Assumptions of the Theis (1) equation affect specific capacity and transmissivity estimated from specific capacity. These assumptions are given below:

Aquifer is homogeneous and isotropic.

Aquifer is horizontal, of uniform thickness, and infinite in areal extent.

Aquifer is confined by impermeable strata on its upper and lower boundaries.

Density gradient in the flowing fluid must be negligible and the viscous resistance to flow must obey Darcy's Law.

Control well penetrates and receives water equally from the entire thickness of the aquifer.

Control well has an infinitesimal diameter.

Control well discharges at a constant rate.

Control well operates at 100 percent efficiency.

Aquifer remains saturated throughout the duration of pumping.

Implications of Assumptions and Limitations of Method.

The simplifying assumptions necessary for solution of the Theis equation and application of the method are never fully met in a field test situation. The satisfactory use of the method may depend upon the application of one or more empirical correction factors being applied to the field data.

Generally the values of transmissivity derived from specific capacity vary from those values determined from aquifer tests utilizing observation wells. These differences may reflect 1) that specific-capacity represents the response of a small part of the aquifer near the well and may be greatly influenced by conditions near the well such as a gravel pack or graded material resulting from well development, and 2) effects of well efficiency and partial penetration.

The values of transmissivity estimated from specific capacity data are considered less accurate than values obtained from analysis of drawdowns that are observed some distance from the pumped well.