Significance and Use
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.
1.1 This test describes a procedure for conducting a specific capacity test, computing the specific capacity of a control well, and estimating the transmissivity in the vicinity of the control well. Specific capacity is the well yield per unit drawdown at an identified time after pumping started.
1.2 This test method is used in conjunction with Test Method D4050 for conducting withdrawal and injection well tests.
1.3 The method of determining transmissivity from specific capacity is a variation of the nonequilibrium method of Theis (1) for determining transmissivity and storage coefficient of an aquifer. The Theis nonequilibrium method is given in Test Method D4106.
1.4 Limitations—The limitations of the technique for determining transmissivity are primarily related to the correspondence between the field situation and the simplifying assumptions of the Theis method.
1.5 The values stated in SI units are to be regarded as standard.
1.6 This standard may involve hazardous materials, operations, and equipment. This standard does not address safety problems 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
D4050 Test Method for (Field Procedure) for Withdrawal and Injection Well Tests for Determining Hydraulic Properties of Aquifer Systems
D4106 Test Method for (Analytical Procedure) for Determining Transmissivity and Storage Coefficient of Nonleaky Confined Aquifers by the Theis Nonequilibrium Method
aquifers; aquifer tests; control wells; hydraulic conductivity; observation wells; specific capacity; storage coefficient; transmissivity; unconfined aquifers
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