Agenda - Environmental Policy Board - 10/15/2018

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Franconia is the Ironton -Galesville Formation. The Ironton and Galesville formations are medium to very coarse -grained sandstones interlaid with thin beds of shale. The formations are separated from the water at the surface by clay layers in glacial material and by the thick shale bed in the Franconia Formation. These units of shale and clay act as—quitards", meaning they have low permeability and slowly transmit water, or retard the flow of water to lower bedrock units. The Minnesota Geological Survey (MGS) is currently reviewing the bedrock geology of this region. A final map from this study will be available in fall 2003. The study identifies the possibility of shallow bedrock valleys where the St. Lawrence formation is absent throughout Anoka County. These shallow valleys can be difficult to identify, as the St. Lawrence is often misinterpreted as Upper Franconia. It is typically present as a cap on high bedrock areas. Well logs from the project site (Figure 19.3) indicate that the St. Lawrence does not exist below the Town Center, but because of common misinterpretations, a thin layer may be present. Through the course of the MGS study, a bedrock valley was identified two miles north of the Town Center site (Figure 19.4). The valley cuts down through all upper bedrock units into the Ironton -Galesville Formation. Bedrock valleys bring quaternary sediments in direct contact with deep bedrock formations. This interaction may result in the quaternary aquifer recharging bedrock aquifers without the typical aquitards that protect these aquifers from surface pollutants. 19b. Soils within the Town Center are highly permeable sand and gravel in the upper 50 feet. These are the soils through which RTC stormwater infiltrates (Figure 12.2). Soil borings on -site indicate the first foot of soil is silt and sand, followed by poorly graded fine to medium -grained sand with traces of gravel. The Natural Resources Conservation Service (NRCS) classifies the soils on site as Hubbard series, Duelm, and Isanti. The Hubbard soils classified on site are coarse sand with slopes that range from 0-12 percent. The Duelm is a loamy coarse sand and the Isanti is a sandy loam. The Isanti is a hydric soil. All soils on site have a permeability that ranges from six -to -twenty inches per hour. The high permeability of the soils increases the potential for shallow groundwater contamination. To reduce this risk, pretreatment of stormwater runoff prior to infiltration and community education programs on household chemical and fertilizer use can be implemented. City wells were tested for tritium as part of the wellhead protection area (WHPA) and drinking water supply management area (DWSMA) delineation for the City of Ramsey. Tritium is a form of hydrogen and can act as an indicator of groundwater age, but does not pose a health risk. Atmospheric tritium levels increased during the 1950's due to testing of atomic bombs. Therefore, tritium levels are used to indicate whether groundwater entered the ground before or after 1950. Public water supply wells with high levels of tritium are classified as—vinerable" to surface processes because of the relatively recent (post-1950) interaction with the surface. Tritium levels in the three city 19-2