(2) Nonequilibrium equations are directly applica-

ble to confined (artesian) aquifers and may also be used

ing tests: *equilibrium *(steady-state flow) and *nonequi-*

with limitations to unconfined aquifers (gravity flow

conditions). These limitations are related to the per-

centage of drawdown in observation wells related to

the water discharged initially comes from aquifer stor-

the total aquifer thickness. Nonequilibrium equations

age adjacent to the well. As pumping continues, water

should not be used if the drawdown exceeds 25 percent

is drawn from an expanding zone until a state of equi-

of the aquifer thickness at the wall. Little error is

librium has been established between well discharge

introduced if the percentage is less than 10.

and aquifer recharge. A state of equilibrium is reached

when the *zone of influence *has become sufficiently en-

larged so that: natural flow into the aquifer equals the

(1) Both *equilibrium and nonequilibrium *methods

pumping rate; a stream or lake is intercepted that will

for analyzing aquifer performance are generally based

supply the well (fig. C-1); or vertical recharge from

on the assumptions that:

precipitation on the area above the zone of influence

equals the pumping rate. If a well is pumped at a con-

stant rate until the zone of drawdown has become sta-

zontal direction from the well and has a constant

bilized, the coefficient of permeability of the aquifer

thickness.

can be computed from *equilibrium *formulas subse-

quently presented.

formation.

(1) In this type of test, the value of k is computed

(2) Although the assumptions listed above would

from a relation between the rate of pumping Q, draw-

seem to limit the analysis of pumping test data, in

down H' at a point P near the well, distance from the

reality they do not. For example, most pervious forma-

well to the point of drawdown measurement r, coeffi-

tions do not have a constant k or transmissibility

cient of storage of the aquifer S, and elapsed pumping

T(T = k x aquifer thickness), but the average T can

time t. This relation permits determination of k from

readily be obtained from a pumping test. Where the

aquifer performance, while water is being drawn from

flow is artesian, stratification has relatively little im-

storage and before stabilization occurs.

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