Groundwater Flow Modeling and the Delineation of Wellhead Protection Areas, Cass County, Minnesota
2002-09
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Groundwater Flow Modeling and the Delineation of Wellhead Protection Areas, Cass County, Minnesota
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2002-09
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Minnesota Rules define wellhead protection as "a method of preventing well contamination by effectively managing potential contaminant sources in all or a portion of a well's recharge area". They further state that this recharge area, or wellhead protection area is the "surface and subsurface area surrounding a well or well field that supplies a public water system, through which contaminants are likely to move toward and reach the well or well field". 1986 Amendments to the Safe Drinking Water Act mandated States to prepare a Wellhead Protection Program and submit it to the Environmental Protection Agency by 1989. Whereas the EPA allows flexibility in each State's specific Wellhead Protection Plan Program, the programs must address certain criteria. Through legislation, the Minnesota Department of Health has established guidance for communities to follow in developing their Wellhead Protection Programs. This guidance provides for ample involvement at the community level, but relies on scientific investigation for the delineation of the wellhead protection area. Using four communities in Cass County, Minnesota, this study demonstrates the value of published data and local expertise in creating groundwater flow models for use in delineating wellhead protection areas. The guidelines established by the Minnesota Department of Health for delineation of wellhead protection areas requires that five criteria must be addressed. These criteria are: Time of travel: The time of travel must be at least ten years. Flow boundaries: The location and influence of flow boundaries must be identified using existing information. Daily volume of water pumped: The daily volume of water pumped must be calculated for each well in the public water supply system. Groundwater flow field: The groundwater flow field must be identified for the aquifer used by the public water supply. Aquifer transmissivity: The aquifer transmissivity must be calculated. Using MODFLOW, a modular three-dimensional finite-difference groundwater flow model, steady state flow models of the four areas were created and calibrated. MODPATH, a particle tracking post-processing package for MODFLOW, was used to determine the well capture zone. Landform assemblages provided a framework for data entry into the model. Minnesota Department of Health Water Well Records supplied stratigraphic information. USGS 1:24,000 quadrangle topographic maps provided calibration data. Assistance from State and County employees and local well drillers addressed specific local geologic and hydrogeologic variables. The resultant flow models and particle traces established wellhead protection areas for the communities studied. These wellhead protection area delineation's address State guidelines and were accomplished in an economic, efficient manner.
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A Thesis submitted to the faculty of the Graduate School of the University of Minnesota by Benjamin Robert Bertsch in partial fulfillment of the requirements for the degree of Master of Science, September 2002.
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