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<br />The above values served as inputs for mixing and dispersion calculations to simulate potential <br />consequences on the alluvial aquifer and on the groundwater (Dakota Formation) aquifer. The <br />calculations assumed no adsorption or attenuation of metals and no chemical reactions. <br />Model calculations used a mixture of measured values, rational assumptions, and worst <br />case assumptions. The authors presumed that this approach is conservative. Measured values <br />were input for CKD volume, leach test results and hydraulic conductivity. Rational assumptions <br />were input for hydraulic gradient (actually, a range of values from reasonable to worst case were <br />tested), dispersion coefficient, and saturated thickness of alluvium and Dakota Sandstone. <br />Conservative and/or worst case assumptions were input for CKD cover composition, leachate <br />flux rate, porosity of the CKD, and metal adsorption and attenuation. The model assumed <br />porous media flow rather than fracture flow. <br />DISCUSSION <br />On the presumption that the input values for hydraulic gradient and hydraulic conductivity are <br />either rational or conservative, the modeling results do represent a conservative approach. The <br />assumption of porous media flow rather than fracture flow seems reasonable; casual field <br />observations from my earlier site inspection show that the formations are gently folded, have no <br />visible faults, and an unremarkable set of orthogonal fractures with apparently wide spacing. <br />Moreover, the pit floors and walls form asemi-confining zone whereby, according to the <br />operator, evaporation, not Flow to groundwater, is the main source of water loss from the pit. <br />In an actual water rock system such as this, adsorption and/or ion exchange involving <br />clays should remove some metals from solution. The model assumed no adsorption or ion <br />exchange; metals attenuation was achieved solely by dilution. Also, high pH solutions of lime <br />should be buffered by COZ and/or clay reactions, yet the model presumed no reactions, only <br />straight line mixing. Overall, these form the basis of a geochemically conservative model <br />because metals and chemicals that would react to attenuate metals and buffer pH in real solutions <br />were presumed to be inert and obey straight line mixing behavior. <br />Verification of [he model results if provided in part through an analysis of the pit water. <br />The pit collects precipitation and dust control water runoff directly from the CKD along with <br />some seepage from the upland irrigation ditch. Except for sulfate, which is higher in the pit <br />water than in the CKD leachate, and thallium and selenium, which aze lower in the pit water than <br />in the leachate, the two waters aze similar in composition. <br />The CKD was leached using the Synthetic Precipitation Leach Procedure (SPLP) (EPA <br />Method 1312). This is a physically aggressive test, even for already powdered materials, <br />because the samples are rolled in a vat for 18 hours to assure maximum contact between solids <br />and solutions. The procedure is only moderately chemically aggressive. The high pH from the <br />leachate tests is apparently due to dissolution of unreacted lime in the solids. Lime, stored in the <br />atmosphere, fairly quickly reacts with atmospheric carbon dioxide to form calcium carbonate <br />(limestone) provided air can reach the lime. Because pH from the A-pit sample was similar to <br />groundwater (about pH 8), it appears that where lime is in contact with rainwater, lime is being <br />rapidly neutralized. <br />