estimate hydraulic conductivities of the three different layers based on the dominant materials in those layers;
• Conceptualize the Missouri River as a head-dependent boundary condition using the Zone feature in MAGNET. The long-term mean stage in this area before the river rises is: 587 ft. The average depth of the river is 30 ft. The leakance (conductivity per unit thickness of the riverbed) is 5 day^-1.
  • Note: The Missouri River Zone feature should exist in both the 1^st aquifer layer and 2^nd aquifer layer. After creating the first zone, you can “copy and paste” it to the next layer (open the Zone Attributes menu and select Copy – see the Help page for more details)
• Do not simulate any other surface drainage (surface drain leakancy = 0).
• Adjust the hydraulic conductivities of the different layers and/or the aquifer recharge in an attempt to reproduce (to the extent reasonably possible) the head contours shown in Figure 3.
• Change the river stage to 593 ft to simulate conditions on June 4, 1976.

Figure 2 – Generalized cross section of Missouri River Valley alluvium showing a fining-upward sequence of sediment mostly of glacial origin.  The present-day River only partially penetrates the alluvial sediment and its width is small compared to the river that scoured the valley during and after glaciation (Grannemann, 1976).

Figure 3 – Hand contoured water table from data collected from 16 wells on April 26, 1976 (Grannemann, 1976).

Figure 4 – Hand contoured water table from data collected at 16 wells on May 3, 1976 after river stage rise of 6 feet (Grannemann, 1976).