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densely and the Darcy velocity was approximately one-half the value used <br />in the CR-A test. The tests were conducted under these different • <br />conditions to provide an indication of the sensitivity of the results to <br />the experimental parameters. <br />The ESM test conducted as described above consists of injecting <br />distilled water into a porous medium already containing moisture at <br />chemical equilibrium with the solid phase. Before effluent begins to <br />flow at the top of the column, the flow process is one of unsaturated <br />f:1ow. An important feature of this process is that the antecedent <br />moisture is displaced ahead of the injected fluid with a thin zone of <br />mixing caused by hydrodynamic dispersion. From a practical standpoint, <br />this result means that the first increment of effluent from the top of <br />the column is comprised entirely of antecedent moisture that has not been <br />diluted or otherwise chemically modified. Furthermore, the chemical <br />composition of the first effluent is independent of length of column, <br />injection rate, bulk density, etc. <br />Equally important is the fact that the displacement of antecedent <br />moisture as experienced in the ESM test also occurs in the field. Thus, • <br />the short column tests conducted here yield the chemical composition of <br />the 'first flush' leachate caused by invasion of ground or river waters <br />into the backfilled refuse, regardless of the length of flow path and <br />residence time. The much longer residence times expected in the field <br />will effect the rate of decline of dissolved constituents in the <br />leachate, but the maximum values are accurately modeled by the short <br />column tests. <br />The electrical conductivity and pH as functions of effluent volume <br />are given in Table 30, Leaching Data for Coal Refuse, and the EC is shown <br />graphically in Figure 18, Electrical Conductivity As A Function of <br />Effluent Volume. It is noted that the EC values for the first increment <br />of effluent are practically identical for both tests. This is'to be <br />expected from the displacement theory mentioned above. The somewhat <br />slower rate of decline for test CR-B results from longer residence time <br />(lower injection rate). <br />(Revised OS/11N4) • <br />2.05-66 <br />