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n <br /> Pam! l <br /> fillâ–º <br /> Figure 27: Photograph of the Economic mill(Woods,1901). <br /> No information regarding how these two mills disposed of tailings was available, but given the time period <br /> it is likely that the mill tailings were discharged to the Arequa Gulch drainage. Inefficiencies in <br /> metallurgical processing in the early 1900s would have resulted in incompletely oxidized sulfide minerals <br /> being contained within the tailings. After deposition of the tailings, the remaining sulfide minerals would <br /> gradually oxidize, resulting in soluble mineral species. These oxidation products, and soluble species <br /> that resulted from the beneficiation processes, could then be mobilized during precipitation events. A <br /> portion of these dissolved species would report to groundwater as a result of recharge following <br /> precipitation. <br /> The Arequa Gulch monitoring wells are constructed in the bottom of the Arequa Gulch drainage, below <br /> the Arequa Gulch Valley Leach Facility (VLF1). Prior to construction of VLF1, tailings from the now- <br /> dismantled Carlton mill, which operated from 1951 into 1962 (Feltz, 1978) were deposited in an un-lined <br /> tailings facility which was located at the confluence of the north and east forks of Arequa Gulch, above <br /> the site of the Arequa mill (Grimstad and Drake, 1983). Figure 28 shows the Carlton mill, lower tailings <br /> facility, and two lined heap leach pads in a 1991 aerial view. The larger heap leach pad in Figure 28 was <br /> built on top of the upper Carlton Mill tailings pile (EPA, 1992). The Carlton mill tailings were relocated <br /> prior to construction of VLF1 (Henry et al., 1996). It is interesting to note that tailings were also removed <br /> from the active stream channel of Arequa Gulch as part of VLF1 construction (Henry et al., 1996). Figure <br /> 29 shows the footprint of VLF1, the Arequa Gulch monitoring wells, and the Precambrian-diatreme <br /> contact on the 1951 USGS 1:24000 topography. <br />