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Ground-Water Quantity 25 <br />River watershed consists of unconsolidated, poorly sorted, <br />valley-fill deposits that range in thickness from a few feet along <br />the valley walls up to at least 140 ft in the valley center. The <br />Troublesome Formation aquifer consists primarily of siltstone <br />with interbedded sandstone and conglomerate, with a thickness <br />of more than 600 ft. Previous estimates of ground-water storage <br />in the alluvial and Troublesome Formation aquifers were made <br />by Apodaca and Bails (1999). <br />Ground-water quantity is discussed in terms of aquifer <br />properties, water-level measurements, and where appropriate, <br />qualitative estimates of ground water available to be withdrawn <br />by wells. Aquifer properties were derived from lithologic well- <br />log data, from results of available aquifer tests (Resource Engi- <br />neering, Incorporated, 1999 and 2000; Wilson, 1965), and from <br />comparison to aquifer properties of similar systems. In addition, <br />two downhole monitors were installed in wells completed <br />in the alluvial and Troublesome Formation aquifers to measure <br />water level and temperature. The monitors were installed in <br />September of 1998 and continuously collected data until mid- <br />September 2001. <br />The quantity of available water in the alluvial aquifer is a <br />function of the areal extent of the aquifer, the saturated thick- <br />ness of the aquifer, and the effective porosity (specific yield) of <br />water in the saturated zone. The extent of the alluvial aquifer <br />overlying the Troublesome Formation was determined from a <br />geographic information system (GIS) coverage of the Fraser <br />River valley geology (Green, 1992; Tweto, 1979). Unconsoli- <br />dated surficial deposits of Quaternary age were aggregated, and <br />a total areal extent of 22 mil was calculated (fig. 14). Informa- <br />tion from well logs obtained from the Colorado Division of <br />Water Resources indicates the maximum alluvial aquifer depth <br />of at least 140 ft in the area of interest upstream from Taber- <br />nash. Data collected from the continuous water-level monitor in <br />the alluvial aquifer at site 9 indicated the average depth to water <br />over the period of study was 8.5 ft below land surface with a <br />minimum of 5.9 ft and a maximum of 9.5 ft below land surface <br />(fig. 15). The minimum depth to water occurred during early <br />spring as snowmelt recharged the alluvial aquifer. The maxi- <br />mum depth to water occurred during the fall and winter as dis- <br />charge of ground water to surface water exceeded recharge, <br />causing the water table to decline. <br />To determine the volume of the saturated alluvial aquifer, <br />the volume of the alluvial aquifer above the water table was <br />subtracted from the total volume of the alluvial aquifer by using <br />a GIS procedure. The total volume of the alluvial aquifer was <br />estimated by generating a generalized map of aquifer thickness <br />from lithologic logs obtained from the Colorado Division of <br />Water Resources for 291 wells (fig. 14). Locations of wells and <br />the thickness of the alluvium were plotted on a map of the areal <br />extent of the alluvial aquifer. Thickness contour lines of 50 ft <br />were then drawn on the map to determine areas where the allu- <br />vial aquifer was either 0 to 50 ft thick or 50 to 100 ft thick. A <br />100-ft maximum thickness was assumed because of the uncer- <br />tainties in the limited data. The assumed 100-ft maximum thick- <br />ness allows for some equalization in the estimate of aquifer vol- <br />ume such that some areas may not reach the full 100-ft depth, <br />while in some areas the full depth is at least 140 ft. An addi- <br />tional assumption was made concerning the geometry of the <br />bottom of the alluvial aquifer. It was assumed that the bottom is <br />not flat but is actually thickest in the center of the valley and <br />tapers off toward the edges of the aquifer boundary in a lenslike <br />morphology. To account for this, the total volume of the area of <br />the 0- to 50-ft thickness was calculated by multiplying the areal <br />extent of the aquifer by one-half the maximum thickness, or <br />25 ft. For the areas of 50- to 100-ft aquifer thickness, the total <br />volume is a combination of the top 50 ft, which would not have <br />any effect from the bottom of the aquifer morphology, plus the <br />bottom 50 ft, which would be affected by the configuration of <br />the bottom of the aquifer. To calculate the total volume, the <br />areal extent of the 50- to 100-ft-thick areas was multiplied by <br />50 ft to represent the upper part of the aquifer that was unaf- <br />fected by the lower boundary. This number was then added to <br />the same areal extent multiplied by one-half the thickness, or <br />25 ft, to represent the bottom one-half of the aquifer where the <br />boundary condition was present. This equates to 75 ft multiplied <br />by the areal extent in the 50- to 100-ft-thick areas for the alluvial <br />aquifer. Saturated thickness of the alluvial aquifer was calcu- <br />lated for minimum, average, and maximum depths to water to <br />yield estimates for the seasonal variation in aquifer storage. To <br />generate the saturated volume of the alluvial aquifer, the depths <br />to water were subtracted from average aquifer thicknesses and <br />the differences were multiplied by aquifer area. This approach <br />yields a maximum saturated volume of the alluvial aquifer <br />equal to about 3.33 x 1010 ft3, an average saturated volume of <br />3.17 x 1010 ft3, and a minimum saturated volume of 3.11 x 1010 <br />ft3. These values result in a seasonal change in saturated volume <br />of the alluvial aquifer of about 7 percent during the 2 years of <br />available data. <br />Alluvial aquifer testing in the Fraser River Basin con- <br />ducted by the USGS in 1960 (Wilson, 1965) indicated a coeffi- <br />cient of transmissivity of 12,000 gpd/ft (gallons per day per <br />foot) or a hydraulic conductivity of 114 ft/d (feet per day), <br />which is in the range of well-sorted sands or glacial outwash <br />(Fetter, 1994). Common specific yields (effective porosity) for <br />aquifers of this type range from 0.19 to 0.27 (Fetter, 1994). <br />Assuming a specific yield of 0.2, the amount of available water <br />in the alluvial aquifer at average saturated volume is estimated <br />to be about 150,000 acre-ft. Seasonal fluctuations in water level <br />indicate that recharge occurs fairly rapidly and that water dis- <br />charged from the alluvial aquifer is replenished during spring <br />snowmelt. Determination of sustainable yields from the alluvial <br />aquifer would require more information regarding the amount <br />of recharge, regional flow, amount of discharge, and the loca- <br />tions and pumping rates of proposed water-supply wells. <br />The Troublesome Formation aquifer in the Fraser River <br />watershed encompasses an area of about 52 mil upstream from <br />Tabernash (fig. 3). Aquifer testing of the Troublesome Forma- <br />tion (Resource Engineering, Incorporated, 1999 and 2000) indi-