Salem Plateau Groundwater Province
The Salem Plateau groundwater province surrounds the St. Francois Mountains and includes all or parts of 49 counties, an area of about 24,760 square miles. The province is most extensive to the north, west and south of the St. Francois Mountains, and relatively small on the east side. Groundwater resources in the Salem Plateau groundwater province are the most extensive in the state. About 46.6 percent of Missouri’s potable groundwater is in this region, a volume of about 233 trillion gallons. All but a very few communities and essentially all of the rural residents in this province rely on groundwater.
There are two major aquifers that underlie this region, the St. Francois aquifer and the Ozark aquifer. As in the St. Francois Mountains groundwater province, the St. Francois aquifer here also consists of the Lamotte Sandstone and the overlying Bonneterre Formation. The aquifer ranges in thickness from less than 200 feet to locally more than 700 feet thick, and averages about 500 feet in thickness. Depth to the top of the aquifer ranges from less than 500 feet near the St. Francois Mountains to more than 5,000 feet in extreme eastern Missouri.
The St. Francois aquifer receives recharge several ways. In the St. Francois Mountains area where the aquifer is unconfined, water from precipitation can move downward into the aquifer. Away from the St. Francois Mountains where the aquifer is overlain by the Ozark aquifer, there is likely at least some interchange of water between these two regional aquifers. Relatively few wells produce from the St. Francois aquifer outside of the St. Francois Mountains groundwater province, so no detailed potentiometric map of the St. Francois aquifer is available.
Most wells that are deep enough to produce from the St. Francois aquifer in this region also produce from the shallower and more prolific Ozark aquifer. Depending on location, yields of from 70 to more than 125 gallons per minute are possible from the St. Francois. Cities in this groundwater province with wells that produce at least part of their water from the St. Francois aquifer include Rolla, Sullivan, Potosi and Lebanon. Sedalia, located just outside of the northwest boundary of the Salem Plateau groundwater province, also has wells that produce water from the St. Francois aquifer as well as the Ozark aquifer.
For many years, mine dewatering in the Old Lead Belt, centered on the Bonneterre-Park Hills area, drained considerable water from the St. Francois aquifer. Mining ceased in the Old Lead Belt many years ago, but began along the Viburnum Trend in Iron and Reynolds counties in the 1960s. Lead here is also mined from the Bonneterre Formation. Mine dewatering in the Viburnum Trend has caused water-level to decline in the St. Francois aquifer several hundred feet in and near the mining area.
The only groundwater-level observation well in the Salem Plateau groundwater province that measures groundwater-level in the St. Francois aquifer is Potosi. De Soto, National Lead and Warsaw observation wells are open to both the Ozark and St. Francois aquifers.
Figure 1 - Thickness of the St. Francois aquifer in the Salem Plateau Groundwater Province. Data Source: USGS, 1994
Overlying the St. Francois aquifer is 100 to 500 feet of low-permeability carbonate rock and shale including the Derby-Doerun dolomites and Davis Formation. Together, they form the St. Francois confining unit. Though these units can yield small quantities of water, they are not considered a significant aquifer. Instead, they greatly limit the interchange of water between the two aquifers. The movement of water between the Ozark and St. Francois aquifers is controlled by the thickness and hydraulic conductivity of the St. Francois confining unit, and the water-level difference between the two aquifers. Where the potentiometric surface of the Ozark aquifer is at a higher elevation than that of the St. Francois aquifer, then water would tend to move downward from the Ozark aquifer, through the St. Francois confining unit, into the St. Francois aquifer. However, where the potentiometric surface of the St. Francois aquifer is higher than that of the Ozark aquifer, then there is an upward flow potential.
Thick Ordovician- and Cambrian-age dolomite and sandstone units comprising the Ozark aquifer overlie the St. Francois confining unit. The Ozark aquifer consists of bedrock units from the top of the Kimmswick Limestone to the base of the Potosi Dolomite. Throughout much of the province the Ozark aquifer is generally 800 to 1,000 feet thick, but it reaches thickness exceeding 2,000 feet locally. It is considered an unconfined aquifer in most of this region.
The geologic units comprising the Ozark aquifer do not have uniform water-yielding characteristics. Generally, the younger formations have lower hydraulic conductivities than the older units. The major exception is the St. Peter Sandstone. The St. Peter underlies much of the extreme northeastern and eastern parts of the province. It can range in thickness from a few feet to more than 100 feet. Where it is sufficiently thick and deep enough to be saturated with water, it generally yields from 10 to about 40 gallons per minute. It is most widely used in this province in southwestern St. Louis County and parts of Jefferson, Perry and Cape Girardeau counties. There are several limestone and dolomite units overlying the St. Peter Sandstone that are considered part of the Ozark aquifer, but are not generally major water yielding formations. These include the Kimmswick Limestone, Plattin Formation, and Joachim Dolomite. The units between the St. Peter and the deeper high-yielding zones, including the Everton, Powell, Smithville formations and the Cotter and Jefferson City dolomites, can supply enough water for private domestic wells. Higher yields are available from these units in the southeastern part of the province, particularly in Cape Girardeau County.
The most water-productive formations in the Ozark aquifer are in the lower part of the sequence. The Roubidoux Formation, Gasconade Dolomite, Eminence Dolomite and Potosi Dolomite generally have the highest yields. Many of the private wells in the province produce from the Roubidoux Formation and Gasconade Dolomite. However, both of these units have been weathered and host many karst features. Where they are at or near land surface and can be rapidly recharged from precipitation, it is best to set casing through them and produce from deeper in the aquifer.
The Ozark aquifer is the most widespread and widely used aquifer in Missouri. It supplies nearly all of the water-supply needs in this province. Depending on well depth and location, private domestic wells a few hundred feet deep can easily produce water ample for domestic purposes, while larger-diameter wells 1,200 to 1,500 feet deep typically can produce from 300 to more than 1,000 gallons of water per minute.
Because of its great importance, many of the groundwater-level observation wells monitor the Ozark aquifer. Observation wells that measure and record water levels in the Ozark aquifer in this province include Camdenton, Ozark Fisheries, all of the Phelps County wells, Bixby, West Plains, Theodosia, Branson, Cooper Creek, Marshfield, Norwood, Akers, Big Spring, Drake, St. Clair, Eureka, Fairview and several more. Some of these wells are open to only part of the aquifer, while others are drilled completely through it.
Figure 2 - Thickness of the Ozark aquifer in the Salem Plateau groundwater province. Data source: USGS, 1990
Large quantities of high-quality groundwater are generally easy to obtain in this province. Minimum construction standards for private domestic wells in much of this region call for at least 80 feet of casing set 30 feet into rock. In the northeastern part of the region, including parts of Phelps, Crawford, Maries, Osage, Gasconade and Franklin counties, Pennsylvanian-age sandstone and shale units overlie the Ordovician-age bedrock. Groundwater quality in the upper part of the Ozark aquifer in this area is different than in other parts of the Ozarks. The sulfate content is commonly elevated, and may exceed the maximum recommended level of 250 mg/L. Thus, where Pennsylvanian strata are present, at least 150 feet of casing is required for a private domestic well. Also, special casing-depth standards apply within one-fourth mile of the shorelines of major lakes including Table Rock, Bull Shoals, Norfolk, Stockton, Pomme De Terre, Truman, Lake of the Ozarks, Clearwater and Wappapello. Private domestic wells within a quarter mile of these lakes must be cased at last 50 feet below lake-bottom elevation. The lowest point in the lake within one-quarter mile of the well is used in calculating the casing depth.
The construction standards for public water supply wells are much more stringent than those for most private wells. There are no minimum casing depths for public water supply wells constructed in bedrock aquifers. The casing depth for each well is determined on a site-specific basis, and depends on the geology at the well site and other factors including the proximity of potential contaminant sources. Public water supply wells commonly contain 300 to more than 600 feet of casing that is sealed full length using neat cement grout (a mixture of Portland cement and water). Engineering plans are required for most of them. Because of their careful design and construction, very few public wells that penetrate deep bedrock aquifers produce water that requires permanent disinfection or any other type of treatment to meet safe drinking water standards.
Groundwater quality in this region is generally very good. The water is generally a moderately mineralized calcium-magnesium-bicarbonate type, which reflects the dolomitic bedrock in the area. Many residents use a water softener to reduce the hardness caused by calcium and magnesium levels, but the water generally requires no treatment.
The Salem Plateau groundwater province is also host to many of Missouri’s largest springs. There are more than 5,000 recorded springs in the state, but only eight of them are considered first magnitude springs. A first magnitude spring must have an average flow that exceeds 100 cubic feet of water per second (64.6 million gallons per day). All of the the first magnitude springs are in the Salem Plateau groundwater province. They are Big Spring (Carter County), Greer Spring (Oregon County), Welch Spring (Shannon County), Alley Spring (Shannon County), Blue Spring (Shannon County), Meramec Spring (Phelps County), Bennett Spring (Dallas County), and Double Spring (Ozark County). The combined flow of these eight springs averages a little more than a billion gallons of water per day.
Although this region contains abundant groundwater resources, the geology in some areas makes groundwater particularly prone to contamination. Permeable residual soils and karst features such as sinkholes and losing streams allow rapid groundwater recharge to occur. In some areas, most of the normal surface-water flow is lost underground in losing streams and sinkholes. Pipeline breaks in losing-stream watersheds and sinkhole collapses beneath wastewater lagoons have occurred in this part of the Ozarks, and caused serious water-quality problems at the springs where the contaminants resurfaced. Proper land use and waste disposal practices are paramount to protecting the wells and springs of this region of Missouri.
Figure 3 - Bennett Spring, Dallas County, average flow about 114 million gallons per day
Figure 4 - Greer Spring (lower outlet) Oregon County, average flow about 222 million gallons per day
Figure 5 - Blue Spring, Shannon County, average flow about 85 million gallons per day
Figure 6 - Alley Spring, Shannon County, average flow about 87 million gallons per day
Figure 7 - Big Spring, Carter County, average flow about 289 million gallons per day
Figure 8 - Meramec Spring, Phelps County, average flow about 100 million gallons per day
Figure 9 - Double Spring, Ozark County, average flow about 82 million gallons per day
Figure 10 - Welch Spring, Shannon County, average flow about 120 million gallons per day
Figure 11 - Goodwin Hollow at Missouri Highway 5 north of Lebanon in Laclede County.
Upstream from this point this losing stream drains more than 72 square miles, but seldom
has any flow. Water lost underground in this drainage provides recharge to Bennett,
Sweet Blue and Ha Ha Tonka springs.
Figure 12 - Sinkhole near Edgar Springs in Phelps County
Figure 13 - Trash disposed in a sinkhole, Laclede County.
Dye tracing shows this sinkhole to provide recharge to Ha Ha Tonka Spring.