Springfield Plateau Groundwater Province
The Springfield Plateau groundwater province occupies the southwestern part of the state and a small region of central Missouri south of the Missouri River. Thick Mississippian-age limestones and cherty limestones form the bedrock surface in the region and overlie the same Ordovician and Cambrian strata found in the Salem Plateau. Mississippian-age strata are also found south of the Mississippi River in St. Louis, Jefferson, Ste. Genevieve, Perry and Cape Girardeau counties. However, the eastern Missouri occurrences of these strata are not considered to be in the Springfield Plateau physiographic province.
As with the Salem Plateau, the sedimentary rock sequence in the Springfield Plateau rests atop Precambrian-age igneous and metamorphic rocks. The hydrogeologic significance of these basement units is that they serve as a confining unit and do not allow a significant interchange of groundwater. The Precambrian rocks are overlain by up to 350 feet of Cambrian-age Lamotte-Reagan Sandstone. This unit in southwestern Missouri is similar to that farther east except it contains less arkosic materials in this area. Its average thickness is about 200 feet. It is overlain by the Bonneterre Formation, which contains up to about 380 feet of dolomite. The unit thins in the western part of the province, and may be locally absent at the state line. The Davis Formation overlies the Bonneterre and reaches a maximum thickness of about 100 feet in the province. In the eastern counties it contains a significant percentage of shale, but to the west the unit is principally limestone (figure 1).
The Derby-Doerun Dolomite rests upon the Davis, and ranges in thickness from zero to 50 feet in the region. In turn, it is overlain by the Potosi Dolomite, which is up to about 50 feet thick. The overlying Eminence Dolomite, the youngest Cambrian-age unit in the sequence, is up to about 350 feet thick in the region. In general, the Eminence and Potosi are thickest in the eastern part of the province and thin locally to as little as 150 feet in combined thickness near the western border of the state.
One of the few sandstone units found in southwestern Missouri overlies the Eminence. It is the Ordovician-age Gunter Sandstone member of the Gasconade Dolomite, and it consists of 5 to about 45 feet of sandstone and sandy dolomite. The remainder of the Gasconade Dolomite, informally subdivided into upper and lower units, ranges from 300 feet thick near the western border of the state to about 450 feet at the eastern margin of the Springfield Plateau. The overlying Roubidoux Formation consists of about 140 to 210 feet of dolomite, dolomitic sandstone, and sandstone. The Jefferson City Dolomite, which averages about 210 feet in thickness, and the Cotter Dolomite, 175 to 400 feet in thickness, are the youngest Ordovician units throughout most of the region, although up to about 80 feet of Powell Dolomite occurs near the Arkansas border. The combined thickness of these three units reaches more than 700 feet in southern Barry County, but thins substantially to the north, decreasing in thickness to about 250 feet in the northern counties of the province.
In most areas of the Springfield Plateau province the Cotter Dolomite is unconformably overlain by the Mississippian-age Compton Formation; Silurian and Devonian-age strata are absent. The exception is in the extreme southwestern part of the state. In southernmost Newton, western Barry, and nearly all of McDonald County the Devonian-age Chattanooga Shale separates the Ordovician and Mississippian strata. The Chattanooga consists of up to about 30 feet of black, organic shale. The shale commonly has an oily smell, and in places small quantities of crude oil have been found with the unit.
Numerous Mississippian-age units crop out across southwestern Missouri. The Compton Formation overlies the Cotter (or Chattanooga in the extreme southwestern part of the area) and consists of 5 to about 50 feet of limestone with minor shale. Locally, a few feet of Sedalia Formation, a carbonate unit similar to the Compton, is found. Above that is the Northview Formation, which is principally a shale and siltstone unit. Above the Northview is the Pierson Limestone, which ranges in thickness from about 5 to about 55 feet. The Reeds Spring Formation, which consists of up to 100 feet of very cherty limestone, overlies the Pierson. Some 30 feet of Elsey Formation, which is lithologically similar to the Reeds Spring, overlies it. The Burlington-Keokuk Limestone, the most widespread and thickest Mississippian-age unit in the region, overlies the Elsey and forms the bedrock surface throughout much of the region. The Warsaw Formation overlies the Burlington-Keokuk in the northwestern part of the province, and consists of 100 feet or so of limestone and minor shale.
Pennsylvanian-age strata rest atop the Mississippian units in the northwestern part of the province, and also occur as isolated outcrops at some of the higher elevations in the central part of the region. The Pennsylvanian materials are mostly cyclic deposits of shale, limestone, and sandstone with some coal beds. These are not significant water-bearing units in southwestern Missouri, but rather form an effective confining unit where they are sufficiently thick and laterally continuous.
Hydrogeology and Groundwater Resources
There are three significant bedrock aquifers in the Springfield Plateau Groundwater Province. Like in the Salem Plateau and St. Francois Mountains, the Bonneterre Formation and Lamotte (Reagan) Sandstone form the St. Francois aquifer in southwestern Missouri. The Precambrian-age igneous and metamorphic rock beneath the St. Francois aquifer has very low hydraulic conductivity and form the basement confining unit. Because of its depth and generally modest yield potential, few wells produce from the St. Francois aquifer in the province. The City of Sedalia, while not in southwestern Missouri, is on the Springfield Plateau and has several wells that bottom in the St. Francois aquifer. In the Sedalia area, the Lamotte (Regan) Sandstone adds probably 100 to more than 200 gallons per minute to the yields of the wells which also are open to the Ozark aquifer. A few industrial wells in the Springfield area are open to the St. Francois as well as the Ozark aquifer, and in the Joplin area some wells that had low production from the Ozark aquifer were deepened to take advantage of what additional yield was supplied by the St. Francois.
The Davis Formation forms the St. Francois confining unit between the St. Francois and Ozark aquifers. The Davis in the eastern part of the province contains more shale than to the west where it is principally a carbonate unit. Nonetheless, it does serve to limit groundwater interchange between the zones above and below it.
The most widely used and important groundwater supply source in the Springfield Plateau groundwater province is the Ozark aquifer. The geologic formations comprising this aquifer are the Derby-Doerun, Eminence, Potosi, Gasconade, Roubidoux, Jefferson City, and Cotter. Most, of course, are dolomite units except for the sandstones of the Roubidoux and Gunter Sandstone member of the Gasconade. This aquifer varies in thickness from locally as little as 600 to 800 feet in northwestern Barton County and parts of St. Clair County, to about 1,600 feet in parts of Stone and Barry counties. The average thickness is about 1,200 feet (figure 2).
Figure 3. Stratigraphic units comprising the Springfield Plateau and Ozark aquifers in southwestern Missouri and yield estimates.
Low-permeability units between the Ozark aquifer and the shallower Springfield Plateau aquifer form an aquitard and greatly limit the vertical interchange of water between the two aquifers. Throughout most of the region, the Compton Limestone and Northview Formation form the Ozark confining unit. Although these units have low hydraulic conductivities, they do allow some water to move through them. The volume of water that can move from one aquifer to the other depends on the hydraulic conductivity of the confining unit, and the water-level differences between the two aquifers. The volume of water movement is relatively small per unit area, but is a much larger amount when the size of the region is considered. The Chattanooga Shale is part of the confining unit in McDonald County and parts of adjacent Barry and Newton counties. Because of the very low hydraulic conductivity of the Chattanooga, the amount of water moving through the confining unit where the Chattanooga is present is likely much less than in other areas.
Mississippian-age strata comprise the Springfield Plateau aquifer, which is widely used as a private water-supply source in this province. Yields of wells producing from the Springfield Plateau aquifer are typically less than about 20 gallons per minute. Like in the Salem Plateau, dissolution of the limestone bedrock by slightly acidic groundwater has created numerous karst features such as sinkholes, losing streams, caves and springs. These karst features can create pathways for rapid groundwater movement. Losing streams and sinkholes are recharge features that rapidly funnel large quantities of surface water underground. The water follows well-defined flow paths through solution enlarged fractures, conduits, and cave openings. Springs provide the outlet points for much of this shallow groundwater. These features are particularly well developed in parts of Greene and Christian counties, so much so that private wells drilled in much of Greene and northern Christian counties since 1987 were constructed to exclude production from the Springfield Plateau aquifer. More recently, similar construction restrictions have been placed on private wells in parts of Jasper and Newton counties due to the widespread presence of excessive lead and cadmium concentrations in shallow groundwater associated with past mining or because of trichloroethylene contamination in a large area just south of Neosho.
Groundwater recharge in this province depends on location and aquifer. The Springfield Plateau aquifer is chiefly recharged by precipitation. Some of the recharge is the gradual downward infiltration of water from precipitation through the soil materials, into the shallow bedrock, until it reaches the water table. There are likely few areas left in the province where the water level in the Springfield Plateau aquifer is at a lower elevation than the potentiometric surface of the Ozark aquifer, so upward leakage of water from the Ozark aquifer into the Springfield Plateau aquifer is not a major source of recharge to the shallower zones.
Recharge to the Ozark aquifer is more complicated. It may seem logical that groundwater flowing through the Ozark aquifer beneath the Springfield Plateau could be provided by recharge to the east, where the Ozark aquifer is unconfined and can be recharged directly from precipitation. However, such is not the case. The direction of groundwater movement in an aquifer can be determined using a potentiometric map. A potentiometric map uses contours lines drawn at particular elevations to show the water-level elevation in an aquifer. In a confined aquifer like the Ozark aquifer in southwestern Missouri, the contour lines show the elevation to which water in a tightly cased well penetrating the aquifer will rise. Groundwater moves down gradient and at right angles to the potentiometric contours. Potentiometric maps of the area show that very little groundwater enters the Ozark aquifer in the Springfield Plateau from the Salem Plateau (figures 4 and 5). A notable exception is the Springfield area, where a large cone of depression created by pumping exists. Potentiometric maps show that some water in the Ozark aquifer in the Salem Plateau in Webster and northeastern Greene counties moves toward the Springfield pumping cone. However, north of the Springfield pumping cone, water in the Ozark aquifer moves northward toward the Sac and Osage rivers. South of the pumping cone it moves southward toward the White River. Potentiometric maps show a major groundwater divide in the Ozark aquifer extending northward in west-central Barry County, across eastern Lawrence County and northwestern Greene County. East of this divide, groundwater in the Ozark aquifer either moves toward the White River valley, or into the Springfield pumping cone. West of the divide, groundwater in the Ozark aquifer moves westerly toward Oklahoma and Kansas. Based on the potentiometric maps, there can be no recharge to the Ozark aquifer in the counties west of the groundwater divide from the Salem Plateau. Essentially all of it must come from downward leakage of water from the Springfield Plateau aquifer, through the Ozark confining unit, and into the Ozark aquifer.
Figure 4. Pre-development potentiometric map of southwestern Missouri (data source: USGS Hydrologic Atlas HA-711-E, Sheet 3). Contour lines show potentiometric surface elevation in feet above mean sea level.
Figure 5. Composite 2006-2007 Potentiometric map of southwestern Missouri (data source: USGS Scientific Investigations Report 2007-5253, USGS Scientific Investigations Map 3003, and U.S. Army Corps of Engineers Planning Assistance to States project [in press]). Contour lines show potentiometric surface elevation in feet above mean sea level.
Groundwater in storage in this province is estimated to be about 122.5 trillion gallons, or about 24.5 percent of the usable groundwater in Missouri. The resource is not evenly distributed between the three bedrock aquifers. The St. Francois and Springfield Plateau aquifers are estimated to contain 4.1 trillion gallons and 5.7 trillion gallons, respectively. The Ozark aquifer is estimated to contain about 112.6 trillion gallons of water, or about 92 percent of the groundwater in the province.
Natural groundwater quality in the Springfield Plateau region is generally very good. The water from all three aquifers typically meets primary and secondary maximum contaminant levels for drinking water. The major differences in water quality between the Springfield Plateau aquifer and deeper Ozark and St. Francois aquifers is the degree of mineralization and the dissolved constituents present. Water from the Springfield Plateau aquifer typically has the lowest levels of mineralization, and is a calcium-bicarbonate type of water, reflective of the limestone that the water has been in contact with. Water produced from the Ozark and St. Francois aquifers generally has a higher total dissolved solids level, and is a calcium-magnesium-bicarbonate type as a result of the dolomite bedrock. Because of the ease of which recharge occurs, water in the Springfield Plateau aquifer is more likely to be affected by waste products, and shallow wells not cased through the aquifer are more likely to produce water containing bacteria. Wells cased through the Springfield aquifer are much less likely to experience water-quality problems associated with surface activities, particularly public water supply wells that are constructed to much more stringent specifications than private wells.
The northwest boundary of the Springfield Plateau groundwater province abuts the Osage Plains groundwater province. This boundary is based on a major water quality change and is termed the freshwater-saline water transition zone. It is approximately where the dominant water quality anion in the three aquifers changes from bicarbonate to chloride. It is also where the total dissolved solids content reaches about 1,000 mg/L, the dividing line between brackish water and freshwater. North and west of this zone the St. Francois, Ozark, and Springfield plateau aquifers produce water that, without extensive treatment, is too mineralized to be considered potable.
Groundwater-Level Decline and Water Use
Nowhere in Missouri has water supply in general, and specifically groundwater use and availability, become such an important issue as in southwestern Missouri. If you consider that the Springfield Plateau groundwater province occupies an area of about 8,700 square miles, or about 12.5 percent of the state, but has nearly 25 percent of the groundwater resources, it would seem that water availability should not present a problem. However, water use in this area is relatively high, the competition for water is greater than in many areas of the state, and the demand for water is not evenly distributed. Depending on county, major groundwater users in southwestern Missouri include industry, agribusiness, municipalities, irrigation, and electrical generation. Major municipal groundwater use areas include the Springfield/northern Christian County area, the Joplin area, and Monett. Groundwater is used extensively for irrigation in northern Jasper, western Dade, and much of Barton counties. Agribusinesses in Barry and McDonald counties also use large quantities of groundwater, primarily for poultry production and processing. Likewise, electrical generation, principally in Jasper and Greene counties, is a major user of groundwater. The most recent water-use data reported for the southwestern Missouri counties show a groundwater use increase of about 37 percent between 2000 and 2006. Nearly all of this water is produced from the Ozark aquifer.
In areas such as southwestern Missouri where groundwater use is quite high and recharge is limited due to geologic conditions, groundwater-level declines have occurred. However, the magnitude of the declines varies greatly across the region. Groundwater-level changes have been documented and quantified two ways. Groundwater levels have been monitored in Missouri for many years using dedicated groundwater-level observation wells. The oldest stations, including several in southwestern Missouri, were installed in the late 1950s. Expansions to the network in 1999-2000 and again in 2007-2008 have increased the number of wells state-wide to more than 140. About 30 of these monitor groundwater levels in the Ozark aquifer in southwestern Missouri. Although the term “static water level” is commonly used, it does not mean that groundwater levels are stationary. It refers to the non-pumping water level in a well open to a particular aquifer. All of the groundwater-level observation wells are non-pumping wells, and continually measure “static water level” at the wells locations. Depending on location, water levels in the Ozark aquifer in southwestern Missouri can be expected to fluctuate as little as 15 feet per year, or more than 150 feet per year (figure 6). This is because groundwater use is heavily influenced by season as well as by weather. Most municipalities use far less water during winter months than during the summer. Industrial water use may be relatively constant or seasonal, depending on the industry. The irrigation season in southwestern Missouri typically lasts less than 3 months, beginning in early July and ending in early September. In a wet year, much less water is needed for optimum crop production than in a dry year, and different crop types have different water needs. Electrical generation, and thus the water needed for electrical production, also varies with season.
Figure 6. Typical yearly Ozark aquifer water-level fluctuations in feet measured from area groundwater-level observation wells. (Single values are wells with less than 2 years of record.)
Groundwater-level changes have also been documented using potentiometric maps. As previously mentioned, a potentiometric map is basically a contour map that shows the elevation to which water levels occur in an aquifer. These maps are constructed by measuring water levels from wells that penetrate the aquifer of interest. When dealing with confined aquifers like the Ozark aquifer in southwestern Missouri, it is very important to have good construction information for the wells whose water-level measurements are used to construct a potentiometric map. To be of the greatest value, the well should have tightly cemented casing set through the Ozark confining unit. Most wells used for potentiometric map measurements are production wells that are pumped at least periodically. The water-level measurement should not be taken until water level has, as much as possible, stabilized after a pumping period. The water-level measurements should all be made in as short a time period as possible. Unlike water-level measurements taken from dedicated observation wells which show water-level changes over a long period of time at a specific location, a potentiometric map shows a snapshot view of water-level distribution in a large area during a specific time period. Because groundwater levels fluctuate throughout the year, a potentiometric map made from data collected during one part of the year may look considerably different than a potentiometric map of the same area made from data collected during a different part of the year. In general, groundwater levels in southwestern Missouri are shallowest in the late spring and early summer, and deepest in the late summer and early fall. They are shallower during relatively wet years when water use is less, and deeper during droughts when more groundwater is produced.
Two potentiometric maps are needed to quantify long-term water-level changes. Until recently, the only regional Ozark aquifer potentiometric map for southwestern Missouri was a pre-development potentiometric map produced by the U.S. Geological Survey (Hydrologic Atlas HA- 711-E) in 1990. The pre-development map was intended to depict groundwater levels in the Ozark aquifer prior to extensive pumping. It made use of the initial water-level information collected mostly from municipal and other wells when they were first drilled in an area. More recently, a series of studies have produced potentiometric maps of parts of southwestern Missouri and adjacent parts of northeastern Oklahoma and southeastern Kansas that, when combined, depict a more current view of the potentiometric surface. Two of the studies are by the U.S. Geological Survey (U.S.G.S. Scientific Investigations Report 2007-5253 and Scientific Investigations Map 3003). The third, still in press, was produced by workers at Missouri State University as part of a Planning Assistance to States project by the Corps of Engineers, in cooperation with the Missouri Department of Natural Resources.
The potentiometric maps from the three studies were scanned, geo-referenced, and combined to produce a composite potentiometric map of the Ozark aquifer that reflects current winter-spring water-level conditions. The pre-development potentiometric surface map was also scanned and geo-referenced to produce a digital version of the map. Using a Geospatial Information System (GIS), water-level values from the most recent potentiometric map were compared to values from the pre-development potentiometric map to produce a water-level change contour map. That map was further processed to produce a series of five maps that depict the magnitude of water- level change across southwestern Missouri. The maps show areas of 1) essentially no water- level decline between pre-development and present (figure 7); 2) areas where less than 100 feet of decline have occurred (figure 8); 3) areas where 100 to 200 feet of decline have occurred (figure 9); 4) areas where 200 to 300 feet have occurred (figure 10); and 5) areas where more than 300 feet of decline have occurred (figure 11).
Figure 8. Areas in southwestern Missouri with less than 100 feet of water-level decline in the Ozark aquifer.
Figure 9. Areas in southwestern Missouri with 100 to 200 feet of water-level decline in the Ozark aquifer.
Figure 10. Areas in southwestern Missouri 200 to 300 feet of water-level decline in the Ozark aquifer.
Figure 11. Areas in southwestern Missouri with more than 300 feet of water-level decline in the Ozark aquifer.
The potentiometric maps show that well over half of southwestern Missouri, particularly the eastern part of the region, has experienced less than 100 feet of groundwater-level decline in the Ozark aquifer. Much of the rest of the region, mostly in its western part, has experienced 100 to 200 feet of decline. The areas of greatest groundwater-level decline in the Ozark aquifer are in and near major pumping centers. The largest area with more than 300 feet of decline is not in Missouri, but in the Miami, Oklahoma area. Combined, all of the 300-ft or greater decline areas in southwestern Missouri are considerably less than that around Miami. In Missouri, the largest areas are in the Springfield and Joplin areas, with smaller areas in McDonald, Barry, Stone, and Taney counties.
Water-level decline in an aquifer is never desirable, but it is a relatively common occurrence, especially in heavily used aquifers. The question that commonly arises is at what point does water-level decline become a serious concern? Long-term observation well data, as well as comparison between pre-development and current potentiometric maps, indicate that there has been at least some water-level decline in much of the Ozark aquifer in southwestern Missouri, but is the degree of decline simply an interesting phenomena, or genuinely of concern? To help answer this question it is necessary to understand how groundwater-level changes affect an aquifer and its ability to yield water.
Aquifers are broadly classified as either confined (also termed “artesian”) or unconfined (also termed “water table”) aquifers. For example, the Springfield Plateau aquifer in most of southwestern Missouri is considered an unconfined aquifer. It has no low-permeability confining unit at its top; recharge from precipitation can enter the aquifer with relative ease. The top of the aquifer is neither land surface nor the top of a particular geologic formation; rather, the top of the aquifer is the water table. The water table is the relatively flat, two-dimensional boundary between the rock above whose pore spaces are not saturated with water and the rock below whose pore spaces are saturated with water. As recharge enters an unconfined aquifer, the water table rises. As water is drained from an unconfined aquifer via streams or pumped from wells, the water table drops. A well drilled into an unconfined aquifer such as the Springfield Plateau aquifer will begin to encounter water at the water table, and water level in the completed well will be approximately the elevation of the water table at that location.
The ability of an aquifer to produce water is dependent on its transmissivity. The transmissivity of an aquifer depends on two other factors. One is the hydraulic conductivity of the aquifer and the other is its saturated thickness. Using English units, hydraulic conductivity can be simply defined as the amount of water that a one square foot cross-section of an aquifer can yield if it is under a hydraulic gradient of 1 (in other words, a potentiometric surface that changes 1 foot vertically for each foot of horizontal change). In an unconfined aquifer, the saturated thickness is the vertical extent of the saturated part of the aquifer, or in other words the distance from the water table to the base of the aquifer. The transmissivity is equal to the hydraulic conductivity multiplied by the saturated thickness, and is the amount of water that a 1-foot wide strip of the aquifer will yield if it is under a hydraulic gradient of 1. If the water level of an unconfined aquifer declines, then the saturated thickness of the aquifer decreases, and thus the transmissivity of the aquifer decreases. If, for example, the saturated thickness of an unconfined aquifer with a uniform hydraulic conductivity decreases from 300 feet (figure 12) to 150 feet (figure 13), then the transmissivity of the aquifer will decrease 50 percent. A well attempting to produce the same volume of water from the partly dewatered aquifer would experience considerably more drawdown than it would have when the aquifer had a saturated thickness of 300 feet. It creates a rather vicious circle: As saturated thickness decreases, transmissivity decreases. As transmissivity decreases, in order to produce the same quantity of water a well must create greater drawdown which further decreases the saturated thickness at the well. Not to mention the increase in pumping costs because the water has to be pumped a greater vertical distance from pumping level in the well to land surface.
Figure 12. Diagrammatic cross-section showing a relatively shallow water table in the Springfield Plateau aquifer.
The Ozark aquifer throughout most of southwestern Missouri is a confined aquifer. The water level in a tightly cased well drilled into a confined aquifer will be some distance above the top of the aquifer. The forces that cause artesian conditions are a combination of elevation change in the aquifer coupled with hydraulic pressure applied from the overlying strata. A confined or artesian aquifer does not have a water table, per se. The elevation to which water will rise above the top of a confined aquifer is called its potentiometric surface. As with the unconfined aquifer discussed above, the transmissivity of a confined aquifer is its hydraulic conductivity multiplied by its saturated thickness. However, the saturated thickness of a confined aquifer is the distance between the base of the upper confining unit, and the top of the lower confining unit. For example, if the vertical distance between the base of the Ozark confining unit and the top of the St. Francois confining unit is 1,000 feet, then that is the saturated thickness of the Ozark aquifer at that location. If the water level in a tightly cased well drilled into the Ozark aquifer at that location is, say, 300 feet above the top of the aquifer, then that would be the amount of artesian head pressure that the aquifer has at that location (figure 14). If the aquifer is pumped heavily for an extended length of time, and the water level declines to 150 feet above the top of the aquifer, the aquifer has lost 150 feet of artesian head, but is still fully saturated, and still has its original transmissivity (figure 15). The most significant change is that the pumping costs will have increased because water has to be moved vertically an additional 150 feet. However, if water level continues to decline to a point where the potentiometric surface drops below the top of the aquifer, then the aquifer will become unconfined at that location. If, say, water level drops to a point where water level is 200 feet below the base of the confining unit (figure 16), then the aquifer will have lost 20 percent of its saturated thickness, and its transmissivity will decrease accordingly.
Figure 14. Diagrammatic cross-section showing a relatively shallow potentiometric surface in the Ozark aquifer.
Figure 15. Diagrammatic cross-section showing a 150-ft decline in artesian head in the Ozark aquifer.
Water-level data has been collected from an observation well at Noel in McDonald County since 1962. This well was drilled in 1936 as part of the municipal water system, and at that time was a flowing artesian well. Water level in the Ozark aquifer at the well site was several feet above ground level and about 56 gallons of water per minute would discharge from the top of the well casing. By 1962, water level had dropped modestly, to about 48 feet below ground level. The Springfield Plateau aquifer is missing at the well site. The Chattanooga Shale is the shallowest bedrock unit and the top of the Ozark aquifer is only 70 feet below land surface. Figure 17 shows the long-term hydrograph of the Noel observation well and the relation of groundwater levels to the Ozark confining unit and Ozark aquifer. Groundwater use in the Noel area has lowered the potentiometric surface more than 400 feet, which has decreased the saturated thickness of the Ozark aquifer by about 330 feet at that location.
Figure 17. Long-term hydrograph and well site geology for the Noel observation well in McDonald County, showing the loss of saturated thickness in the Ozark aquifer due to high groundwater-use rates.
With an unconfined aquifer, any significant water-level decline will have at least some effect on the ability of the aquifer to yield water. Even though a modest loss of saturated thickness is usually acceptable, it is prudent to match the long-term regional withdrawal rate of water from an unconfined aquifer with its long-term regional recharge rate. Water use from the Springfield Plateau aquifer in southwestern Missouri is relatively low. The aquifer is not capable of sustaining yields suitable for large scale irrigation and public water supply. Most of the water that is artificially removed from the aquifer is through private domestic wells. Observation wells that monitor water-level changes in this aquifer show water-level fluctuations to be more closely tied to recharge from precipitation and drainage of water from the aquifer to supply springs and streams, and not fluctuations due to pumping.
The ability of a confined aquifer to yield water is not affected by a drop in its potentiometric surface so long as the potentiometric surface remains above the top of the aquifer. If that condition is met then the aquifer remains fully saturated and its transmissivity is not decreased. Of course, the cost of pumping water will have increased because the pumping level in the well is a greater vertical distance from land surface, so a larger horsepower pump may be required to produce the same volume of water that was pumped when the potentiometric surface was higher. A larger horsepower pump may not fit into the existing well if its diameter is too small, so in some instances a significant drop in the potentiometric surface of a confined aquifer may require a larger diameter well to be drilled to achieve the same production rate.
As previously mentioned, unlike the unconfined Springfield Plateau aquifer which receives direct recharge from precipitation, the Ozark aquifer in southwestern Missouri receives most of its recharge by downward leakage from the Springfield Plateau aquifer through the Ozark confining unit. The volume of water that can move downward through the confining unit depends on the vertical hydraulic conductivity of the confining unit and the water-level difference between the upper and lower aquifers. If the water table elevation of the Springfield Plateau aquifer is at a higher elevation than the potentiometric surface of the Ozark aquifer, then there is a downward flow potential between the aquifers. If the potentiometric surface of the Ozark aquifer is at a higher elevation than the Springfield Plateau aquifer water table elevation, then the direction of water movement across the confining unit would be upward. If we assume that the water table elevation in the Springfield Plateau aquifer remains constant, and the potentiometric surface of the Ozark aquifer declines, the rate of recharge through the confining unit actually increases because the head difference between the aquifers has increased. The maximum head difference that will impact the recharge rate is reached when the potentiometric surface drops below the base of the confining unit. At this point, the head change that impacts the vertical leakage rate through the confining unit has reached its maximum. Even with a relatively large difference in water level between the two aquifers, the leakage rate through the confining unit is not great enough to significantly impact water level in the Springfield Plateau aquifer.
Transmissivity is one of two hydrologic coefficients that describe the water yielding characteristics of an aquifer. A second coefficient is the storage coefficient or storativity of the aquifer. Using English units, the storativity of an aquifer is defined as the volume of water that can be taken into or released from storage per square foot of area with a water-level change of one foot. Confined aquifers typically have relatively small storage coefficients, in the range of 0.001 to 0.00001. The storage coefficient for an unconfined aquifer is called its specific yield. The specific yields of unconfined aquifers are much larger than the storage coefficient of confined aquifers, generally between 0.01 and 0.3. For this reason, the drawdown created by a well pumping from a confined aquifer of a given transmissivity will be much greater than that created by an identical well pumping from an unconfined aquifer having the same transmissivity. For example, if a confined aquifer has a storage coefficient of 0.0001, then a square foot of that aquifer would produce only 0.0001 cubic foot of water (or 0.00075 gallons of water) for each foot of water-level decline. An unconfined aquifer with a specific yield of 0.1 would produce 0.1 cubic foot of water (or 0.75 gallons of water) per square foot of area with each foot of water-level decline. If the potentiometric surface of a confined aquifer declines to the top of the aquifer and the aquifer becomes unconfined, then the storage coefficient actually changes in the unconfined part of the aquifer.
A criterion that can be useful in evaluating the impact of water-level decline on a confined aquifer is whether the aquifer is still fully saturated. A comparison of the pre-development potentiometric map of the Ozark aquifer in southwestern Missouri (USGS Hydrologic Atlas HA-711-E, Sheet 3) with a map depicting the elevation of the base of the Ozark confining unit (USGS Hydrologic Atlas HA-711-E, Sheet 1) shows that prior to widespread pumping, nearly all of the Ozark aquifer in southwestern Missouri was under confined conditions (figure 18).
Figure 18. Pre-development confined (blue) versus unconfined (yellow) areas in the Ozark aquifer. Green depicts the outcrop area of formations comprising the Ozark aquifer.
A similar comparison of the 2006-2007 potentiometric surface map with the base of the Ozark confining unit depicts the areas where the Ozark aquifer is currently unconfined (figure 19). Most of the areas currently unconfined are those where groundwater-level declines in the Ozark aquifer are the greatest, such as the Springfield/northern Christian County area, parts of northern and southwestern Barry County, McDonald County, and the Joplin area in southern Jasper and northern Newton counties.
Figure 19. Current (2006-2007) confined (blue) versus unconfined (yellow) areas in the Ozark aquifer. Green depicts the outcrop area of formations comprising the Ozark aquifer.
The effects of well interference, which is created by multiple closely spaced pumping wells, accentuate the problems of groundwater-level decline in southwestern Missouri. The areas where more than 300 feet of groundwater-level decline have been documented are typically areas containing numerous high-yield wells that are relatively close together. When a well is pumped, the water level in the well drops to a lower level. It is the change in water level between the well and the adjacent aquifer that induces water to move toward the well. The greatest amount of drawdown is in the pumped well, and the drawdown decreases with distance away from the pumped well. This drawdown pattern is in the shape of a cone, and is commonly referred to as a pumping cone or cone of depression. The distance from the pumped well that is affected is called the radius of influence. The size and depth of the pumping cone as well as the radius of influence are controlled by several factors including the transmissivity of the aquifer, its storage coefficient, the pumping rate, and length of pumping period. If wells are spaced far enough apart, their pumping cones do not overlap. If wells are spaced too closely together, their pumping cones will significantly overlap. This results in well interference. When wells interfere with each other, it results in greater drawdown in both wells than they would normally experience during the same pumping periods. If the wells are drilled to similar depths, and their pumps are placed at similar depths, then neither well has an advantage. However, if two wells are drilled into the same aquifer and have greatly dissimilar depths, then the deepest well with the deepest pump setting may cause the water level in the shallower well to decrease to the point where it is below the pump intake or even the bottom of the well, rendering the shallower well unusable, at least while the deeper well is operating (figure 20).
Figure 20. Diagrammatic cross-section of the Ozark aquifer showing how the pumping cone of a deep, high capacity well can affect shallower nearby wells.
A term that is sometimes used to describe the water removal rate that should not be exceeded from a particular aquifer is called its safe yield. Unfortunately, there is not a clear definition of safe yield. Terms like transmissivity, storage coefficient, porosity, and saturated thickness have technical definitions, but safe yield is somewhat philosophical. One definition of safe yield is the amount of water that can be removed from an aquifer annually without creating an undesirable effect. An undesirable effect could be many things including a change in water quality, a decline in aquifer water level, or well interference. Once again, scale plays an important factor. If the total volume of water produced from an aquifer in a certain area is not greater than the recharge to that aquifer in the same area, then in that sense the safe yield would not be exceeded. However, if all of the water were produced in a relatively small area, then there may be water-level decline in that area as well as problems with well interference. Since both of these are considered undesirable effects, then in that area the safe yield was exceeded.
Figure 21. Long-term hydrograph and well site geology for the Golden City observation well in Barton County, showing the effects of seasonal water use for irrigation on water level in the Ozark aquifer.
Because drawdown is the natural response of an aquifer to pumping, it cannot be eliminated except by not pumping water from the aquifer. In many instances, drawdown is temporary. Figure 21 shows the long-term hydrograph for the Golden City observation well. The major seasonal water-level fluctuations are due to irrigation in the area. Even through water levels fluctuate considerably, during most years the water-level declines do not drop the water level below the base of the confining unit. Only during very dry years when irrigation is greatest does the Ozark aquifer in Golden City become temporarily unconfined. Water level in the observation well and adjacent aquifer drops while wells in the area are pumping, and recover to near pre-pumping level by the start of the next irrigation season. Groundwater cannot be extracted from an aquifer without creating some type of change, either temporary or longer term. Drawdown and well interference can be minimized by pumping wells at rates that generate minimal drawdown, and by spacing wells so that their pumping cones do not appreciably overlap. This will help minimize well interference, but will not guarantee that groundwater levels will not decline over time. The only way to ensure that groundwater levels do not decline is to produce no more water than what is replaced by recharge on a long-term basis. In a confined aquifer like the Ozark aquifer in southwestern Missouri, the rate of recharge is very low per unit of area, but much higher if larger areas are considered. It is not realistic to expect a well to produce a volume of water that does not exceed the recharge occurring above its pumping cone. It is much more realistic to consider the total volume of water pumped in a larger area and the volume of recharge that occurs in the same area.
Despite the documented water-level changes in the Ozark aquifer, it is still a high-quality, viable water-supply source throughout southwestern Missouri, and will likely continue to be so. The use of surface water has lessened dependence on the Ozark aquifer in several high use areas such as Springfield and Joplin. Further expansion of surface water supply in the areas of highest water demand, and the areas where Ozark aquifer water-level decline is greatest, will do much to help preserve this vital aquifer for future generations.