SIMILARITIES IN CHEMICAL TRANSPORT IN CARBONATE AQUIFERS AND FRACTURED ROCK

Carbonate rocks are often regarded as hydrogeologic environments that share little in common with other indurated formations or crystalline rock. Fractures in crystalline rock are pathways for fluid movement and chemical transport, just as dissolution enhanced features in carbonate rocks serve as conduits for flow. Unlike fractures in crystalline rock, however, dimensions of dissolution enhanced features in some carbonate rocks may range from millimeters to tens of meters, resulting in complex responses to ground-water recharge, flow, and chemical transport. Formation properties frequently used to characterize some fractured rock settings, such as hydraulic conductivity, transmissivity, and dispersivity, cannot necessarily be applied to carbonate rocks over dimensions where individual conduits dominate hydraulic and chemical responses. Nevertheless, comparisons of controlled tracer tests conducted in both carbonate and fractured crystalline rocks show dramatic similarities in the breakthrough curves of conservative tracers used in these tests. Tracer experiments were conducted in limestone of the Biscayne Aquifer in Florida and the Madison Aquifer in South Dakota. The results of these tests, conducted over dramatically different distances and travel times, were similar to the breakthrough curves from tests conducted in crystalline rock in central New Hampshire. In particular, the slopes of the declining limb of logarithmic plots of concentration versus time were similar. It is hypothesized that the range of variability in the fluid velocity controls the slope of the declining limb of the breakthrough curves. The similarity in the slopes of the declining limbs of the breakthrough curves from various geologic settings suggests that individual conduits or fractures may control the first arrival and the majority of the mass arrival of a chemical constituent; however, carbonate and fractured rocks both have a distribution of smaller conduits or fractures with velocities ranging over many orders of magnitude that control the residual chemical mass moving through these formations. Understanding the fate of the residual chemical mass moving through carbonate and fractured rocks is extremely important in assessing the longevity of contaminants that have been introduced into the subsurface.