Procedure for Conducting Pumping Tests part (4)
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.
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 :
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
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