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Ground Water <br />Monitoring and on-site investigations have identified four aquifer systems at the Edna <br />Mine. These units are: 1) Trout Creek Sandstone, 2) Wadge Coal and overburden, 3) <br />Quaternary alluvium, and 4) replaced spoils. With the exception of the Quaternary <br />alluvium and replaced spoils, it is thought that each of the identified aquifers are <br />hydrologically discontinuous with each other, due to the relatively impermeable nature of <br />the intervening strata. <br />The consolidated (bedrock) aquifers of Twentymile Pazk basin are generally rechazged at <br />their outcrops near the edge of the basin. Flow direction from the recharge area is toward <br />the center of the basin. Ground water cannot flow out of the basin because bedding <br />generally dips toward the center of the basin at all locations in the basin. Locally, some of <br />the smaller, laterally discontinuous aquifers dischazge into the alluvium of Trout Creek, <br />Middle Creek, and Foidel Creek. <br />The alluvial aquifers are locally recharged by the bedrock aquifers, as well as by direct <br />infiltration from precipitation, irrigation return flows, and surface runoff. The alluvial <br />water is discharged into the adjacent stream or consolidated aquifers, or may flow out of <br />the basin via the Trout Creek alluvium which dischazges into the Yampa River alluvial <br />system. <br />The Trout Creek Sandstone, which is thought to be a regionally significant source of <br />ground water, is usually characterized by sodium-sulfate type water with an average total <br />dissolved solids (TDS) concentration of about 1040 milligrams/liter (mg/1). <br />The Wadge Coal and overburden units are marginally acceptable as aquifers at the Edna <br />Mine site; however, they are included in this discussion because this unit as a whole is <br />considered to be an aquifer at other mine sites within Twentymile Park. Aquifer <br />properties were determined from single-hole aquifer tests. These tests indicate that the <br />transmissivity of the coal and overburden varies from 0.7 ftz/day to 39 ft2/day. These <br />values indicate that the aquifer is marginally useful as a water source in this azea. <br />The Wadge Coal water chemistry is calcium magnesium bicarbonate to sodium sulfate, <br />with TDS values varying between 490 mg/I and 570 mg/1. Ground water quality in the <br />Wadge overburden decreases in the downdip direction. Up-gradient monitoring locations <br />indicate that the water is of moderately good quality, characterized by low Sodium <br />Absorption Ratio (SAR) values and moderate levels of total dissolved solids. The <br />down-gradient monitoring wells, however, show increasingly higher levels of total <br />dissolved solids. This trend applies primarily to the Moffat Area. <br />The vertical migration of water between the Trout Creek Sandstone and the Wadge Coal, <br />due to mining related disturbances has not occurred, primarily due to the thick sequence <br />of relatively impermeable interburden. <br />C-1980-001 Permit Renewal 5 Findings Page S of 33 Pages <br />