R2000-258 AUTHORIZING AGREEMENT - Laserfiche WebLink

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Summarv This proposed plan is being submitted in response to a request for proposals sent by the City of Decatur to evaluate a digital ground-water flow model developed for the City by Layne GeoSciences, Inc. (LGI) that estimates water level drawdown in the vicinity of the City's emergency well field. The Illinois State Water Survey (ISWS) proposes to conduct an evaluation of the modeling report submitted to the City by LGI, and to evaluate the model itself using the available data for the 84 days when the Emergency well field was in operation during November 1999-February 2000. The findings,to be published as a contract report, will describe the accuracy of the model, identify and comment on each component of the model (boundary conditions, geologic data, water level data, hydraulic properties, etc.), evaluate the modeling approach taken by LGI, identify information missing in the report, and suggest ways to improve the model if improvement is necessary. Typically, a ground-water flow model is less accurate near the boundaries where assumptions may have a stronger influence on the results. Therefore, though there may be measurable errors near the boundaries, the model may do an adequate job of representing conditions near the pumping wells. Therefore, an evaluation of the model error based on the distance from the pumping wells, will be included in the report. This assessment is useful in determining if the model may be better at predicting water level declines closer to the well fields versus near the boundaries where drawdown will likely be small and impacts minimal. The last portion of the report will propose recommendations for the City that, depending on the results of the error analysis, may include: collection of additional field data to improve the ' geologic understanding of the connections between aquifers and possible connections with the Sangamon River in the area, development of a long term observation well network, and\or development of a new, fully 3-dimensional flow model. The proposed duration of the project shall be 4 months, starting at the time we receive the model code from the City. A technically reviewed draft of the report will be available to the City � 8 weeks into the project. The remaining time for the project will allow the ISWS to publish the contract report through our normal editorial process. Project Goal The project goal is to determine the accuracy of the model developed by LGI in , simulating the 1999-2000 drawdown in the Mahomet aquifer. � Evaluation of Model Accuracv Accuracy of a ground-water flow model can be defined in many ways. The RFP from the City of Decatur states, " The City of Decat�.u� is requesting proposals for a follow up study to determine the accuracv of the model created by Layne." and "The goal of this work is to review the model created by Layne and determine its accuracv based on the 1999-2000 operation." 2