Environmental Engineering Reference
In-Depth Information
aquifer storage and recovery (ASR) sites are in operation around the U.S.
These projects range from a single well to networks of 30 wells, with recov-
ery capacities ranging from 500,000 gallons per day from single wells to
100 million gallons per day from well fields.”13
13
Recharge wells are designed
to replace water in an aquifer or underground structure by flowing water
backward into a well, thereby recharging the aquifer. In ASR wells, water
is both injected and removed, depending on seasonal cycles and water use
obligations. This type of well sets the precedent for an aquifer UPHES instal-
lation, although aquifer UPHES cycles are much more frequent. Modified
AR and ASR wells designed for direct injection operations have been pro-
posed.
14
Figure 4.11 and Figure 4.12 show options for modified wells to
increase recharge flow capacity. ASR wells can be cost-effective and can be
easily integrated with existing water utility facilities using well fields.
Essentially, these concepts serve to increase possible injection flow by
increasing the completed surface area in contact with the aquifer or by
increasing the well diameter. Horizontal screen pipes, radial screen pipes, or
horizontally dug wells are examples of well designs that increase injection
flow capacity. These installations can increase the surface area interfacing
the aquifer and the radius of influence of a well to achieve higher injection
rates than with a traditional well.
Another option for increasing well injection flow may be the use of an
infiltration pit dug near the bottom of a well. An infiltration pit could be
used to increase to surface area of contact of the well to the aquifer in both
saturated and unsaturated regions. This unused and unproven option may
complicate well completion procedures and increase cost. Figure 4.13 illus-
trates the infiltration pit well concept.
The infiltration pit option presents different implementation challenges,
depending on whether it is used in a confined or unconfined aquifer.
The left half of Figure 4.13 shows a characteristic mound of injection in
the unconfined case. One design consideration here is where to place the
pump turbine unit. The water level in the well changes significantly during
pumping and injection modes. To alleviate the problem of a “dry” pump-
ing situation, an extension pipe that reaches toward the bottom of the com-
pletion is installed. Alternately, during turbine operation, it is desirable to
allow free flow of water at the exit of the turbine. To accomplish this, a short
pipe that dumps water into the air above the water level in the well is pro-
posed. This allows maximum water velocity through the turbine, increas-
ing generating efficiency. In the unconfined aquifer, the situation is more
difficult. It is likely not possible to operate the turbine so that its exit water
dumps into free air. Thus, the velocity of water through the turbine may
not be ideal.
It is difficult to compare the performance of an infiltration pit well to the
other completion options. The factors involved include pit size and dynamic
flow patterns in the wells. Field testing of such well completions is needed to
add to our knowledge about UPHES systems.
