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GEOCHEMICAL MODELING CONSTRAINTS The principle ways by which the composition of water can change along a flow path include: (1) dilution or mixing with other waters of different chemical compositions, thus changing [he mixture to an average of the two waters; (2) precipitation of minerals from dissolved constituents in the water, thus removing some material from the fluid; (3) dissolution of mineral solids from rocks along the flow path, thus adding constituents to the water, and (4) exchange of elements between dissolved solids in the water and minerals along the flow path, thus removing one element from the water while adding another element. In order to determine where a groundwater sample comes from, i[ is necessary to know or to approximate what has happened along the flow path using known compositions of the minerals and waters in that flow path to constrain the outcome of a geochemical model and narrow down the field of chemical possibilities. Where only one of these processes is involved, geochemical modeling can be relatively simple; interactions involving two or more of the processes make modeling more difficult. A standard presumption is that the simplest model is considered the best model. For any geochemical model to be useful, it must fit with the physical and chemical conditions. Geochemical models may cover a range of possible chemical conditions, but only insofar as standard analytical error and natural variability will allow. Chemical interactions should be constrained by the availability of local waters and minerals. The physical conditions at the West Elk Mine indicate that groundwater likely would be available in the underground workings. The quantity of such water that may have been present is not known, but the likelihood of its presence seems nonetheless assured. Because surface meteoric water in this case would have to travel through alluvium derived from local cock and through somewhat fractured rock disturbed by underground mining, contact between the meteoric water and the rock would have been higher than through an equivalent section of undisturbed rock. The amount of contact between the water and the rock, and [he degree to which minerals along the meteoric water flow path may have dissolved and contributed to the composition of the water are not known, but the likelihood of the presence of dissolved constituents in the meteoric water seems again to be nonetheless assured. Thus, a principle assumption of the model was this: I(fa:rl[ water moved,from the farrlts• to the seep, i[ should have encountered an tmknown amount ojmeteoric water which, in turn, contained an unknown quantity of dissolved constituents from [he overlying stratigraphic section. The physical movement of the fault waters to the underground sump likely would have mobilized fine particles in the ditches and in the sump sediments. If the fault waters were not in chemical equilibrium with these sediments, and if the sediments could dissolve quickly enough to dissolve in [he fault water, this would have changed [he composition of the fault waters. Thus, a second assumption of the model was: The warm fault waters may have dissolved some of the Jine sediment benveen the,faul[s and the floor of the underground s•:unp. The simplest mechanism for the sump waters to have moved to the Edwards Portal seep would have been through the 250 to 500 foot thick wall of coal that separates the West Elk workings from the Bear Mine workings underground. Sane sediment filtering and possibly