2017-01-09_REVISION - M1973007SG

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MEMORANDUM �.--- B B A water consultants BISHOP-BROGDEN ASSOCIATES, INC. To: Brandon Heser,Grant Smith,Bruce Humphries From: Chris Sanchez Subject: Transit Mix—Daniels Pit—Proposed New Settling Pond and Potential Ground Water Interaction with Existing Schlage PCE Ground Water Contamination Plume Job: 9107.02 Date: January 5,2017 Introduction The purpose of this memorandum is to respond to concerns raised by the Division of Reclamation,Mining and Safety(DRMS)regarding the interaction of a proposed new settling pond and an existing ground water contamination plume and related remediation activities. Transit Mix intends to construct a new settling pond within its existing Daniels Pit mining permit area. The proposed settling pond is located such that infiltration of water from the settling pond into the local ground water system may interact with the Schlage contamination plume and related remediation activities. In its letters dated November 23, 2016 and December 7,2016,the DRMS staff commented on the possible interaction of the proposed settling pond with the ground water system. This memorandum addresses the DBMS comments and concerns. Project Description Transit Mix desires to construct a new settling pond for the purposes of wash fines disposal within an existing mined area, located as shown in Figure 1. The pond will replace an existing settling pond, also located as shown in Figure 1. The new pond is necessary because the existing pond has nearly filled with wash fines as a result of ongoing mining processes. The new settling pond will be contained by a new embankment to be constructed across the floor of the existing pit.Wash fines will be delivered to the settling pond in a water slurry.Wash fines will settle out of the water slurry in the pond, and the water will infiltrate into the alluvial aquifer. The water level in the pond will increase with time as more fines accumulate and as fine grained material starts to impede infiltration into the aquifer. The estimated life of the new pond is 5 to 10 years, depending on mining production rates. Economically recoverable sand and gravel has already been mined at the location of the proposed settling pond. Mining progressed to the bottom of the economically recoverable sand and gravel deposits,but did not extend to the bottom of the alluvial deposit. Mining stopped in fine grained material which was not economical to mine.In general,the alluvial deposit consists of an upper sand,a fine-grained unit containing clay, and a lower sand unit which overlies shale bedrock. Mining progressed through a portion of,but not all the way through,the upper sand.A portion of the upper sand,the fine-grained unit(including clay),and the lower sand remain present and unmined beneath the current floor of the pit. Presented in Attachments IA and 1B is a geologic cross section,provided by Geosyntec,showing the upper and lower sand and the fine-grained unit (including clay) between the two sand units. This cross section shows the pre-mined condition. For reference, the SW-26 and SW-32-A/B boreholes are located within the proposed settling pond area. 80110 :0. fax303-806.8953