R2002-156 AUTHORIZING AGREEMENT AMENDMENT

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Resource Problem: Stormwater and Water Quality Management Goal: Utilize Best Management Practices to better manage runoff in residential areas and in tum meet compliance for NPDES Phase II requirements. Goal: Lower nutrient levels delivered to Lake Decatur. Action: Develop and enforce ordinance that requires recommended Best Management Pra.ctices for the site. Action: Increase a«•areness of stormwater inlets and the floodplain area of Finley Creek. Action: Improve water quality through nutrient management on cropland focusing on nitrogen applications Findings: Development Landscape changes caused by development can produce changes in the natural environment. Under natural conditions, a stream might have flooded once a year,however under developing conditions it is likely to flood more frequently. Impervious cover such as roads, buildings and rooftops impede water from filtering into the soil. The unmanaged runoff ca.uses peak flows in the stream. The stream cha.nnel then becomes enlarged causing scouring of the stream bottom and streambank sloughing. Research has shown that impervious cover of as little as 10 to 15 percent of a watershed can have significant adverse effects on channel conditions. Proper stormwater planning and design can reduce peak runoff raLes and the damages to the environment. Floodplains The base flood elevation(BFE) is the elevation that the base flood would reach every one hundred years. This area is the 100-year floodplain and has the potential of flooding to the BFE once every one hundred years or a 1%probability of flooding ea.ch year. However,this does not mean that this magnitude of flooding cannot occur more often. Within the floodplain are the floodway and the flood fringe. The floodway includes the stream channel and overbank area. Beyond the floodway is the flood fringe that makes up the rest of the floodplain. Fill or structures put in the floodplain can alter the base flood elevation and raise it to a higher level because the fill displaced the flood storage area. Water Quaiity I,ake Decatur serves as a drinking water source for the City of Decatur and village of Mt. Zion. IEPA has rated Lake Decatur as a Priori 1 Restorative. The L , ty alce met the re uirement f r . q o the hi hest cate o g g �� Pnority 1 and was classified as restorative because it is a primary public water supply source having "Partial" drinking v��ater supply use support or was identified as having Maximum Conta.mination Level MCL � ( )violations. Illinois has a drinking water standard of 10 milligrams per liter(10 mg/1) for nitrate(N). This standard is mandator5�for public water supplies. Lake Decatur has exceeded this standard from 1979 to 1993 and 1996 to 2002. Typically�, nitrate levels rise above the mandator�• standard in late spring through summer � and late winter. The nitrate concentrations in Lake Decatur come from the nitrate loading on the land. The �� City of Decatur has monitored the I,a1ce Decatur watershed trying to understa.nd nitrate yields in the ' watershed. The goal is to reduce the nitrate levels by implementing BMPs on the land and reduce nitrogen loading. An example from the Lake Decatur. Illinois Case Study Nitrate Reduction for SDWA Compliance in an Agricultural Watershed (1996) stated, "...a 25 percent reduction in N fertilizer II application over the entire watershed was projected to result in a nitrogen concentration of 8.9 mg/1 in water flowing into Lake Decatur after a one-year storm, compared with 11.6 mg/1 for the base run with li current application rates." OS/31/02 �