Agenda - Council - 11/13/1984 - Laserfiche WebLink

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I I i I I i I I I i I I I I I I I I I~r. HarolcI P. Cahlll, Jr., P.E. Page ground water flow directions. Therefore, all homes shown on U.S. Geological Survey (USGS) topographic maps within a three mile radius of the landfill and not within the city limits of Anoka or separated from the lanOfill by the Mississippi or Rum Rivers were counted, which yieldeO a %oral of 6942 people. Available well logs indicate that nearly all residential wells in the area are finisneO either within the uppermost bedrock (usually the Franconia formation), sanOs and gravels in the lower portion of the unconsolidated material, or sand and gravel layers within the. till unit beneath the surficial sands in the area. All three of these units were considered as aquifers of concern for reasons which will be discussed later. The value of 6942 people was thus arrived at by multiplying 1B27 wells x ~.8 people - 6942 people. well In addition, three Anoka city wells are located within a three mile radius of the landfill. We estimate that these wells serve approximately 779D Anoka city residents. Every other site which was scoreO in both this eno previous rounds of evaluations was scored according to these criteria, so the landfill is in the correct position with respect to the other sites in the State. Therefore, MPCA staff are not recon~nending that this calculation be changed. The second aspect of Hickok's analysis centers on the ability of the local geology to provide sufficient protection for area wells from contaminant migration. Your comments state that based on Hickok's review of well logs in the potentially impacted downgra- dient area, 75 percent are completed in bedrock (Franconia formation or lower). They go on to conclude that these wells are protected from any contaminant migration since they are hydraulically sepa- rated from the shallow aquifer by the St. Lawrence formation (a shaly, silty dolomite which serves as an equitarO) enO/or by interbeaded clays in the drift. MPCA staff accept Hickok's char- acterization of the St. Lawrence formation as an aquitard; however, evidence examined by MPCA staff suggests that the St. Lawrence formation is eroded and may not be present over the entire area. lne St. Lawrence formation does appear to be present in some well logs southeast of the landfill, but it occurs only sporadically. Logs fro~ the two wells on the landfill site which pentrate to bedrock (monitoring well 6A and the landfill shop water supply well), both describe the first bedrock formation below the site as a sandstone. The shop well penetrates 75 feet of what is described as white sandstone, well 6A pentrates 16 feet of brown and light and light greenish brown waterbearing sandstone. These descrip- tions most closely resemble those of the Franconia formation, and tend to support the conclusion of erosion of the St. Lawrence for- mation. NPCA staff feel that the St. Lawrence formation does not guarantee the degree of protection to wells finished in bedrock that Waste Management, Inc. has assumed in its analysis.