Anniina Kittilä, ETH Zürich, Institute of Geophysics, Geothermische Energie u. Geofluide. Sonneggstrasse 5, 8092 Zürich, Switzerland e-mail: anniina.kittila(at)erdw.ethz.ch
Pumping test is a field study to gain information about hydraulic characteristics of aquifers. The principle is that water is pumped from a well with either constant or variable rate (discharge measured), and the drawdown in the well and in nearby observation wells is measured. The hydraulic characteristics of an aquifer can then be calculated with appropriate equations based on time-drawdown and distance-drawdown measurements. Basically there are two types of pumping tests: single well and multiple well. In single well pumping tests there is only one well to pump and take measurements from, and in multiple well pumping tests one well is pumped and water level is measured in both the pumped well and observation wells or piezometers nearby. Most often the pumping rate is constant, but sometimes variable discharge rate is used, deliberately or due to the characteristics of the pump (Kruseman & de Ridder 2000, OhioEPA 2006). Other pumping configurations also exist, but the focus of this article is in constant rate pumping tests. Pumping tests are not purely methods to measure flow rate in an aquifer (through a well), but are used to determine transmissivity and hydraulic conductivity, for example. The local flow rate can be measured with constant pumping rate (flow rate), adjusted so that the water level in the well stays stable and does not fluctuate. Pump run-times and ratings or the pumping capacity can also be used to estimate inflow flow rate to mines, but the information should be handled carefully because factory parameters of the pump, its capacity and rating might have changed due to, for example, fluctuations in power supply and clogging of the pump pipes (Wolkersdorfer 2008).
Description of the method
Single well pumping tests can be used to determine transmissivity, hydraulic conductivity, and yield of an aquifer, well loss, or optimizing rate and pump setting for a multiple well pumping test. Multiple well pumping tests, on the other hand, can be used to define the overall hydrogeological setting of the studied area, including transmissivity, storativity and specific yield. They can also help in predicting rates of groundwater flow, determining groundwater zones, or designing remediation activities. (OhioEPA 2006)
There is no specific duration that a pumping test should last, but in general, the type of the aquifer, the degree of desired accuracy, and the objectives of the test affect the duration (Kruseman & de Ridder 2000, OhioEPA 2006). It is not recommended, however, to shorten the duration of the pumping due to economic reasons, because the cost of running the pump longer compared to the total cost of the test is low. Additionally, the data is more reliable if pumping is continued at least until steady or pseudo-steady flow is attained (Kruseman & de Ridder 2000). Although it is not absolutely necessary to continue the pumping until steady state of flow has been reached, because some of the methods used to interpret the data can analyse unsteady-state data, it is good to aim at steady state. That way reliable results can be obtained and information on the aquifer characteristics is accurate. Kruseman & de Ridder (2006) also point out that pumping for a longer time could reveal previously unknown boundary conditions, or in the case of fractured formation, will reveal the specific flows that develop during the test.
Steady state of flow has been reached when the cone of depression has stopped expanding. In that point the recharge of the aquifer equals the pumping rate (flow rate). The cone of depression develops vary fast in the beginning of the pumping test, because the pumped water originates from the aquifer storage immediately around the well. Subsequently, as the cone expands horizontally, its expansion and deepening slows down because a larger volume of the aquifer storage becomes available. This apparent stabilization should not be mistaken for steady state, as measurements would be still inaccurate. In average, steady state can be reached in leaky aquifers after approximately 15 to 20 hours of pumping, in confined aquifers after 24 hours, and in unconfined aquifers after 3 days. However, steady state can occur in some occasions in a couple of hours, or in weeks, or might never be reached. (Kruseman & de Ridder 2006)
Variations in discharge associated with pump motor or engine could happen even with the most reliable pump, which certainly is not desirable when conducting a constant rate pumping test. In addition to daily temperatures affecting the combustion ratio of engine-powered pump, and thus also the engine revolutions per minute, the increase in pumping lift as the water level near the pumping well drops decreases the rate of discharge at a given level of power, for both electric motors and combustion engines (Osborne 1993). This is why the actual discharge-rate is recommended to be controlled separately by placing a valve on the discharge pipe (Osborne 1993, Kruseman & de Ridder 2000). During pumping it is good to measure the discharge (flow rate) at least every hour (Kruseman & de Ridder 2000), but in the beginning of pumping measurements and possible adjustments should be done at 5, 10, 20, 30 and 60 minutes (OhioEPA 2006). The discharge rate measurement can be done with commercial water meter connected to the discharge pipe, but if it is not available or possible, there are other suitable methods too, such as flumes or weirs, or container of known volume (Kruseman & de Ridder 2000, OhioEPA 2006).
The effect of drawdown is measured from the changes in the water level in the pumping well (and in observation wells). However, the static water level should always be measured before the pumping test is started (Osborne 1993, Kruseman & de Ridder 2000, OhioEPA 2006). Monitoring the water level for some days prior to the test, for example twice a day, should already give sufficient information on the natural changes in hydraulic head, both long-term regional trends and short-term local variations. Also especially in longer pumping tests the atmospheric pressure should be observed, both during the baseline measurements and during the test to correct the water level trend (Kruseman & de Ridder 2000). The water level measurements during the test should be taken with as good accuracy as possible, which is affected by both measurement frequency and measuring device. The measurement time interval should be more frequent in the beginning of the test, when water levels are changing most rapidly and as pumping continues, the intervals can be gradually lengthened as the changes are slower too (Osborne 1993, Kruseman & de Ridder 2000, OhioEPA 2006). In the first 5 minutes the time intervals should be, for example, 30 seconds, increasing to 5 minutes until an hour has passed, to 20 minutes until 2 hours has passed, and to 60 minutes for the rest of the test, although in single well pumping tests the intervals should be shorter in the first 5-10 minutes because the very early drawdown data could reveal well-bore storage effects (Kruseman & de Ridder 2000). Similar guidelines for intervals, although more frequent, exist for observation wells and piezometers. The suggested intervals do not need to be strictly followed, because local conditions, available personnel, etc. determine the best practice (Kruseman & de Ridder 2000). In addition to drawdown measurements, the water levels can be measured after the pump has been shut down and the water levels start to rise, which gives more reliable data as the water table will recover in constant rate. This is known as a recovery test (Osborne 1993, Kruseman & de Ridder 2000, OhioEPA 2006).
To identify the hydraulic characteristics of an aquifer, the first step after the pumping test is to compare the drawdown behaviour with various theoretical models. The drawdown is best to interpret from semi-log plot (time in logarithmic scale), although the construction of log-log plot is also recommended in order to gain the best insight of the hydraulic characteristics. The choice of right theoretical model is very important, because using wrong model to calculate the hydraulic characteristics could result in erroneous interpretations of the real aquifer. Analysing the data requires expertise (Kruseman & de Ridder 2000).
Pumping test is one of the most reliable methods to study hydraulic properties of aquifers, which has the potential to give accurate estimates of more than one aquifer parameter at once. This is, however, only the case when pumping tests are conducted in a proper way and the data is interpreted correctly (Osborne 1993). It is also good to remember that pumping tests are based on models that are assumed to represent the characteristics of a real aquifer system, because the characteristics are not fully and precisely known. This, or choosing a wrong model, might lead to unknown errors in interpreting the results (Kruseman & de Ridder 2000). A disadvantage in single well pumping tests is that they usually do not recognize impermeable or recharge boundaries, or interconnection between other aquifers or surface water, unless they are in very close proximity to the well being pumped. (OhioEPA 2006). Additionally, many existing wells are not suitable for conducting pumping test, so preliminary information of the well should be carefully evaluated (Osborne 1993, Kruseman & de Ridder 2000).
The performance of a pumping test depends on many factors, including the capacity of the pump, the suitability of the well, and the aquifer properties. The suitability of a well and the location are discussed in the next chapter. The requirements for a suitable pump can be simplified so that: i) the pump should be capable of operating continuously at a constant rate for the period of the desired test, and ii) the capacity of the pump and the rate of discharge should be high enough to be able to create a measurable drawdown even in observation wells dozens of metres from the pumped well. Sometimes also the observed water levels need to be corrected. That is if external influences have been significant enough to affect the water level, and the need to do so can be determined by analysing trends in the hydraulic head combined with atmospheric pressure data (Kruseman & de Ridder 2000).
An environmental aspect of pumping tests usually arises from the discharge of pumped water. Firstly, the pumped water should be prevented from returning to the aquifer, but secondly, if the groundwater is contaminated, the discharge may need to be pumped to separate containers and treated properly (OhioEPA 2006).
In order to understand the aquifer and the pumping test results better, some hydrological and geological information from the site should be collected (Kruseman & de Ridder 2000, OhioEPA 2006):
- The geological characteristics that may influence the groundwater flow (lithological, stratigraphic, and structural features)
- Aquifer type (confined, unconfined, or leaky) and confining layers
- Aquifer dimensions (thickness and extent)
- Groundwater flow (gradients and regional trends)
- Location and properties of existing wells in the area.
The benefit of being able to use an existing well is the saving in costs, but the existing well might not be the most suitable for pumping test, and could reduce the accuracy of the results. Drilling a new well increases the cost of the pumping test, but this gives a possibility to study the hydrogeological characteristics of the surrounding environment and choose the best site for the well. Few points should be kept in mind when choosing the well site: i) the hydrogeological conditions do not change in the vicinity of the well, and the site is representative of the aquifer, ii) the site should not be near infrastructure or activities that cause tremors, possibly fluctuating the groundwater level, iii) no discharging wells should be nearby, and iv) the gradient of the water table should be low (Kruseman & de Ridder 2000). In the design of a pumping well its diameter, depth, and measurements of the screened interval need to be taken into account. The well diameter needs to be large enough to accommodate the pump, and it should be drilled to the bottom of the aquifer, if possible. The exact length and top and bottom depths of the well screen are decided based on the lithology and groundwater level, but a general rule is that it should be extending over at least 80 percent of the aquifer thickness (Kruseman & de Ridder 2000, OhioEPA 2006).
Kruseman, G.P. & De Ridder, N.A. 2000. Analysis and evaluation of pumping test data, 2nd edition. International Institute for Land Reclamation and Improvement (ILRI), Publication 47, 372 p.
OhioEPA 2006. Pumping and slug tests. Technical Guidance Manual for Ground Water Investigations, Chapter 4, Revision 1, Ohio Environmental Protection Agency, 45 p.
Osborne, P.S. 1993. Suggested operating procedures for aquifer pumping tests. U.S.EPA. Office of Research and Development, EPA Ground Water Issue, 23 p.
Wolkersdorfer, C. 2008. Water Management at Abandoned Flooded Underground Mines. Springer, 465 p.