Agenda - Council - 02/09/1999 - Laserfiche WebLink

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RECONSTRUCTION OF TRUNK HIGHWAY 47 ENVIRONMENTAL ASSESSMENT RESPONSES TO WRITTEN COMMENTS Metropolitan Council Dischargc volumes and pollutant loading calculation were not provided in thc EA because final plans for storm water detention/treatment measures have not bccn completed, and these measures will have an i~npact on the anticipated water quality and quantity discharged from the project area after construction. Based on cra-rent plans, runoff volumes generated by impervious sm'faces and areas adjacent to the roadway that discharge to the system are anticipated to increase fi'om the existing 3,060 xn3 (2.5 acre- feet) to 5,084 m'~ (4.0 acre-feet) after construction of the proposed improvements (as determined using Soil Conservation Service methodology for a 2.5 inch, 24-hem' storm event). Thc current design incorporates as "urban" roadway section (i.e. cm'b and gutter) to convey water from approximately 60 percent of the project corridor to three detenlion/trcatment ponds designed to meet WMO and Nationwide Urban Runoff Program (NURP) standards. The remaining portions of the project corr/dor -- that cannot be cm~vcyed to treatment ponds due to topographic constraints -- will be constructed to "rural" roadway sections, where water is conveyed and Ireatcd via dilches with ditch blocks. It should be noted that the sandy soils in the project area make it likely that much of the surface water being conveyed in thc ditches/ditch blocks or held in ponds will infiltrate into the soil, reducing thc overall volume of water being discharged ('rom the project area. The details of the convcyance/trcatmcnts facilities and the exact size of thc proposed detention ponds arc unknown at this time as design plans are still being finalized. However, final design plans submitted for project permitting will incorporate Best Management Practices (BMPs) and design features as required to conform to the NPDES permit and I,ower Rum River WMO requirements. 2 Thc amount of salt and sand required to be applied to roadways for winter de-icing is based lm'gely on weather conditions, however, Mn/DOT has identified and implemented strategics for minimizing the water quality impacts of these operations on all roadways they maintain, including TH 47. Similar measures have been implemented by local governments to decrease pollution and to minimize costs associated with applying thc materials. Examples of mitigation strategics to minimize the impacts of road de-icing materials include: a) b) c) Minimize thc amount of salt and sand used to thc extent possible, Usc a salt/sand mixture with a litdc salt as possible. Prcwet salt and sand prior to use to increase thc amount lhat sticks to Ibc road surface.