The Complex Dakota Aquifer: Managing Groundwater in Nebraska
David C. Gosselin, F. Edwin Harvey and Charles Flowerday

Origins and chemistry of the groundwater

The Dakota aquifer of the central Great Plains is a key secondary aquifer in many parts of Nebraska. The aquifer is an important source of water for municipal, industrial and domestic supplies in the northern, eastern and southeastern parts of the state. Although the groundwater flow through the aquifer can be characterized generally as moving northeast from the Rocky Mountains to the Missouri River, locally, the flow systems are hydrologically complex. As development increases in eastern Nebraska, various natural-resource agencies need more detailed information on these flow systems to develop water-management strategies and policies.

Important to this effort is understanding the movement of water into and through the groundwater system. This task naturally falls to us and other researchers at the Conservation and Survey Division of the Institute of Agriculture and Natural Resources at the University of Nebraska-Lincoln, the state’s geological survey.

Our biggest challenge is that groundwater monitoring data does not exist at the scale necessary to develop appropriate management strategies on the county or city level. However, we can use an archive of groundwater chemistry data and recently obtained isotopic analyses to develop broad, general, first-order management strategies.

The aquifer’s geology

The Dakota aquifer and its water-bearing equivalents comprise one of the most extensive aquifer systems in North America, extending from the Arctic Circle to New Mexico, and from the Rocky Mountain front to the eastern Great Plains of Minnesota and Iowa. Younger Cretaceous rocks confine the Dakota aquifer over most of its extent. These units have eroded along the elevated Rocky Mountain front and in the outcrop and subcrop belt in the eastern Great Plains.

The Dakota aquifer of the central Great Plains region consists of the Dakota Formation. In a Geological Society of America Special Paper (no. 287), Brian Witzke and Greg Ludvigson defined a general sequence for this formation that includes a lower sandstone-dominated Nishnabotna Member and an upper mudstone-dominated Woodbury Member. The Woodbury Member is the primary hydrostratigraphic (water-bearing) unit in northeastern and east-central Nebraska. Its thickness ranges from 15 to 70 meters in the east to about 120 meters westward. Although Woodbury strata are dominated by mudstones, shales and siltstone, very fine- to fine-grained, channel-shaped sandstone bodies are present ranging in thickness from less than 1 meter up to 25 meters. This variability leads to challenges when looking for a water supply.

In east-central Nebraska, the Woodbury Member is not recognizable, and the continuous sandstone bodies of the Nishnabotna Member are the primary hydrostratigraphic units. The thickness of the Nishnabotna in Nebraska is highly variable and probably less than 75 meters. Farther west and to the south, the Nishnabotna and Woodbury members cannot be differentiated. In his 1969 Ph.D. dissertation, Maurice Veatch indicated that the sandstone bodies are geometrically complex and discontinuously interbedded with shales in southeastern Nebraska.

The Dakota Formation is completely or partially overlain by Upper Cretaceous marine shales or Quaternary deposits consisting of glacial till and loess.

Origin of the groundwater

A critical component to managing water resources is understanding the source of the groundwater extracted from a well. There are three distinct sources for the groundwater in the Dakota. These include modern precipitation, saline brines (NaCl) from underlying formations, and sulfate-rich waters. Based on these three water sources and our current understanding of the geochemical evolution of the various water types in the Dakota, we suggest the following general management strategies.

Northern and Northeastern Areas: calcium-sulfate and calcium-sodium-sulfate type waters. Isotope analyses have shown that some of this water is very old and, being under a thick confining layer, will not recharge quickly in the northern part of the state. The groundwater extracted from wells in this area is being removed from long-term aquifer storage. This water supply is not easily renewable and should be managed accordingly.

Water in the northeastern part of the state (Dakota, Thurston, Cuming, Burt and Dodge counties) varies widely in chemical composition that results from the interaction between calcium sulfate

(Ca (±Na)SO4) waters from the confined aquifer and locally derived calcium bicarbonate (CaHCO3) waters related to precipitation. These complex chemical variations are associated with the subcrop contact between the Dakota Formation and overlying Cretaceous shales and areas where the Dakota is overlain by Pleistocene glacial till or by loess.

The complex distribution of water types is related to the transition of the Dakota from a predominantly confined aquifer (one bounded above and below by relatively impermeable geologic units) to an unconfined aquifer (one in which the upper surface is at atmospheric pressure and fluctuates with the addition or subtraction of water). The extent to which the various waters interact is a function of dispersion and the time that the water has been in the aquifer. In addition, changes in flow direction as a result of glaciation may have caused waters with different compositions to interact. In relative terms, where the confining unit is thin or has been removed, the Dakota can receive a relatively greater amount of local recharge from precipitation infiltrating from the surface. The recharge can interact with groundwater that is regionally derived from the confined aquifer. Where the aquifer is largely derived from the regional groundwater flow system, development of the aquifer will be limited. The more recently recharged area has better potential for increased development. At the same time, this development can only happen if increased usage does not exceed local recharge and lead to mining of the aquifer.

Where the aquifer is under varying degrees of confinement, changes in groundwater chemistry could indicate the extent to which a well is being influenced by water from the regional confined aquifer or local flow system. Because the chemistry of the water from the regional and local flow systems are distinct, changes in the chemistry of a well from CaHCO3 toward a chemistry of Ca(±Na) SO4 water may indicate that the well is starting to mine the aquifer.

Calcium-magnesium bicarbonate (Ca (±Mg) HCO3) type waters In the eastern part of the state, the water is recharged locally and relatively recently by precipitation. This supply should be adequate for moderate development but may become a concern during drought. These waters occur where the Dakota aquifer is unconfined.

The Central and Southeastern Areas: Mixed Groundwater Types. In the central area (Saunders, Seward and Lancaster counties), and the southeastern area (Seward, Fillmore, Saline, Jefferson and Gage counties), the waters are a mixture of modern, dilute recharge and moderately to highly saline waters. This groundwater most often forms from the interaction of two distinct water types: meteoric water (recently from the atmosphere) and either CaSO4 and CaNaSO4 type water, or NaCl type water.

In areas such as Lancaster County, where local meteoric recharge waters have likely displaced NaCl water downward, the vertical chemistry profile will be complex. It will progress from relatively fresh CaHCO3 type water near the top to more saline, mixed groundwater types to NaCl waters at depth. If the fresher groundwater is extracted faster than it is recharged, then the water chemistry will change as the interface between fresh and saline waters moves. In those areas where saline water could encroach on the drinking water, chemical analysis of those wells can offer some idea when those kinds of problems might begin. If the water chemistry changes, so too should the pumping strategy. This monitoring is particularly important for supplies of smaller communities where such supplies may be overextended during a drought.

In Lancaster County, saline water has discharged at the surface in places, creating the saline wetlands along Little Salt Creek and Rock Creek. These wetlands attracted early settlers who anticipated a thriving salt business. The source of this salinity is probably below the Dakota in the Pennsylvanian rocks. Development has affected the distribution and occurrence of these unique wetlands and also resulted in saline water encroachment on fresher water supplies similar to those Lincoln faced early in its history.

Going local

Our main message is that one size (or strategy) does not fit all where Dakota groundwater management is concerned. To convey this message, the Conservation and Survey Division will be using our locally run natural resources districts to distribute the information in this article. The districts are set up to address local groundwater management issues as they relate to the variations in natural conditions and to development pressures that may affect the Dakota aquifer. In their management of water resources, natural resources districts and other water-management groups must consider these differences, not only related to the Dakota, but also related to the mostly unconfined Ogallala (or High Plains) aquifer. Conditions of the aquifers vary enough across that state that imposing a single management model will not work. With the help of the state survey, local groups can access relevant expertise that will allow them to more effectively manage their water resources.

Origins and chemistry of the groundwater

The chemistry and origin of Nebraska’s groundwater varies by regions: northern, northeastern, eastern, central, southern and western, where the Dakota aquifer is used as a primary or secondary source of water. In addition to assessing the major ion chemistry of the groundwater, researchers from the state’s geological survey — the Conservation and Survey Division of the Institute of Agriculture and Natural Resources at the University of Nebraska-Lincoln — have also obtained strontium-87 and stable isotope of oxygen (oxygen-18) and hydrogen data for selected samples. We express the isotopic ratios as a per million deviation from modern sea water.

Evolution and source of saline waters in the central and southern areas:
The saline — in this case NaCl — waters occur in the vicinity of Lincoln, as well as near the contact between the Greenhorn-Graneros formations and the Dakota Formation in western Jefferson County. These waters have total dissolved solids (TDS) that range from 2,100 to 44,000 milligrams per liter. Groundwater can only evolve to brine rich in NaCl if it encounters highly soluble chloride minerals, typically associated with evaporative deposits (evaporites). It is unlikely that the right conditions existed for evaporites during the deposition of the Dakota Formation.

In 1887, a 751-meter (2,463 feet) exploratory well was drilled in what is now west Lincoln, identifying three saline zones. Zone 1 was in the lower part of the Dakota, and the other two zones were in formations in the Pennsylvanian Shawnee Group and the Kansas City Group. Waters from these zones flowed to the surface under artesian conditions. In their 1939 Nebraska Geological Survey Paper 115, Condra and Reed indicated that the salt water from the Pennsylvanian zones could not be the source of the saline waters
in the Dakota because these zones were separated from the Dakota by “thick, impervious zones.” However, in South Dakota, other investigators have indicated that pre-Cretaceous limestones can be a significant source of water for the Dakota aquifer and others argue for vertical leakage within fracture zones. Fractures and faults are common in the pre-Cretaceous rocks of eastern Nebraska. More specifically, the east-to-west trending Denton Arch, which crosses the middle part of Lancaster County, provides physical pathways that make vertical migration of water from older units feasible.

The majority of the Dakota NaCl waters from Lancaster County have sodium and chlorine concentrations and other geochemical ratios similar to the saline groundwater in the Shawnee Group. Saline waters from the central area near the Lincoln portion of the aquifer have strontium isotopic compositions that are consistent with derivation from documented saline brine zones in the Pennsylvanian limestones. The available major ion chemistry data, strontium isotope data and geochemical modeling support our currently favored hypothesis that saline Dakota waters are derived from saline waters similar to those documented in underlying Pennsylvanian rocks. Additional support for our interpretation is the geologic history of the Pennsylvanian rocks in Nebraska summarized in a 1993 Educational Circular by Marvin Carlson, in which salt beds were deposited along with limestones as seaways became more restricted in western Nebraska. The saline waters in Jefferson County have sodium-to-chloride ratios and other geochemical indicators consistent with halite dissolution. However, these waters are a northward extension of the saline waters in Kansas that have a Permian source.

Evolution of calcium bicarbonate and calcium-magnesium bicarbonate waters in the eastern and northeastern areas:
Waters from the eastern area are calcium-bicarbonate (CaHCO3) and calcium-magnesium- bicarbonate (CaMgHCO3) waters having TDS less than 500 milligrams per liter, and similar waters are in the northeastern area. These types of waters are produced by the interaction of local precipitation and the silicate minerals in the Dakota Formation, an interpretation supported by a variety of geochemical indicators and geochemical modeling. Although the dissolved solids are derived by local groundwater interaction, the distinct strontium-87 values for the two different groups of waters from the northeastern area indicate that these waters are at different stages of evolution or have interacted with different geological materials related to local groundwater flow paths.

Evolution of calcium sulfate and calcium-sodium sulfate waters in the northern area:
In northern Nebraska, calcium sulfate (CaSO4) type waters predominate along with some calcium-sodium sulfate (CaNaSO4) waters. Both of these water types are associated with the confined part of the Dakota aquifer. The CaNaSO4 waters generally occur south of the calcium-dominated waters. The average TDS in the both types are 1,100 and 1,400 milligrams per liter, respectively.

Oxygen and hydrogen isotopes suggest that these waters are meteoric, but the average yearly atmospheric temperatures during recharge were potentially 15 degrees Celsius colder than current conditions. Reconnaissance carbon dating suggests residence times for these waters on the order of thousands to tens of thousands of years. The source of these older waters may have been in eastern Nebraska during a colder climate. An alternative idea is that they have migrated long distances. In either case, the calcium sulfate waters have had relatively long residence times in the aquifer and represent groundwater from the regional flow systems. The calcium-sodium sulfate waters are calcium sulfate waters that have evolved to more sodium-rich compositions by interacting with sediments that had been in contact with saline brines sometime in the past.

David C. Gosselin, F. Edwin Harvey and Charles Flowerday

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Gosselin is an associate professor and research hydrogeologist/geochemist. Email him at: Harvey is an associate professor, research hydrogeologist and supervisor of the groundwater chemistry laboratory. Flowerday is editor and publications officer.

All authors work at the Conservation and Survey Division, Institute of Agriculture and Natural Resources, University of Nebraska-Lincoln.

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