Agenda - Council - 09/25/1990 - Laserfiche WebLink

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I I ! I I $. I I I I I I I $. I I I I I On p. P-132 it states that the flow direction in the bedrock aquifer could not be determined on-site due to limited water level data. On p. P-125 it states that four bedrock piezometer have been installed at the site. Table P.F.1 lists four wells screened in the bedrock (MW-4C, MW-11C, MW-34C and MW-35C), but then notes that two were not installed. The actual number of bedrock wells or piezometers at Site P was not readily apparent. A minim_urn of three points are needed to determine groundwater flow direction, so ff t~.-~-e-e--or more wells exist, the . groundwater flow direction could be estimated. Tables P.F. 1 and P.F.3 indicate that well MW-1BB is completed in buried outwash, but cross-section H-H' (Figure P.E. 12) shows that MW-1BB is completed in Superior Till. This apparent conflict was not resolved. On p. P-146 it states that "all vertical flow velocities are at least two to three orders of magnitude less than horizontal flow velocities." This is not true - in general horizontal velocities range from 0.28 to 2.26 feet/day (table P.F.9) while vertical velocities range from 0.004 to 0.6 feet/day (Table P.F. 10). Most horizontal velocities are approximately one order of magnitude (10' times) greater than vertical velocities; not two to three orders of magnitude (100 - 1,000 times) greater. The significance of this point is that there is a relatively strong vertical component to flow direction which could poss~ly influence transport of potential groundwater contamination. There is no site specific discussion with regard to the hydrogeologic properties of the bedrock beneath Site P. The Regional Hydrogeology section stated that "the St. Lawrence Fm. is considered a confining bed because of its low hydraulic conductivity... The Franconia-Ironton-Galesville formations (which underlie the St. Lawrence) from a common aquifer... [However,] the Franconia is sometimes classified as an aquitard (p. P-89)." The Site Geology section indicates that "the St. Lawrence overlies the Franconia Fm. to an estimated depth of 40 feet; however, 6