Missouri River Alluvial Aquifer Ground-Water Protection (28.3
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The Missouri River alluvial aquifer in the Kansas City metropolitan area supplies all or part of the drinking water for more than 900,000 people in ninety municipalities and public water-supply districts and is the only aquifer in the area that can supply large quantities of ground water for public and industrial use. Because of the importance of this resource to the metropolitan area, a comprehensive ground-water protection plan is being developed for the Missouri River alluvial aquifer. Hydrogeologic data collected and compiled for over 1400 locations in the 475 km2 study area were entered into a geographical information system and interfaced with a ground-water flow model and a particle-tracking program to determine the contributing recharge areas for public-water-supply well fields as a basis for this plan.
The floodplains of the Missouri River, Kansas River, Blue River, Little Blue River, and Fishing River are underlain by alluvial deposits of clay, silt, sand, gravel, cobbles, and boulders that form the alluvial aquifer and lie atop shale, limestone and sandstone bedrock. Several abandoned alluvial channels are hydraulically connected to the Missouri River alluvial aquifer and exist as a result of changes in the course of the Missouri River and its tributaries during previous glacial and interglacial periods. The aquifer thickness ranges from less than 1 to about 70 meters. Average thickness is about 25 meters.The potentiometric surface is free to move up or down over time in the unconfined Missouri River alluvial aquifer and is the boundary across which recharge from precipitation flows into the aquifer. Recharge has been estimated in several previous studies to be between 2 and 25 percent of precipitation. Because the study area has low local relief, topography has little influence on the areal distribution of recharge. Rather, the vertical hydraulic conductivity of soils directly controls the rate of infiltration. Local events such as flooding, irrigation, well pumping, and dewatering during construction can alter ground-water flow directions.
Reported hydraulic conductivity values for the aquifer are between 0.1 m/day and 1400 m/day, transmissivity values are as large as 7400 m2/day,and specific yield is between 0.15 and 0.2. Ground-water flow between the aquifer and the valley walls and bedrock is thought to be minimal in comparison to the total flow of ground-water in the aquifer because both the valley wall and bedrock units have estimated hydraulic conductivities between 0.003 m/day and 3 m/day.
A ground-water flow model of the Missouri River alluvial aquifer in the Kansas City metropolitan area was developed for the ground water protection plan using the USGS model MODFLOWARC a modified version of MODFLOW that reads and writes files used by ARC/INFO a geographic information system. The model used a uniform grid size of 150 meters by 150 meters and contains 310, 400 cells in 4 layers, 485 columns, and 160 rows. Hydrogeologic data from within the study area were entered into the GIS and assigned to each model cell by interpolation.The model was calibrated to both quasi-steady state hydraulic head data from the January 1993 synoptic water level measurement and transient hydraulic head data from flood data of August 1993, and synoptic water level measurements from October 1993, and February 1994. The quasi-steady state calibration was used to assess model geometry, confirm the conceptual model of ground-water flow, test the appropriateness of simulated boundary conditions, and obtain approximate transmissivity and recharge arrays. The RMS error for the quasi-steady state calibration was 1.15 m. The transient calibration was used to fine-tune the model hydraulic properties through a period of prolonged aquifer drainage from August 1993, just after the peak of the flood of 1993, to February 1994, when river stage and ground-water levels had approached typical conditions for that time of year. The October 1993 RMS error was 0.71 meters and the February RMS error was 0.80 meters. Sensitivity analysis indicates that the model is most sensitive to increases and decreases in calibrated hydraulic conductivity values and least sensitive to decreases in vertical conductance between layers 1 and 2 and increases in river conductance.
Ground-water flow was simulated for five different well pumping rate/river stage scenarios to bracket the range of conditions expected to occur. These scenarios include: (1) low pumping rates and low river stage; (2) low pumping rates and high river stage; (3) quasi-steady state conditions of January 1993; (4) high pumping rates and low river stage; and (5) high pumping rates and high river stage. The 1-, 5-, 10-, 100- and 1000-year CRAs to each public water-supply well field for each of the five scenarios were determined with the USGS particle tracking program MODPATH.
Ground-water flow model results indicate: (1) The fluctuation of river stage in the Missouri and Kansas Rivers, and to a lesser extent, the Blue, Little Blue and Fishing Rivers has a larger effect on regional ground-water gradients than well pumping; (2) In the absence of pumping, ground-water flow within the alluvial aquifer typically is away from the valley walls, toward the Missouri River and down the river valley; and (3) A sudden increase in river stage can temporarily reverse the direction of ground-water flow.
The effect of well pumping and river stage on the total CRA of each well field in the study area is different because of the unique qualities of each well field with respect to: the orientation of the well field to the geometry of the aquifer, the alluvial valley walls, the rivers, and other pumping wells; the magnitude and spatial orientation of the hydraulic properties of the aquifer in the vicinity of each well field; and the rate of pumping from each well field. The ground-water flow model and the particle-tracking program results combined these effects in determining the CRA of each well field.Several conclusions can be made based on the results of particle tracking analysis for the Missouri river alluvial aquifer in the study area.
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Contact address:
Brian P. Kelly, Hydrologist
U.S. Geological Survey
401 NW Capital Drive
Lee's Summit, MO 64086
PHONE: 816-554-2414
FAX: 816-554-9273
EMAIL: bkelly@usgs.gov
U.S. Department of the Interior, U.S. Geological Survey
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Updated: September 3, 2003
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URL: http://missouri.usgs.gov/indep/kelly/mo-alluvial-gw/summary.htm