Special Problems of Ground-Water Monitoring in Karst Terranes

Reliable monitoring of ground-water quality in any terrane is difficult. There are many ways in which violation of sound principles of monitoring-network design and good sampling protocol makes it easy to acquire data that are not representative of the water or pollutants within an aquifer. In karst terranes it is especially easy for irrelevant data, which inadvertently misrepresent conditions within the aquifer, to be obtained.

The special problems of monitoring ground water in most karst terranes can be grouped into four major categories of problems that are rarely as significant in other terranes. These categories are the following: 1. Where to monitor for pollutants: The only relevant locations are at springs, cave streams, and wells that have been shown by tracing tests to include drainage from the facility to be monitored—rather than at wells to which traces have not been run but which were selected because of their convenient downgradient locations. Wells located on fracture traces and fracture-trace intersections and wells located randomly can be successfully used for monitoring, but only if there is a positive trace from the facility to them. Often successful monitoring can only be done several kilometres away from the facility. 2. Where to monitor for background: The only relevant locations are at springs, cave streams, and wells in fractured rock—in which the waters are geochemically similar to those to be monitored for pollutants but which have been shown by tracing tests not to drain from the facility—rather than at wells selected because of their convenient locations upgradient from the facility site. This, too, may have to be done several kilometres away from the facility. 3. When to monitor: Before, during, and after storms or meltwater events—rather than regularly with weekly, monthly, quarterly, semiannual, or annual frequency. 4. How to determine reliably and economically the answers to Problems 1, 2, and 3: Reliable monitoring of ground water in karst terranes can be done, but it is not cheap or easy.

These problems exist because many of the assumptions made for monitoring flow in granular media are not valid for karst terranes. Implicit assumptions made for monitoring in karst terranes with the strategy recommended herein can be stated axiomatically, but they are valid only about 95% of the time.

The monitoring strategy recommended herein is not applicable universally, but it is applicable in most karst aquifers, especially, all those that drain to springs. It is not applicable in terranes that are merely recharge areas of regional aquifers.