2019-10-22_REVISION - M1988112 (2)

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INSTRUMENTATION Piezometers: Piezometer readings from July, 2018 through May, 2019 are presented graphically on Figure 2. The piezometers all indicate dry conditions or minor amounts of water present at the bottom of the casings due to moisture condensation. This behavior is consistent with historic observations and indicates that the sub-drains are functioning. Low phreatic conditions are maintained within the dam embankment. No abnormalities are indicated in the measurements. Underdrains: Discharge rates from the underdrain system, as measured flows to the seepage collection pond from June, 2018 through May, 2019 are presented in Table 1. Figure 3 shows a graph of the discharge rates for the 6-year period between May 2014 and May 2019. Prior to June 2018, flow rates generally ranged from 37.5 to 42.5 gpm, with an average of approximately 39 gpm. The recorded flow rates for this past year from June 2018 through May 2019, ranged from 28.5 to 34.5 gpm, with an average of approximately 33 gpm, or about 6 gpm lower than historic averages. The cause for this reduction in underdrain flow rates is unknown, and may be attributable to historic drought conditions in the region. It is also possible that drain flows may be partially restricted due to increased clogging by sediment or precipitates. It is recommended that the pipes be cleaned and inspected again to confirm that the reduction in flows is due to drought conditions and is not due to pipe clogging. Table 1. San Luis TSF Underdrain Flow Measurements: July 2018—May 2019 Date Flow m 6/28/2018 28.5 7/31/2018 33.0 8/31/2018 34.0 9/27/2018 34.0 10/25/2018 34.5 11/29/2018 34.0 12/31/2018 33.5 1/31/2019 33.0 2/27/2019 32.8 3/31/2019 32.5 4/30/2019 31.5 5/30/2019 34.0 3 2019 Dam Safety Inspection Report San Luis Tailings Dam