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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 stream-laid <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 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 /> 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 />