CARRYING ON JIM'S LEAD WITH A COMBINATION OF MULTIPLE TRACER DATA AND MODELING

Tracing in karst has largely been restricted to the use of injected dyes to determine pathways. Currently more quantitative tracing and analysis of breakthrough curves is being done. Traced velocities in conduits (mean = 2 km/day) give more than just reason to doubt porous medium velocities in unconfined carbonates.

In 1996 an ASTM paper dedicated to Jim Quinlan defined a karst aquifer and actually had calculated sizes of conduits or channels that could sustain turbulent flow at minimal conduit velocities. A few millimeters to a few centimeters is enough to transport contaminants long distances rapidly.

Calculated proportions of flux in the matrix, fractures and conduits vary little between different carbonate aquifers. The matrix porosity is typically 50 - 500 times the conduit porosity, but >94% of the flux is always in conduits, with and the largest amount of storage in the matrix. Information from conduits is vital. Although a rapid traced velocity can infer the presence of conduits and the location of conduits can be reasonably assumed to be in troughs in potentiometric maps. This approach is easier than days and days of geophysics trying to detect conduits

If appropriate modeling codes are used that can accept tracing parameters, excellent results can be obtained and the locations of conduits confirmed, sometimes literally. The horizontal and vertical hierarchy of conduits makes filling in the details of a system more difficult, but contaminants, geochemistry, and natural and anthropogenic isotopic tracers can help. Tracing in between main conduits presents challenges (most wells end up here) but it may only represent a small amount of the flux.