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1 ' NORTH TAFT HILL EXPANSION SITE Ground Water Modeling Report ' PROJECT DESCRIPTION The North Taft Hill Expansion site is operated by Western Mobile Northern, Inc. (WMN), a ' subsidiary of Lafarge Corporation. The site is located in Section 33, Township 8 North, Range 69 West in Fort Collins, southwest of the intersection of Taft Hill Road and CR 54G (formerly Hwy 287). North Taft Hil] Road borders the site to the east and the Cache la Poudre River is ' approximately'/, mile to the south. Figure No. l is a vicinity map of the subject property. Iviining at this site is expected to last approximately 3 to 7 years with the proposed end land use ' of an open groundwater lake. The site will be dewatered to enable dry mining. A dewatering trench will be constructed around the perimeter of the pit and gravity flow water to a collection ' pond. Water will be pumped from this location back to the Poudre River. The dewatering operation has the potential to impact wells in the surrounding area. No groundwater will be exposed at the site until an approved substitute supply plan is obtained from the State Engineer's Office. To estimate the potential impacts of the dewatering operation at the North Taft Hill Expansion ' site, a finite difference groundwater model was developed using a widely accepted computer program, Groundwater Modeling System (GMS). GMS was used to create a conceptualized model of the groundwater flow through the project area. GMS is a comprehensive computer modeling package that interfaces with a lazge number of groundwater modeling codes, including MODFLOW. This modeling effort for the North Taft Hill Expansion site utilized the MODFLOW package in GMS. Model input pazameters consisted of water level data from ' existing groundwater monitoring programs, well borings, and information of the alluvial aquifer from USGS maps. A slurry wall has been constructed on the site adjacent to the north of the North Taft Hill Expansion site. This slurry wall has altered the groundwater from its original state. The enclosed groundwater analysis completed for the subject site includes this slurry wall as its ' baseline steady state. PREVIOUS GROUNDWATER STUDIES Grant Environmental, Inc. completed a groundwater study on the site to the north with the slurry wall. The results presented in this study predicted that groundwater levels immediately upgradient of the slurry wall will increase approximately 2 feet, and the groundwater levels immediately downgradient will decrease 2 feet. Monitoring wells were constructed around the perimeter of the slurry wall. Groundwater levels were recorded before and after the construction of the wall. The groundwater level monitoring began in August 1999 and is still being recorded on a monthly basis aC that site. 1