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aquifers in this azea, consists primarily of clay, sand, and lenticular, discontinuous gravel layers. Stream flows are the <br />• primary sowce of recharge to the alluvial aquifers. <br />Alluvial deposits in the Twentymile Park area display characteristics depositional patterns. Major deposition <br />occurs in aeeas where reduced stream gradients or a meandering stream pattern results in reduced flow velocities. <br />Alluvial deposition is limited in areas where stream gradients are relatively steep and in areas where the stream <br />channel has eroded to or intersects resistant bedrock. Consequently, the alluvial deposits do not tend to be <br />continuous. The locations of significant alluvial deposits in the Foidel Creel drainage in the vicinity of the permit <br />area are shown on Map 15, Alluvial Valley Floors and Water Rights. <br />Mapped alluvial deposits in Foidel Creek tend to be limited in extent. Several shallow wells completed in these <br />deposits indicate that they aze usually less than 20 feet thick and the depth to ground water is less than 10 feet. The <br />limited extent of the Foidel Creek alluvial deposits, and the lack of base flow in the creek indicates that these <br />deposits are relatively insignificant to the area ground water hydrologic system. <br />Near the confluence with Middle Creek, stream gradients are relatively low and more extensive floodplain has <br />developed. The floodplains of Foidel and Middle Creeks merge in this area. Drilling and shallow well completion <br />in this area indicates that the alluvial deposits may reach 30 feet in thickness. The alluvial deposits in this area <br />have been studied in some detail and documented in the Energy Mine No. 3 permit application submitted by <br />Colorado Yampa Coal Company (historic permit number 76-16, and current permit number C-84-062). <br />Unconsolidated streamlaid deposits occur in Middle Creek from its confluence with Foidel Creek, downstream to <br />the confluence with Trout Creek. Along this section of Middle Creek, which is approximately 2.1 miles long, <br />alluvial deposits are fairly extensive with thicknesses up to 30 feet. <br />• Alluvial deposits are also found within the floodplain of Fish Creek. The location of these deposits is shown on <br />Map 15, Alluvial Valley Floors and Water Rights. The configuration and depositional characteristics of the <br />alluvial deposits along Fish Creek aze similar to those in other area streams. However, unlike Foidel and Middle <br />Creeks, discharge from confined aquifers, particularly the Twentymile Sandstone, constitute a significant <br />proportion of the total recharge to the Fish Creek alluvial aquifer. Boettcher (1972) estimates that 25% of the total <br />flow in Fish Creek above the old USGS Station 0944100 upstream from any of TCC or CYCC's operations may be <br />attributed to ground water discharge. The contribution of ground water aquifers to Fish Creek within the <br />Twentymile Park Basin is considered in a later section. <br />Water quality data indicate that prior to 1981 there were not significant influences from discharges from distwbed <br />areas to Foidel Creek upstream from these alluvial monitoring wells, particularly the S-10 well. The S-10 well <br />data shows very little change in water quality over the period of 1979 to 1985. This data indicates that alluvial <br />water quality in this locality has probably not been influenced by mining activities. Water quality is characterized <br />by sodium bicarbonate dominance and total dissolved solids concentrations of about 750 mg/1. <br />The water quality data from the 001-S-5 and 001-S-6 wells indicate that water quality in the Foidel Creek alluvium <br />in this locality is typically of mixed type with a tendency for dominance of sodium and sulfate. Total dissolved <br />solids concentrations are in the range of 700 to 1900 mg/l. The data from these wells suggest some influence from <br />mining related activities, particularly at solute concentrations show a slightly increasing trend over the period of <br />record. <br />The water quality data from the alluvial wells might be expected to show seasonal fluctuations reflecting recharge <br />of the alluvium during spring runoff ,but this is not readily observable. Plots of selected chemical pazameters <br />• versus time (Figures 4-b through 4-f, Ground Water Quality Data) show significant variation but do not show any <br />consistent trends. This suggests that ground water flow in the alluvium is relatively stagnant, with limited flushing <br />dwing the spring runoff period. This is consistent with the relatively clayey nature of the Foidel Creek alluvium, <br />which tends to limit the rate of recharge and ground water movement. <br />PR 03-06 2.04-23 05/07/03 <br />