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Mr. Zach Trujillo <br />December 17, 2015 <br />Page 14 <br />and Jubb Creek drainages. Rule 4.05.13 (1) (b) requires that points of compliance be <br />established, in addition to other monitoring points required by the Division. Please propose <br />locations for alluvial groundwater points of compliance downgradient from the area affected by <br />the activity proposed with PR -4. <br />Response: Please see the response to Item 84. <br />ITEM 86. Five alluvial monitoring wells are proposed with PR -4. Their approximate locations <br />are shown in Figure I as pink symbols with green labels; the red polygon represents the <br />approximate outline of the proposed Collom pit. The completion information for these wells is <br />provided in Exhibit 26, Item 1, and is satisfactory. No upgradient monitoring wells are <br />proposed, since, as is explained in Volume 15, Rule 4, Page 14, shallow groundwater is not <br />present upgradient of the proposed Collom pit. With this in mind, impacts will be assessed <br />relative to baseline data only. <br />The only proposed monitoring point on Little Collom Gulch (MLC -04-01) is located more than <br />three miles downgradient of the proposed pit. The time taken for affected water to travel <br />between the proposed pit and the MLC -04-01 may be roughly estimated using an equation for <br />average linear velocity from Fetter (reference 3 below): <br />Assuming: <br />• Hydraulic conductivity, K = 3 ft./day (ref. table 2.04.7-40). <br />• Hydraulic gradient, dh/dl = -0.04 (equal to the average surface gradient <br />estimated from map 10B ((6540-7300)118,500). <br />• Effective porosity, ne = 25%. <br />The average linear velocity, v.,, would be around 0.5 ft/day, at which rate the time taken for <br />impacted water to reach the monitoring point location would be over a hundred years. This <br />estimate is only as accurate as the estimated hydraulic conductivity, which is notoriously difficult <br />to measure accurately; nevertheless, the point is that a single, distant, monitoring point would <br />yield very little information about the impacts of the Collom pit on downgradient water quality. <br />A sequence of monitoring points, much closer to the disturbance would allow for the monitoring <br />of any future contaminant plume, as well as the extent of natural attenuation over time and <br />distance. Furthermore, well tests at these new locations would improve the accuracy and <br />precision of the aquifer properties, and allow for an improved prediction of the PHC. <br />Similar arguments apply to downgradient monitoring in the Collom Gulch and West Fork of <br />Jubb Creek drainages, although the flow paths are more complex and the estimated parameter <br />values are less accurate. <br />A Touchatk,ne Energv'C;c)operiti%,e <br />