Groundwater Occurance in Upper Bedrock Units,Johnson County, Kansas

Groundwater Occurance in Upper Bedrock Units, Johnson County, Kansas

Created by:
Julie WestHoff

May 2, 2008

Contents
Hydrologic Setting Hydrologic and Geologic Properties
Water Resources Ground-Water Contamination
References

Hydrologic Setting

Johnson County, Kansas lies within the Western Interior Plains Confining System (USGS, 1996) as shown on Figure 1. The bedrock units found in Johnson County are not part of a regional aquifer system but are part of a confining unit (Figure 2) between the Great Plains aquifer system and the Western Interior Plains aquifer system (USGS, 1996). Figure 3 shows the major regional rock units. The Pennsylvanian age limestone deposits can act as potential aquifers, but leakage and vertical water movement through the confining unit is very small, due to the thick shale layers found in the region. The Pennsylvanian rocks found in Johnson County can be used to provide water for domestic and stock purposes with very low well yields. These units are not used to provide water for large municipal water supplies. The bedrock units are inconsistent and highly variable in terms of finding sustainable yields.

Johnson County lies within the Physiographic region known as the Central Lowland Province, specifically, the Osage Cuesta physiographic area. The Cuestas of the Central Lowland Province in Johnson County are Pennsylvanian Age limestones and shales that dip gently to the west and northwest. The cuestas are characterized by a series of east-facing ridges (or escarpments), between which are flat to gently rolling plains. Each escarpment is capped by the more-resistant limestone, while the gentle slopes are underlain by thick layers of shale. The steep faces of the cuestas range in height from approximately 50 feet to 200 feet (www.kgs.ku.edu/Physio/osage.html).Streams within Johnson County flow as tributaries to major rivers such as the Kansas River to the north, Missouri River to the northeast, and the Marais des Cygnes River to the southeast. Tributaries to the major streams are typically formed from valley-fill sediments of silt and clay; therefore, the transmissibility is low.

The climate is Johnson County is considered modified humid continental. Average humidity in eastern Kansas in July ranges from 45 to 50 percent. In the winter, the relative humidity is nearly 70 percent.(O’Connor, 1971). The average annual normal precipitation is 40.17 inches and the mean annual temperature is 55.5 degrees F measured at Olathe. KS from 1971-2000 (oznet.ksu.edu/climatic data). The mean annual evapotranspiration is 15 to 25 inches per year (Hanson, 1991). The mean annual runoff is 8 to 10 inches per year (Sophocleous, 1998). The range of mean annual recharge is 5 to 10 inches per year to aquifer outcrop areas in Johnson County (USGS, 1996). Precipitation is the major source of recharge to the region.

Johnson County, KS has an area of 478 square miles with a population of 516,731 (as of 2006).

Figure 1. Regional Aquifer Systems (USGS, 1997)

Figure 2. East-West Cross Section Showing Confining Unit (USGS, 1997)

Figure 3. Regional Geologic Map (USGS, 1997)

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Hydrologic and Geologic Properties

Paleozoic sedimentary rocks overlie Precambrian rocks in Johnson County. The Paleozoic rocks range in thickness from 2150 feet in southeastern Johnson County to 2550 feet in southwestern Johnson County. Regionally, the rocks dip generally westward and northwestward at about 20 feet per mile and near surface rocks dip gently northwestward at 12 feet per mile on average (O’Connor, 1971). The Paleozoic rocks range in age from Cambrian to Pennsylvanian. The bedrock surface in Johnson County (Figure 4) is formed on sedimentary sequences of limestone and shale, with or without sandstone or minor amounts of coal. These rocks were deposited in cyclic sequences known as cyclothems (Fgiure 5). Marine transgression and regression is believed to be the main cause of the cyclic sedimentation.

Figure 4. Geologic Map of Johnson County, KS (O'Connor, 1971)

Figure 5. Pennsylvanian Cyclothem Sequence (USGS, 1997)

The Pennsylvanian rocks found in Johnson County are not considered principal aquifers in the state mainly due to the presence and thickness of the shale units. The Kansas City Group and Lansing Group of the Pennsylvanian System are part of the Western Interior Plains confining system. The confining unit varies in thickness across the region. However, the confining unit may transmit small quantities of lateral flow where there is slight intrinsic permeability. For Johnson County, the intrinsic permeability of the Pennsylvanian rock is estimated at approximately 1 x 10-13 ft2 (USGS 1996) which converts to a hydraulic conductivity of 7.98 x 10 -7 cm/sec (0.002 ft/day). The thickness of the confining unit is between 800 and 1000 ft in Johnson County (USGS 1996).

Bedrock permeability is generally due to secondary porosity features such as fractures and joints. In sandstones, groundwater moves through both pore spaces and fractures. In limestone, it moves through solution cavities, joints, and fractures. Limestones that crop out in Johnson County have the potential to yield water to wells due to the movement of water within the zones of weathering at shallow depths but these units are inconsistent in their yields and can go dry during periods of droughts.

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Water Resources

The principal aquifer for the region and the source of much of the public water supply for Johnson County is the unconsolidated Kansas River alluvium. The bedrock units are unable to provide water with adequate yields. The estimated well yield for consolidated rock groups the Douglas, Lansing and Kansas City Group is 0 to 50 gpm, with typical yields of less than 10 gpm (Figure 6). The quality of the water decreases with depth, becoming very saline below 250 feet in units beneath the Kansas City Group (O’Connor, 1971). Wells installed in the Pennsylvanian rocks are typically used for agricultural purposes. Domestic use is limited to areas where a public water supply is not available. With the increasing urbanization in Johnson County and growing public water supplies, less use of groundwater is anticipated in the future.

Figure 6. Estimated Well Yields and Water Quality of Pennsylvanian and Older Bedrock Units (O'Connor, 1971)

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Ground-Water Contamination

The chemical quality of the water pumped from wells ranges from poor to excellent quality but typically bedrock wells will have more mineralization than ground water pumped from unconsolidated Pleistocene deposits. The chemical quality of groundwater from the limestone units has been characterized as very hard with high iron content (O'Connor, 1971). The shallow limestone wells are susceptiple to bacteria from surface sources such as septic systems; therefore, the sanitary quality of groundwater may be poor. In addition, surface contamination from urban, populated areas can migrate into the bedrock units. Potential sources of contamination to shallow bedrock aquifers include, among others, petroleum hydrocarbons, pesticides, herbicides,and metals. Because of meager yield, relatively poor chemical water quality, and susceptibility to bacterial contamination, groundwater obtained from bedrock aquifers has been minimal in Johnson County.

Regulatory agencies responsible for protection or cleanup of contaminated groundwater for Johnson County, KS include the US Environmental Protection Agency (EPA) Region 7, Kansas Department of Health and the Environment (KDHE), and Johnson County Environmental Department. In the event a chemical release occurs at a site, the regulatory agency, typically KDHE or EPA, may require an assessment of the groundwater underlying the site. While the area is not underlain by a major aquifer system,understanding which bedrock units have the potential to store and transmit water within the upper Pennsylvanian age rocks is important in assessing risk to the groundwater from a release in Johnson County,KS.

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References

Hanson,RL. 1991. Evapotranspiration and Droughts, in Paulson, R. W. Chase, E.B, Roberts, R.S., and Moody, D. W. Compilers, National Water Summary 1988-89- Hydorlogic Events and Floods and Droughts: US Geological Survey Water Supply Paper 2375, p. 99-104 USGS, http://geochange.er.usgs.gov/sw/changes/natural/et

O’Connor,H. 1971. Geology and Groundwater Resources of Johnson County, Northeastern Kansas State Geological Survey, The University of Kansas, Bulletin 203. KGS

Sophocleous,M. 1998, Perspectives of Sustainable Development of Water Resources in Kansas, Kansas Geological Survey, Bulletin 239.

US Geological Survey, 1996, Geohydrology and Simulation of Steady-State Flow Conditions in Regional Aquifer Systems in Cretaceous and Older Rocks Underlying Kansas, Nebraska, and Parts of Arkansas, Colorado, Missouri, New Mexico, Oklahoma, South Dakota, Texas, and Wyoming, USGS Professional Paper 1414-C

US Geological Survey, 1997. Groundwater Atlas of the United States, Segment 3, Kansas Missouri Nebraska, Hydrologic Investigations Atlas 730-D. USGS