ASTM D5920-96(2005)

Assumptions:

The control well discharges at a constant rate, Q.

The control well, observation wells, and piezometers are of infinitesimal diameter.

The unconfined aquifer is homogeneous and really extensive.

Discharge from the control well is derived initially from elastic storage in the aquifer, and later from gravity drainage from the water table.

The geometry of the aquifer, control well, observation wells, and piezometers is shown in Fig. 2. The geometry of the test wells should be adjusted depending on the parameters of interest.

Implications of Assumptions:

Use of the Neuman (1) 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.

If drawdown is large compared with the initial saturated thickness of the aquifer, the late-time drawdown may need to be adjusted for the effect of the reduction in saturated thickness. Section 5.2.3 of Test Method D4106 provides guidance in correcting for the reduction in saturated thickness. According to Neuman (1) such adjustments should be made only for late-time values.

FIG. 2 Cross Section Through a Discharging Well Screened in Part of an Unconfined Aquifer

1.1 This test method covers an analytical procedure for determining the transmissivity, storage coefficient, specific yield, and horizontal-to-vertical hydraulic conductivity ratio of an unconfined aquifer. It is used to analyze the drawdown of water levels in piezometers and partially or fully penetrating observation wells during pumping from a control well at a constant rate.

1.2 The analytical procedure given in this test method is used in conjunction with Guide D4043 and Test Method D4050.

1.3 The valid use of the Neuman method is limited to determination of transmissivities for aquifers in hydrogeologic settings with reasonable correspondence to the assumptions of the theory.

1.4 The values stated in SI units are to be regarded as standard.

1.5 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.