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<br /> <br />VER11CAL HYDRAUliC CONDUClTVITY MEASUREMENTS IN THE DENVER BASIN, COLORADO <br /> <br />estimated by the USGS using computer modeling is <br />summarized in Table 1 and includes estimates for both <br />horizontal discharge through the four aquifers as well as <br />interflow between the aquifers. For this paper, the impor- <br />tant values are the vertical flow estimates, which are signif- <br />icant volumes ranging between 1350 and 5200 acre-ftlyr. <br />The USGS also used estimates for vertical hydraulic con- <br />ductivity in the modeling efforts that resulted in the esti- <br />mated water budget shown in Table 1 (Robson, 1987). <br />These estimates of vertical hydraulic conductivity, listed in <br />Table 2, were arrived at through calibration of the model <br />wherein aquifer parameters were adjusted until model <br />results closely matched known conditions of the aquifer <br />and are reasonable values for the type of geologic materi- <br />als. Subsequent modeling efforts, such as those used for <br />the Senate Bill 74 (SB-74) investigations (CWCB, 1996) and <br />the South Metro Study (Black and Veatch, et al., 2004), <br />which stems off of the SB-74 efforts, have arrived at equiv- <br />alent values for vertical hydraulic conductivity through sim- <br />ilar groundwater modeling. <br /> <br />The USGS model also assumed that there was vertical <br />connection within the aquifers and that the aquifers would <br />remain confined for an extended period of time. Recent <br />rapid water level decline rates . in excess of 30 ft/yr that <br />continue to decline at this rate on the west side of the <br />basin where the aquifers rise near the Front Range, even <br />after the water levels drop below the tops of the aquifers, <br />would suggest that the vertical connection within the <br />aquifers is less than originally assumed. <br />This conceptual model treats the Denver Basin aquifers <br />as a relatively simple seven-layer system based on each <br />designated aquifer along with the separating confining lay- <br />ers. In reality, the real-world aquifer system consists of <br />many more individual layers of varying geometry and with <br />varying degrees of interconnection as previously described. <br />It is likely that each individual water-bearing sandstone or <br />conglomerate layer, in effect, can be treated as an individ- <br />ual aquifer. <br />Not only is a scientifically based understanding of verti- <br />cally hydraulic conductivity within the sedimentary sequence <br /> <br />Table 1. <br />Model derived water budget for the four main Denver Basin aquifers. Positive numbers indicate flow into the aquifer and negative <br />numbers indicate flow out of the aquifer. Based on a transient-state 20-yr groundwater model. Adapted from Robson, 1987. <br /> <br /> Precipitation Groundwater <br /> Recharge Discharge Net Inter-aquifer flow <br /> (acre-feet per year) (acre-feet per year) (acre-feet per year) <br />Dawson 29,400 -24,200 -5,200 <br />Denver 4,000 -5,350 1,350 <br />Arapahoe 2,050 -5,900 3,850 <br />laramie-Fox Hills 4,200 -4,200 0 <br />Total 39,650 -39,650 0 <br /> <br />Table 2. <br />Model-derived vertical hydraulic conductivity values. A from Robson (1987); B from Barkmann and Edington, 2001). <br /> <br />Source <br /> <br />Method <br /> <br />Robson, 1987 <br /> <br />Model Derived <br /> <br />Edington <br /> <br />Model Derived <br /> <br />l\ <br /> <br />layer <br /> <br />Vertical <br />Hydraulic <br />Conductivity (em/see) <br /> <br />Laramie confining layer <br />Arapahoe-Denver confining layer <br />Denver-Dawson confining layer <br /> <br />o <br /> <br />9.17xlO-9 <br />1.23xl0-8 <br /> <br />Denver Basin confining layers <br /> <br />1 .76x1 0-7 <br /> <br />175 <br /> <br />The Rocky !'-.1ountain Association of Geologists <br />