1993-01-20_REVISION - M1988112 (4)

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ADEQUACY RESPONSES TO COLORADO DIVISION OF~~~ ~~°r MINING AND GEOLOGY ADEQUACY COMMENTS JAN 2 01993 TO TECHNICAL REVISION NO. 08 1113/93 D,v~~,~: COMMENT 1. BMG has provided sensitivity analyses of porosity and dispersivity in order to demonutrate the adequacy of the nenvork which is appreciated. The single point of discharge utilized for both analyses, howeNer, was the mid- poira between Wells M-12 and M-13 and is ident~ed ats "a worst-case scenario ". While this Wray be one such scenario, the one drat concerns the Division more is a point of limited discharge from the sputhtivest part of rite collection pond, between Wells M-13 and M-9. BMG should supply a plot of appropriate containment (sicJ plumes originating front this area in order to more completely demonstrate tTre adequacy of rite monitoring nenvork. RESPONSE: The reason that the main axis of the hypothetical plume was selected as the mid-point between wells M-12 and M-13 is that this location provided the maximum transverse distance from the main axis o'f a hypothetical plume to a well and, therefore, would be the most difficult to detect. Therefore, we considered that to be the worst-case scenario. However, to evaluate the Division's concern, we have changed the axis of the plume so that the source is the southwest corner of the collection pond, This change in the location of the source of the hypothetical contaminant plume is shown in Figure 3. Since the aquifer hydraulic characteristics are the same far both locations, the hypothetical plume would not change based on the source location. The hypothetical plumes for the porosity sensitivity analysis for this case are shown in Figure 4 and the hypothetical plume for the dispersivity sensitivity analysis is shown in Figure 5. These figures show that well M- 9 will adequately monitor downgradient of a potentlial leak in the southwest corner of a collection pond. As Figure 3 shows, well M-13 would be upgradient of a potential leak in the southwest corner of the collection pond and would not observe such a leak. COMMENT 2. BMG leas indicated that Figure 1 presents the results of the porosity analysis and Figure 2 dre dispersivity analysis. 77re reverse appears true. In tare Figure 1, fire plot of a plume under rite influence of 20% porosity appears to be 6ltre rather than red plot; rite 35% porosity plot rite red rather than blue.