PUMPING TESTSPUMPING TESTS

Advanced Center for Water Resources Development and Advanced Center for Water Resources Development and Management (ACWADAM)Management (ACWADAM)Plot 4, Plot 4, LenyadriLenyadri society, society, SusSus road, road, PashanPashan, Pune, Pune--411021. 411021.

020020--2587153925871539Email: Email: acwadam@vsnl.netacwadam@vsnl.netWebsite: Website: www.acwadam.orgwww.acwadam.org

Some common questions from wellSome common questions from well--owners about aquifers…owners about aquifers…

In hydrogeology, once the aquifers have In hydrogeology, once the aquifers have been delineated it is necessary to been delineated it is necessary to find out how an aquifer will respond find out how an aquifer will respond to pumping out the water from to pumping out the water from aquifer storage?aquifer storage?

A wellA well--user may be interested in user may be interested in knowing:knowing:

•• How much water his well can How much water his well can supply? supply?

•• What should be the capacity of the What should be the capacity of the pump to be fitted on his well? pump to be fitted on his well?

•• What is the range of What is the range of TransmissivityTransmissivityand and StorativityStorativity of the aquifer? of the aquifer?

A pumping test means pumping a wellA pumping test means pumping a well

In order to answer questions In order to answer questions about the aquifer and wells about the aquifer and wells tapping the aquifer, certain tapping the aquifer, certain tests are conducted in the field, tests are conducted in the field, by pumping out water from a by pumping out water from a well.well.

Such tests are known as Such tests are known as “Pumping Tests”.“Pumping Tests”.

The water table or The water table or potentiometricpotentiometric surface declines surface declines when we pump out the water when we pump out the water from the aquifer storage. This is from the aquifer storage. This is measured along with how much measured along with how much water is pumped out.water is pumped out.

What is a pumping test?What is a pumping test?

Basic procedure of a Basic procedure of a pumping test pumping test involves: involves: water is pumped from water is pumped from a well (pumping well) a well (pumping well) its impact on the its impact on the pumping well as well pumping well as well as on the aquifer it as on the aquifer it taps is ascertained by taps is ascertained by •• observing the change observing the change

in water levels in wells in water levels in wells tapping the aquifertapping the aquifer

•• and measuring how the and measuring how the rate at which the water rate at which the water is pumped out from the is pumped out from the wellwell

Type of the pumping testsType of the pumping tests

Pumping tests are Pumping tests are divided into two divided into two main types:main types:

1.1. Well testWell test2.2. Aquifer Aquifer

performance testperformance test

Well testWell testIn a well test, a well is In a well test, a well is pumped and observations pumped and observations are made in the pumping are made in the pumping well only. well only.

Here, Here, the well the well is tested is tested rather than the aquifer.rather than the aquifer.

The aim of this test is The aim of this test is To determine the To determine the capacity of the well to capacity of the well to supply water. supply water.

OrOrTo determine the To determine the yyieldield

of the well. of the well.

Aquifer Performance TestAquifer Performance TestAs the name implies, an As the name implies, an aquifer performance test aquifer performance test is is conducted to quantify the conducted to quantify the properties of the aquifer, i.e. properties of the aquifer, i.e. the aquifer characteristics. the aquifer characteristics.

Observations are made in the Observations are made in the pumping well as well as in pumping well as well as in observation wells around the observation wells around the pumping well. pumping well.

The aim of this test is: The aim of this test is: to estimate the to estimate the performance of the performance of the aquifer and aquifer and estimate aquifer estimate aquifer properties like properties like TransmissivityTransmissivity and and StorativityStorativity. .

A pumping test requires equipment…A pumping test requires equipment…

Equipment for Pumping TestEquipment for Pumping TestMain equipment required Main equipment required

for a pumping test is a for a pumping test is a pumping assembly: pumping assembly:

Pump and motor Pump and motor Pipes to pump out Pipes to pump out water from a dug well water from a dug well or bore well. or bore well. A measuring tape or A measuring tape or electronic water level electronic water level recorder to measure recorder to measure water levels.water levels.A device to measure A device to measure the pumping rate like a the pumping rate like a container of known container of known capacity or a flowcapacity or a flow--metremetre. . A good watch to A good watch to measure measure time.time.

Mechanics of pumping or Mechanics of pumping or Well HydraulicsWell Hydraulics

What happens in a well being What happens in a well being pumped and in the surrounding pumped and in the surrounding aquifer is aquifer is descripeddescriped as as “mechanics of pumping” or “mechanics of pumping” or “well hydraulics” .“well hydraulics” .

Prior to pumping the water Prior to pumping the water level in the aquifer including level in the aquifer including pumping well is referred to as pumping well is referred to as “static water level” (SWL) “static water level” (SWL)

SWL is represents the level SWL is represents the level above which water in a well above which water in a well will not rise at any particular will not rise at any particular time because there is no time because there is no recharge into or discharge from recharge into or discharge from the aquifer storage. the aquifer storage.

Static and Pumping Water LevelStatic and Pumping Water Level

When pumping begins, water When pumping begins, water levels in the pumping well and levels in the pumping well and in wells nearby (observation in wells nearby (observation wells) declines.wells) declines.

Any level measured during the Any level measured during the process of pumping is called process of pumping is called pumping water levelpumping water level..

Now, the pumping water level Now, the pumping water level in the pumping well stands at in the pumping well stands at a lower elevation (deeper) as a lower elevation (deeper) as compared with the water compared with the water levels in the surrounding levels in the surrounding aquifer. aquifer.

Such water level can be Such water level can be referred to as “Head”. referred to as “Head”.

SWL

Hydraulic GradientHydraulic Gradient

A head difference exists A head difference exists between the pumping well and between the pumping well and the surrounding aquifer.the surrounding aquifer.

Therefore, a hydraulic gradient Therefore, a hydraulic gradient is created from the is created from the surrounding aquifer towards surrounding aquifer towards the pumping well (according to the pumping well (according to Darcy’s law Darcy’s law -- from from higher higher head head to to lower headlower head).).

Under the influence of this Under the influence of this artificial hydraulic gradient the artificial hydraulic gradient the water stored in the aquifer water stored in the aquifer surrounding the pumping well surrounding the pumping well starts moving towards the starts moving towards the pumping well and finally into pumping well and finally into the pumping well as the pumping well as inflow inflow (known as aquifer contribution (known as aquifer contribution “q”).“q”).

Dewatering of aquiferDewatering of aquiferAs pumping continues, As pumping continues, inflow inflow waterwater from the aquifer is also from the aquifer is also pumped out along with water pumped out along with water stored in the pumping well. stored in the pumping well.

More and more water is now More and more water is now derived from the aquifer derived from the aquifer dewatering the volume of the dewatering the volume of the aquifer surrounding the pumping aquifer surrounding the pumping well. well.

Dewatering of aquifer volume (due Dewatering of aquifer volume (due to discharge from aquifer storage) to discharge from aquifer storage) results in lowering the water level results in lowering the water level (i.e. Head) in pumping well as well (i.e. Head) in pumping well as well as over aquifer surface area where as over aquifer surface area where dewatering of openings under the dewatering of openings under the hydraulic gradient has taken hydraulic gradient has taken place. place.

DrawdownDrawdown

Now, if we have Now, if we have observation wells observation wells surrounding the pumping surrounding the pumping well, the decline of water well, the decline of water level will be reflected in level will be reflected in these observation wells. these observation wells.

The difference between The difference between the static water level and the static water level and the pumping water levels the pumping water levels in different wells is in different wells is known as “known as “drawdowndrawdown”.”.

Cone of DepressionCone of Depression

A shape of an inversed A shape of an inversed cone results if you join the cone results if you join the drawdown in the pumping drawdown in the pumping and observation wells (i.e. and observation wells (i.e. head distribution within head distribution within the aquifer).the aquifer).

This inverted cone is This inverted cone is called as called as Cone of Cone of DepressionDepression . .

The cone of depression The cone of depression expands with continued expands with continued pumping. pumping.

Aquifer contributionAquifer contribution

The expansion of cone of The expansion of cone of depression depends depression depends upon: upon:

The rate of The rate of pumpingpumping-- QQ

The aquifer The aquifer contributioncontribution-- q from q from the aquifer to the the aquifer to the well (as inflows into well (as inflows into the well)the well)

Drawdown depends upon quantity of inflowDrawdown depends upon quantity of inflowFor instance, For instance, •• if pump discharge Q > aquifer if pump discharge Q > aquifer

contribution q more and more contribution q more and more portion of aquifer volume is portion of aquifer volume is dewatered, indicated by dewatered, indicated by continuous drawdown in continuous drawdown in pumping well as well as in the pumping well as well as in the observation wells. observation wells.

•• if Q nearly matches with q i.e. if Q nearly matches with q i.e. Q = q, drawdown in the Q = q, drawdown in the pumping well and in the pumping well and in the observations wells remains observations wells remains nearly constant (as the inflow nearly constant (as the inflow from aquifer “q” is in excess or from aquifer “q” is in excess or equal to the pump discharge equal to the pump discharge Q) indicated by no further Q) indicated by no further deepening of cone of deepening of cone of depression in the pumping depression in the pumping well.well.

Relation between the outflow and inflowRelation between the outflow and inflowQ=Outflow (pumping)Q=Outflow (pumping)q=Inflow (aquifer water flowing q=Inflow (aquifer water flowing

into well) into well)

ThenThen•• Q > q implies INEQUILIBRIUM Q > q implies INEQUILIBRIUM

or UNSTEADY STATE or UNSTEADY STATE CONDITIONCONDITION

•• Q = OR < q implies Q = OR < q implies EQUILIBRIUM or STEADY EQUILIBRIUM or STEADY STATE CONDITIONSTATE CONDITION

Pumped Water Level & DrawdownPumped Water Level & DrawdownThe distance from centre of The distance from centre of pumping well to periphery pumping well to periphery of the cone of depression is of the cone of depression is known as “radius of known as “radius of influence”influence”

After the pumping is After the pumping is stopped, the level of water stopped, the level of water in pumped well is known as in pumped well is known as “pumped water level”. “pumped water level”.

The difference in static The difference in static water level and pumped water level and pumped water level is known as water level is known as “Total Drawdown” in this “Total Drawdown” in this condition.condition.

Recuperation or recoveryRecuperation or recovery

In this condition the water from the In this condition the water from the surrounding aquifer continues to surrounding aquifer continues to flow towards the pumping well flow towards the pumping well under the influence of the (artificial) under the influence of the (artificial) hydraulic gradient towards the hydraulic gradient towards the pumping well. pumping well.

After pumping is stopped, due to After pumping is stopped, due to continued aquifer contribution continued aquifer contribution qq, , water level in the pumping well as water level in the pumping well as well as in the aquifer surrounding well as in the aquifer surrounding the pumping well rises. the pumping well rises.

This is because the portion of the This is because the portion of the aquifer which was earlier dewatered aquifer which was earlier dewatered during pumping starts during pumping starts resaturatingresaturatingdue the water inflowing towards the due the water inflowing towards the pumping well (indicated by rise in pumping well (indicated by rise in water level in pumping and water level in pumping and observation wells). observation wells).

This process is known as “recovery This process is known as “recovery or recuperation”.or recuperation”.

Recovery and the cone of depressionRecovery and the cone of depressionDuring the process of recuperation During the process of recuperation or recovery, more and more or recovery, more and more desaturateddesaturated portion of the aquifer portion of the aquifer gets gets resaturatedresaturated. . This is indicated by a rise in water This is indicated by a rise in water in areas away from pumping well in areas away from pumping well also. also. Due to rise in water level the Due to rise in water level the hydraulic gradient towards hydraulic gradient towards pumping well becomes gentler pumping well becomes gentler with time, thereby reducing the with time, thereby reducing the rate of the aquifer contribution q. rate of the aquifer contribution q. Now, the water required for this Now, the water required for this resaturationresaturation process is derived by process is derived by dewatering the peripheral areas of dewatering the peripheral areas of cone of depression cone of depression i.e. cone of i.e. cone of depression continues to expand in depression continues to expand in periphericalperipherical areas even after the areas even after the pump is switched off. pump is switched off. Slowly, with time, the water level Slowly, with time, the water level rises in the aquifer and finally get rises in the aquifer and finally get stabilized at a new static water stabilized at a new static water level (which may be fractionally level (which may be fractionally lower as compared to the original lower as compared to the original S.W.L.) S.W.L.)

Conducting pumping tests…some practical Conducting pumping tests…some practical aspectsaspects

The wells in the vicinity of chosen The wells in the vicinity of chosen pumping wells should not be pumped pumping wells should not be pumped at least 48 hours prior to a pumping at least 48 hours prior to a pumping test. test.

This ensures that water levels in the This ensures that water levels in the aquifer surrounding the pumping aquifer surrounding the pumping wells are as close to the static water wells are as close to the static water level as possible and that there are level as possible and that there are no artificial hydraulic gradients.no artificial hydraulic gradients.

The dimensions (depth and diameter The dimensions (depth and diameter / length & breadth) of the pumping / length & breadth) of the pumping well are measured. well are measured.

Nature of the aquifer to be tested is Nature of the aquifer to be tested is ascertained on the basis of well ascertained on the basis of well inventory data: inventory data:

•• rock type, rock type, •• unconfined / confined / leaky unconfined / confined / leaky •• thickness of the aquifer thickness of the aquifer

A measuring point (MP) is selected A measuring point (MP) is selected and marked on the pumping well and marked on the pumping well head and observation well head with head and observation well head with respect to which all water levels are respect to which all water levels are measured.measured.

Pumping test data collectionPumping test data collection

During pumping (drawdown part) the water levels are measured During pumping (drawdown part) the water levels are measured with respect to the MP at regular time intervals. The time interwith respect to the MP at regular time intervals. The time interval val could be could be •• 1 min or 2min or 5min or 20 minutes and so on. 1 min or 2min or 5min or 20 minutes and so on. •• The time interval is subjective and mostly dictated by The time interval is subjective and mostly dictated by hydrogeologicalhydrogeological

field conditions. field conditions.

In a well test, the drawdown is recorded in the pumping well onlIn a well test, the drawdown is recorded in the pumping well only y and in aquifer performance test drawdown is recorded in pumping and in aquifer performance test drawdown is recorded in pumping well as well as in observation wells. well as well as in observation wells. During the drawdown measurements the discharge of the pump (Q) During the drawdown measurements the discharge of the pump (Q) is also measured (preferably with a simple drum/bucket of knownis also measured (preferably with a simple drum/bucket of knownvolume using the stopwatch method). volume using the stopwatch method). Pump discharge should be measured frequently as it tends to varyPump discharge should be measured frequently as it tends to varydepending upon the efficiency of the pump, drawdown and voltage depending upon the efficiency of the pump, drawdown and voltage fluctuations. fluctuations. Once the pumping from pumping well is stopped, the recovery or Once the pumping from pumping well is stopped, the recovery or repurcationrepurcation measurements are also made in pumping well as well as measurements are also made in pumping well as well as in observation wells at fixed intervals. in observation wells at fixed intervals. During the pumping test the water level in pumping well (if it iDuring the pumping test the water level in pumping well (if it is dug s dug well) goes down exposing the walls of the well below water tablewell) goes down exposing the walls of the well below water table for for observation, regarding how and at what depth water is flowing inobservation, regarding how and at what depth water is flowing into to the well from the aquifer (i.e. information on the the well from the aquifer (i.e. information on the inflow zonesinflow zones).).

Interpretation of pumping test dataInterpretation of pumping test data

Pumping test data are interpreted Pumping test data are interpreted for:for:

•• Estimating aquifer properties like Estimating aquifer properties like TransmissivityTransmissivity and and StorativityStorativity..

•• Estimating well characteristics, Estimating well characteristics, especially the well yield or specific especially the well yield or specific capacity.capacity.

Aquifer performance testsAquifer performance testsObservations in pumping and Observations in pumping and observation wells.observation wells.

In an aquifer performance test, In an aquifer performance test, the drawdown measurements the drawdown measurements are made with respect to fixed are made with respect to fixed time intervals. The pump time intervals. The pump discharge is also checked discharge is also checked several times during pumping several times during pumping period. The recovery period. The recovery measurements, after the pump measurements, after the pump being shutoff, are also made at being shutoff, are also made at fixed intervals.fixed intervals.

The main aim this test is to The main aim this test is to estimate aquifer characteristics estimate aquifer characteristics i.e. i.e. TransmissivityTransmissivity and and StorativityStorativity. .

There are many methods of There are many methods of interpreting pumping test data. interpreting pumping test data. These are given in various text These are given in various text books on groundwater.books on groundwater.

An example of interpreting aquifer test dataAn example of interpreting aquifer test dataWe will consider a very simple graphical method given by We will consider a very simple graphical method given by Cooper and Jacob (1964) to estimate T and S.Cooper and Jacob (1964) to estimate T and S.The water level in pumping well / observation wells (below MP) The water level in pumping well / observation wells (below MP) is plotted against time since pumping started is plotted against time since pumping started tt on semi on semi logarithmic graph paper:logarithmic graph paper:•• Water level (as drawdown s) on arithmetic scale and time on log Water level (as drawdown s) on arithmetic scale and time on log scale. scale.

The first few points reflect the effect of well storage (segmentThe first few points reflect the effect of well storage (segment 1) 1) Segment 2 indicates the effect of contribution from aquifer in tSegment 2 indicates the effect of contribution from aquifer in the form of inflow he form of inflow (in pumping well during pumping) (in pumping well during pumping) In case of observation well data, points on a straight line indiIn case of observation well data, points on a straight line indicate the effect of cate the effect of dewatering of the aquifer in the form of drawdown.dewatering of the aquifer in the form of drawdown.The third segment which also falls on a straight line, may indicThe third segment which also falls on a straight line, may indicate: ate:

•• A gentler slope as compared with segment 2 indicating an increasA gentler slope as compared with segment 2 indicating an increase in aquifer e in aquifer contribution (q) as compared to the pump discharge Q. This indiccontribution (q) as compared to the pump discharge Q. This indicates that the ates that the aquifer is receiving recharge from some external source during paquifer is receiving recharge from some external source during pumping (may be umping (may be river, lake, canal etc.), often leading to a steady state conditriver, lake, canal etc.), often leading to a steady state condition.ion.

•• Sometimes the slope of the segment 3 Sometimes the slope of the segment 3 steepenssteepens as compared with segment 2. This as compared with segment 2. This indicates the limited extent of the aquifer, showing sudden incrindicates the limited extent of the aquifer, showing sudden increase in the rate of ease in the rate of drawdown (i.e. aquifer contribution q is much smaller than the pdrawdown (i.e. aquifer contribution q is much smaller than the pump discharge Q).ump discharge Q).

A timeA time--drawdown plot… STEADY STATEdrawdown plot… STEADY STATE

A time drawdown plot

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

1 10 100 1000

Time in minutes

Dra

wdo

wn

in m

bgl

Segment 1

Segment 2

Segment 3

Time drawdown plot

0

1

2

3

4

5

6

7

1 10 100 1000

Time in minutes

Dra

wdo

wn

in m

bgl

A timeA time--drawdown plot… DEWATERED drawdown plot… DEWATERED WELLWELL

Segment 1Segment 2

Segment 3

TransmissivityTransmissivity from the Cooper Jacob formulafrom the Cooper Jacob formula

The The TransmissivityTransmissivity can be obtained can be obtained using the using the semilogsemilog graph (as given in graph (as given in the above examples) and the the above examples) and the following procedure:following procedure:•• The value of The value of ∆s∆s which is the which is the

drawdown over ONE LOG CYCLE of drawdown over ONE LOG CYCLE of time “t’ is calculated. time “t’ is calculated.

•• Segment 2 is generally used to obtain Segment 2 is generally used to obtain the the ∆s∆s value used in following value used in following equationequation

T= (2.303 x Q)/ 4∏∆s is the original T= (2.303 x Q)/ 4∏∆s is the original equation where the final equation isequation where the final equation is

T= (264 x Q) / T= (264 x Q) / ∆s∆s•• Here, T is directly obtained in Here, T is directly obtained in

mm22/day directly/day directly•• Q= Pump discharge in mQ= Pump discharge in m33 per minper min•• ∆s∆s= slope of segment 2 over one log = slope of segment 2 over one log

cycle of time t expressed in cycle of time t expressed in metresmetres..

If Q (pump discharge) is If Q (pump discharge) is common for both the cases common for both the cases and is equal to 1 mand is equal to 1 m33/min, /min, we havewe have

•• For graph 1 (steady For graph 1 (steady state), state), ∆s∆s is is approximately 0.5 mapproximately 0.5 m

T= (264 x 1) / 0.5T= (264 x 1) / 0.5i.e. T= 528 mi.e. T= 528 m22/day/day

•• For graph 2 (unsteady For graph 2 (unsteady state), state), ∆s∆s is approximately is approximately 15 m15 m

T= (264 x 1) / 15T= (264 x 1) / 15i.e. T= 17.6 mi.e. T= 17.6 m22/day/day

StorativityStorativity from the Cooper Jacob formulafrom the Cooper Jacob formula

For For StorativityStorativity S the data S the data of drawdown in OBs well at of drawdown in OBs well at distance “r” from the distance “r” from the pumping well is necessary.pumping well is necessary.StorativityStorativity S is given by S is given by the equation the equation S=2.25 TtS=2.25 Tt00 / r/ r2 2 where, where, •• T= T= TransmissivityTransmissivity in min m22 / /

minmin•• tt00= intercept of straight = intercept of straight

line on the time axis line on the time axis •• r = distance of r = distance of

observation well from observation well from pumping wellpumping well

(S is given in fraction)(S is given in fraction)

For an observation well at a For an observation well at a distance of 50 m from the distance of 50 m from the pumping well:pumping well:

Graph 1 shows tGraph 1 shows t0 0 of about 60 of about 60 min and therefore:min and therefore:

S= 2.25(528/1440)60 / 2500S= 2.25(528/1440)60 / 2500i.e. i.e. S= 0.0198S= 0.0198

Graph 2 shows tGraph 2 shows t0 0 of about 40 of about 40 min and therefore:min and therefore:

S= 2.25(17.6/1440)40 / 2500S= 2.25(17.6/1440)40 / 2500i.e. i.e. S= 0.000049S= 0.000049

Well testsWell testsObservations are made in the Observations are made in the pumping well pumping well itself (no observation itself (no observation wells)wells)The main aim of the well test is to estimate the yielding capaciThe main aim of the well test is to estimate the yielding capacity ty of the well or capacity to derive water form the aquifer known aof the well or capacity to derive water form the aquifer known as s “specific capacity”“specific capacity”in case of a bore well, the water stored in the form of water in case of a bore well, the water stored in the form of water column is small in quantity. Therefore, during pumping, initiallcolumn is small in quantity. Therefore, during pumping, initially y the stored water is pumped out and later on the pump discharge the stored water is pumped out and later on the pump discharge is entirely composed of water derived from the aquifer. Under is entirely composed of water derived from the aquifer. Under such condition the capacity of the bore well to derive water frosuch condition the capacity of the bore well to derive water from m the aquifer i.e. specific capacity of bore well is given by equathe aquifer i.e. specific capacity of bore well is given by equation tion Specific capacity C = Q / s whereSpecific capacity C = Q / s where•• Q= discharge of the pump in Liters per minute (Q= discharge of the pump in Liters per minute (LpmLpm))•• s= drawdown in pumping well after certain time has lapseds= drawdown in pumping well after certain time has lapsed

Therefore, specific capacity is Therefore, specific capacity is discharge per unit of drawdowndischarge per unit of drawdown. . •• For example, a bore well is pumped at rate Q= 500 For example, a bore well is pumped at rate Q= 500 lpmlpm for 2 hours for 2 hours

with a resultant drawdown s= 5m,with a resultant drawdown s= 5m,•• Specific capacity of the bore well is C = Q/s = 500 Specific capacity of the bore well is C = Q/s = 500 lpmlpm / 5 m / 5 m •• i.e. 100 i.e. 100 lpmlpm per per metremetre of drawdownof drawdown

What does specific capacity mean…What does specific capacity mean…The specific capacity value of The specific capacity value of

C= 100 C= 100 lpmlpm /m implies:/m implies:

•• that the same that the same borewellborewell will will yield about 6000 yield about 6000 litreslitres in in one hour, resulting in a one hour, resulting in a drawdown of 1 m.drawdown of 1 m.

•• It will yield about 12000 It will yield about 12000 litreslitres in two hours with a in two hours with a drawdown of about 2 m.drawdown of about 2 m.

•• If it is pumped at the rate of If it is pumped at the rate of 200 200 lpmlpm, the well will draw , the well will draw down to 2 m in one hour.down to 2 m in one hour.

Thus, in case of a bore well, Thus, in case of a bore well, the specific capacity can be the specific capacity can be used to predict the level of used to predict the level of drawdown for different drawdown for different pump discharge rates.pump discharge rates.

Which bore well has a greater Specific capacity?

Which is larger:Q/s1 or Q/s2?

SolutionSolution

Well 1 has a greater specific Well 1 has a greater specific capacity than well 2.capacity than well 2.

If Q= 100 If Q= 100 lpmlpms1= 5ms1= 5ms2= 10m thens2= 10m then

C of well 1= 20 C of well 1= 20 lpm/mlpm/mC of well 2= 10 C of well 2= 10 lpm/mlpm/m

Specific capacity of a large Specific capacity of a large diameter dug welldiameter dug well

In case of large diameter dug wells (which are common in In case of large diameter dug wells (which are common in most parts of India), the specific capacity estimation most parts of India), the specific capacity estimation method used for bore wells is method used for bore wells is notnot applicable. applicable. The amount of water stored in a The amount of water stored in a dugwelldugwell is much larger as is much larger as compared to that in a bore well. Therefore, the pump compared to that in a bore well. Therefore, the pump discharge in case of dug well is composed of water partly discharge in case of dug well is composed of water partly derived from: derived from: •• the well storage and the well storage and •• water derived from the aquifer. water derived from the aquifer.

The resultant drawdown in a dug well is also due to the The resultant drawdown in a dug well is also due to the effect of the above two factors.effect of the above two factors.Therefore, the normal method of using the ratio Therefore, the normal method of using the ratio pump pump discharge / drawdown discharge / drawdown may not exactly represent the may not exactly represent the capacity to derive water from the aquifer per unit of capacity to derive water from the aquifer per unit of drawdown.drawdown.

Slichter’sSlichter’s method of estimating the method of estimating the specific capacity (yield) of a dug wellspecific capacity (yield) of a dug well

In order to estimate the specific In order to estimate the specific capacity of a dug well, the capacity of a dug well, the (recuperation) recovery data is (recuperation) recovery data is used for analysis. used for analysis.

Recovery data does not involve Recovery data does not involve the effect of well storage on the effect of well storage on pump discharge (as the pump is pump discharge (as the pump is shut off) and contains the effect shut off) and contains the effect of the contribution of water from of the contribution of water from the aquifer only. This effect is in the aquifer only. This effect is in the form of rise in water level in the form of rise in water level in pumping well. pumping well.

The specific capacity is calculated The specific capacity is calculated using using Slichter’sSlichter’s Recovery Recovery Formula:Formula:

C = (2.303(A / t’)) (logC = (2.303(A / t’)) (log1010 (s1/ s2)) (s1/ s2)) where where C= Specific capacity of dug well C= Specific capacity of dug well in min m33/min per /min per metremetre of drawdown of drawdown or or lpm/mlpm/m of drawdown of drawdown A= cross sectional area ∏rA= cross sectional area ∏r22 (in (in mm22) ) t’= time since pumping stopped t’= time since pumping stopped in minutesin minutess1= total drawdown in meterss1= total drawdown in meterss2= residual drawdown values in s2= residual drawdown values in meters at respective time values meters at respective time values since pumping stopped (t’)since pumping stopped (t’)

Method for estimating specific capacity of dug well Method for estimating specific capacity of dug well using the using the SlichterSlichter methodmethod

Measure the total drawdown in the well (s1) at the end of Measure the total drawdown in the well (s1) at the end of pumping.pumping.Similar to a drawdown test, measure the water level rise in Similar to a drawdown test, measure the water level rise in the well and estimate the residual drawdown s2 at different the well and estimate the residual drawdown s2 at different times (t’).times (t’).Residual drawdown (s1) can be estimated by subtracting Residual drawdown (s1) can be estimated by subtracting the rise in water level over a fixed time from the total the rise in water level over a fixed time from the total drawdown (s1) drawdown (s1) The ratio s1/s2 is calculated for each value of time t’ for The ratio s1/s2 is calculated for each value of time t’ for which values of s2 are measured. which values of s2 are measured. A graph of t’ values (YA graph of t’ values (Y--axis) is plotted against the ratio s1 axis) is plotted against the ratio s1 / s2 ( X/ s2 ( X--axis) on a axis) on a semilogsemilog paper.paper.A straight line is plotted through these points and the slope A straight line is plotted through these points and the slope of the line (change in the value of t over one logof the line (change in the value of t over one log--cycle of cycle of the ratio (s1/s2) is estimated.the ratio (s1/s2) is estimated.

Drawdown and residual drawdown Drawdown and residual drawdown measurement during recoverymeasurement during recovery

Specific capacity dataSpecific capacity data

Slichter's plot for Specific capacity

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s1/s2

t'

∇t'= 220 min

Specific capacity exampleSpecific capacity exampleUsing the plot Using the plot (previous slide) the (previous slide) the equation:equation:C= (2.303(A / t’))/(logC= (2.303(A / t’))/(log1010

(s1/ s2)) reduces to(s1/ s2)) reduces to

C= (2.303A)/C= (2.303A)/∇∇tt’’Where Where ∇∇tt’’= change in the = change in the

value of tvalue of t’’ over one log over one log cycle of the ratio s1/s2cycle of the ratio s1/s2

If the cross sectional area If the cross sectional area (A) of the well= 20 m(A) of the well= 20 m22

∇∇tt’’= 220 minutes= 220 minutes

Specific capacity can Specific capacity can be calculated as:be calculated as:

C= (2.303(20)/220)1000C= (2.303(20)/220)1000

C=C= 209.36 209.36 lpm/mlpm/m of of drawdowndrawdown

Thank you for your patience.Thank you for your patience.