The aquifers 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
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.
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.
and chemistry of the groundwater
The chemistry and origin of Nebraskas 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 states 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:
Evolution of calcium bicarbonate and calcium-magnesium
bicarbonate waters in the eastern and northeastern areas:
Evolution of calcium sulfate and calcium-sodium
sulfate waters in the northern area:
David C. Gosselin, F. Edwin Harvey and Charles