2020-09-09 CC Packet - Laserfiche WebLink

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2.Fracture Flow Delineation Method – Prairie du Chien Aquifer Since the Prairie du Chien aquifer is capable of rapidly transmitting water through its secondary porosity features (fractures and solution cavities), an additional delineation effort was required to account for this flow. The Minnesota Department of Health has developed a guidance for delineating the fracture flow component to the delineation of wells in this type of setting. The methodology is outlined in greater detail in Guidance for Delineating Wellhead Protection Areas in Fractured and Solution-Weathered Bedrock in Minnesota (Minnesota Department of Health, December 2011). Since Well 2 is fully completed within the Prairie du Chien aquifer, the first step of the process was to delineate a fixed radius capture zone for the Prairie du Chien aquifer around Wells 2. Table 5 shows the values used to create the fixed radius calculation, using the MDH’s guidance for fracture flow delineations. A 1-year fixed radius and a 5-year fixed radius (with a 5-year upgradient extension) was calculated. The results are shown in Table 5. Sine only one well was being calculated for the fracture flow analysis, no overlap calculations with nearby wells were required. A 5-year upgradient extensions was added to represent a 10-year capture area. The angle of the ugradient extension was based on observed groundwater flow direction. The upgradient extension was then varied 10 degrees in either direction of the flow gradient (90 degrees from North), to account for possible uncertainties regarding flow direction. The fixed radius delineations and their upgradient extensions were combined to create a composite fracture-flow delineation area for the Prairie du Chien bedrock, as shown in Figure 2. This is the area that, combined with the 10-year porous flow area delineated in Figure 1, will serve as the groundwater capture zone for the Centerville well. The composite 10-year groundwater capture area is shown on Figure 3, which represents the wellhead protection area (WHPA). A composite 1-year capture zone was also created and displayed on Figure 3, known as the emergency response area (ERA). C.Uncertainties relating to the accuracy of the calculated wellhead protection area boundaries Using computer models to simulate ground-water flow necessarily involves representing a complicated natural system in a simplified manner. These simplifications are a result of incomplete knowledge or understanding of part of the natural system and the limitations of mathematical models implemented in groundwater modeling computer codes. The necessary simplifications give rise to uncertainty in the model results. A reasonable attempt to account for the most significant causes of model uncertainty was made in the delineation of the WHPA. The technical report for the Metropolitan Model (Metropolitan Council, 2014) outlines some of the model uncertainties that exist within the original MODFLOW model. Wherever possible, locally obtained values of hydraulic conductivity were used to more accurately represent conditions in and around the Centerville wells. The main uncertainty which would have the greatest impact on the size and shape of the delineated capture zone is the hydraulic conductivity in the Prairie du Chien aquifer. Conductivity can vary greatly in this aquifer, sometimes over short distances, based on the number of fractures and solution cavities that are present. The bedrock matrix itself is not highly permeable, so the presence of fractures and cavities is essential for high capacity production wells. The aquifer pumping test conducted at Well 2 in 1998 offers an excellent base value for modeling the wellhead protection capture zone. To account for the potential variability of the hydraulic conductivity around the well, two additional model runs were conducted for the capture zone analysis. One model run increased the conductivity by 50%, while the other model run decreased the conductivity by 50%. A composite of all three model runs was used to create the porous 10