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Alluvial Aquifers <br />Ground water exists under unconfined conditions within alluvial deposits of the major surface drainages in <br />Twentymile Park. These aquifers consist of shallow deposits of unconsolidated stream -laid material, and are located <br />within the floodplains of the major drainages. In the permit and adjacent areas, such aquifers are located in the <br />floodplains of Foidel, Middle, and Fish Creeks. The unconsolidated stream -laid material, which comprises the <br />alluvial aquifers in this area, consists primarily of clay, sand, and lenticular, discontinuous gravel layers. Stream flows <br />are the primary source of recharge to the alluvial aquifers. <br />Alluvial deposits in the Twentymile Park area display characteristic depositional patterns. Major deposition occurs <br />in areas where reduced stream gradients or a meandering stream pattern results in reduced flow velocities. Alluvial <br />deposition is limited in areas where stream gradients are relatively steep and in areas where the stream channel has <br />eroded to or intersects resistant bedrock. Consequently, the alluvial deposits do not tend to be continuous. The <br />locations of significant alluvial deposits in the Foidel Creek drainage in the vicinity of the Permit Area are shown <br />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 are 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 insignificant relative to the area ground water hydrologic system. <br />Near the confluence with Middle Creek, stream gradients are relatively low and a more extensive floodplain has <br />developed, with the floodplains of Foidel and Middle Creeks merging in this area. Unconsolidated streamlaid <br />deposits occur in Middle Creek from its confluence with Foidel Creek, downstream approximately 2.1 miles to the <br />confluence with Trout Creek. Drilling and shallow well completion in this area indicates that the alluvial deposits <br />are fairly extensive and may reach 30 feet in thickness. The alluvial deposits in this area have been studied in some <br />detail and documented in the Energy Mine No. 3 permit application submitted by Colorado Yampa Coal Company <br />(historic Permit Number 76-16, and current Permit Number C -84 -062). <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 alluvial <br />deposits along Fish Creek are similar to those in other area streams. However, unlike Foidel and Middle Creeks, <br />discharge from confined aquifers, particularly the Twentymile Sandstone, constitute a significant portion of the <br />total recharge to the Fish Creek alluvial aquifer. Boettcher (1972) estimates that 25 percent of the total flow in Fish <br />Creek above the old USGS Station 0944100 upstream from any of TC's or CYCC's historic 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 disturbed <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 TDS concentrations of about 750 mg/1. The water quality data from the <br />001 -S -5 and 001 -S -6 wells indicate that water quality in the Foidel Creek alluvium in this locality is typically of <br />mixed type with a tendency for dominance of sodium and sulfate. TDS are in the range of 700 to 1,900 mg/l. The <br />data from these wells suggest some influence from mining - related activities, particularly at solute concentrations <br />show a slightly increasing trend over the period of 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 parameters <br />versus time (Figures 4 -b through 44, 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 />during 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 />TR13 -83 2.04 -23 11/03/14 <br />