Week 6 beginning November 2 nd Section 1.6
Note: Much of this section is derived from Chapter 9 of Clark, L (1988).
1.6.1. Introduction
A pumping test is a controlled pumping of a well so that the response of the production well and the growth of the draw down cone can be measured. The analysis of the pumping test can then be used to evaluate the aquifer characteristics (Clark,
1988:3).The pumping test can be used to determine the probable yield of a well. In the pumping test, the water level in the well is depressed to an amount equal to the safe working head for the subsurface. The water level is kept constant by making the pumping rate equal to the percolation into the well. The quantity of water pumped in a known time suggests a probably yield of the well of a given diameter (Raghunath,
1983: 216-217.).
The values of the aquifer characteristics are needed to predict the effect of a particular pumping policy on the groundwater regime, and are obtained by means of pumping tests on test wells. Test wells, with their satellite observation boreholes, are expensive installations and their number in any particular investigation is likely to be limited by budget constraints. In a uniform aquifer, test wells can be widely spaced – several or many kilometres apart, but in an aquifer which is variable, or in a situation where there are several inter-fingering aquifers, as is the case in the Gaza coastal aquifer, test wells may have to be close together before the results of their pumping tests can be used to predict aquifer behaviour with any confidence. Observation boreholes are also used to obtain water samples for analysis in order to measure the variation in regional groundwater quality (Clark, 1988:7).
Pumping tests of wells and boreholes are carried out for three main reason:

To measure the well performance;

To estimate the well efficiency or the variation of well performance with the discharge rate;

To measure the aquifer characteristics of storativity, hydraulic conductivity and tranmissivity.
The overall reason is, therefore, to evaluate the well hydraulics of the well and the aquifer characteristics.
Under natural conditions, groundwater flows through aquifers from areas of high hydraulic head to areas of low head where water is discharged natural as springs or into other surface water features. A pumping test is designed to cause a disturbance in this natural flow of groundwater, but under very controlled conditions. The effects can then be analysed to obtain values of well performance, well efficiency or the aquifer characteristics.
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The most common tests are

Step draw down test to measure the well efficiency, to measure the well performance and to measure the variation of well performance with the discharge rate.

Constant discharge tests to measure the well performance and to measure the aquifer characteristics.
The disturbance to groundwater flow is observed by measuring the changes in the water levels in an array of observation boreholes spatially drilled around the production well
(Figure 1.6.1).
Figure 1.6.1.Borehole array for a test well.
Source:Clark (1988)
The site measurements involved are those of time through the tests, water levels in the boreholes and the discharge rate of the production well.
Pumping lowers the water level in the well and the resultant head difference between the aquifer and the well induces flow into the well. The effects spread out radially forming the characteristic cone of depression discussed in the previous section.
The analysis of constant discharge tests are dependent on the comparison of the observed behaviour of the groundwater levels with the theoretical behaviour in a perfect aquifer, as defined by the Theis equation discussed later in this section.
The traditional method of comparison is by graphical supposition of data curves over type curves (Figure 1.6.2). These graphical methods of analysis have largely been replaced by computer analysis. Programmes are now available to model water table responses to pumping rates, taking into account well storage, variations in aquifer storage, permeability and leakage as well as normal aquifer hydraulics.
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Figure 1.6.2. Computed and actual curves for Herodion 2 well (42/9)
Source: Guttman (1998).
1.6.2.Step draw down tests
The primary aim of a step draw down test is to evaluate the well performance rather than the aquifer performance. The drawdown in the pumped well is measured while the discharge rate is increased in steps. The discharge rate in each step is kept constant through the step, and the change in rate between steps is made as quickly as possible.
Observation boreholes are not used, though measurements in such wells, during a test may be useful in designing future constant discharge tests.
The factors to be decided in the design of a step test are:

The number of steps

Duration of steps

Discharge rate of steps

Whether the steps should follow each other without a break.
A step draw down test measures the variation of specific capacity with discharge rate, data which can be analysed to differentiate the proportion of the draw down which is aquifer loss and that which is well loss. The test value can also give the value of the aquifer transmissivity and an indication of the storativity. The data on the specific capacity of the well are of great practical value, because they give a basis on which to choose the pump size and pump setting for the well in long term production.
An analysis of a test involves the plotting of the specific drawdown (the reciprical of specific capacity) and the discharge rate for each step, i.e., each step produces one data
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point on the graph (Figure 1.6.3), In order for the relationship between specific capacity and discharge rate deduced from such analysis to be valid, a minimum of four steps are required, because three points do not adequately define a trend. Six steps is the optimal number, because above that number the logistics of the test become difficult.
Figure 1.6.3 Step draw down test data showing that more than three stages are needed to draw a valid specific draw down /discharge curve.
Source: Clark (1988)
The most important feature of the step test is the number of steps rather than their lengths, but step should be long enough to allow well storage effects seen at the beginning of each step to dissipate. The steps are most commonly between 60 and 120 minutes long. There is little advantage in extending the steps beyond 120 minutes, but it does help the interpretation if the steps are of equal length. The length of time taken for each step is measured on a log scale, as if each step were a separate constant discharge test.
The maximum discharge rate of the test should be as great as the well or pump will give and, in either case – if possible – greater than the expected long-term production rate. The discharge rates for the other steps have to be chosen to give equal discharge increments through the test. These rates have to be chosen before the test, and the pumping equipment should be calibrated during the equipment test, to give the required rates. At the beginning of each step, the discharge rate is set and then left.
1.6.3 Pumping Tests of the Deir Sharaf Well No. 2a.
Students should study very carefully Section 4, Assessment of the Properties of the Upper
Beit Kahil Aquifer and the performance of the DSW2a well, in Aliewi, A., Wishahi, S.,
Abdul Jaber,Q and Rabi,A (1995) Design, Construction and Chemical Analysis of Deir
Sharaf Well No.2a and Assessment of the Properties of the Upper Beit Kahil Aquifer in
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the Nablus Area, Palestinian Hydrology Group, Jerusalem 35- 50. This gives a clear practical example of how these tests are applied for a Palestinian well.
1.6.4. Aquifer Assessment of the Herodion and Bani Naim well fields
The results of pumping tests carried out in these well fields are not yet in the public domain as there are still sites at which the drilling company is working. However it is possible to evaluate the specific capacity, that is, the pumping rate over steady draw down, of the wells. This allows the engineers to identify the areas of regional importance for further groundwater utilization in these well fields. Figure 1.6.3. represents the zones of specific capacity values and Table 1.6.1 details the specific capacity of each zone.
Figure 1.6.3. The Zonation of the Herodion and Bani Naim Well Fields.
Source: Alliewi & Jarrar (2000)
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Table 1.6.1 Specif Capacity within each zone of the Herodion & Bani Naim Well Fields.
Zone
4
5
6
1
2
3
Average value of specific capacity in m 3 /day/m
1440
145
240
345
75
312
Zone 1 in the south west of the Herodion well field was selected by the Israeli Mekorot
Company for Hebron 1 and 2. These wells supply Hebron City and represent that part of
Article 40 of the Oslo 2 Accords that required Israel to augment its supply to the
Palestinian population. Also in this zone is the PWA 1 well. This zone has the best specific capacity. Zone 4 just to the south of Bethlehem, with JWC4, Hundaza and the
PWA 3 wells also has a high specific capacity. Early drilling in the Bani Naim wellfield
(Zone 6) indicates high specific capacity. Although close to Zone 1, Hebron Municipal well in Zone 2 does not have a high specific capacity.
An explanation for the differences in specific capacity in the Herodion wellfield is the karstic nature of the aquifer. If an area is highly karstic, as in zone 1, then the potential for further groundwater development is high. However if, as in Zone 5, there is relatively less karstic phenomena, there will be a low value for its specific capacity. Thus the geogreaphic location in the well field is not significant, but the very local geohydrologic detail is. Lower values of specific capacity will mean a more rapid decline of the water table for higher pumping rates. This will probably cause those parts of the aquifer to dewater and thus be of no further use (Aliewi & Jarra, 2000).
There have been several Israeli and palestinian sponsored attempts to model the Eastern
Basin of the Mountain Aquifer and these will be studied later in the course. These make use of all data available in as greatert detail as is practical to evaluate the realisable potential of an aquifer.
1.6.5. Thiem, Theis & Boulton Analyses.
It is important, when analysing pumping tests that they are based on a number of assumptions:
 that the aquifer is infinite in extent
 that the aquifer is homogeneous,isotropic and of uniform thickness
 that the piezometric surface or water table is horizontal
 that groundwatrer flow is horizontal and uniform throughout the whole aquifer thickness
 that the well discharge rate is constant.
Such assumptions rarely relate to reality.The first significant analysis of the significance of the cone of depression was by Thiem (1906) who assumed that after a long period of pumping the cone of depression reaches a steady state. Thiem demonstrated that the
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drawdown in the cone at steady state is proportional to the log of the radius from the pumped well. This Thiem equation enabled the determination of aquifer transmissivity by a constant-discharge test. However, as there is no change in storage at a steady state, the equation tells us nothing about storativity.
In 1935 Theis developed an equation for non-steady state flow to a well. This equation incorporates transmissivity, storativity and the duration of pumping. In addition to the standard assumptions (p.59), Theis also assumed that the aquifer was confined, the well was infinitely thin and that water was released instantly from storage as the head declined. The Theis analysis demonstrates that in an ideal aquifer, the drawdown in any point in a cone of depression will follow the form of the curve illustrated in Figure 1.6.4.
Figure 1.6.4, Pumping test curves to show the effect of a leaky aquifer.
Source: Clark (1988).
Developments in well hydraulic analysis since Theis have been mainly concerned with taking into account the more common departures from the ideal state. Aquifers, for example, are rarely totally confined, and therefore leakage occurs during pumping in the area of reduced hydrostatic head. The effect of leakage is to reduce the drawdown within the cone of depression. Leakage is detected in a pumping test by the observation that the drawdown data from observation wells follow a curve below that illustrated in Figure
1.6.4.
If the drawdown is significant, then the well behaves as if the aquifer transmissivity decreases through the test, and the data curve lies above the type curve. The drawdown/time curves of tests in unconfined or phraetic aquifers are commonly sinusoidal in form as illustrated in Figure 1.6.5. This kind of curve could not be interpreted until Boulton (1963) showed that it was the result of delayed yield.
Immediately after a pump is switched on and the water level in the well drops, water is released from aquifer storage by pressure release. The drawdown follows a Theis curve but the storage is the confined storage coefficient. Later, water is gradually released from
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Figure 1.6.5. Pumping-test curves to show the effect of delayed yield.
Source: Clark (1988). storage by gravity drainage and drawdown begins to follow a new Theis curve, with the storage being the unconfined specific yield. The time taken for the well behaviour to change from one state of storage to another can vary from a few minutes to a few hours.
The well response in the early phase of delayed yield is very similar to that in a leaky aquifer.
The Boulton analysis of delayed yield only applies to an unconfined aquifer, but similar effects are observed in confined dual-porosity aquifers, as in the Mountain Aquifer with limestones that have both primary and secondary porosities (see Section 1.2). When pumping starts in an aquifer with solution channels, for example, there is an immediate response from the fissures, but then, as the water leaks from the solid rock to the fissure, the porosity of this solid rock may dominate the well performance.
An aquifer can be abruptly terminated by a geological fault, which acts as a barrier and stops groundwater flow to the well from that direction. On a drawdown/time curve of a well in such an aquifer, the drawdown follows a Theis curve until the fault is reached, and then the rate of drawdown increases and the data curve rises above the Theis curve, as illustrated in Figure 1.6.6. The maximum effect of a straight fault is apparently to halve the aquifer transmissivity.
A recharge boundary has the opposite effect to a fault. In an unconfined aquifer, the groundwater usually discharges to a spring into a surface stream and provides the base flow to the stream (see "Water for Jordan" video). If pumping from a well lowers the water table below the elevation of the stream, then the base-flow increment from groundwater is intercepted, stopped and may be replaced by a net loss of river flow. This base flow interception can result in surface streams drying up completely. The effect on the water level in the pumped borehole is similar from that from aquifer leakage. The
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Figure 1.6.6. Pumping test curves to show the effect of recharge and barrier boundaries.
Source: Clark (1988). drawdown/time curve follows a Theis type curve until the boundary is reached by the cone of depression. At that point, the data curve falls below the Theis type curve. When the recharge balances the discharge, a steady-state condition prevails and water levels no longer change with time.
The water level reactions predicted by the Theim, Theis and Boulton analyses are those in the aquifer or in the observation boreholes. They are not strictly applicable to the pumping well, because the well itself affects the reaction to pumping. The three analyses, assume, for example, that the pumping well is infinitely thin, yet the water in the well, the well storage, affects the drawdown over the first few minutes of a pumping test. The pump is removing water from the well storage making the drawdown less than would be predicted from the aquifer characteristics alone. This can seriously affect a pumping test analysis, because well storage affects the Theis curve in the first few minutes when it is at its steepest and most susceptible to change.
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