1992-08-18_REVISION - M1988112 (2)

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1 H •.' I~ F• ri t: I F7 E T I C'=:. I h! r.. = a _ ~~ = ; :? :_: E• •~ P . 9 • • - Tt is our understanding that one purpose of the current study was to develop the necessary data and design a groundwater containment system below the tailings Facilities. Since no value for the storage coefficient of the aquifer was established, we cannot model well interference effects or design an effective groundwater Containment system. No such design is attempted in the document. The report also addresses the suitability of the existing wells to monitor and intercept contamination, should it reach the groundwater System. The consultant conducted some contaminant transport modeling and determined that the existing wells would intercept any contaminant release. However, we have not been able to reproduce the modeling completed by the consultant and request that additional information and supporting data on the modeling process be provided. In the absence of additional information, it apperirs to us triat groundwater releases in key areas of the tailings facility, the center area of the tailings embankment and the collection pond area, may not be picked up in the existing wells. Lacking any data on aquifer anisotropy, we can assume that groundwateN flow is perpendicular to the equipotential lines delineated on Figure 7 of the report. By constructing "reverse flow" lines upgradient and perpendicular to the equipotential lines, we can estimate where water moving by advective flow may have originated. Such lines drawn upgradient from the three monitor wells are depicted on the attached sketch. These reverse flow lines suggest that monitor wells M-9 and M-13 are most effective in identifying contamination which may originate north of the main body of the tailings embankment. similarly, well M-12 is most effective in identifying contamination originating from the south end of the tailings. None of the wells appear particularly well suited to identify contamination which may originate from the collection pond, or center of the tailings embankment. It is well understood that dispersion effects will cause a contaminant plume to widen down-gradient. with this in mind, the "reverse flow lines" may be thought of as the axis of plupnes which widen to the west. The degree to which the plume will widen as it moves, at least in the modeling, is dependent on the values for dispersivity utilized in the computer code. Although we have not been able to reproduce the transport conditions depicted in trie consultants report, they are apparently determining that the plumes will widen adequately down gradient to be recognized in the monitor wells. To accomplish this, a lateral dispersivity of 100 feet, and a transverse dispersivity of 30 feet were utilized in the calculations. These are assumed values, since no measurements were made of dispersivity in the field. Zn fact, dispersivity is often estimated since it is quite difficult to effectively measure. However, the values used riere appear inappropriately nigh, especially if used for evaluating Contaminate plumes between the monitor wells and collection pond.