1990-03-27_REPORT - C1981013 (5)

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Alluvial deposits along the Purgatoire River are complex. The headwaters of the river are underlain by volcanic rocks and sedi- ments. As it flows to the east, the river cuts its way through many different geologic formations including Pleistocene glacial-fluvial sediments, foothills belt of Paleozoic-Mesozoic strata, terrestrial sediments of the Tertiary, and marine deposits of the Pierre Shale. Channel shape and structure of the Purgatoire also vary within each of these formations as does the water quality. A natural deteriora- tion of water quality can be expected as the stream flows out of the igneous terrains, through sodic sediments, and into marine sediments. Alluvial deposits in the area of the mines range from their current location to older terraces of forty to fifty feet above the Purga- toire. Buried channels filled with alluvium incised into the underlying Raton Formation have also been found. These alluvial deposits support typical hydrophytic vegetation characteristic of floodplains and contain ground water hydrologically connected to the Purgatoire River. 3 . 1 Surface Water Surface water availability is directly related to precipitation received in the drainage. The climate summary, as described in the mine permit application, indicates total calculated precipitation for the mines should be 16.92 inches. The Purgatoire River valley ranges in width from a few hundred to a few thousand feet. Use of the river in the area of the mines is limited to stock watering, flood irrigation on bottomland terraces, water supply for the New Elk Mine and preparation plant, and industrial supply for mining at both mines. Appendix 2 presents water uses for mine operations during 1989 which were calculated at 70. 0 ac-ft. It is also estimated that approximately 6000 ac-ft per year are diverted from the upper Purgatoire River Valley for irrigation use. -12-