Southeast Lowlands Groundwater Province
Figure 1 - Physiograpic map of the Southeastern Lowlands groundwater province. (Data source: U.S. Geological Survey)
The Southeastern Lowlands groundwater province is a 3,916 square mile area in the southeastern part of Missouri (Fig. 1). This area has an extremely high agricultural production. Excellent soils, a favorably long growing season and the highest average annual precipitation in the state are major factors contributing to its agricultural excellence, but the groundwater resources in this region are what makes it consistently a major grain and fiber producer. Like in most areas of the state, corn, soybeans and other small grains are produced here, but rice and cotton are also extensively produced in this region.
Other Missouri groundwater provinces may have higher total volumes of groundwater in storage, but the Southeastern Lowlands contains the greatest volume of groundwater per unit area of any of Missouri’s groundwater provinces. Parts of the St. Francois and Ozark aquifers are usable in the northwestern part of this province between Crowley's Ridge (a northwest-trending line of low hills across northern Stoddard and Scott counties) and the Ozark Escarpment (the abrupt topographic change where the flat alluvial plain meets the hills of the Ozarks.) However, most of the usable groundwater is contained in thick deposits of shallow alluvium and deeper Tertiary- and Cretaceous-age unconsolidated sands. About 15.2 percent of the state’s groundwater, an estimated 75.8 trillion gallons, is found in this southeastern corner of Missouri (Fig. 2).
At first glance, the groundwater resources in this province may seem infinite. Groundwater is very abundant here and in most places easily obtainable, but the resource is certainly not limitless. If groundwater use exceeds recharge to the aquifer, the amount of groundwater in storage will decrease and groundwater levels will decline. Despite the large volume of groundwater use, significant water-level declines have not occurred. In fact, historically, this lowland area has had more problems due to excessive water than inadequate water. When first settled, much of this province was characterized by tupelo gum and bald cypress swamps (Fig. 3). Its agricultural value was only realized after massive drainage projects were finished in the early to middle 1900s. A series of north-south drainage ditches were constructed within the Little River watershed to channel surface water and groundwater draining from the alluvial aquifer south into Arkansas, then east into the Mississippi River. The Headwaters Diversion Channel was constructed across the northern part of the lowlands parallel to the Ozark Escarpment. It captures runoff from the Castor and Whitewater Rivers, which drain the Ozark province north of the lowlands, and channels it into the Mississippi River just south of Cape Girardeau. Formerly, this water flowed south into the Little River and south across the poor-draining lowlands. Two large reservoirs, Clearwater and Wappapello, were constructed across the St. Francois and Black rivers, respectively, to capture spring runoff and release it more slowly later in the year when conditions are drier. The results of these drainage projects created the most prolific agricultural area of the state (Fig. 4).
The Southeastern Lowlands is a structurally active area. Few Missourians have not heard of the New Madrid seismic zone and the earthquakes that devastated the region in the early 1800s. Structural deformation began in this area many millions of years ago, and downwarping created a northeast trending structural trough. As subsidence occurred in the older bedrock, a thick sequence of younger Cretaceous and Tertiary sediments were deposited on the older bedrock surface as shallow seas transgressed over this area. The bedrock strata are not nearly horizontal here like they are in much of the state, but rather dip to southeast, becoming deeper as you move south. There is nearly 3,000 feet of relief between the top of the Paleozoic rocks in southeast Pemiscot County and the Ozark Escarpment in the northern end of the lowlands. Fig. 5 is a north-south geologic cross-section of the Southeastern Lowlands.
The Ozark and St. Francois aquifers underlie this area, but only in the area west and north of Crowley’s Ridge and the Benton Hills are they considered usable aquifers. Many private domestic and public water supply wells in this part of the province produce from the deeper bedrock rather than the shallow alluvial aquifer. The alluvial aquifer, though very productive, contains water that has excessive dissolved iron and manganese. The Ozark aquifer that underlies it has a lower yield, but generally produces better quality water. Bedrock dips away from the St. Francois Mountains, and south and east of Crowley's Ridge the Ozark aquifer becomes too deep for most water wells to access. Water quality is also known to naturally deteriorate east and south of Crowley's Ridge, but the location of the freshwater-salinewater transition zone is not precisely known.
The greatest groundwater resources in this region are the aquifers associated with the Cretaceous, Tertiary and Quaternary sedimentary deposits. The Cretaceous-age McNairy Formation forms an aquifer that overlies the bedrock surface throughout most of the lowlands south and east of Crowley's Ridge. The unit crops out along Crowleys Ridge, and in parts of Stoddard and Butler counties it underlies the alluvial materials. Thus, relatively shallow wells can access the unit in the northern part of the province, but in southern Dunklin and Pemiscot counties wells 2,000 feet deep or greater are needed. Yields vary considerably, but generally range between 150 and 750 gallons per minute where the sands are clean and relatively thick. Several thick low-permeability units including the Clayton and Owl Creek formations and the Porters Creek Clay overlie the McNairy and form a confining unit. Thus, many wells drilled into the McNairy south and east of Crowley's Ridge are flowing artesian wells. In the northern part of the lowlands, private domestic wells as well as public water supplies use the McNairy. To the south, where the unit is deep in the subsurface, its users are almost exclusively municipalities and rural water districts.
The quality of water from the McNairy varies with location. In the northern part of the province and along Crowley's Ridge, the water is harder and contains more calcium, magnesium, bicarbonate, and iron than it does farther south. To the south, it typically is very soft, contains little iron, calcium, and magnesium but contains greater quantities of sodium and potassium. In part of eastern Stoddard, northern New Madrid, southern Scott, and western Mississippi counties, the McNairy yields water that is a sodium-chloride type, that is, without extensive treatment, too highly mineralized for most uses. The McNairy receives recharge along Crowley's Ridge where the Porters Creek Clay is absent, and where it is directly overlain by alluvium along the west side of Crowley's Ridge.
The Wilcox Group, a series of Tertiary-age sands unit containing minor clay and lignite, overlies the Porters Creek Clay and forms the Wilcox aquifer. The unit thins to a featheredge in the northern part of the lowlands along Crowley's Ridge and thickens to the south, reaching a maximum thickness of about 1,400 feet in the southern end of the bootheel. It underlies the shallow alluvial aquifer throughout about 60 percent of the Southeastern Lowlands. The water in the Wilcox is mostly a calcium-bicarbonate or calcium-magnesium-bicarbonate type, and it commonly contains relatively high levels of iron and manganese. Yields of wells that produce from it are generally good, and range from less than 200 gallons per minute near its outcrop area to more than 1,700 gallons per minute where the unit is thick. Municipalities and rural public water supply districts mostly use the Wilcox. It typically has lower and more constant iron and magnesium levels than the alluvial aquifer, making it more attractive for public water supply than the alluvial aquifer because of easier treatment. Ample water is available for agricultural use from the shallow alluvial aquifer so it is rarely necessary to produce water from the Wilcox for irrigation. The Wilcox receives recharge from rainfall on Crowley's Ridge, and leakage from the overlying alluvial aquifer, which is in direct contact with the Wilcox throughout most of the area.
The Southeast Lowlands alluvial aquifer is the most prolific and widely used aquifer in this groundwater province. This aquifer underlies about 92 percent of the Southeastern Lowlands, and consists of unconsolidated clay, silt, sand and gravel that were deposited by the ancestral Mississippi and Ohio rivers as well as the Black, St. Francis, and Little rivers. West of Crowley's Ridge the alluvium is typically less than 150 feet thick, but in parts of Mississippi, Dunklin, and Pemiscot counties, it can exceed 250 feet in thickness (Fig. 6). It is absent on Crowley's Ridge.
Figure 6 - Thickness of the Southeastern Lowlands alluvial aquifer.
Properly constructed wells in the alluvial aquifer are capable of producing large volumes of water with little drawdown. Wells drilled into the aquifer commonly produce 1,000 to 2,000 gallons of water per minute. Nearly 50 years of water-level measurements from several observation wells show that water levels fluctuate between wet and dry seasons, but there is no indication of significant long-term water-level decline in this aquifer. Numerous towns use water from the alluvial aquifer, but most of the water produced from it is used for agricultural purposes. Much of the land between Crowley's Ridge and the Ozark Escarpment in Butler and Ripley counties is flood irrigated and used for rice production (Fig. 7). Corn and other small grains along with cotton dominate the rest of the area and are generally watered using center pivot irrigation systems (Fig. 8). The U.S. Department of Agriculture 2002 Census of Agriculture showed 881,582 acres of irrigated land in the Southeastern Lowland counties. This represents over 85 percent of the total irrigated land in Missouri. The census contained no information on water application rates, but if an average application rate of 12 inches of water per year is assumed, then irrigation in this area would use about 287 billion gallons of water annually.