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1991 Symposium: The Dakota Aquifer in Kansas


Geohydrology of the Dakota Aquifer in Washington and Republic Counties

Alan Wade
Kansas Geological Survey, Lawrence, KS 66047

As part of the Dakota Aquifer Program, a local study of the geohydrology of the aquifer in north-central Kansas has been made. In this area, northeast of the Republican River valley, there is negligible subsurface information in the form of oilfield well logs. Eight test holes were therefore recently drilled and logged in this area by the Kansas Geological Survey. It was found that the aquifer is approximately 300 ft thick, consisting of mudstone and loosely-cemented sandstone in a proportion of 2:1, and it dips to the west at approximately 10 ft per mile. The aquifer consists almost exclusively of the Dakota formation; it is confined at the top by the Graneros Shale (where present) and shales of the Permian Wellington Formation at the base. The Kiowa formation comprises an insignificant proportion of the aquifer in the subsurface and pinches out completely before reaching the outcrop belt. Sandstones are not generally laterally continuous except at the base of the Dakota formation where a sandstone was discovered in each test hole. This sandstone is therefore the best conduit for continuous lateral flow within the aquifer. Sandstones in the lower 200 ft of the Dakota formation are generally clean and have high permeabilities whereas sandstones within 100 ft of the top are muddy and very fine grained and therefore have a lower permeability.

Field measurements of water levels, depths to water from well-drillers logs and stream surface elevations were used to determine the form of the potentiometric surface of the basal sandstone and the vertical head gradient at various locations. Ground-water flow within the basal sandstone is governed by two different types of flow system: A regional flow system containing saline water derived from underlying Permian sediments discharges into the Salt Creek watershed in southern Republic County and the Mill Creek watershed in northern Washington County. This water flows into the area from the west due to a statewide eastward-sloping potentiometric surface in the aquifer. The creeks act as drains, allowing local flow systems to control ground-water flow east of Salt Creek and south of Mill Creek. In addition, there are local flow systems recharged by rainfall in less transmissive upper sandstones throughout the outcrop area of the aquifer. The potentiometric surface of both systems is a subdued form of the topography.

There is no evidence of regional flow into the basal sandstone in the area south of Mill Creek in Washington County. On the contrary, water with TDS less than 500 ppm has been pumped from the basal sandstone in T5S R1E via irrigation wells for several years. This area was modelled in plan-view as a steady-state system to determine whether local recharge alone could account for the relief on the potentiometric surface of the basal sandstone. The hydraulic conductivity used in the model was 77 f/day. This was estimated from a pumping test performed on an irrigation well in T5S R1E. It was found that the potentiometric surface could be sustained with a recharge rate through the confining mudstones of only 0.25 in/yr, and an inflow of ground water from the north-west of only 5% of the total flow through the sandstone. The vertical hydraulic conductivity of the confining mudstones, estimated from the pumping test, is easily sufficient to transmit a recharge rate of this order of magnitude. Cross-sectional models incorporating an upper, low-conductivity sandstone layer supported the findings of the plan-view model.

Present rates of pumping of wells in the model area were estimated at 800 ac-ft/yr based on DWR appropriations. Using the model it was found that pumping the aquifer at this rate had little effect on water levels. This is supported by the fact that there have been no reports of significant drops in water levels in this area. Latest simulations of an increase in pumping rates suggest that the aquifer would support water usage of double the present rate with no detrimental effects on ground water quality or quantity. However, pumping beyond this level could begin to bring in saline water in significant proportions from the north-west.

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Kansas Geological Survey, Dakota Aquifer Program
Original symposium held October 1991
Electronic version placed online April 1996
Scientific comments to P. Allen Macfarlane
Web comments to webadmin@kgs.ku.edu
URL=http://www.kgs.ku.edu/Dakota/vol3/symp/wade.htm