Page 132 - Fair, Geyer, and Okun's Water and wastewater engineering : water supply and wastewater removal

JWCL344_ch03_061-117.qxd 8/17/10 7:48 PM Page 94

94 Chapter 3 Water Sources: Groundwater
waste disposal (or artificial recharge) and for water supply in the same area. The recirculation
can be minimized by locating the recharge well directly downstream from the discharge well.
The critical value of discharge and optimum spacing for no recirculation can be evaluated.
The permissible distance, r , between production and disposal wells in an isotropic,
c
extensive aquifer to prevent recirculation is given by
r 2Q>( >TI) (3.38)
c
3
where r is in ft or m, Q is the equal pumping and disposal rate in gpd or m /d, T is the
c
3
transmissivity in gpd/ft (m /d/m), and I is the hydraulic gradient of the water table or
piezometric surface.
3.13.2 Location of Aquifer Boundaries

In many instances, the location and nature of hydraulic boundaries of an aquifer can be
inferred from the analysis of aquifer-test data. The effect of a boundary when it reaches
an observation well causes the drawdowns to diverge from the curve or the straight-line
methods. The nature of the boundary, recharge, or barrier is given by the direction of
departures. An observation well closer to the boundary shows evidence of boundary ef-
fect earlier than does an observation well at a greater distance. The theory of images
can be used to estimate the distance to the boundary. The analysis can be extended to
locate multiple boundaries.
For the estimation of the formation constants, only those observations should be used
that do not reflect boundary effects, that is, the earlier part of the time-drawdown curve.
For the prediction of future drawdowns, the latter part of the curve incorporating the
boundary effects is pertinent.

3.14 CHARACTERISTICS OF WELLS
The drawdown in a well being pumped is the difference between the static water level and
the pumping water level. The well drawdown consists of two components:
1. Formation loss, that is, the head expended in overcoming the frictional resistance
of the medium from the outer boundary to the face of the well, which is directly
proportional to the velocity if the flow is laminar
2. Well loss, which includes (a) the entrance head loss caused by the flow through the
screen and (b) the head loss due to the upward axial flow of water inside the screen
and the casing up to the pump intake. This loss is associated with the turbulent flow
and is approximately proportional to the square of the velocity.

The well drawdown D can be expressed as
w
D BQ CQ 2 (3.39a)
w
where B summarizes the resistance characteristics of the formation and C represents the
characteristics of the well.
For unsteady flow in a confined aquifer, from Eq. 3.18,
B (264>T) log(0.3Tt>r S) (3.39b)
2
w
This shows that the resistance of an extensive artesian aquifer increases with time as
the area of influence of the well expands. For relatively low pumping rates, the well loss
may be neglected, but for higher rates of discharge it can represent a sizable proportion of
the well drawdown.