Worthington Groundwater
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Rapid changes in water level and electrical conductivity

Borehole methods

Boreholes are the standard technique in porous medium aquifers (such as sand) for measuring aquifer characteristics. In carbonate aquifers they are important for learning about flow in the matrix, in fractures, and in the smaller channels. A single borehole has a low probability of intersecting major channels, so borehole tests are best supplemented by well-to-well or well-to-spring tracer tests and by spring monitoring, both of which yield important information on the larger channels.

The most useful borehole tests for characterizing carbonate aquifers are those which information on the distribution of permeability or water quality within the borehole. These include downhole video or borehole imaging (with the ability to view horizontally and thus see the channels), sensitive flow metering (e.g. heat pulse flow meter), and electrical conductivity (EC) profiling during pumping tests. Pumping tests are useful, as are continuous water level and EC measurements. Many carbonate aquifers have rapid changes in water level and water quality following rain, on the scale of hours, so continuous measurement is appropriate, and is straightforward with modern instrumentation.

The image shows short-term changes in water level and electrical conductivity following rain at a monitored depth of 21-26 m below the surface at a PCB-contaminated site. Further details can be found in Test methods for characterizing transport in a glaciated carbonate aquifer.

We have discussed borehole testing methods for karst in Borehole tests for megascale channeling in carbonate aquifers (Proceedings of the 6th conference on limestone hydrology and fissured media, La Chaux-de-Fonds, Switzerland, 1997) and in a 1999 paper A comprehensive strategy for understanding flow in carbonate aquifers