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The Dakota Aquifer


 Groundwater Resource Evaluation of the Lower Dakota Aquifer

The Dakota Aquifer is first aquifer to be studied by the Iowa Geological and Water Survey under the auspices of the 2008 Water Resources Management program.  It is used for rural and public water supplies in western Iowa (Figure 1).  The Dakota is composed of riverine sandstone deposits 200 to 300 feet thick, and generally yields 100 to 500 gallons per minute (gpm) of fair to poor quality water.  The aquifer is confined by 200 to 400 feet of glacial till and younger Cretaceous-age strata.

 Map showing Area of occurrence and significant use of the Dakota Aquifer in western Iowa

Figure 1.  Area of occurrence and significant use of the Dakota Aquifer in western Iowa (modified from Iowa’s Groundwater Basics by Jean C. Prior, et al., 2003, Iowa Dept. of Natural Resources, Iowa Geological Survey Educational Series 6, 83 pages). The study area is highlighted in red.

 

Recharge to the Dakota Aquifer is by downward percolation through the confining units above it.  Regional groundwater flow in the aquifer is from north to south and discharge is to lower reaches of major rivers in western Iowa.  The Dakota is the youngest and shallowest of the major bedrock aquifers in Iowa (Figure 2).

 

Figure 2.  Cross-sectional view of Iowa’s major aquifers and aquitards from northwest to southeast (modified from Iowa’s Groundwater Basics by Jean C. Prior, et al., 2003,  Iowa Dept. of Natural Resources, Iowa Geological Survey Educational Series 6, 83 pages).  The Dakota Aquifer is highlighted in red.

 

Iowa’s groundwater resources are stored in shallow unconsolidated aquifers and in five deeper bedrock aquifers that are generally separated by widespread confining beds, or aquitards, that slow the movement of water between the aquifers.  The unconsolidated aquifers include alluvial sand and gravel deposits found along stream valleys and in ancient buried river valleys, and sand and gravel deposits found within glacial drift.  The bedrock aquifers are usually sandstone, siltstone, limestone, or dolomite, and sometimes are a combination of all of these rock types.  The major bedrock aquifers in Iowa were deposited between 75 to 550 million years ago (mya), and include, from shallow to deep: the Cretaceous (Dakota), Mississippian, Silurian-Devonian, Cambrian-Ordovician (Jordan), and Dresbach (Mt. Simon).

The Dakota Aquifer is composed of two members: thinly bedded and well sorted Woodbury Member shales and very fine- to fine-grained sandstones, and the underlying thickly bedded and poorly-sorted Nishnabotna Member fine- to very course-grained sandstones (Figure 3).  These deposits formed in riverine environments 100 mya. 

 

Figure 3.  The Dakota Aquifer is composed of two members: Woodbury Member  fine-grained sandstones, and underlying Nishnabotna Member fine- to very course-grained sandstones. The Lower Dakota Aquifer includes the contiguous sandstones of the Nishnabotna and Woodbury members.  Woodbury sandstones form a minor aquifer which grade to confining shale layers, while the Nishnabotna sandstones form a major aquifer capable of high yields in some areas.

 

Woodbury rocks form a minor aquifer with low to moderate yields, which grades to a confining layer, while Nishnabotna rocks form a major aquifer capable of yielding greater than 1,500 gpm in some areas.  Because of the greater continuous areal extent and higher yield potential, the initial geologic and hydrologic evaluations concentrate on the lower part of the Dakota Aquifer within the 16 counties in northwest Iowa (Gannon et al., 2008), and (Rowden, 2008). 

For the initial water quality evaluation of the Dakota Aquifer (Rowden, 2008), the upper and lower members of the aquifer are not distinguished because there is currently not enough well data to determine if there are consistent differences in water quality between the upper and lower members.  It appears that there may be differences, and that a major factor influencing the water quality may be the depth of the aquifer and the type of materials overlying it.  In general, the lower part of the Dakota has greater yield potential, but probably poorer natural water quality.  For practical purposes, domestic supplies often use the upper portion of the aquifer because drilling costs are lower, and they do not need large yields.  Public and industrial users that need greater yields must use the lower portion of the aquifer, even if the water quality is poorer.

The individual sandstone beds within the Dakota Aquifer range from less than 10 feet to more than 150 feet in thickness, and while the cumulative thickness of the sandstone also varies widely, it generally ranges from 200 to 300 feet in thickness throughout much of the study area.  The sandstones are confined over most of the study area by 200 to 400 feet of clay-rich glacial till as well as by thick shale, siltstone, thin chalky limestone, and lignite (low-grade coal).  Most wells developed in the aquifer range from 100 to 600 feet deep in the area.  The confining beds underlying the aquifer include Dakota shales, undifferentiated Paleozoic rocks, and Precambrian crystalline rock (Figure 4).

Figure 4.  Cross-sectional view from northwest to southeast of the stratigraphic units in the Dakota study area (modified from Iowa’s Groundwater Basics by Jean C. Prior, et al., 2003,  Iowa Dept. of Natural Resources, Iowa Geological Survey Educational Series 6, 83 pages).  The southeastward dip of the geologic units is vertically exaggerated.

 

The groundwater resource evaluations conducted by the Iowa Geological and Water Survey are part of the 2008 State Water Plan work in progress to delineate the occurrence, movement, availability, use, and chemical quality of groundwater from Iowa’s major aquifers for better and sustainable management of Iowa’s groundwater resources.  As more wells are completed in the aquifers of Iowa and more stratigraphic, construction, and water-quality data are interpreted and entered into our databases, our knowledge of these valuable resources will improve and our evaluation of them will be refined.

Lower Dakota Aquifer

Dakota Aquifer