5.2. ERMUSR 02-12-2008 - Laserfiche WebLink

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5.3 Saturated Thickness and Permeability of Unconsolidated Deposits The maximum possible saturated thickness of the unconsolidated deposits is the vertical distance between the top of the Eau Claire Formation and the water table. The elevation of the water table varies from about 1,000 feet above mean sea level in the northern portion of the wellfield area to the stage elevation of the Mississippi River (approximately 860 feet, above mean sea level). This suggests that the maximum possible saturated thickness of the unconsolidated deposits is in the range of approximately 125 to 250 feet. Provided that the permeability is sufficiently high, saturated thickness is this range indicate that wells in the unconsolidated deposits would have relatively high yields. The actual saturated thickness, however, is the thickness of continuous unconsolidated deposits below the water table that are permeable (generally sand or coarser). In most locations, there will be layers of clay, which will result in the actual saturated thickness being lower than the possible saturated thickness Unconsolidated deposits that are mostly sand and gravel can be expected to have high permeabilities: it the range of 30 to 100 feet per day. Silts and clayey silts generally have permeabilities less than 10 feet/day and clays can be expected to have permeabilities less tan 1 foot/day. In order for an area to be considered a likely candidate for further evaluation as a paired well site, the unconsolidated deposits in and around that area must be at least as permeable as medium sand. The following table summarizes the estimates of saturated thickness and permeability at the well sites: P:\Mpls\23 MN\71\2371105 Water Supply Alternative Study\FinalDeliverables\Alternatives_Report_final.doc 40