2017-01-13_REPORT - M1977004

• Permit No: M1977004
• IBM Index Class Name: Report
• Doc Date: 1/13/2017
• Doc Name: Geotechnical Report
• From: Homestake Mining Company
• To: DRMS
• Annual Report Year: 2016
• Email Name: DMC
• Media Type: D
• Archive: No

Homestake 2016 Geotechnical Observations
Pitch Reclamation Project Indian and Tie Camp Rock Dumps
2016 Observations- Field observations indicate that the repairs performed in 1995 were
successful and no new areas of surficial instability were observed from 1996 through 2016.
In 2011, Piezometer iC10300 was replaced due to sediment buildup in the casing making it
difficult to measure the rise in the water level. It was abandoned and replaced by Piezometer
IC10300R, located approximately 30 feet west of Piezometer IC10300. The piezometer was
terminated at 210 feet bgs and completed with 20 feet of screen and an 8 -inch steel well
protective casing set in concrete.
In 2016, the water level increased by 7.21 feet, which is largely the same as the 6.96 feet
increase in 2015, In 2014, the water level in Piezometer IC1030OR rose by 17.9 feet
compared with 11.1 feet in 2013 and 0.2 feet in 2012. Regardless of the wide range in water
level rise, each year the water level drops back to normal levels in the early fall each year.
Toe of Rock Dump — Technical Revision #7
Site Description: Four piezometers were installed near the toe of the Indian Rock Dump in
June 2016 to advance uranium treatment pilot testing using injection of soluble phosphate -
based reagents to create an emplaced reactive zone near the base of the dump. The
piezometers were installed in the downgradient portion of the dump where the original valley
bottom narrows and flow through the rock pile converges within a zone approximately 150
feet wide (Figure 3). Three piezometers (RD -01, RD -02, and RD -03) were installed
approximately 20 feet apart along a transect normal to groundwater flow, and a fourth
piezometer (RD -04) was installed approximately 60 feet downgradient of this transect. The
piezometers were terminated at approximately 35 feet bgs. were completed with steel
protective casings, and set in concrete. Boring logs and water levels collected from
piezometers after completion indicate approximately 15 feet of saturated dump rock and
native alluvium, underlain by bedrock. Each piezometer was constructed with a 20 -foot
screen interval spanning saturated alluvium and dump rock.
2016 Observations: A total of 25,000 gallons of water were extracted from Piezometers
RD -02 and RD -03, amended with reagent, and reinjected into Piezometer RD -01 from July
24 through August 3, 2016 (Phase 1 -10,000 gallons) and from August 22 through August
31, 2016 (Phase 2 -15,000 gallons).
The water level drop in the piezometers near the toe of the Indian Rock Dump ranged from
0.60 to 1.4 feet during the monitoring period (late July through October, 2016) as shown in
Figure 6. At the completion of Phase 1, water level rises of 0.54 feet in Piezometer RD -01
and 0.27 feet in downgradient Piezometer RD -04 were observed, whereas water levels in
Piezometers RD -03 and RD -04 both dropped by 0,20 feet. Based on the general trend, the
0.2 -foot drop in RD -03 and RD -04 does not appear to be the result of pumping, but rather the
natural seasonal decrease in water level with baseflow discharge. Phase 2 was
characterized by consistent water level drop in each piezometer.
Homestake Pitch Reclamation Project Page 4
2016 Annual Geotechnical Report December 31, 2016
M-1977-MHR 2015 AnnualGeotechnicalReportText_Final.doc