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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 Creels 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 />disc(}arae from confined aquifers, particularly the T-kventymile 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 />T,,,enrymilc 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 />i�'y sodium bicarbonate dominance and TDS concentrations of about 750 m --/I. The water quality data from the
<br />00i--"-5 and 001_- ,-< .velis indicate that i -nater quality in the Foidel Creek alluvitun in this locality Is typiczlly of
<br />Emi: ect lytic- -, ith a tendency for dornillance of sodium and sulfa1tc. TDS are iii the i'ai}ge 1,900
<br />�.; r; .i',-..�,r„�=y __ ��','_ _,i,� �t-,SC ,_ ;}P �iiel�ilrn:'(: 'i€.r:� ;i�i3�il_'c'-�l,l�t.�i ,r�lt ;'�t4'�. '£€i`is.,jiCs.;.i-., .-r �4i1•,i1�:, Jt"y,�;-7io, ' _ _
<br />of -,.he alluvium duIi-g spring runoff, but this 1s ncE :-eadiiy observaole. Mots of Selected chemical paaameters
<br />w?'sLIS11ii'5 F 1rt'_res ?-b :ilio?' h 4-:i Ch-ound `tV�atei' b1lal.r,' Da`LG l SliO sioi+ll alit "Iarlatl�irl It�7't rte 12ow; SiiG§7i ;}:r
<br />^ei1.•�1= li.�.. Tiiiw
<br />V.r`%". miall—�D_., _ I_i. J a .
<br />1€n -11", L! e�ltlUll_ L
<br />r!ur"ng Elie iurjo l ijHrird. THIS "s consistent with -ale relative -1 � cia-, ey : ature cj rheoidel Creai- '9'1111-vit_i;€i_
<br />:_ich ,encs m '((riff ''L, --rate Cr 'e l;at'Je and ground water 1 .c='eli;cli1.
<br />iii(-ii5 n.0'1 11iiP'3%1
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