Procedure for Conducting Pumping Tests part (3)
The pump and power unit should be capable of operating continuously at a constant discharge for a period of at least a few days.
There are several factors to be considered when determining the type of pump to be used and the depth at which it should be set, including:
1) well diameter
2) desired pumping rate
3) total dynamic head including the pumping water level, the above ground head (if applicable) and all friction losses in the casing, pipes, fittings, etc.;
4) reliability of power source; and
5) horsepower requirements.
An even longer period may be required for unconfined or leaky aquifers, and especially for fractured aquifers. In such cases, pumping should continue for several days more. The capacity of the pump and the rate of discharge should be high enough to produce good measurable drawdowns in piezometers as far away as, say, 100 or 200 m from the well, depending on the aquifer conditions.
Discharging the pumped water:
The water delivered by the well should be prevented from returning to the aquifer of the same well. This can be done by conveying the water through a large-diameter pipe, say over
a distance of 100 or 200 m, and then discharging it into a canal or natural channel.
Piezometers:
Bore wells used to only measure the water levels nearer to the pumping wells are called as piezometers. The water levels measured in piezometers represent the average head of the nearby aquifer. Piezometers should be placed not too near the well, and not too far from it, also.
Depth of the piezometers:
The depth of the piezometers is at least as important as their distance from the well. In an isotropic and homogeneous aquifer, the piezometers should be placed at a depth that coincides with that of half the length of the well screen. For example, if the well is fully penetrating and its screen is between 10 and 20 m below the ground surface, the piezometers should be placed at a depth of about 15 m.
The type of aquifer:
When a confined aquifer is pumped, the loss of hydraulic head propagates rapidly because the release of water from storage is entirely due to the compressibility of the aquifer material and that of the water. The drawdown will be measurable at great distances from the well, say several hundred metres or more. In unconfined aquifers, the loss of head propagates slowly. Here, the release of water from storage is mostly due to the dewatering of the zone. A leaky aquifer occupies an intermediate position.
Transmissivity:
When the transmissivity of the aquifer is high, the cone of depression induced by pumping will be wide and flat . When the transmissivity is low, the cone will be steep and narrow. In the first case, piezometers can be placed farther from the well than they can in the second.
The duration of the test:
The duration of the pumping test depends on the purpose of the well, the type of aquifer and any potential boundary conditions.
Theoretically, in an extensive aquifer, as long as the flow to the well is unsteady, the cone of depression will continue to expand as pumping continues. Therefore, for tests of long duration, piezometers can be placed at greater distances from the well than for tests of short duration.
The discharge rate :
During the aquifer test, discharge should be measured accurately and frequently enough to verify that a constant discharge rate is being achieved. Waste of the discharge should be avoided.
If the discharge rate is high, the cone of depression will be wider and deeper than if the discharge rate is low. With a high discharge rate, therefore, the piezometers can be placed at greater distances from the well.
Control of the pumping rate:
Control of the pumping rate during the test is important. Because it allows for reliable drawdown data to be collected to determine the yield of the well and aquifer properties.
Controlling the pumping rate by adjusting the pump speed is generally not satisfactory.
It is better to use a gate valve to adjust the pumping rate to keep it constant.
The discharge pipe and the valve should be sized so that the valve will be from ½ to ¾ open when pumping at the desired rate.
The valve should be installed at a sufficient distance from the flow measurement device to avoid any impacts from turbulence.
Measuring the discharge of pumped water accurately is also important and common methods of measuring discharge include the use of an orifice plate and manometer.
Aquifers with stratification:
Homogeneous aquifers are rare in nature. Most of the aquifers are stratified to some extent. Stratification causes differences in horizontal and vertical hydraulic conductivity, so that the drawdown observed at a certain distance from the well may differ at different depths within the aquifer. As pumping continues, these differences in drawdown diminish. Moreover, the greater the distance from the well, the less effect stratification has upon the drawdowns.
Fractured rock :
Deciding on the number and location of piezometers in fractured rock poses a special problem, although the rock can be so densely fractured that its drawdown response to pumping resembles that of an unconsolidated homogeneous aquifer; if so, the number and location of the piezometers can be chosen in the same way as for such an aquifer.
The measurements to be taken:
The measurements to be taken during a pumping test are of two kinds:
- Measurements of the water levels in the well and the piezometers.
- Measurements of the discharge rate of the well.
Ideally, a pumping test should not start before the natural changes in hydraulic head in the aquifer are known
- both the long-term regional trends and the short-term local variations.
So, for some days prior to the test, the water levels in the well and the piezometers should be measured, say twice a day.
END OF PART 3
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