WATER WELL SPECIFIC CAPACITY TESTING FOR AQUIFER TRANSMISSIVITY   

Water well specific capacity may be used to estimate aquifer transmissivity.  Transmissivity derived from specific capacity is useful because much specific capacity data is available from water well driller's completion reports.  I have used such data for projects in Montana and Colorado, where driller's reports may be obtained from state agencies.  Other states also make driller's data available to the public.  Often, driller's reports contain the only aquifer data available for an area prior to performing more detailed aquifer tests (e.g., step tests, leaky aquifer tests, fractured aquifer tests, injection tests).  Rough estimates of transmissivity calculated from driller's specific capacity data can be a useful, cost-effective reconnaissance tool in groundwater investigations.

A value for transmissivity may be obtained from the Cooper-Jacob equation:

Q/s = T/[264 log10(Tt/2693rw2S)+165]

where:

Q/s = specific capacity, gpm/ft

   T = transmissivity, gpd/ft

   S = aquifer storativity, dimensionless

  rw = well radius, ft

   t = time after pumping started, minutes

This equation cannot be solved directly for T.  It must be solved by successive approximations.  I have written a Python program to perform the calculation.  It is presented in Figure 1.  This program makes it possible to use numerous specific capacity values if they are available for a groundwater investigation.  I have used it in subdivision water supply investigations and a mine dewatering study.