Transmissivity can be estimated from multi-well aquifer-test data with the distance-drawdown method after a quasi-steady-state has been established (Cooper and Jacob, 1946). Most of the water released from storage originates beyond the wells that are being analyzed once quasi-steady-state conditions are established. A quasi-steady-state is reached after dimensionless time (u) is less than 0.1 at the well furthest from the pumping well. This would be t > 2.5*r² S/ T, where t is time, r is radial distance from pumping well, S is storage coefficient, and T is transmissivity in consistent length-time units. Quasi-steady-state conditions typically occur within 500 ft of the pumping wells after 6 hours of pumping from many confined aquifers. Site specific estimates for a Theis aquifer can be estimated with an analytical distance-drawdown tool.
Distance-drawdown is a simple graphical method (Weissman and others, 1977) that solves for transmissivity (T) of confined aquifers and hydraulic conductivity (K) of unconfined aquifers. The workbook DistanceDrawdown_Pumping-2019.xlsm revises the original workbook from Halford and Kuniansky (2002) to function with Excel 2013+ (Figure 1). Drawdown at distances from the pumping well are specified and plotted on a semi-log plot as drawdowns in confined aquifers and as saturated thickness squared in unconfined aquifers. A straight line initially is regressed to distance-drawdown data with the “GROSS FIT” button. Fit between straight line and plotted data can be refined visually to ignore outliers with the “ADJUST LINE” button. Transmissivity and hydraulic conductivity are reported with a user defined number of significant digits.
DistanceDrawdown_Pumping-2019.xlsm and explanatory PDF can be downloaded with the following link.
References
Cooper, H.H., and C.E. Jacob. 1946. A generalized graphical method for evaluating formation constants and summarizing well field history. American Geophysical Union Transactions 27: 526–534. https://doi.org/10.1029/TR027i004p00526
Halford, K.J. and E.L. Kuniansky 2002, Documentation of spreadsheets for the analysis of aquifer pumping and slug test data, USGS OF 02-197 https://pubs.usgs.gov/of/2002/ofr02197/
Weissman, Warren, Jr., Knapp, J.W., Lewis, G.L., and Harbaugh, T.E., 1977, Introduction to Hydrology: Harper and Row, NY, 704 p.