PUMPING TESTS
Advanced Center for Water Resources Development and Management (ACWADAM) Plot 4, Lenyadri society, Sus road, Pashan, Pune-411021. 020-25871539 Email:
[email protected] Website: www.acwadam.org
Some common questions from wellowners about aquifers… In hydrogeology, once the aquifers have been delineated it is necessary to find out how an aquifer will respond to pumping out the water from aquifer storage? A
well-user may knowing:
be
water
interested
his
well
in
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How much supply?
can
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What should be the capacity of the pump to be fitted on his well?
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What is the range of Transmissivity and Storativity of the aquifer?
A pumping test means pumping a well
In order to answer questions about the aquifer and wells tapping the aquifer, certain tests are conducted in the field, by pumping out water from a well. Such tests are known as “Pumping Tests”. The water table or potentiometric surface declines when we pump out the water from the aquifer storage. This is measured along with how much water is pumped out.
What is a pumping test? Basic procedure of a pumping test involves: water is pumped from a well (pumping well) its impact on the pumping well as well as on the aquifer it taps is ascertained by • observing the change in water levels in wells tapping the aquifer • and measuring how the rate at which the water is pumped out from the well
Type of the pumping tests
Pumping tests are divided into two main types: 1. 2.
Well test Aquifer performance test
Well test
In a well test, a well is pumped and observations are made in the pumping well only. Here, the well is tested rather than the aquifer. The aim of this test is To determine the capacity of the well to supply water. Or To determine the yield of the well.
Aquifer Performance Test
As the name implies, an aquifer performance test is conducted to quantify the properties of the aquifer, i.e. the aquifer characteristics. Observations are made in the pumping well as well as in observation wells around the pumping well. The aim of this test is: to estimate the performance of the aquifer and estimate aquifer properties like Transmissivity and Storativity.
A pumping test requires equipment…
Equipment for Pumping Test Main equipment required for a pumping test is a pumping assembly:
Pump and motor Pipes to pump out water from a dug well or bore well. A measuring tape or electronic water level recorder to measure water levels. A device to measure the pumping rate like a container of known capacity or a flowmetre. A good watch to measure time.
Mechanics of pumping or Well Hydraulics
What happens in a well being pumped and in the surrounding aquifer is descriped as “mechanics of pumping” or “well hydraulics” . Prior to pumping the water level in the aquifer including pumping well is referred to as “static water level” (SWL) SWL is represents the level above which water in a well will not rise at any particular time because there is no recharge into or discharge from the aquifer storage.
Static and Pumping Water Level
When pumping begins, water levels in the pumping well and in wells nearby (observation wells) declines. Any level measured during the process of pumping is called pumping water level. Now, the pumping water level in the pumping well stands at a lower elevation (deeper) as compared with the water levels in the surrounding aquifer. Such water level can be referred to as “Head”.
SWL
Hydraulic Gradient
A head difference exists between the pumping well and the surrounding aquifer. Therefore, a hydraulic gradient is created from the surrounding aquifer towards the pumping well (according to Darcy’s law - from higher head to lower head). Under the influence of this artificial hydraulic gradient the water stored in the aquifer surrounding the pumping well starts moving towards the pumping well and finally into the pumping well as inflow (known as aquifer contribution “q”).
Dewatering of aquifer
As pumping continues, inflow water from the aquifer is also pumped out along with water stored in the pumping well. More and more water is now derived from the aquifer dewatering the volume of the aquifer surrounding the pumping well. Dewatering of aquifer volume (due to discharge from aquifer storage) results in lowering the water level (i.e. Head) in pumping well as well as over aquifer surface area where dewatering of openings under the hydraulic gradient has taken place.
Drawdown
Now, if we have observation wells surrounding the pumping well, the decline of water level will be reflected in these observation wells.
The difference between the static water level and the pumping water levels in different wells is known as “drawdown”.
Cone of Depression
A shape of an inversed cone results if you join the drawdown in the pumping and observation wells (i.e. head distribution within the aquifer). This inverted cone called as Cone Depression .
is of
The cone of depression expands with continued pumping.
Aquifer contribution The expansion of cone of depression depends upon:
The rate of pumping- Q The aquifer contribution- q from the aquifer to the well (as inflows into the well)
Drawdown depends upon quantity of inflow
For instance, • if pump discharge Q > aquifer contribution q more and more portion of aquifer volume is dewatered, indicated by continuous drawdown in pumping well as well as in the observation wells. • if Q nearly matches with q i.e. Q = q, drawdown in the pumping well and in the observations wells remains nearly constant (as the inflow from aquifer “q” is in excess or equal to the pump discharge Q) indicated by no further deepening of cone of depression in the pumping well.
Relation between the outflow and inflow Q=Outflow (pumping) q=Inflow (aquifer water flowing into well)
Then • Q > q implies INEQUILIBRIUM or UNSTEADY STATE CONDITION
• Q = OR < q implies EQUILIBRIUM or STEADY STATE CONDITION
Pumped Water Level & Drawdown
The distance from centre of pumping well to periphery of the cone of depression is known as “radius of influence”
After the pumping is stopped, the level of water in pumped well is known as “pumped water level”.
The difference in static water level and pumped water level is known as “Total Drawdown” in this condition.
Recuperation or recovery
In this condition the water from the surrounding aquifer continues to flow towards the pumping well under the influence of the (artificial) hydraulic gradient towards the pumping well. After pumping is stopped, due to continued aquifer contribution q, water level in the pumping well as well as in the aquifer surrounding the pumping well rises. This is because the portion of the aquifer which was earlier dewatered during pumping starts resaturating due the water inflowing towards the pumping well (indicated by rise in water level in pumping and observation wells). This process is known as “recovery or recuperation”.
Recovery and the cone of depression
During the process of recuperation or recovery, more and more desaturated portion of the aquifer gets resaturated. This is indicated by a rise in water in areas away from pumping well also. Due to rise in water level the hydraulic gradient towards pumping well becomes gentler with time, thereby reducing the rate of the aquifer contribution q. Now, the water required for this resaturation process is derived by dewatering the peripheral areas of cone of depression i.e. cone of depression continues to expand in peripherical areas even after the pump is switched off. Slowly, with time, the water level rises in the aquifer and finally get stabilized at a new static water level (which may be fractionally lower as compared to the original S.W.L.)
Conducting pumping tests…some practical aspects
The wells in the vicinity of chosen pumping wells should not be pumped at least 48 hours prior to a pumping test. This ensures that water levels in the aquifer surrounding the pumping wells are as close to the static water level as possible and that there are no artificial hydraulic gradients. The dimensions (depth and diameter / length & breadth) of the pumping well are measured. Nature of the aquifer to be tested is ascertained on the basis of well inventory data: • • •
rock type, unconfined / confined / leaky thickness of the aquifer
A measuring point (MP) is selected and marked on the pumping well head and observation well head with respect to which all water levels are measured.
Pumping test data collection
During pumping (drawdown part) the water levels are measured with respect to the MP at regular time intervals. The time interval could be • 1 min or 2min or 5min or 20 minutes and so on. • The time interval is subjective and mostly dictated by hydrogeological field conditions.
In a well test, the drawdown is recorded in the pumping well only and in aquifer performance test drawdown is recorded in pumping well as well as in observation wells. During the drawdown measurements the discharge of the pump (Q) is also measured (preferably with a simple drum/bucket of known volume using the stopwatch method). Pump discharge should be measured frequently as it tends to vary depending upon the efficiency of the pump, drawdown and voltage fluctuations. Once the pumping from pumping well is stopped, the recovery or repurcation measurements are also made in pumping well as well as in observation wells at fixed intervals. During the pumping test the water level in pumping well (if it is dug well) goes down exposing the walls of the well below water table for observation, regarding how and at what depth water is flowing into the well from the aquifer (i.e. information on the inflow zones).
Interpretation of pumping test data
Pumping test data are interpreted for: • Estimating aquifer properties like Transmissivity and Storativity. • Estimating well characteristics, especially the well yield or specific capacity.
Aquifer performance tests
Observations in pumping and observation wells. In an aquifer performance test, the drawdown measurements are made with respect to fixed time intervals. The pump discharge is also checked several times during pumping period. The recovery measurements, after the pump being shutoff, are also made at fixed intervals. The main aim this test is to estimate aquifer characteristics i.e. Transmissivity and Storativity. There are many methods of interpreting pumping test data. These are given in various text books on groundwater.
An example of interpreting aquifer test data
We will consider a very simple graphical method given by Cooper and Jacob (1964) to estimate T and S. The water level in pumping well / observation wells (below MP) is plotted against time since pumping started t on semi logarithmic graph paper:
• Water level (as drawdown s) on arithmetic scale and time on log scale. The first few points reflect the effect of well storage (segment 1) Segment 2 indicates the effect of contribution from aquifer in the form of inflow (in pumping well during pumping) In case of observation well data, points on a straight line indicate the effect of dewatering of the aquifer in the form of drawdown. The third segment which also falls on a straight line, may indicate:
• A gentler slope as compared with segment 2 indicating an increase in aquifer contribution (q) as compared to the pump discharge Q. This indicates that the aquifer is receiving recharge from some external source during pumping (may be river, lake, canal etc.), often leading to a steady state condition. • Sometimes the slope of the segment 3 steepens as compared with segment 2. This indicates the limited extent of the aquifer, showing sudden increase in the rate of drawdown (i.e. aquifer contribution q is much smaller than the pump discharge Q).
A time-drawdown plot… STEADY STATE A time drawdown plot Time in minutes 1
10
100
1000
0
Segment 1 0.1
Drawdown in m bgl
0.2
Segment 2 0.3
0.4
0.5
0.6
Segment 3 0.7
A time-drawdown plot… DEWATERED WELL Time drawdown plot Time in minutes 1
10
100
0
Segment 1 1
Segment 2
Drawdown in m bgl
2
3
4
5
6
7
Segment 3
1000
Transmissivity from the Cooper Jacob formula
The Transmissivity can be obtained using the semilog graph (as given in the above examples) and the following procedure: •
•
The value of ∆s which is the drawdown over ONE LOG CYCLE of time “t’ is calculated. Segment 2 is generally used to obtain the ∆s value used in following equation
T= (2.303 x Q)/ 4∏∆s is the original equation where the final equation is
T= (264 x Q) / ∆s
• Here, T is directly obtained in m2/day directly • •
Q= Pump discharge in m3 per min ∆s= slope of segment 2 over one log cycle of time t expressed in metres.
If Q (pump discharge) is common for both the cases and is equal to 1 m3/min, we have • For graph 1 (steady state), ∆s is approximately 0.5 m T= (264 x 1) / 0.5 i.e. T= 528 m2/day • For graph 2 (unsteady state), ∆s is approximately 15 m T= (264 x 1) / 15 i.e. T= 17.6 m2/day
Storativity from the Cooper Jacob formula
For Storativity S the data of drawdown in OBs well at distance “r” from the pumping well is necessary. Storativity S is given by the equation S=2.25 Tt0 / r2 where, • T= Transmissivity in m2 / min • t0= intercept of straight line on the time axis • r = distance of observation well from pumping well
(S is given in fraction)
For an observation well at a distance of 50 m from the pumping well: Graph 1 shows t0 of about 60 min and therefore: S= 2.25(528/1440)60 / 2500 i.e. S= 0.0198
Graph 2 shows t0 of about 40 min and therefore: S= 2.25(17.6/1440)40 / 2500 i.e. S= 0.000049
Well tests
Observations are made in the pumping well itself (no observation wells) The main aim of the well test is to estimate the yielding capacity of the well or capacity to derive water form the aquifer known as “specific capacity” in case of a bore well, the water stored in the form of water column is small in quantity. Therefore, during pumping, initially the stored water is pumped out and later on the pump discharge is entirely composed of water derived from the aquifer. Under such condition the capacity of the bore well to derive water from the aquifer i.e. specific capacity of bore well is given by equation Specific capacity C = Q / s where • Q= discharge of the pump in Liters per minute (Lpm) • s= drawdown in pumping well after certain time has lapsed
Therefore, specific capacity is discharge per unit of drawdown. • For example, a bore well is pumped at rate Q= 500 lpm for 2 hours with a resultant drawdown s= 5m, • Specific capacity of the bore well is C = Q/s = 500 lpm / 5 m • i.e. 100 lpm per metre of drawdown
What does specific capacity mean… The specific capacity value of C= 100 lpm /m implies: • that the same borewell will yield about 6000 litres in one hour, resulting in a drawdown of 1 m. • It will yield about 12000 litres in two hours with a drawdown of about 2 m. • If it is pumped at the rate of 200 lpm, the well will draw down to 2 m in one hour. Thus, in case of a bore well, the specific capacity can be used to predict the level of drawdown for different pump discharge rates.
Which bore well has a greater Specific capacity? Which is larger: Q/s1 or Q/s2?
Solution Well 1 has a greater specific capacity than well 2. If Q= 100 lpm s1= 5m s2= 10m then C of well 1= 20 lpm/m C of well 2= 10 lpm/m
Specific capacity of a large diameter dug well
In case of large diameter dug wells (which are common in most parts of India), the specific capacity estimation method used for bore wells is not applicable. The amount of water stored in a dugwell is much larger as compared to that in a bore well. Therefore, the pump discharge in case of dug well is composed of water partly derived from: • the well storage and • water derived from the aquifer.
The resultant drawdown in a dug well is also due to the effect of the above two factors. Therefore, the normal method of using the ratio pump discharge / drawdown may not exactly represent the capacity to derive water from the aquifer per unit of drawdown.
Slichter’s method of estimating the specific capacity (yield) of a dug well
In order to estimate the specific capacity of a dug well, the (recuperation) recovery data is used for analysis. Recovery data does not involve the effect of well storage on pump discharge (as the pump is shut off) and contains the effect of the contribution of water from the aquifer only. This effect is in the form of rise in water level in pumping well.
The specific capacity is calculated using Slichter’s Recovery Formula:
C = (2.303(A / t’)) (log10 (s1/ s2)) where C= Specific capacity of dug well in m3/min per metre of drawdown or lpm/m of drawdown A= cross sectional area ∏r2 (in m2) t’= time since pumping stopped in minutes s1= total drawdown in meters s2= residual drawdown values in meters at respective time values since pumping stopped (t’)
Method for estimating specific capacity of dug well using the Slichter method
Measure the total drawdown in the well (s1) at the end of pumping. Similar to a drawdown test, measure the water level rise in the well and estimate the residual drawdown s2 at different times (t’). Residual drawdown (s1) can be estimated by subtracting the rise in water level over a fixed time from the total drawdown (s1) The ratio s1/s2 is calculated for each value of time t’ for which values of s2 are measured. A graph of t’ values (Y-axis) is plotted against the ratio s1 / s2 ( X-axis) on a semilog paper. A straight line is plotted through these points and the slope of the line (change in the value of t over one log-cycle of the ratio (s1/s2) is estimated.
Drawdown and residual drawdown measurement during recovery
Specific capacity data Slichte r's plot for Spe cific capacity 200 180 160 140
t'
120
∇t'= 220 min
100 80 60 40 20 0 1
10
s1/s2
Specific capacity example
Using the plot (previous slide) the equation: C= (2.303(A / t’))/(log10 (s1/ s2)) reduces to C= (2.303A)/∇t’ Where ∇t’= change in the value of t’ over one log cycle of the ratio s1/s2
If the cross sectional area (A) of the well= 20 m2 ∇t’= 220 minutes
Specific capacity can be calculated as: C= (2.303(20)/220)1000
209.36 lpm/m of drawdown
C=
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