Groundwater Quantity of the Lower Dakota Aquifer
While Iowa is not facing an immediate
water shortage, increased demand for groundwater by agriculture, industries, and
municipalities have raised concerns for the future of the resource. However, the
information necessary for decision makers to answer basic questions regarding
how much water can be withdrawn from Iowa’s aquifers on a sustainable basis were
not available. The 2007 Iowa General Assembly, recognizing this lack of
information, approved funding for the first year of a multi-year evaluation and
modeling of Iowa’s major bedrock aquifers by the Iowa Geological and Water
Survey (IGWS). The first aquifer studied was the Dakota Aquifer in a
sixteen-county area of northwest Iowa.
An intensive one-year investigation of the geology and hydrogeology of the Lower
Dakota Aquifer was undertaken to provide a more quantitative assessment of the
groundwater availability, and to construct a groundwater flow model that can be
used as a planning tool for future water resource development. The Lower Dakota
Aquifer includes most of the basal member of the Dakota Formation, the
Nishnabotna Member, and sandstones in the overlying Woodbury Member that
directly overlie and are in hydrologic connection with the lower sandstones. A
series of maps were produced, including the elevation of the top of the Lower
Dakota Aquifer and its thickness, as well as maps of the thickness of the
shale-dominated Cretaceous rocks that lie above the aquifer, the thickness of
the Quaternary materials that lie at the land surface, the bedrock geology of
the region, the limits of the geologic units that lie below the aquifer, and
others. These maps defined the geometry of the Lower Dakota Aquifer and the
other major geologic packages that defined the model.
The hydrologic characteristics of the geologic layers included in the modeling
of the Lower Dakota Aquifer were investigated. An important component of this
study was a network of approximately 60 wells, used to evaluate water levels.
Key to the investigation were eleven observation wells which had time series
data. These data were used for the transient model development. Other important
tasks performed to develop an understanding of the hydrology of the study area
included collection, compilation, and analysis of available geologic and
hydrologic data, and collection, compilation, and estimation of the major points
of groundwater withdrawals within the study area.
With this information a numerical groundwater flow model of the Lower Dakota
Aquifer was developed using four hydrogeologic layers as shown in Figure 1. The
model was created using Visual MODFLOW version 4.2. Hydrologic processes
examined in the model include net recharge, hydraulic conductivity, specific
storage, flow through boundaries, no flow boundaries, well discharge, river
boundary, and groundwater upwelling.
The modeling approach involved the following components:
1. Calibrating a pre-development steady-state model using water level data from
historic records and wells approximately 10-miles from major pumping centers.
2. Calibrating a transient model using water-use data from 2001 through 2006.
Simulated water levels were compared to observed time-series water level
measurements.
3. The calibrated model was used to predict additional drawdowns through 2028
for low, medium and high usage simulations. An additional simulation was run to
predict the additional drawdown for a 2-year drought using 161 new irrigation
permits.
The hydraulic properties of the Lower Dakota Aquifer were shown to vary
considerably in both the lateral and vertical direction, and were obtained for
modeling primarily from aquifer pump test analyses. Based on aquifer test
results, the hydraulic conductivity of the aquifer ranges from 22 to 81 feet per
day, with an arithmetic mean of 47 feet per day. The lateral distribution of the
hydraulic conductivity in the Lower Dakota Aquifer is shown on Figure 2.
Transmissivity values range from 2,700 to 12,000 feet squared per day, and are
controlled primarily by the aquifer thickness. The storage coefficient of the
Lower Dakota Aquifer ranges from 1.8 x 10-5, to 2 x 10-3. The arithmetic mean
storage coefficient is 3.3 x 10-4.
Recharge to most of the Lower Dakota Aquifer is through relatively thick
confining beds that include Cenozoic (Pleistocene) glacial till and upper
Cretaceous shale units. Due to the relatively thick confining units, the rate of
recharge to the lower Dakota is very small. Calibrated recharge rates varied
from 0.15 inches per year to 0.05 inches per year over most of the study area. A
calibrated recharge rate of 3 inches per year was used in the Sioux City area
due to thin or absent confining units.
The calibrated model provided good correlation for both steady-state and
transient conditions. Root mean square errors of 14.8 and 9.4 feet were
relatively small errors over an area of 8,100 square miles. Simulated water
level changes are most sensitive to recharge in the steady-state model, and
pumping rates in the transient model.
The Lower Dakota Aquifer has tremendous development capacity. The current summer
time usage was estimated to be approximately 31.6 mgd. This withdrawal is well
below the development potential for the aquifer. The actual volume of
groundwater available for development depends on location. However, both the
Storm Lake and Cherokee areas are producing water at or near the sustainability
threshold of the Lower Dakota Aquifer. Additional groundwater availability is
the eleven major pumping centers is shown on Figure 3.
