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Mr. Zach Trujillo <br />November 20, 2015 <br />Page 12 <br />ITEM 85. CCC has shown that there is the potential for activity permitted under PR -4 to negatively <br />impact the quality of alluvial groundwater in the Collom Gulch, Little Collom Gulch and Jubb Creek <br />drainages. Rule 4.05.13 (1) (b) requires that points of compliance be established, in addition to other <br />monitoring points required by the Division. Please propose locations for alluvial groundwater points of <br />compliance downgradient from the area affected by the activity proposed with PR -4. <br />Response: A response for this comment will be submitted at a later date under a separate cover letter. <br />ITEM 86. Five alluvial monitoring wells are proposed with PR -4. Their approximate locations are <br />shown in Figure 1 as pink symbols with green labels; the red polygon represents the approximate outline <br />of the proposed Collom pit. The completion information for these wells is provided in Exhibit 26, Item 1, <br />and is satisfactory. No upgradient monitoring wells are proposed, since, as is explained in Volume 15, <br />Rule 4, Page 14, shallow groundwater is not present upgradient of the proposed Collom pit. With this in <br />mind, impacts will be assessed relative to baseline data only. <br />The only proposed monitoring point on Little Collom Gulch (MLC -04-01) is located more than three <br />miles downgradient of the proposed pit. The time taken for affected water to travel between the proposed <br />pit and the MLC -04-01 may be roughly estimated using an equation for average linear velocity from <br />Fetter (reference 3 below): <br />Kdh <br />1VX = <br />Assuming: <br />• Hydraulic conductivity, K = 3 ft/day (ref. table 2.04.7-40). <br />• Hydraulic gradient, dh/d1= -0.04 (equal to the average surface gradient estimated from <br />map lOB ((6540-7300)/18,500). <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 impacted <br />water to reach the monitoring point location would be over a hundred years. This estimate is only as <br />accurate as the estimated hydraulic conductivity, which is notoriously difficult to measure accurately; <br />nevertheless, the point is that a single, distant, monitoring point would yield very little information about <br />the impacts of the Collom pit on downgradient water quality. A sequence of monitoring points, much <br />closer to the disturbance would allow for the monitoring of any future contaminant plume, as well as the <br />extent of natural attenuation over time and distance. Furthermore, well tests at these new locations would <br />improve the accuracy and precision of the aquifer properties, and allow for an improved prediction of the <br />PHC. <br />Similar arguments apply to downgradient monitoring in the Collom Gulch and West Fork of Jubb Creek <br />drainages, although the flow paths are more complex and the estimated parameter values are less <br />accurate. <br />Please propose additional downgradient alluvial monitoring points in the Collom Gulch, Little Collom <br />Gulch and West Fork of Jubb Creek drainages closer to the affected area. <br />Response: A response for this comment will be submitted at a later date under a separate cover letter. <br />AN EQUAL OPPORTUNITY/AFFIRIWATIVEAC;TION EMPLOYER <br />A Touchstone Energy'Cooperative <br />