Nonequilibrium-type test

(1) In this type of test, the value of к is computed from a relation between the rate of pumping Q, draw­down H’ at a point P near the well, distance from the well to the point of drawdown measurement r, coeffi­cient of storage of the aquifer S, and elapsed pumping time t. This relation permits determination of k from aquifer performance, while water is being drawn from storage and before stabilization occurs.

(2) Nonequilibrium equations are directly applica­ble to confined (artesian) aquifers and may also be used with limitations to unconfined aquifers (gravity flow conditions). These limitations are related to the per­centage of drawdown in observation wells related to the total aquifer thickness. Nonequilibrium equations should not be used if the drawdown exceeds 25 percent of the aquifer thickness at the wall. Little error is introduced if the percentage is less than 10.

b. Basic assumptions.

(1) Both equilibrium and nonequilibrium methods for analyzing aquifer performance are generally based on the assumptions that:

(a) The aquifer is homogeneous and isotropic.

(b) The aquifer is infinite in extent in the hori­zontal direction from the well and has a constant thickness.

(c) The well screen fully penetrates the pervious formation.

(d) The flow is laminar.

(e) The initial static water level is horizontal.

(2) Although the assumptions listed above would seem to limit the analysis of pumping test data, in reality they do not. For example, most pervious forma­tions do not have a constant к or transmissibility T(T = к x aquifer thickness), but the average T can readily be obtained from a pumping test. Where the flow is artesian, stratification has relatively little im-

Nonequilibrium-type test

C-l

 

portance if the well screen fully penetrates the aqui­fer; of course, the derived permeability for this case is actually kh. If the formation is stratified and kh >_ kv, and the flow to the well is gravity in nature, the com­puted permeability к would be <kh and >kv.

(3) Marked changes of well or aquifer perfor­mance during a nonequilibrium test indicate that the physical conditions of the aquifer do not conform to the assumptions made in the development of the formula for nonsteady flow to a well. However, such a departure does not necessarily invalidate the test data; in fact, analysis of the change can be used as a tool to better determine the flow characteristics of the aqui­fer.

C-2. Pumping test equipment and proce­dures. Determination of k from a pumping test re­quires: (a) installation of a test well, (b) two, and pref­erably more, observation wells or piezometers, (c) a suitable pump, (d) equipment for sounding the well
and adjacent piezometers, and (e) some means for accu­rately measuring the flow from the well.

a. Test and observation wells. The test well should fully penetrate the aquifer to avoid uncertainties in­volved in the analysis of partially penetrating wells, and the piezometers should be installed at depths below any anticipated drawdown during the pumping test. The number, spacing, and arrangement of the ob­servation wells or piezometers will depend on the char­acteristics of the aquifer and the geology of the area (figs. C-2 and C-3). Where the test well is located ad­jacent to a river or open water, one line of piezometers should be installed on a line perpendicular to the river, one line parallel to the river, and, if possible, one line away from the river. At least one line of piezometers should extend 500 feet or more out from the test well. The holes made for installing piezometers should be logged for use in the analysis of the test. The distance from the test well to each piezometer should be meas-

Nonequilibrium-type test

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Nonequilibrium-type test

PIEZOMETERS

 

PROPOSED CONSTRUCTION SITE

 

>P

 

U. S. Army Corps of Engineers

 

Nonequilibrium-type testNonequilibrium-type test

ured, and the elevation of the top of each accurately determined. Each piezometer should be capped with a vented cap to keep out dirt or trash and to permit change in water level in the piezometer without cre­ating a partial vacuum or pressure. The test well and piezometers should be carefully installed and devel­oped, and their performance checked by individual pumping or falling head tests in accordance with the procedures discussed in chapter 5 of the main text.