USING LOW FLOW SAMPLING DATA TO ESTIMATE HYDRAULIC CONDUCTIVITY IN MODERATELY PERMEABLE AQUIFERS

Hydrogeologists sample wells to determine aquifer water quality, and conduct slug tests to estimate aquifer permeability. G.A. Robbins et al. (2009), “Determining Hydraulic Conductivity Using Pumping Data from Low-Flow Sampling”, GROUND WATER 47 (2): 271–276, had the brilliant idea to combine these two activities, that is conduct low flow sampling and use the resulting data on flow rates and drawdowns to estimate aquifer hydraulic conductivity (K). This was done by fitting a straight line to the data pairs for low flow rates and drawdowns, as Henry Darcy did with his lab data. Despite the obvious advantages of obtaining both aquifer water quality and K from field measurements originally performed only to get water quality, adoption of this method by our industry has been slow.

Low flow data from multiple shallow wells in three Michigan glacial aquifers have been examined for suitability for this method. These glacial aquifers, while inherently variable, were mostly too permeable for the wells to have significant drawdowns due to low flow sampling; hence, this method was not appropriate for these wells. So far, one well has adequate drawdown to apply this method. While we intend to cross-check the K value against slug tests performed in the same well, we generated “slug out” results by monitoring water-level recovery after low flow pumping stopped. This adds only a small effort to the low flow sampling regimen.

We are currently low flow sampling additional wells, and will cross-check our “slug out” results against the Robbins et al. method and actual slug tests in those wells that have significant drawdown. Again, the method only works in less permeable aquifers where drawdowns are adequate to reasonably define K values.