Environmental Engineering Reference
In-Depth Information
precipitation or from a body of surface water such as a river or lake.
15
Conined
aquifers have water tables above their upper boundaries; thus a well dug into
a confined aquifer may find pressurized water or even artesian flow to the
surface.
The storage coefficient is an important characteristic that distinguishes
confined and unconfined aquifers. Confined aquifers have very low storage
coefficient values (generally less than 0.01 and as little as 10
-5
). These val-
ues indicate that a confined aquifer stores water using the mechanisms of
aquifer matrix expansion and the compressibility of water; both typically are
quite small quantities. Unconfined aquifers have storage coefficients (spe-
cific yields) normally above 0.01, and they release water from storage by the
mechanism of actually draining the pores of the aquifer, releasing relatively
large amounts of water.
Both unconfined and confined aquifers are candidates for aquifer UPHES
installation. Confined aquifers have the advantage of being much deeper
(farther below the surface) than unconfined aquifers. However, the specific
yields of confined aquifers are decidedly lower than those of unconfined
aquifers. Alternately, while unconfined aquifers have high specific yield
capacities, they are generally much shallower or closer to the surface. Here
again, we see a design trade-off between a high head, low low option and
a low head, high low option. Another important note is that water quality
requirements are more stringent for unconfined aquifers. Table 4.2 gives a
qualitative comparison of the two types of aquifers.
Considering the minimum requirements for an aquifer UPHES system,
for a 200 foot (60.1 meter) thick aquifer, a transmissivity of 2.6 ft
2
/min (40.3
cm
2
/s) translates to a hydraulic conductivity of 0.013 ft/min (0.0066 cm/s).
Table 4.3 summarizes the typical ranges of hydraulic conductivity and trans-
missivity values for different geologic materials. Based on the ranges in the
table, unconsolidated gravel and sand, sedimentary limestone, dolomite,
TABLE 4.2
Qualitative Comparison of Aquifer Types
UnconinedAquifer
ConinedAquifer
Hydraulic conductivity
Medium to high
Low to medium
Storage coefficient
Medium to high
Low
Transmissivity
Medium to high
Low to medium
Depth to water
Low to medium
Low to high
Specific yield
High
Low to medium
Advantages
Existing irrigation wells,
high flow yield
Very high head potential, water
quality specifications easily met
Disadvantages
Stringent water quality
specificationss, water rights
difficult to obtain, depth to
water typically shallow
Low low yield, advanced
completion more difficult to use
