Horsetooth Seepage Chemistry: Analysis of Seepage Water Chemistry before and during Safety of Dams Modification Construction at Horsetooth Dam, Colorado-Big Thompson Project, Ft. Collins, Colorado, 1999-2001

摘要

This report compares seepage and well water chemistry for samples collected at Horsetooth Dam during 2001 with data collected in previous years. The purposes of this evaluation were to note any significant changes in seepage or groundwater chemistry that have occurred since the reservoir surface level was lowered to restricted elevation of approximately 5360 feet for repair construction activities; and to assess historical seepage data with respect to mass loading and void formation from soluble mineral dissolution. Evaluation of all available chemistry data strongly suggest that mineral dissolution has been occurring in subsurface water flows beneath Horsetooth Dam from Horsetooth Reservoir, and that the dissolution of gypsum anhydrite and limestone (calcite and dolomite) contributed to increased seepage flows at Horsetooth Dam. The chemistry data strongly suggest that the decision to repair Horsetooth was timely. Four recent well samples show significant differences in chemistry associated with reservoir surface lowering.