Procedure for Conducting Pumping Tests part (4)

Procedure for Conducting Pumping Tests part (4)

Water-level measurements:
The water levels in the well and the piezometers must be measured many times during a test, and with as much accuracy as possible. Because water levels are dropping fast during the first one or two hours of the test, the readings in this period should be made at brief intervals. As pumping continues, the intervals can be gradually lengthened. After the pump has been shut down, the water levels in the well and the piezometers will start to rise - rapidly in the first hour, but more slowly afterwards. These rises can be measured in what is known as a recovery test.

Duration of the pumping test:
The question of how many hours to pump the well in a pumping test is difficult to answer because the period of pumping depends on the type of aquifer and the degree of accuracy desired in establishing its hydraulic characteristics. At the beginning of the test, the cone of depression develops rapidly because the pumped water is initially derived from the aquifer storage immediately around the well.

Conversion of the data:
The water-level data collected before, during, and after the test should first be expressed in appropriate units. The measurement units of the International System are recommended, but there is no fixed rule for the units in which the field data and hydraulic characteristics should be expressed.
Transmissivity, for instance, can be expressed in m2/s or m2/d. Field data are often expressed in units other than those in which the final results are presented.
Time data, for instance, might be expressed in seconds during the first minutes of the test, minutes during the following hours, and actual time later on, while water-level data might be expressed in different units of length appropriate to the timing of the observations.
It will be clear that before the field data can be analyzed, they should first be converted: the time data into a single set of time units (e.g. minutes) and the drawdown data into a single set of length units (e.g. metres), or any other unit of length that is suitable.

Pump regime - General guidance:

For Confined aquifers:

Transmissivity is more important than storativity: observation wells are not always needed (although accuracy lost without them!).
For Unconfined aquifers: Storativity much larger, and has influence over transmissivity estimates: observation wells important as is larger test duration.
Care is needed if aquifer only partly screened.

Measurement intervals to be considered:
Water levels measurements for pumping well could be taken as the following :

Similarly, for observation wells, water level measurement can be taken as the following:

After the pump has been shut down, the water levels in the well will start to rise again. These rises can be measured in what is known as recovery test.
If the pumping rate was not constant throughout the pumping test, recovery-test data are more reliable than drawdown data because the water table recovers at a constant rate.
Measurements of recovery shall continue until the aquifer has recovered to within 95% of its pre-pumping water level.
Amongst the arrangements to be made for pumping test is a discharge rate control. This must be kept constant throughout the test and measured at least once every hour, and any necessary adjustments shall be made to keep it constant.

Basic Assumptions :
We need to make assumptions about the hydraulic conditions in the aquifer and about the pumping and observation wells. All geological formations are horizontal and of infinite horizontal extent.
The potentiometric surface of the aquifer is horizontal prior to the start of the pumping. The potentiometric surface of the aquifer is not changing with time prior to the start of the pumping.
All changes in the position of the potentiometric surface are due to the effect of the pumping well alone. The aquifer is homogeneous and isotropic. All flow is radial toward the well. Groundwater flow is horizontal. Darcy’s law is valid. Groundwater has a constant density and viscosity.

Proper discharge of the pumped water:
Proper discharge of the pumped water is important to ensure there is no damage due to erosion, flooding or sediment deposits in streams.
For land disposal, direct the water from the pumping well in a down-hill direction at a sufficient distance from the pumping well. This will prevent re-circulation of the pumped water into the well or aquifer and will preserve both the pumping water level and the integrity of the pumping test.

Collecting water samples for analysis:
A pumping test is a good time to collect water quality samples to assess the chemical, physical and bacterial properties of the water.

Water samples should be collected when conditions have stabilized.

Hydrofracturing :
If hydrofracturing (fracking) has been used to increase the productivity of the well, it may advisable to wait up to a week before conducting the pumping test.

Pumping Test Report:
The formal report for a pumping test should be submitted at the end of the work.

This report should contain the following:
• information on the well (i.e., the well construction report, type of well and a diagram showing the well’s location on the property, etc.);
• information on field procedures and personnel involved in the test,
• information on the hydrogeologic setting, including references to mapped aquifers.
• pumping test information including the date of the pumping test, all data on the pump type, depth of pump setting, pumping rates, method of flow measurement, observations made during the pumping test, duration of the test, available drawdown, specific capacity, method of water level measurements and water levels/times recorded during the pumping test and recovery period;
• analysis and assessment of the pumping test data including an assessment of the long-term sustainable yield and potential impacts to neighbouring wells and/or streams.

Borehole Performance Curves
Borehole performance curves are best plotted on a graph of water level against pumping rate.
Water levels are used (in metres below datum) instead of drawdowns so that seasonal variations can be plotted on the same graph if the borehole is tested again at a different time of year.

Multiple Production Wells:
For cases in which there are multiple production wells, all such wells must be monitored during the test. In addition, the test must be conducted in a way that will obtain information pertinent to the operational needs of the entire wellfield. If wells may have to be operated simultaneously in order to meet demand, the test must be designed to produce data representative of these conditions.

Limitations of pumping tests:

Analysing groundwater levels and pumping rates measured during pumping tests provide some indication of the behaviour or state of ‘health’ of the aquifer or groundwater system. These tests undoubtedly provide valuable information and help to understand the groundwater system. However, the decisions should be based on a wider understanding of the regional geology, hydrogeology and environment. View publication

END OF PART 4

Procedure for Conducting Pumping Tests part (3)

Procedure for Conducting Pumping Tests part (3)

The pump : -
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

Procedure for Conducting Pumping Tests part (2)

Procedure for Conducting Pumping Tests part (2)

Planning Stage:
Designing and planning a pumping test is critical prior to testing. Lack of planning can result in delays, increased costs, technical difficulties and poor or unusable data.

Some things to consider in the pre-planning stage are:
1) time of year the pumping test should be done
2) natural variations in the groundwater levels that occur during the test
3) informing others who may be affected
4) depth of pump setting and type of pump
5) pumping duration
6) pumping rate
7) control and measurement of the pumping rate
8) frequency of measurements of the water levels
9) measuring water levels in neighbouring wells and/or streams
10) discharge of pumped water
11) collection of water samples for water quality analysis
special conditions to be aware of e.g., salt water intrusion in coastal aquifers

Materials required for conducting pumping tests:
For conducting pumping tests and analysing the data, the following items may be required:
1) generator
2) submersible pump
3) discharge pipe, connections
4) flow measurement device(s)
5) tape measure(s), steel tape(s) and carpenter's chalk
6) pressure transducer(s), cables, data logger(s)
7) electric water-level sounder(s) and batteries
8) watches/stopwatches
9) barometric sensor/ thermometer
10) pH and conductivity meters
11) sample bottles
12) toolkit, , wires
13) data collection forms, log book, permanent-ink pens
14) computer, calculator
15) graph paper (semilog, log) and/or computer software
16) references, standard operating procedures
17) manufacturer's operating manuals for equipment
18) maps (site, geologic and topographic), cross section(s).

Well-Inventory analysis:

Well-inventory is one basic step. Well inventories are also conducted as part of most of the environmental investigations.
Different types of wells are studied for recording their yielding capacities, main aquifers contributing to yield, etc. The nature and period of their use and sustainability are also recorded. The hydrostatic heads of the aquifers are monitored on a monthly basis through shallow dugwells (monitoring stations), piezometers, deep wells, etc, in the areas. Water samples are collected from selected wells and analysed to determine the variation of water quality over time and space.
Before conducting a pumping test the geological and hydrological information of the area should be collected.
1. The geological characteristics of the subsurface (i.e. all those lithological, stratigraphic, and structural features that may influence the flow of groundwater).
2. The type of aquifer and confining beds. 3.The thickness and lateral extent of the aquifer and confining beds.
4. The aquifer may be bounded laterally by barrier boundaries of impermeable material in the lithology (e.g. the bedrock sides of a buried valley, a fault, or simply lateral changes of the aquifer material);
Data on the groundwater-flow system: horizontal or vertical flow of groundwater, water table gradients, and regional trends in groundwater levels.

Details of any existing surrounding wells in the area.

Selecting the well for the pumping tests:
Well should be suitable for the test.

1) The hydrogeological conditions should not change over short distances and should be representative of the area under consideration, or at least a large part of it;
2) The site should not be near railways or motorways where passing trains or heavy traffic might produce measurable fluctuations in the hydraulic head of a confined aquifer;
3) The site should not be in the vicinity of existing discharging wells;
4) The pumped water should be discharged in a way that prevents its return to the aquifer.

The gradient of the water table or piezometric surface should be low;
Manpower and equipment must be able to reach the site early and easily.

New Exploratory and observations wells:
If there is no existing well in a region, bore wells are drilled for pumping test purpose. Sometimes, bore well drilled for drinking water supply purpose are tested to know the hydrological properties, by conducting pumping tests.

Well diameter:
Before conducting the pumping test the dimensions of the well should be measures.

Radius for circular wells. length and width for rectangular wells.
The depth also should be measured. if it is new well, during the drilling operations, samples of the geological formations that are pierced should be collected and described lithologically.


Records should be kept of these lithological descriptions, and the samples themselves should be stored for possible future reference.

Well screen: for bore wells, the casing pipe length should be measured.


END OF PART 2

Procedure for Conducting Pumping Tests part (1)

Procedure for Conducting Pumping Tests part (1)

What is a pumping test ?
A pumping test consists of pumping groundwater from a well, usually at a constant rate, and measuring water levels in the pumped well and any nearby wells (observation wells) or surface water bodies during and after pumping.

A pumping test is a practical, reliable method of estimating well performance, well yield, the zone of influence of the well and aquifer characteristics (i.e., the aquifer’s ability to store and transmit water, aquifer extent, presence of boundary conditions and possible hydraulic connection to surface water).

Aquifer test and aquifer performance test (APT) are alternate designations for a pumping test. In petroleum engineering, a pumping test is referred to as a drawdown test.

Purpose of conducting Aquifer Tests:
Hydrogeological studies include determination of aquifer parameters by conducting pumping tests on dug / bore / tube wells and analysis of pumping test data.

Basically, pumping tests are conducted for a wide variety of reasons, including the following:

1) To determine the reliable long-term yield (or ‘safe’ yield) of a borehole.
2) To assess the hydraulic performance of a borehole, usually in terms of its yield-drawdown characteristics. How much drawdown does it take to yield a certain amount of water?
3) To derive the hydraulic properties of the aquifer.
4) Pumping tests are the classic (and perhaps the only) way to derive in situ aquifer hydraulic properties, such as transmissivity and the storage coefficient, or to reveal the presence of any hydraulic boundaries.
5) To test the operation of the pumping and monitoring equipment,
6) To determine the effects of abstraction on neighbouring abstractions (sometimes referred to as derogation).
7) To determine the environmental impact of the abstraction.
8) To provide information on water quality. Is the water quality suitable for the intended use? Are there likely to be any problems such as drawing in saline or polluted water after extended periods of pumping?
9) To optimize operational pumping regimes.
10) To help determine the correct depth at which the permanent pump should be installed in the borehole.

Common types of pumping tests
The common types of pumping tests conducted include the following:

Constant-rate tests:
In this test it is necessary to maintain pumping at the control well at a constant rate. This is the most commonly used pumping test method for obtaining estimates of aquifer properties. These tests are carried out by pumping at a constant rate for a much longer period of time than the step test, and primarily designed to provide information on the hydraulic characteristics of the aquifer. Information on the aquifer storage coefficient can be deduced only if data are available from suitable observation boreholes.

Step-drawdown tests :
These tests proceed through a sequence of constant-rate steps at the control well to determine well performance characteristics such as well loss and well efficiency. Step tests are designed to establish the short-term relationship between yield and drawdown for the borehole being tested. It consists of pumping the borehole in a series of steps, each at a different discharge rate, usually with the rate increasing with each step. The final step should approach the estimated maximum yield of the borehole

Recovery tests :
These tests use water-level (residual drawdown) measurements after the termination of pumping. Although often interpreted separately, a recovery test is an integral part of any pumping test. Recovery test are carried out by monitoring the recovery of water levels on cessation of pumping at the end of a constant-rate test (and sometimes after a step test). It provides a useful check on the aquifer characteristics derived from the other tests but is valid only if a foot-valve is fitted to the rising main; otherwise water surges back into the borehole.

Preliminary studies:
When planning a pumping test, it is useful to gather together all the information that can be found about the aquifer and the borehole itself.

Basic geology:
Are the rocks crystalline basement, volcanic, consolidated sediments or unconsolidated sediments? Groundwater occurs in these rocks in different ways, and behaves in different ways.
Aquifer configuration:
Is the aquifer confined, unconfined or leaky?
Borehole construction:
How deep is the borehole, and of what diameter?
Has solid casing, screen or gravel pack been installed?
Installed equipment:
If a pump is already installed in the borehole, what are its type and capacity, and at what depth is the pump’s intake? Can the pumping rate be varied?
Historical or background water levels:
Information about the historical behaviour of the groundwater level is very useful.
Does the water level vary much from wet season to dry season?
In the period before the test takes place, is the water level already falling or rising or is it stable? What is the current water level?
Local knowledge:
Residents often have a surprisingly good understanding of how the groundwater in the area behaves. For example, how does the water level respond to rainfall?
Can borehole yields be maintained?
Is the water safe for drinking, and does the water quality change over time?


END OF PART 1