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Significance and Use
The well discharges at a constant rate, Q, or at steps of constant rate Q1, Q2 ... Qn.
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
1.1 This test method covers an analytical procedure for determining the transmissivity of a confined aquifer. This test method is used to analyze data from the recovery of water levels following pumping or injection of water to or from a control well at a constant rate.
1.2 The analytical procedure given in this test method is used in conjunction with the field procedure in Test Method D4050.
1.3 Limitations—The valid use of the Theis recovery method is limited to determination of transmissivities for aquifers in hydrogeologic settings with reasonable correspondence to the assumptions of the Theis theory (see 5.1).
1.4 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory limitations prior to use.
2. Referenced Documents (purchase separately) The documents listed below are referenced within the subject standard but are not provided as part of the standard.
D653 Terminology Relating to Soil, Rock, and Contained Fluids
D4043 Guide for Selection of Aquifer Test Method in Determining Hydraulic Properties by Well Techniques
D4050 Test Method for (Field Procedure) for Withdrawal and Injection Well Tests for Determining Hydraulic Properties of Aquifer Systems
D4105 Test Method for (Analytical Procedure) for Determining Transmissivity and Storage Coefficient of Nonleaky Confined Aquifers by the Modified Theis Nonequilibrium Method
D4106 Test Method for (Analytical Procedure) for Determining Transmissivity and Storage Coefficient of Nonleaky Confined Aquifers by the Theis Nonequilibrium Method
D4750 Test Method for Determining Subsurface Liquid Levels in a Borehole or Monitoring Well (Observation Well)
ICS Number Code 93.160 (Hydraulic construction)