Mississippi River Valley Alluvial Aquifer

GO 571 Hydrogeology, Spring 2010

Wesley C. Smith

Hydrologic Setting

Regional Area

The Mississippi River Valley alluvial aquifer encompasses an area of approximately 32,000 square miles (Figure 1). It is present in seven states and is the uppermost aquifer in the Mississippi embayment aquifer system. Major rivers that drain the relatively flat alluvial plane include the Mississippi, St. Francis, White, Arkansas, Yazoo and Quachita Rivers (Ackerman, 1996).

Figure 1. Regional extent of alluvial aquifer (Ackerman, 1996)

Climate

The climate in the Mississippi River Valley alluvial aquifer geographic area is mild and humid. Mean annual air temperature and annual precipitation vary from 14 degrees C and 47 inches in the northern portion of the area to 19 degrees C and 52 inches in southern areas (Ackerman, 1996).

Hydraulic Properties

Geology

Aquifer medium is composed of unconsolidated Quaternary alluvium and terrace deposits consisting mostly of sand and gravel size sediments that gradually fine upward (Figure 2). These sediments are overlain by deposits of clay and silt that form a semi confining layer. The average thickness of the confining layer is 30 ft but the unit is highly variable and even missing in some areas of the alluvial aquifer (Ackerman, 1996). The base of the aquifer is a sub-Quaternary eroded bedrock surface (Fisk, 1944).

Figure 2. General Lithology (Gonthier, 1998)

Aquifer thickness ranges from 25 to greater than 150 ft (Renken, 1998). On average, the aquifer is 100 feet thick and thins from north to south (Ackerman, 1996). Generally hydraulic conductivity ranges from 200 to 245 feet per day and varies depending on the local makeup of the aquifer medium. Storage coefficients range from .0001 in confined portions of the aquifer to .30 in unconfined portions (Ackerman, 1996).

Map of aquifer thickness: See Thickness.

Figure 3. Map showing cross section locations (modified from Ackerman,1989)

Figure 4. Cross Section C-C' (Ackerman,1989)

Recharge

Aquifer recharge occurs from leakage of the confining unit, adjacent aquifers and rivers and from precipitation where the confining unit is absent (Ackerman, 1989).

Surface Water Interaction

Although several rivers are present within the Mississippi River alluvial aquifer the relationship between aquifer and river is dependent on how deep the river and/or surface body is cut into the confining layer. The deeper the river is cut into the confining layer and into the aquifer the more river-aquifer interaction will take place (Ackerman, 1996). (Note on figure 4 how some of the rivers shown in cross section penetrate the alluvial aquifer and some do not. See figure 3 for cross section locations.) Figure 5 below from Akerman, 1996 shows hydrographs for four wells, all within 500 feet of the adjacent river stage gauge. The top two wells show good river-aquifer interconnectedness and the bottom two show very little river-aquifer interaction.

Figure 5. Hydrographs for select wells showing river-aquifer interaction (Ackerman,1996)

Water Resources

Aquifer Use

Land use within the Mississippi River Valley alluvial aquifer region is 80 percent agricultural, approximately 16 percent forest, 3 percent surface water and 1 percent urban (Gonthier, 1998).

Figure 6. Land use (Gonthier, 1998)

Due to the land use, large scale pumping has had a great effect on the Mississippi River Valley alluvial aquifer system. Rivers received most of the outflow from the aquifer system before modern development, but now they have become a major source of inflow due to large scale pumping (Ackerman, 1989). Water from pumping is mostly used for irrigation water for rice, soybeans, corn, etc. and for aquiculture, specifically catfish farming (Renken, 1998). Average annual pumping from the aquifer is about 7Bgal/d (Gonthier, 1998).

Hydrographs showing pumping over time: See Pumping.

Groundwater Contaminants

Due to the large amount of agricultural land use major environmental concerns include surface water and groundwater pollution from non-point sources, specifically pesticides and fertilizers. Approximately 33 million pounds of pesticides are placed on land in the Mississippi River Valley alluvial aquifer region each year (Gonthier, 1998).

Chemical Character

Excessive hardness and high iron and manganese concentrations are the major inhibitors for widespread public water use. Generally speaking calcium is the dominant cation averaging 56% of the cation equivalents followed by magnesium, sodium and iron (Gonthier, 1998). Bicarbonate is the dominant anion averaging 84% of the anion equivalents followed by sulfate and chloride (Gonthier, 1998). Generally water from the aquifer can be characterized as a calcium bicarbonate type with dissolved-solid concentrations that are usually less than 500 milligrams per liter (Ackerman 1996).

Conclusion

The Mississippi River Valley alluvial aquifer, present in seven states, consists of sand and gravel size Quaternary alluvial and terrace deposits. Recharge occurs from leakage of the confining unit, from surface water bodies, adjacent aquifers and from precipitation where the confining unit is absent. Water extracted from the aquifer is mostly used for agricultural purposes; large scale pumping has changed flow and the relationship between the aquifer and surface water bodies over time.

References

Ackerman, D.J., 1989a, Hydrology of the Mississippi River Valley alluvial aquifer, south-central United States - A preliminaryassessment of the regional flow syatem: U.S. Geological Survey Water Resources Investigation Report 88-4028, 74p.

Ackerman, D.J., 1996, Hydrology of the Mississippi River Valley alluvial aquifer, south-central United States: U.s. Geological Survey Professional Paper 1476-D, 68p.

Fisk, H.N., 1944, Geological Investigation of the alluvial valley of the lower Mississippi River: U.S. Department of the Army, Mississippi River Commission, 78p.

Gonthier, G.J., 1998, Quality of Groundwater in Pleistocene and Holocene subunits of the Mississippi River Valley alluvial aquifer: U.S. Geological Survey Water Resources Investigations Report 03-4202, 96p.

Renken, R.A., 1998, Ground water atlas of the United States - Arkansas, Louisiana, Mississippi: U.S. Geological Survey HA 730F (http://pubs.usgs.gov/ha/ha730/ch_f/index.html)See Source

Page Created to fulfill requirements of GO 571, Hydrogeology: See Hydro.

Emporia State Univerity, Earth Science Department: See Earth.