A reservoir model of tracer transport for karstic flow systems

A reservoir approach to analyzing tracer transport in steady-state karstic groundwater systems is evaluated. Tracer transport is dictated by a central flow system made up of a series of reservoirs or pools. These reservoirs may also have a ‘side capacity’ that represents less integrated hydraulic features that act to delay tracer as it moves through the system. The composite system results in a relatively extended tracer signal following a rapid first arrival, a behavior often observed in tracer experiments performed in karst. The reservoir model is an alternative to the often-used advection-dispersion model augmented with a first-order exchange between mobile and immobile fractions of the flow system (the MIM model). A well-known field tracer test is revisited using both the reservoir and MIM approach. The reservoir model indicates that ‘immobile’ zones actually represent only a small delay in tracer transport, but these delays, in sum, lead to a significant tail on the breakthrough curve. It is suggested that a reservoir analysis of tracer breakthrough in karst systems may be more relevant to physical descriptions of karstic groundwater systems than models such as MIM, which are based upon solute transport in porous media.