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Cresson Project Hvdrogeochemistry AdrianBrown <br />rate of 7,000 gpm (Vivian, 1941). The flow has decreased since, and today is approximately <br />1,500 gpm. Water from the Carlton Tunnel flows under permit to Fourmile Creek, immediately <br />downstream of its confluence with Cripple Creek (Plate 7). <br />The flow from the Carlton Tunnel portal passes through a sediment pond system, which when <br />constructed in 1989 had a retention capacity of 1 million gallons. Flow from the tunnel was measured at <br />the portal from 1989 through 1996. The monthly flows for this period are presented together with the <br />precipitation in the District in Plate 6. <br />In 1996 major modifications were made at the Carlton Tunnel portal. The four settling ponds were taken <br />out of service and cleaned of sediment, a fifth pond was installed to raise the capacity of the ponds to <br />1.25 million gallons, and the flow measurement flume and sampling location were moved to a location <br />on the portal bench, immediately downstream of the settling ponds. The reported flow readings in 1996 <br />show a reduction from those measured at the portal, and measured flows in subsequent years are <br />consistently lower than flows prior to these changes (Plate 7). <br />In 2008 further modifications were made to the Carlton Tunnel portal pond system to enhance the <br />sediment - capture performance of the system. A sixth sediment pond was added near the portal of the <br />tunnel, increasing the total pond volume area to 1 acre, and increasing the total pond storage capacity to <br />1.6 million gallons. <br />For the purposes of evaluating the regional ground water system, the flow from the Carlton Tunnel <br />portal is required. To achieve this, a correction has been applied to the measured flow, as follows: <br />1. An increase in the flow measured downstream of the ponds, calibrated by comparing the flow <br />above the ponds and below the ponds after the modifications made in 1996. <br />2. A further increase in the flow measured downstream of the ponds after the modifications in <br />2008, calibrated by matching the flow below the ponds before and after the change. <br />The Carlton Tunnel portal flows are shown by month in Plate 7. This evaluation concludes that: <br />Time period 1992 -2002 2003 -2011 <br />Average flow (gpm) 1691 1503 <br />Infiltration area (square mile) 8.7 8.7 <br />Infiltration rate (inch /year) <br />5.9 5.2 <br />Average precipitation (inch /year) 19.47 19.07 <br />Infiltration rate (% precipitation) 30% 27% <br />Review of the Carlton Tunnel portal flow data in Plate 7 shows that a substantial reduction in average <br />flow occurred in 2002. This change appears to be due to three things: <br />1. Interception of infiltration by the AGVLF liner systems. All incident precipitation on the <br />double- and triple -lined AGVLF is collected and recycled within the leaching system. The total <br />area covered by the VLF in 2011 was approximately 400 acres, of which the 160 acres which <br />1385L.20120125 6 <br />