Assumptions:

The well discharges at a constant rate, Q, or at steps of constant rate Q₁, Q₂ ... Q_n.

Well is of infinitesimal diameter and is open through the full thickness of the aquifer.

The nonleaky aquifer is homogeneous, isotropic, and areally extensive.

Discharge from the well is derived exclusively from storage in the aquifer.

The geometry of the assumed aquifer and well are shown in Fig. 1.

Implications of Assumptions:

Implicit in the assumptions are the conditions of radial flow. Vertical flow components are induced by a control well that partially penetrates the aquifer, that is, not open to the aquifer through the full thickness of the aquifer. If vertical flow components are significant, the nearest partially penetrating observation well should be located at a distance, r, beyond which vertical flow components are negligible. See 5.2.1 of Test Method D4106 for assistance in determining the minimum distance to partially penetrating observation wells and piezometers.

The Theis method assumes the control well is of infinitesimal diameter. The storage in the control well may adversely affect drawdown measurements obtained in the early part of the test. See 5.2.2 of Test Method D4106 for assistance in determining the duration of the effects of well-bore storage on drawdown.

Application of Theis Recovery Method for Unconfined Aquifers:

Although the assumptions are applicable to artesian or confined conditions, the Theis solution may be applied to unconfined aquifers if (A) drawdown is small compared with the saturated thickness of the aquifer or if the drawdown is corrected for reduction in thickness of the aquifer and (B) the effects of delayed gravity yield are small. See 5.2.3 of Test Method D4106 for guidance in treating reduction in saturated thickness and delayed gravity drainage in unconfined aquifers.

FIG. 1 Cross Section Through a Discharging Well in a Nonleaky Aquifer