2015-10-14_REVISION - C1981019 (2)

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Chris Gilbreath Page 10 Please propose locations for alluvial groundwater points monitoring points required by the Division. of compliance downgradient from the area affected by the activity proposed with PR-4. ITEM 86. Five alluvial monitoring wells are proposed with PR-4. Their approximate locations are shown in Figure 1 as pink symbols with green labels; the red polygon represents the approximate outline of the proposed Collom pit. The completion information for these wells is provided in Exhibit 26, Item 1, and is satisfactory. No upgradient monitoring wells are proposed, since, as is explained in Volume 15, Rule 4, Page 14, shallow groundwater is not present upgradient of the proposed Collom pit. With this in mind, impacts will be assessed relative to baseline data only. The only proposed monitoring point on Little Collom Gulch (MLC-04-01) is located more than three miles downgradient of the proposed pit. The time taken for affected water to travel between the proposed pit and the MLC-04-01 may be roughly estimated using an equation for average linear velocity from ??ℎ ?=− Fetter (reference 3 below): ??? ? ? Assuming:  Hydraulic conductivity, K = 3 ft/day (ref. table 2.04.7-40).  Hydraulic gradient, dh/dl = -0.04 (equal to the average surface gradient estimated from map 10B ((6540-7300)/18,500) .  Effective porosity, n = 25% . e The average linear velocity, v, would be around 0.5 ft/day, at which rate the time taken for impacted x water to reach the monitoring point location would be over a hundred years. This estimate is only as accurate as the estimated hydraulic conductivity, which is notoriously difficult to measure accurately; nevertheless, the point is that a single, distant, monitoring point would yield very little information about the impacts of the Collom pit on downgradient water quality. A sequence of monitoring points, much closer to the disturbance would allow for the monitoring of any future contaminant plume, as well as the extent of natural attenuation over time and distance. Furthermore, well tests at these new locations would improve the accuracy and precision of the aquifer properties, and allow for an improved prediction of the PHC. Similar arguments apply to downgradient monitoring in the Collom Gulch and West Fork of Jubb Creek drainages, although the flow paths are more complex and the estimated parameter values are less accurate. Please propose additional downgradient alluvial monitoring points in the Collom Gulch, Little Collom Gulch and West Fork of Jubb Creek drainages closer to the affected area.