2008-03-05_REVISION - M1997054 (24)

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i The 3-acre sandstone quarry contained within the existing permit area is located east of Tallahassee Creek and north of the existing sand and gravel pit. No aspect of this amendment application proposes to affect or change any mining or reclamation practices previously specified for the sandstone quarry. The purpose of this Ground Water Impact Analysis Report is to conservatively estimate the impacts to the prevailing hydrologic balance of the ground water flow in the vicinity of the site as a result of the dewatering operation. This report will also provide recommendations for monitoring and mitigation of adverse impacts to the surrounding area during the mining operation. Due to the lack of currently available data, this report will not directly quantify the anticipated impacts following reclamation, but will make preliminary recommendations for pre-reclamation mitigation should future information show that such measures are necessary. OVERVIEW OF EXISTING HYDROGEOLOGIC CONDITIONS GENERAL. CHARACTERIZATION In January 1998, Gilmer Geophysics, Inc. prepared a Hydrogeology Summary Report to detail the existing geologic/hydrogeologic conditions encountered by the area in and surrounding the Parkdale Aggregate Mine. This report detailed the geologic characteristics of the glacial outwash aquifer on which the sand and gravel pit is located, in addition to the bedrock aquifer underlying the sand and gravel deposit at the same location. A Geologic Map of Parkdale Colorado was included as an attachment to the Hydrogeology Summary Report that qualitatively details the predominant underlying strata of the mine site and surrounding areas. The Hydrogeology Summary Report and accompanying attachment have been included in Appendix B of this report to provide a more thorough description of the geologic conditions at the mine site, as well as to facilitate ease of reference. The existing Parkdale sand and gravel pit mines aggregate from the glacial outwash aquifer at a location approximately 1,000 feet north of the northern bank of the Arkansas River. The deposit from which the aggregate is extracted is comprised of sand and gravel of Quaternary age, and contains an aquifer that is "bounded on the north and east by Tallahassee Creek, on the west by Precambrian and Mesozoic rocks, and on the south by the Arkansas River; faulting adds structural boundaries within the bedrock aquifer system." (Gilmer Geophysics, 1998) In the immediate vicinity of the site the outwash aquifer ranges in width from 700 feet to 4,000 feet, and extends to depths of "...at least 67 feet, depending on location." (Gilmer Geophysics, 1998) In general, glacial outwash aquifers are formed by the deposition of aggregate in response to the abrasive forces of glacial advance and retreat. These aquifers typically can be characterized by their non-homogeneous, high transmissivity nature. The saturated thickness of the glacial outwash aquifer in the area of the Parkdale sand and gravel pit varies from a "feather edge" on the western extents of the deposit, to approximately 20 feet in more central locations. The historic hydrologic regime of the Tallahassee Creek Drainage Basin that provides the vast majority of the outwash aquifer's source water is typically comprised of seasonal snowmelt runoff earlier in the spring followed by periodic stormwater runoff later in the year. The fluvial Parkdale Aggregate Mine I Overview of Existing Hydrogeologic Conditions