Geology Reference
In-Depth Information
Observation Well (Ob Well)
To measure drawdown within the cone of depression caused by discharging
well. As a general rule, the observation wells should be placed at a distance
of 1.5 to 2.0 times the thickness of the aquifer. More than one observation
well, if available, would also give the directional inhomogeneities, if any.
The position of observation wells should be considered with respect to:
(a) type of aquifer, (b) hydraulic conductivity, (c) well penetration, (d)
geohydrologic boundaries, (e) discharge and (f) stratification. Observation
wells should not be located near a recharge area, as the data from these wells
may not reflect actual storage coefficient
The operation for pumping test involves prior information at the test site:
(a) measurement of static water level (before pumping begins), the depth
below a measuring point or ground level
(b) the lithological characteristics and the details of well depth, length of
casing inserted, the discharge of the well (say at the time of drilling)
(c) general water table, the performance of nearby wells.
EQUIPMENT REQUIRED
A truck-mounted test-pumping unit should comprise a submersible pump
(with three-phase motor) and a (diesel) power generating set (since
uninterrupted power may not be available at the test site as in most parts of
India) and tripod/hydraulic arrangement for smooth lowering of pump and
pipes. The pump and power unit for a pumping test should be capable of
operating continuously at constant discharge rate for longer periods of time.
Suitable measurement devices for time, discharge and water level. The time
intervals may be best noted through a stopwatch, and if several wells have
to be monitored then all the watches have to be matched to a common time.
Measurement of Water Level
Best device is automatic water level recorder since this gives a continuous
record of changes in water level measurements. The other types of devices
in usage are (a) chalked steel tape, (b) whistle tape, (c) hollow cylinder, (d)
digital tape and (e) electrical tape. These can be used accurately for
measurement in the observation wells since the water levels are not disturbed
by changes in pumping rate and inflows into the well (as in the case of
fractured rocks).
Since the water levels decline at a faster rate during the early stages of
pumping, it is required that the frequency of measurements is high in the
beginning of the test. The yield test is generally run for 10-15 hours
continuously and till constant draw down is maintained in the well. Thus, the
specific capacity of the well is its yield per unit draw down and for the given
pumping period. This may also vary with the wet and dry seasons.
