2010-04-20_REVISION - M1977300 (9)

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Environmental Protection Plan, Schwartzwalder Mine 15-7 • Preliminary design considerations for the funnel and gate PRB installation for the Schwartzwalder Mine include the system location, aquifer thickness, groundwater velocity, and groundwater geochemistry. The most practical and cost effective location is downgradient (east) of Sump 1, where the valley narrows and the alluvium is constricted. Groundwater currently daylights at ground surface, because flow in the aquifer exceeds the carrying capacity of the aquifer at this bottleneck. Here, the width of the caisson funnel would be 60 feet for full capture of the aquifer. An asymmetrical wingwall (longer on the north [creekside] than on the south [hillside]) would effectively funnel groundwater into the gate, while isolating the system from surface water. Aquifer calculations for the preliminary design were based on two different hydraulic conductivity values: 9.88x10"3 cm/sec from single well tests conducted by HydroGeo in 1982 and 6.0x10"3 cm/sec from a multiple well aquifer pumping test conducted by Adrian Brown Consultants in 1999. Assuming a 17-ft thick aquifer (based on drilling records from adjacent wells), a hydraulic gradient equivalent to the surface gradient, and a porosity of 0.15, the natural flow velocity (Darcy velocity) ranges from 4.0 to 6.7 ft/day, and the aquifer transmits 3.2 to 5.5 gpm at the considered location. Groundwater velocity is an important factor in the design of the system, because high groundwater velocities require larger reactive cells to achieve adequate residence times. Table 15-1. Aquifer Properties for Preliminary Pl4B Design Hydraulic Conductivity Hydraulic Conductivity Units from ABC from HydroGeo Len Y-th 1 to GW flow) 60 60 .................. ft .................... Aquifer thickness .......................................... ...................._. .....................................................................1_? 17 _ ....................................... ft ....................................... Aquifer hydraulic 6.00E-03 9.88E-03 cm/sec conductivity ................................................................... .........,......................................................... .......... ............................................... • Aquifer hydraulic 17.0 28.0 ft/day conductivity ... .......... _................................................................................................................................. ..................................................... ........................ .... _..... .................-............... .................. ...._ Porosity ............... 0.15 ........... 0.15 ................................ . .........................- .......................................................... ..... Hydraulic gradient 0.0357 0.0357 Flow volume 620 ................................................................................... 1020 ....................................................................................................................... ft3/day ........ .... .......................................................................................................... Flow volume ....................................................... 3.22 .... ...................................... 5.30 ....................................... .. ;.......................................................... ........ gpm ...... . ....................................................... .... . _. Flow volume ............... ............. .......................... . 0.007 0.012 cfs Flow velocity (natural) 4.05 6.67 ft/day The capital cost of installing a permeable barrier includes the following items: • Cost of the reactive medium (ZVI) • Cost of the emplacement (funnel and gate caissons) • Technology licensing costs • Cost of disposal of spoils and restoration of ground surface (assumed surface disposal and backfill acceptable, no offsite disposal required). The preliminary design for the Schwartzwalder funnel and gate PRB system assumed a 10 ft x 10 ft x 17ft deep PRB gate, with a 50-ft funnel (60 total width), as shown in Figure 15-2. Construction of the funnel and gate would include the following components: o Pre-excavate the alignment of the funnel and gate down to 10 feet from the ground surface. o Install sealable sheet piling (Waterloo Barrier system). Gate formed by a cofferdam. o Seal the funnel and gates. o Cofferdam will be excavated and supported in order to accommodate the loading of the reactive • media. o Place the reactive media into the excavated cofferdam. 4109C.100419 Whetstone Associates