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TETRA TECH RMC ~1s. ~1adoline Wullacc <br />f'at:c 2 <br />Slapc' .Cntbilitti• <br />According to the text of Section 3.4 -Area njAJl'ected Lands in the amendment application, <br />"Lafarge will mine 60 fi;ct from Idaho Creek..." The justification for this stems to have been based <br />on a letter (includ; d with the original application) from Mr. M. Edwazd Glassgow, P.F.., geologist, <br />of Scott, Cox & Associates, Inc. dated August 22, 2001. ~Vc have reviewed NU. Glassgow's <br />assumptions of material friction angle and his recommendation regarding stable slopes sts being a <br />3:1 ratio of horizontal to vertical and find them to represent an acceptable basis for determining an <br />appropriate setback hom Idaho Creek. <br />however, in a letter accompanying the amended application, and dated October l5, 2003, Mr. <br />Glassgow presents a different opinion that we bdieve is appropriate for Idaho Creek, based on "a <br />somewhat increased risk ojinstahility due to the continued presence oja source ojwctter and the <br />jacr that it is likely that the grow:dwater ut61e will slope upwards toward the elevatiat o(the water <br />surfare in the waterway." ror mitring adjaceru to waterways, IVir. Glassgow recommends a <br />minimum setback of 50 feel wherein no mining takes place, then recommends that excavation of <br />aggregate and overburden leave an unmined 3:1 slope that extends to bedrock (presumably another <br />GO feet of horizontal distance to the bottom of the slope based on a maximum depth to bedrock of <br />20 feet). According to their own expert's recommendation. Lafarge should be restricted to not <br />excavating any material from an area within 50 feet of the bank of Idaho Creek, and should be <br />restricted to not mining any material below a 3:1 slope projected from that 50-foot limit to the <br />bottom of the mine. If Lafarge instead intends to mine to a vertical face, we recommend that the face <br />be set back from Idaho Creek a minimum dislanee of 120 feet. This would be appropriate for depths <br />to bedrock oC up to about '3 feet. If Lafarge can demonstrate to the Ditch Companies' satisfaction <br />that the depth to bedrock in the vicinity of Idaho Creek is less than Z3 feet, I believe that a reduced <br />setback would be appropriate. <br />Increased Scenaee (rata Idaho Creek <br />~t'e could not find any mention in the amended application assessing, or proposing mitigation of, the <br />potential for inducing additional leakage from Idaho Creek due to dewatcring of the mine site. The <br />dewatering of the mine site will undoubtedly Lowe-r the groundwater table below Idaho Crock. <br />~~'hether this results in additional leakage from Idaho Creek depends on the difference between the <br />condition p.^:~ir to mining and the duwatered condition. Lafarge should provide for review, all depth <br />to water table inti~rmation it has obtained during its exploration of the site. The information needed <br />would be the location of each of the borings, the depths to water after groundwater levels stabilized <br />in flit drill holes. and flit dates of the depth to groun•+u•ater measurentcnts. Cmce we have been <br />provided flit data, we will review flit infomtalion to assess how adequately it rcpresens the pre- <br />mininccondition, will offer an opinion regarding the probability that mining of the site could induce <br />additional seepage from Idaho Creek, and will offer susgcstions for mitigaticg any potential losses. <br />