2008-03-05_REVISION - M1997054 (24)

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geomorphology of the watershed, combined with existing agricultural/irrigation demands, dictates that for much of the year Tallahassee Creek has no surface flow along the reach adjacent to the Parkdale Aggregate Mine. The watershed is comprised of approximately 296.4 square miles of contributing area, and its relative steepness indicates an increased susceptibility to episodic flooding events, but the expanse of the watershed provides a means of attenuation, which in many cases will result in reduced runoff peak surface flows at the basin outlet. Additionally, the generally steep drainage basins also tend to limit the capacity of infiltration throughout the watershed, and thus limit the quantity of surface water available for conversion into groundwater. 1 11 depth of the alluvial aquifer. A small surface water diversion on Tallahassee Creek just upstream of the Parkdale Pit irrigates an area west of the site. Return flows from the irrigation of the fields help to recharge the aquifer during the irrigation season. As a result of the highly variable flow of Tallahassee Creek, in combination with the adjacent irrigation practices, the ground water flow in the area is also expected to be variable. During the spring runoff and irrigation season, ground water levels will typically be found higher than during the winter or non-irrigation season. The Arkansas River flows year round with base flows of about 300 cfs and peak flows of nearly 4,000 cfs during 2-year runoff events and over 7,000 cfs during 100-year runoff events, per the reporting of United States Geological Survey (USGS) Gauge Number 07094500, at Parkdale, Colorado. Contributing to these flows is ground water discharged into the river from aquifers located at various locations in the Upper-Arkansas River Watershed, including the glacial outwash aquifer underlying the Parkdale Aggregate Mine site. For the purposes of this analysis, the Arkansas River will act as a constant head boundary. This assumption is standard in the development of boundary conditions in hydrogeologic investigations. As explained in Groundwater Science by Charles R. Fitts, "At fixed head boundary conditions the (hydraulic) head is known. These are usually where the groundwater is in direct contact with a surface water like a lake or a river." In this case, the Arkansas River would act as a recharge boundary up gradient from the saturated zone. SURROUNDING STRUCTURES According to the Colorado DWR Colorado's Decision Support Systems (CDSS) database there are several permitted or decreed wells located in the vicinity of the Parkdale Pit. Figures 1 and 2 of Appendix C show the permitted location of these wells. These wells have total depths greater than the depth of the alluvial aquifer, though they are permitted as exempt tributary ground water wells. The DWR database also shows the static water level of the wells to be at depths greater than the Only one of the wells, well no. 186110, is located on the north side of the Arkansas River and/or is ' within 1,000 feet of the pit. The other permitted wells in the area are located on the south side of the river (beyond the constant head boundary), the closest of which is approximately 1,500 feet from the pit. ' Parkdale Aggregate Mine I Overview of Existing Hydrogeologic Conditions