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3.9 Well Hydraulics 71
computed. The problems of direction of motion, dispersion and molecular diffusion, and
the slow movement of groundwater limit the applicability of this method. The method is
impractical for a heterogeneous aquifer that has large variations in horizontal and vertical
hydraulic conductivity.
The drop in head between two equipotential lines in an aquifer divided by the dis-
tance traversed by a particle of water moving from a higher to a lower potential deter-
mines the hydraulic gradient. Changes in the hydraulic gradient may arise from either a
change in flow rate, Q, hydraulic conductivity, K, or aquifer thickness, b (Eq. 3.6). If no
water is being added to or lost from an aquifer, the steepening of the gradient must be due
to lower transmissivity, reflecting either a lower permeability, a reduction in thickness, or
both (Eq. 3.8).
Of the currently available methods for the estimation of formation constants, aquifer
tests (also called pumping tests) are the most reliable. The mechanics of a test involve
the pumping of water from a well at a constant discharge rate and the observation of
water levels in observation wells at various distances from the pumping well at different
time intervals after pumping commences. The analysis of a pumping test comprises the
graphical fitting of the various theoretical equations of groundwater flow to the observed
data. The mathematical model giving the best fit is used for the estimation of formation
constants. The main advantages of this method are that the sample used is large and re-
mains undisturbed in its natural surroundings. The time and expense are reasonable. The
main disadvantage of the method concerns the number of assumptions that must be
made when applying the theory to the observed data. Despite the restrictive assumptions,
pumping tests have been successfully applied under a wide range of conditions actually
encountered.

3.9 WELL HYDRAULICS

Well hydraulics deals with predicting yields from wells and in forecasting the effects of
pumping on groundwater flow and on the distribution of potential in an aquifer. The re-
sponse of an aquifer to pumping depends on the type of aquifer (confined, unconfined,
or leaky), aquifer characteristics (transmissivity, storage coefficient, and leakage),
aquifer boundaries, and well construction (size, type, whether fully or partially
penetrating) and well operation (constant or variable discharge, continuous or intermit-
tent pumping).
The first water pumped from a well is derived from aquifer storage in the immedi-
ate vicinity of the well. Water level (that is, piezometric surface or water table) is low-
ered and a cone of depression is created. The shape of the cone is determined by the
hydraulic gradients required to transmit water through aquifer material toward the
pumping well. The distance through which the water level is lowered is called the
drawdown. The outer boundary of the drawdown curve defines the area of influence of
the well. As pumping is continued, the shape of the cone changes as it travels outward
from the well. This is the dynamic phase, in which the flow is time dependent (non-
steady), and both the velocities and water levels are changing. With continued with-
drawals, the shape of the cone of depression stabilizes near the well and, with time, this
condition progresses to greater distances. Thereafter the cone of depression moves par-
allel to itself in this area. This is the depletion phase. Eventually the drawdown curve
may extend to the areas of natural discharge or recharge. A new state of equilibrium is
reached if the natural discharge is decreased or the natural recharge is increased by an