Accepted Papers - 3.pdf - UNESCO

H eff = H R – H aq – H f …….. ……… ….. (2)
A simple guideline can be inferred from this is
“Higher the recharge head, lower the static head of
aquifer and lesser the frictional head losses higher
is the Effective head and higher is the recharge rate”.
The Frictional Head Losses H f : Following are the
points causing frictional loss of head in a recharge
well –
� Filter pit Media
Every recharge well must have a filter pit to reduce
the turbidity of water entering the well to reduce
the chocking of screens, filter pack (i.e. gravel pack)
and formation. The sand media bed used in the filter
pit will cause frictional head loss (henceforth termed
as H fpm ). This is dependent upon the permeability of
the sand media and its grain size and depth of media
bed.
� Filter pit Underdrain system
At the bottom of the sand media there has to be an
underdrain system for collection of percolated water
and direct it inside the well pipe assembly and to
retain the filter pit media. This causes some frictional
head loss (henceforth termed as H fpu ).
� Well casing
When the water flows down the casing friction with
the casing wall will cause frictional head loss
(henceforth termed as H fc ).
� Screens
When the water passes thru the slots of well casing
frictional head loss takes place which is dependent
on the permeability of the screens used (Henceforth
termed as H fs ). Higher the screen permeability
lesser is the frictional head loss.
� Filter pack (Gravel pack)
The gravel packing around the screen will cause
some frictional head loss depending upon its
permeability and grain size. (Henceforth termed as
H fg ).
� The Aquifer formation
The water has to then travel thru the aquifer
formation and there could be significant head loss
367
due to friction of the formation material. This is
dependent upon the permeability of the aquifer
material, the aquifer thickness and aquifer depth
from the well wall which is variable. This is
henceforth termed as H fformation .
• Hydraulic equilibrium and radius of
influence
When the water enters the aquifer it would have
already suffered all the frictional losses as explained
above and will have residual effective head pressure
H effR =H R - H aq – H fpm -H fpu -H fc -H fs -H fg .
The water then will keep penetrating the
aquifer till the time its residual effective head is
completely decayed by the formation friction. In
other words the penetration will continue till the
residual effective head becomes zero. This will be
state of hydraulic equilibrium and recharging beyond
this point in the aquifer will not take place. Hence
this point will be the extreme point of recharge
influence and hence the distance of this point to the
center of the well can be termed as “The Radius of
Influence”.
It can be inferred from this discussion that “the
radius of influence of recharge is the distance
between a point away from the wall of the well (i.e.
outer surface of the gravel pack) at which the total
frictional head of the aquifer formation of thickness
b equals the residual effective head and the center
of the well casing”.
Cone of Recharge, Draw up and radius of
influence
A cone of depression gets formed in a pumping
well and draw down occurs when the pumping is
started. Similarly in a recharge well an inverted cone
is formed and draw up (henceforth termed as H du
takes place whereby the water level rises up above
the static water level and radius of influence is the
radius of the cone or the distance of the point of no
draw up to the center of the well. The formula for
estimating the rate of recharge based on cone of
recharge is as follows:
Q R = KbH du /(528*log(r o /r w ))
Where,
Q R = Rate of recharge in gpm
K = Hydraulic conductivity in gpd/ft 2