Summary of Ground Water Conditions in the Grand Prairie and Bayou Meto Study Areas of Eastern Arkansas
 

Rice started being grown in the Grand Prairie in 1904. The area was quickly recognized as an excellent area for growing rice. The Prairie was a treeless plain with gentle relief, moderate climate, abundant groundwater, and a claycap that was resistant to percolation of irrigation water. The rice was irrigated from the shallow Mississippi River Alluvial Aquifer.

As early as 1910, more water was being withdrawn from the aquifer than was being naturally recharged. Water levels began to decline. By 1954, a cone of depression had developed in the water table surface that was 65 feet deep. From 1910 to 1958, the water table dropped an average of one foot per year, and declines continue in many areas today. The data in this paper focuses on the Grand Prairie and Bayou Meto areas of eastern Arkansas. These study areas represent a discrete hydrologic groundwater basin, with recharge around the boundaries moving inward toward the areas of highest use.

The Mississippi River Alluvial Aquifer is at or near the surface in all of eastern Arkansas and portions of seven other states. The aquifer stretches from near Cairo, Illinois to New Orleans, Louisiana. The aquifer was formed when rivers, swollen with glacial meltwater migrated back and forth across the delta.

The Mississippi River Valley is an area of downwarping, or settling, followed by the sedimentation of successive layers of unconsolidated sands, clays, silts and lignite. The primary aquifer is the Mississippi River Alluvial Aquifer found near the surface. The alluvium bottoms out in the Cook Mountain Formation, which is a gumbo clay. Below the Cook Mountain is another aquifer with potable water; the Sparta Sand Aquifer. North of I-40, the Sparta Sand, Cane River and Carrizo Sands become less distinguishable and are grouped to form the Memphis Sand.

An east-west cross-section of the alluvial aquifer (figure 1) from the Arkansas River to Marvell illustrates the shape of the water table and variability of the different layers. The alluvium is essentially confined between a clay cap above and the Cook Mountain clay below. The shape of the water table in the dewatered zone illustrates the effects of large withdrawals exceeding the rate of recharge. Recharge is moving toward Stuttgart from the Arkansas River on the western border, and from the White River on the eastern border. Over 67 % of the recharge to the Grand Prairie Study Area originates from the White and Arkansas Rivers. Another 15 % comes in from the Fall Line to the north and 18% percolates through the clay cap. It takes almost a year for water entering the aquifer from the White River to travel to Stuttgart, if it is not intercepted by wells on the way.

Figure 1

Click on Image to enlarge. (24KB)

The Mississippi River Alluvial Aquifer consists of interfingered lenses of clay, silt, sand and gravel. In the heart of the Grand Prairie, the alluvial aquifer is almost uniformly covered by a clay layer that varies from 2 feet to 98 feet thick. In the Grand Prairie, aquifer thickness varies from 38 to 165 feet thick with an average of 100 feet. The alluvial aquifer with coarse sand and gravel in the basal portion of the unit, allow for the rapid movement of water. Conductivity values (ease of movement through a porous media) average 270 feet per day. The saturated thickness on the western side, near Stuttgart is less than ten feet. Saturated thickness is greater as one goes eastward. For approximately five miles, the aquifer contains only about 20-25 feet of saturated material, increasing to over 50 feet near the White River. When 50% of a water table aquifer has been depleted, it may be designated as "critical" by the State of Arkansas. The shape of the water table (figure 2) is a classic case of remote recharge sources in response to a zone of excessive withdrawals. Water cannot move through the aquifer fast enough to keep up with demand. Therefore, water is removed from storage and a cone or trough of depression results. The trough of depression stretches from just west of DeWitt to near England. Towns in the heart of the depression include: Lonoke, Carlisle, Hazen, Stuttgart, Almyra, and DeWitt. Historically, water levels were less than 20 feet below the surface. The depth to water in the trough of depression, today, is over 120 feet.

Figure 2

Click on Image to enlarge. (37KB)

While the cone of depression has evolved over nine decades, the growth has not been uniform. In (figure 3), the 110 foot potentiometric (water table) contour was selected for different years to show the growth rates of the trough of depression. The 110 foot contour is approximately equal to 100 feet to water. The progression illustrates the areas of the greatest growth which is on the northwest limb of the trough, moving toward Lonoke at a rate of 3300 feet per year. Growth, southward toward Dewitt has averaged 1500 feet of advance per year. Side growth of the trough, east and west has averaged 450 feet per year. In 1938, only 6,700 acres were within the 110 foot potentiometric contour and in 1996, 140,000 acres were contained within the contour.

Figure 3

Click on Image to enlarge. (39KB)

The elevation of the water table varies seasonally and annually in response to withdrawals and precipitation. By measuring the depth to water at approximately the same time each year, we can compare the data and denote change. This graph compares spring 1991 water levels to the spring of 1996. Some areas have rebounded: south of DeValls Bluff and Casscoe, north of Lodge Corner and Gillett. All other areas declined. The areas of greatest decline in the past five years have been between Carlisle and Lonoke, and between Slovak and Stuttgart. This area is of concern, because it represents further growth of the trough of depression toward Lonoke and further erosion of the storage in the "Humnoke Hump" or mound.

The well hydrograph in figure 4 is located five miles northwest of Lonoke and represents the more recent growth in the evolution of the trough of depression in the Grand Prairie and Bayou Meto Study Areas. This hydrograph covers the period from 1937 to 1989. The water level in this well has declined 55 feet in 52 years. Depth to water has gone from 50 feet to water in 1937 to 105 feet to water in 1989. While spring levels are up and down over time, the overall trend is downward. This hydrograph is typical of the initial overdraft impacts when starting with a near saturated aquifer and mining water for several decades.

Figure 4

Click on Image to enlarge. (17KB)

Hydrographs from the Stuttgart area represent water levels in in the heart of the trough of depression and illustrate the impacts of overdraft when most of the saturated thickness has been removed by overpumping. The steep, linear curve of declines is over as illustrated in the previous graphic. When saturated thickness diminishes, water levels tend to fluctuate more erratically. During the last 34 year period, 1961 to 1996, the water table has only declined 3 feet. Periods of decline have been followed by temporary rebound and vice versa. The overall long term trend is still downward but at a slow rate.

It has been estimated, depending on many factors such as well construction techniques, that 25 to 30 feet of saturated thickness is the minimum required to maintain a well at 500 gallons per minute (gpm). Saturated thickness in the study area in the spring of 1992 were less than 20 feet in the heart of the cone. Many data points were less than 5 feet of saturated thickness. A large elongated zone contained less than 40 feet of saturated thickness.

Several projections have been made on the longevity of the Mississippi River Alluvial Aquifer in the Grand Prairie. A large area covering most of the Grand Prairie and Bayou Meto are projected to have insufficient saturated thickness to support wells for irrigation in the year 2022. These projections predicted the growth of the cone of depression moving toward Lonoke and England. These predictions are now coming true.

Figure 5 shows the location, depth and time drilled for Sparta Sand/Memphis Sand wells in the Grand Prairie. The depleted areas in the alluvial aquifer are readily apparent. Deep wells, up to 1000 feet have been drilled into the Sparta Sand for decades as the shallow alluvial aquifer became depleted and unreliable. Over 200 wells in the Sparta Sand are depicted on the graphic and arranged by the decade in which they were drilled. Sparta wells completed in the seventies are in the heart of the trough of depression and toward the southern end. Wells completed in the eighties are fairly scattered throughout the problem area. The wells drilled in the nineties have been mostly on the northwestern front of the trough. Note that the red dots (1990-1996) only cover a six year period compared to ten year increments for the other colors. The growth of the cone of depression in the alluvium has accelerated in the '80's and '90's northward by Hazen, Carlisle, and Lonoke as depicted by the growth of deep well development.

Figure 5

Click on Image to enlarge. (43KB)

The Sparta Sand has been identified by many as the long term alternative groundwater supply to the depleted alluvial aquifer. Nothing could be further from the truth. The alluvial aquifer has 30 times the porosity (or ability to store water), the Sparta Sand has, per unit volume. The ease at which water can move through an aquifer is called conductivity. Conductivity values for the alluvial aquifer average 270 ft/day, while the Sparta equals 35 ft/day. In other words, it is eight times easier for water to move through the alluvium than the Sparta Sand. In addition to aquifer differences, there will be competition for water in the Sparta Sand by municipalities, industry and agriculture. Arkansas state law places priority on public water supply over industrial and agricultural uses.

The Sparta Sand was modeled by the US Geological Survey in 1988. Results of the computer simulation predicted declines of 80 feet in the potentiometric surface between 1992 and 2005 in the Grand Prairie if irrigation withdrawals continue to increase. The Sparta Sand Aquifer is not predicted to maintain irrigated agriculture in the Grand Prairie.

< Back to Ground Water