| Two new simulation models have been developed to investigate questions involving the earliest stages of conduit development within karst aquifers. Results of single conduit modeling under a wide variety of initial conditions show a consistent pattern of kinetic control on limestone dissolution rates that varies as functions of time as well as position along the conduit. Slow, higher order surface reaction rates (close to equilibrium), diffusion rates, and rapid, lower order reaction rates (far from equilibrium) are found to be limiting steps at various times and locations. Under no conditions of circular conduit development did the rate of CO{dollar}sb2{dollar} hydration limit dissolution.; Under the influence of geologically reasonable carbon dioxide pressures and head gradients, minimum diameters of flowpaths that will form into conduits are on the order of 100 microns, while those only a few times larger will form conduits easily. Competition for flow between potential flowpaths is thus a strong function of initial size. Not only have constrictions been observed to lower rates of passage growth, but the effect is sensitive to the position of the constriction.; Network flow modeling suggests that important patterns of selective enlargement occur even during the very early, laminar flow stages of passage development. In the absence of variations in initial fracture size, major flowpaths to develop are those along the most direct path between flow entrance and exit. Those passages most closely aligned with the hydraulic gradient will also be favored for enlargement.; Runs with randomly varying initial fracture widths within the network suggest that with a large variation in widths the initial fracture distribution will be a major influence on the pattern of flow routes that ultimately develops. Of the hypotheses proposed in the literature to explain maze (non-selective) vs. branchwork (selective) pattern of cave development, the most strongly supported is that a number of alternate flowpaths can form where flows are forced to seek alternate routes around constrictions. Suggestions that (1) maze caves form as a result of phreatic, artesian flows, and (2) flowpath selectivity will only begin once turbulent flow conditions are reached were not supported by simulation results. |
