Environmental Engineering Reference
In-Depth Information
Water level is monitored during the slug test using either a manual water-level
sounder or a pressure transducer, depending on the rate of water-level recovery. For
low-permeability settings, manual measurements can often be made quickly
enough to capture the initial rapid phase of water-level recovery and can easily
be made frequently enough during the slower phase of recovery. A pressure
transducer is a far better choice for wells installed in sand or coarser sediments
where the entire recovery can be completed in a matter of seconds. The transducer
is suspended prior to the test at a depth greater than the reach of the slug to avoid
damage to the transducer, and early enough that the well has recovered to the static
water level following displacement of water during immersion of the transducer.
The combined length of the slug and rope needs to be carefully measured to ensure
that the slug does not slam into the pressure transducer as it is rapidly lowered into
the well. If the slug is completely submerged during deployment, the known slug-
displacement volume can be used to estimate the initial rise (or drop) of the water
level during the test. Calculation of the maximum water-level change can then be
compared with the measured value. A substantial difference between calculated and
measured water-level change may indicate a procedural problem or a problem with
the piezometer construction. It also is important to ensure that the piezometer water
level does not go below the top of the screen or the top of sand pack during the
entire test. For this reason, single-well response tests are not recommended for
water-table wells.
The average (or bulk) hydraulic conductivity ( K b ,ms 1 ) of the material
surrounding the piezometer screen (or sand pack, if present) can be estimated
from the recorded water-level data:
r 2
K b ¼ π
=
ð
FT b
Þ
(3.66)
where r (m) is the radius of the inside of the well casing, F (m) is a shape factor
representing the dimension and geometry of the groundwater flow field around the
screen, and T b (s) is the basic lag time of the piezometer (see below for definition).
The “sample volume” of this method is approximately equal to a sphere with a
radius similar to the length of the well screen, L (m). F is a function of L and R , the
radius of the outer surface of the well screen or the sand pack, if present. Numerous
equations have been suggested to estimate F for different types of piezometers
under different conditions (see Butler 1998). In most cases, if L / R is not substan-
tially smaller than 4, the formula of Hvorslev ( 1951 ) as cited by Freeze and Cherry
( 1979 :341) gives a convenient means to approximate F :
F
¼
2
π
L ln LR
=
ð
=
Þ
(3.67)
T b is determined by plotting head versus time on a semi-logarithmic plot
(Fig. 3.43c ). For convenience, head is normalized as:
HH 0 ¼
=
ð
h
h s
Þ
=
ð
h 0
h s
Þ
(3.68)
Search WWH ::




Custom Search