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<br />(3) The direction and magnitude of the ground water gradient were assumed to be <br />constant. Any actual change in the direction of the ground water gradient would <br />tend to disperse a plume more. <br />For the purpose of the analytical model, a conservative assumption was made that there would <br />be no attenuation of cyanide, either in the unsaturated zone or in the Santa Fe aquifer, there <br />would be no adsorption of cyanide, either in the unsaturated zone or in the Santa Fe aquifer and <br />that there would be no decay of cyanide, either in the unsaturated zone or in the Santa Fe <br />aquifer. <br />This analysis was designed solely to demonstrate that under a hypothetical scenario of a single ~ ' <br />point discharge, no change in ground water direction and discharge occurring at the midpoint <br />between wells M-12 and M-13 contaminants in the aquifer would stil[he observed by the <br />existing monitoring well system. Therefore, aworst-case scenario was simulated. <br />Figure 1 presents the plume that would result from a hypothetical source using the values <br />presented in TR-O8 (base case) (black lines). Figure 1 also presents the results of the porosity <br />sensitivity analysis, assuming a porosity of 20 percent ~itl,tlre same loirgituclinal and transverse <br />dispersivity as was presented in the TR-O8 submittal (redlines). The main effegt of this plume <br />at a porosity of 20 percent, as compared to the base case, is that the plume is slightly elongated <br />due to the higher sge~page velocity (18.3 fUyr). Conversely, Figure 1 shows drat at a porosity <br />of 35 percent ue'lines), the plume hasn't travelled as great a distance, due to the smaller <br />seepage velocity (10.4 ac-ft/yr). However, Figure 1 shows that, for all tl>iree cases, the <br />monitoring wells are capable of monitoring contaminants in the aquifer that hypothetically could <br />emanate from the tailings facility or collection pond. <br />Figure 2 shows the same base case as presented in Figure 1 (black lines) and also the <br />hypothetical contaminant plumes that would result from using a longitudinal dispersivity of 30 <br />feet and a transverse dispersivity of 10 feet (blue lines), and a longitudinal dispersivity of 200 <br />feet and a transverse dispersivity of 60 feet (red lines). All cases used the porosity value <br />presented in TR-08 (25 percent). The main difference in the shape of the contaminant plume <br />at dispersivity values of 30/10, as compared to the base case (dispersivity of 100430), is that the <br />plume travels a shorter distance along the main axis (for the same time frame) and tfre plume <br />is narrower. When the dispersivity is increased (to 200/60) and coiTrpared to the base case, the <br />plume is wider and elongated along the main axis. <br />These sensitivity analyses indicate that even varying the parameters within ttre expected range <br />of values that could be reasonable for the Santa Fe Formation, the results are all the same -the <br />existing monitoring well system can adequately monitor for contaminants downgradient of the <br />collection pond and tailings facility. It should also be recognized that as more data are collected, <br />the direction of the gradient could vary as much as 15 to 30 degrees from its ptleserrt position, <br />making the analyses presented above more conservative. <br />-3- <br />