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V, Surface Water Hydrology - Rules 2.04.5, 2.04.7, 2.05.3(4) 2.05.6(3) and Information on surface water can be found in Volume I, Tab 7, Section VII of the permit renewal application and Volume III, Tab 7 of the original permit. The Seneca II Mine is located within two adjacent drainage basins: Grassy Creek and Fish Creek. Both of these are sub-basins of the upper Yampa River basin in northwestern Colorado. A discussion of surface water quantity conditions at the Seneca II Mine is provided in the Probable Hydrologic Consequences discussion (Section IX) of this document. Drainage and sediment control features are described below for each drainage basin. Grassy Creek A major portion of the Seneca II Mine is located within the Grassy Creek basin. Surface runoff originating in the western part of the permit area flows into Little Grassy Creek, a tributary of Grassy Creek. A large sediment pond, the Wadge Impoundment (NPDES 002), is located near the mouth of Little Grassy Creek and controls runoff and sediment from a 2100 acre area of which 860 acres are to be disturbed by mining. This pond has been designed with a large permanent pool of 267 acre-feet. Runoff from the 10 year, 24 hour design storm is 12 ac-feet. Information submitted in the permit renewal application demonstrates the pond is an efficient sediment control structure although less than 24 hours of detention is provided given the principal spillway design. The pond receives inflow from upslope spoils aquifers which have developed in the old Wadge spoils. Surface runoff detained in the pond is considered to dilute the concentrations of total dissolved solids (TDS) in spoils inflow and may serve to improve water quality in general. This pond is in compliance with the requirements of Rule 4.05.6 and continues to meet National Pollution Discharge Elimination System (NPDES) effluent limitations. A second pond is located in the Grassy Creek basin and serves to control runoff and sediment from the shop and office area. The Northwest Impoundment (NPDES 003) drains an area of 30.5 acres and has a permanent pool of 5.1 acre-feet. This pond also receives discharge from a truck-washing facility after the water has been treated. An increase of total dissolved solids on the order of 1000 mg/1 (from 1500 to 2500) has been observed between 1983 and 1986 in the effluent from this pond. This increase is attributable to the truck-wash facility. A review of the pond performance indicated a reduction in the size of the principal spillway to a 6 inch orifice is necessary to provide sufficient detention time to increase sedimentation within the pond and produce effluent within NPDES limitations. Peabody Coal Company has committed to installing the 6 inch orifice at the Northwest Impoundment principle spillway by June 1, 1987. With this revised design, the pond is in compliance with the design standards of Rule 4.05.6 and should continue to meet NPDES permit requirements. -11-