2018-01-16_REPORT - M1977004 - Laserfiche WebLink

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1 Homestake Mining Company 2017 Geotechnical Observations Pitch Reclamation Project Indian and Tie Camp Rock Dumps ' 2017 Observations: Visual inspection of the area in 2017 revealed no evidence of settling or slumping of the rock dump material and the drainage channels are functioning as designed. The drainage on the western side of the upper portion of the Indian Rock Dump,just above ' the sericite stockpile, where the rock dump intersects native ground, underwent minor erosion prior to 2012. A low gradient diversion ditch, was built in 2012, allowing water from the channel to be diverted and dispersed at lower velocities. Boulders were placed above the steep section of the channel and rip-rap was placed below the boulders as energy dissipaters, such that flow velocities are reduced and sediment retention is enhanced. Observations in 2017 indicate that this repair continues to minimize erosion in this area. ' Additional Indian Rock Dump Piezometers Site Description: In 2017, one shallow piezometer(RD-05)was installed with a total depth of ' 36 feet near the toe of the Indian Rock Dump and four piezometers (RD-06, RD-07, RD-08, and RD-09) were installed at the Indian Rock Dump 10300 level at total depths of 206.5, 210, 208, and 212 feet respectively (Figure 2). RD-05 was installed along the transect, normal to groundwater flow, that includes the RD-01, RD-02, and RD-03 piezometers ' installed in 2016. The four new injection wells were installed along an upgradient transect normal to groundwater flow adjacent to existing monitoring well IC-10300R. Each piezometer was constructed with a 20-foot screen interval located at the bottom of the ' piezometer coincident with anticipated static water levels. 2017 Observations: Phosphate and tracer amended water was recirculated within the Indian ' Rock Dump as part of the 2017 in situ source zone treatment program. From mid-August - through September 2017, approximately 1,124,000 gallons of water were recirculated within the Indian Rock Dump, with extraction from RD-02 and RD-05 and injection into RD-06, RD 07, RD-08, and RD-09. On September 23, 2017, IC-1030OR was added to the injection ' system as an additional injection point. Water levels in piezometers used for injection fluctuated within a range of approximately 10 ' feet during active injections (Figure 5). The short-term water level variations reflect rapid water level changes during injection activities, and when the injection system was stopped for monitoring and maintenance activities. Water levels associated with RD-02 and RD-05, the two piezometers used for extraction in 2017, suggest minimal water level change (7.8 and 7.5 feet, respectively) during extraction (Figure 6). The higher water level change for RD-02 and RD-05 during extraction than surrounding piezometers (RD-01, RD-03, RD-04) could be a result of the pumps being close to the water level surface. (i.e., splashing of water within the well providing an artificially high-water level). Mounding of subsurface water near injection piezometers dissipated soon after termination of injection, and extraction of water appeared to have little influence on the overall subsurface water level. The return to ' steady-state levels is illustrated by water levels collected in October and November 2017 for piezometers used for injection (RD-06, RD-09, IC300R) and extraction (RD-04) (Figure 5 and Figure 6). ' Indian Rock Dump Inclinometer Installation To monitor Indian Rock Dump slope stability during and after 2017 phosphate injection activities, three inclinometers (IN-1, IN-2, and IN-3) were installed in August 2017 along a drainage ditch located approximately halfway between the base of the Indian Rock Dump and the 10300-bench (Figure 2). The IN-1, IN-2, and IN-3 boreholes were advanced Pitch Reclamation Project Page 5 2017 Annual Geotechnical Report M-1977-004HR