correcting gw levels using be

7/30/2019 Correcting GW Levels Using BE

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Abstract

Te main concepts or identiying and removing ba-

rometric pressure eects in conned and unconned

aquiers are described. Although is commonly known

that barometric pressure changes can eect water level

readings, ew articles and procedures are provided to

correctly manage piezometric data.Knowing the barometric eciency reduces errors in

calculating piezometric suraces and drawdowns in the

piezometers during pumping tests. Stallman (1967)

suggested urthermore, that air movement through the

unsaturated zone and the attendant pressure lag, could

help to better describe the aquier properties. Rasmus-

sen and Craword (1997) described how barometric

sidering water level changes and atmospheric pressure,

some aquiers show an inverse reletionship: increase in

barometric pressure creates declines in observed water le-

vels and vice versa. wo important consequences are pie-

zometric map interpretation over large areas and/or with

low gradient and pump test analysis. For example, when

dealing with pumping tests and piezometers we have tocorrectly remove barometric pressure infuences to cal-

culate trasmissivity values. Te long duration test should

induce drawdowns in the order o 20-30 cm in the pie-

zometer. o achieve this, the pump discharge should be

high and/or the borehole closer to the producing well.

Moreover, it is widely recognised that the attraction o

Moon and the Sun can infuence piezometric levels and

Correcting water level data for barometric pressurefluctuationsTheoretical approach and a case history for an unconfined karst aquifer (Otavi, Namibia)

Alessio Fileccia*


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sen and Craword (1997) described how barometric Moon and the Sun can infuence piezometric levels and

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to the height o the water column above datum. Fur-

thermore the well response varies due to casing, wellcompletion and aquier geometry.

Casing

Te borehole responds with a time lag to the external

pressure due to:

Well storage

Tickness o the mud cake on the walls

Length and type o screens

Te last issues are known as skin eects.

Te lag time can be as much as hours or days. Tick

mud cakes and short screen lengths slow down the rea-

ching o equilibrium between the water levels on either

side o the well. It is something similar to the pumping

o a bike inner tube: we orce the air through the valve

(well screen) using a hand pump (external atmosphere)into the rubber inner tube (the borehole)1.

Aquier

We can consider two cases:

Phreatic aquier

I the static water level is near to surace and/or the un-

A particular case is reported during severe storms. When

rainall is high the decreasing barometric pressure over-laps the water inltrating vertically through the unsa-turated part, trapping and pushing the air towards the

water table and the screens into the well. Tis behaviourcan induce quick variations, higher than the barome-tric2. At he equilibrium the atmospheric pressure (pa) isthe same in every point o the aquier (pw) and in the

well (g. 2). Te water percolating downwards induces

a rise in the air pressure trapped above the water table(dpa) increasing also the pressure on the water by anequivalent amount (dpw).2 Te equilibrium at the wellis given by:


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substituting:

(6) dpw = dpa + ( h h)

and

(7) dpw dpa = ( h h)

being

(8) dpa = dpw + ds where ds is the eective stress onthe grains

then dpa > dpw and h < h

Te relationship is an inverse one, increases in in ba-rometric pressure create declines o water levels in the

wells. It is important to stress that or such aquiers the-re is a time lag between the drop in the water level inthe well and his new equilibrium with the aquier (timelagged response). Tis lag is in the order o a ew hoursor days and higher in case o thick and rigid aquitardsor unsaturated layers.

dpa = atmospheric pressure variation; dpw = water pres-

sure variation (at the aquier top)

when dpa > 0 also H >0 proving that an increase in theentrapped air leads to a rise in the water column insidethe well (i opened to the atmosphere).Tis particular type o piezometric rise bears no rela-tion to groundwater recharge, but can be mistaken orit when associated to rainall events. In this case the in-

duced pressure change generates well- water level thatdo not represent actual groundwater potential.

Semiconne/conned aquierFor this aquier the transmission o atmospheric pres-sure is instantaneous both to well and aquier, beingunctions o the degree o connement, matrix rigidityand specic weight o water.Te pressure at the aquier top is supported partly romthe rock skeleton and partly rom the water thus in-troducing a time lag or the equilibrium to be reached(hours or days).On the contrary, the level in the well is reached withoutlag.


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es between 0 and 1 and can be expressed as the ratio

in hydraulic head change over a change in barometricpressure during the same time interval:

(9) BE = W/B with

W = change in hydraulic head

B = change in atmospheric pressure (meters o waters)

= specic weight o water ( = g)

A general approach is to obtain BE rom an arithmeti-

cally scaled plot o water level changes as a unction o

concurrrent atmospheric pressure changes over a su-

ciently long period o time.

Te slope o a least squares line t through the data is

the barometric eciency.

Te data time step can be in the order o 15 to 60 min-

utes or a period o a ew days

ed method to be used when BE can be infuenced by

pressure fuctuations other than barometric.Going back to equation (9) the static water level in the

well (H) measured with a common dip meter is com-

prehensive o the barometric pressure (B) and the water

level (W)Substituting in the equation (1):

(10) W/B = (H-B) / B = (H/B) 1

when H/B = 0 then BE =100% and does not aectthe total head within the aquier (e.g. unconned aqui-

er with thick and/or rigid unsaturated zone)

when H/B = 1, the air pressure travels ast through

the soil and BE = 0 (shallow unconned aquier)

I we consider the aquier water compressibility

(Lohman, 1972):

(11) BE = n b / S as a percentage

n = porosity ; b = aquier thickness; S = storage coe-

cient

Introducineg the Specic Storage (Ss), the eective po-


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When the equilibrium is rapid (BE = 0) and H/B = 1

we are acing an unconned aquier with a thin unsatu-rated zone and/or highly permeable.One has to consider a piezometric map over a large area

with dierences in surace elevation or a nearly fat plainwith low gradients.Failure to account or these changes can result in errorsin the calculation o the magnitude and direction o thehydraulic gradient. Tese errors can also be increased i

we add earth or sea tides and fuid density variations.o describe in more detail the subject, one has to con-sider that the rate o air movement within the vadosezone is a direct unction o vertical pneumatic diusiv-ity, vertical permability and moisture content (Weeks,1979).

As a general rule we have that:

the advantage in the calculation o the barometriceciency is to better describe the type o aquier,the permability o the unsaturated zone and the

well eciency

BE varies between 0 and 1; linear regression slopesgreater than 1 indicate that actors in addition to


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height above sensor and barometric pressure.

Compensation is perormed in the oce when data aredownloaded using readings rom another sensor thatmeasured only barometric pressure at the surace.Tese instruments are more convenient then the ventedtype, due to lack o compensation tube.

Gauge sensors (vented transducer)wo dierent steel cylinders are connected by a com-

pensation sot tube. Te sensor is seated in the lowercylinder and lowered into the borehole below the watertable. Te upper part is xed near the top o the casingand houses the batteries. Tese sensors record only the

water column height. Fig. 8 depicts the two sensor typeswith reerence to SS models.

have introduced the Barometric Response Function

(BRF) that revealed to be extremely useul or a betteraquier characterization.

Continuous water level recorders

From Bernoulli equation the piezometric head is:

(13) Ht = Hz + p /

Hz = elevation headp / = pressure head = w g (w ground water density; g gravity)


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Precision

Te error o measurement in relation to other readingso the same phenomena. Normally indicated as the sum

o errors as a percentage or a given temperature. I , or

example, we oresee water level fuctuations o 1 m du-

ring the recording time, we can use a 3 m range sensorwith accuracy o 0,1%. Te error will be 0,3 cm over the

entire pressure range and the sensor may be lowered 1 m

below the initial water level.

Records obtained by the sensorsTe piezometric head is obtained by the sensor at screen

level and elevation z above datum.

Reerring to equation (13) the readings (Pmeas) rom a

vented transducer are:

(14) Pmeas = (w Hp + Pbar) Pre

Where:

w Hp = water column pressure above sensor

Pbar = barometric pressurePre = reerence pressure, constant (usually 10,19 m)

readings by a simple water level dipper are equivalent to

those made by an absolute transducer (non vented) andundergo barometric pressure variations.

In case o unconned surcial aquiers the barometric

pressure does not change in the well and in the aquier,

while with thick and rigid aquiers this imbalance has a

time lagged response through the unsaturated zone.

Te measured water column height in the well (Hp) is

dierent rom that o the aquier and will reach equilib-

rium ater hours or days.

Barometric removal techniques

Tese techniques have been examined by several investi-

gators and are ocused on some mathematical methods

that are beyond the object o this paper and have been

treated in detail by Chien, Rojstaczer, Furbish etc.

A relative simple equation or residual (corrected) head,removes barometric infuences on measured ground wa-

ter levels (Rasmussen, Craword, 1997) :

(15) R = W + BE (B C)

R = corrected head

W = water level in the well


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Same cautions should be observed during pumping

tests, particularly when analyzing piezometers, remo-ving external stresses that can change the dinamic wa-ter level other than the pump discharge. In the same

case barometric eects can exceed articial stresses and,being applied over large areas, small dierences o 2-3cm produce water level changes o as much as 20-30cm. Conned aquiers have a sudden response to ba-

rometric variations that are easily removed. Te same

does not apply or unconned aquiers showing a timedelayed response.

A suggested simplied method to reduce the error is:

Record the trend in barometric pressure and water

levels prior to pumping (4-5 days)

Plot the data, barometric pressure versus water le-vels to evaluate barometric eciency; alternativelyduring the test, one can use a well outside the in-

fuence o the pump to monitor the atmosphericpressure

When all variables are measured in the same pressure

units (meters o water) with a non vented transducer,

sponse Fuynction (BRF) as a more eective mean or

characterizing the longer term response (Rasmussen,Craword 1997; Spane 2002)

Te BRF is used to remove the infuence o the barome-

tric pressure rom the water level records and the dia-

gnostic plots obtained can reveal:

Aquier and aquitard type

Degree o aquier connement

Skin eect

Estimating the time - lag response between barometric

pressure changes and water level responses in a well,

can be accomplished using regression deconvolution

method (Furbish, 1991). A simpler way makes use o

the sotware BECO (Sandia National Laboratories)

allowing to calculate the corrected, adjusted, values andto trace the BE as a unction o time since the imposed

load. Tis procedure helps understanding why in some

situations the initial BE is dierent rom the nal one.

In an aquier rehabilitaiton project, e.g. the BRF obtai-

ned or various boreholes can be useul to demonstrateaquitard properties or aquier connement. Tings that

are not always clear rom lithologic logs alone.


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Te sloping curve (g. 9A) shows that the pressure rise

is rapid or the well but delayed in time or the aquier;BE diminishes slowly until reaching a new equilibrium.

Keeping in mind the barometric eciency denition,

BE = W/B, W is high then decreases while B

is going up.

Skin efect

Te above mentioned description assumes a 100% well

eciency. Actually the barometric fuctuation is re-duced at the well ace due to the presence o the mud

cake, type and length o screens and well storage. Te

eect is to increase the time or the well to reach the

equilibrium with the aquier, consequently also BE is

varying with time.

In g. 9A a steep ascending limb shows good well e-

ciency and thereore the hydrogeological parametersobtained rom a pumping test are more reliable.

Spane suggests that aquiers do not show important

time lag when > 10m2 /day (10m-4 m2 /s) and the

well storage is negligible ater 5 minutes approximately.

Increasing the well diameter and decreasing the aquier

storage leads to a rise in the time lag (see gentle slope

in g. 10).


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In practice there could be also the situation or which

the aquier model behavior overlaps the skin eect mo-del, leading to a composite pattern. An advantage o

using this technique arises when comparing various

BRF rom dierent wells or or the same well, screenedat dierent interval depths.

Such approach can reveal the reliability o the water

points or a piezometric map.

Fig. 11 shows the similar trend or our dierent bo-reholes. Te agreement clearly indicates that the aquier

system is not changing too much despite the wells areseparated by 1-1.5 km distance.

Te short term response with high BE values decreasing

with time compared to the lithology, indicates an un-

conned aquier with thick unsaturated zone.

analyzed in the hydrogeological practice. Recent stu-

dies have proved the existence o tight bounds betweenhydrogeological parameters and earth tides. As a conse-quence they can be applied or a better scheduling andinterpretation o eld tests.Solar and lunar gravitational attraction exerts earth andocean deormations.When the Sun and the Moon passover a point on the earth surace they generate a strainin the rock, changing the volume o the ractures, thus

increasing the general porosity and decreasing its pie-zometric head. Ater the passage o the celestial bodiesthe gravitational attraction diminishes, and so doas theoverall aquier porosity but the piezometric head incre-ases. A lower aquier rigidity corresponds to a higherdeormation and hydraulic head (Hsieh, 1987). Tesephenomena are known as earth tides and are characte-rized by being periodic. Due to their complex motionsthe tides also are complex and have many componentso dierent requencies and amplitudes.Each tidal component is likely to have a dierent in-fuence due to the vector orce applied, i.e. the moonon the horizon exerts orce in a dierent direction than

when it is overhead.Reerring to rock ractures, a tangential orce will have


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cal evaluations. N2 also is oten neglected because the

large signal to noise ratio is a potential source o error.From able 1 it is easily seen that the components haveperiods o 12 to 25.8 hours and requencies o 0.9 to2 cycles per day, while daily deormations reach sometenths o centimeters.Recent investigations have tried to extend data tohydrogeological studies. Te reason is that these orcesare uniorm in magnitude over large areas and so are the

eects on the conned aquiers, without inducing addi-tional lateral fow. Tis particular approach or studyingbounds between earth tides and aquier properties canbe summarized in our dierent steps (Merritt, 2004):

Calculation o diusivity (/S)

Calculation o specic storage or conned aquiers(Ss)

Calculation o trasmissivity () and storage (S) orconned aquiers

Calculation o vertical hydraulic conductivity (Kv)

Bredehoet (1967) showed that the fuctuation o thehydraulic head in a well, due to earth tide, is a unctiono the specic storage (Ss) and this last parameter can

In deep unconned aquiers with rigid vadose zone,

the pressure fuctuation is transmitted rapidly to thewell but displays a time-lagged response at the watertable, because air must move into or out o the over-lying vadose zone to transmit the change in pressure

During an aquier test, drawdowns in the piezom-eter should be more than 0.2 m to be reliable andnot infuenced by barometric fuctuation

Te magnitude o water level change is a unction

o aquier connement, matrix rigidity and specicweight o the water

Generally speaking the elastic aquier behaviordecreases with the increase o the overburden andbarometric eciency

A simplied method to calculate BE or connedaquiers is to prepare a plot o barometric pressurevalues versus water levels or a certain time (BE =W/B)

BE varies between 0 and 1 Te barometric response unction (BRF) is a unc-

tion o time since the imposed load

Harmonic components derived rom earth tidesproved to be related to some hydrogeological pa-rameters


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In all examples we notice that:

Tere is a good correlation between measured andcorrected values, even i with lower amplitude

Tere still is a variation diminishing in the correctedvalues; being excluded skin eects phenomena thisbehaviour could be ascribed to other non barome-tric eects (earth tides, double porosity)

Te initial barometric eciency values are quite si-

milar (0-55-0.61)

In g. 18 is depicted the barometric response unction(BRF) that characterises the water level response overtime to a step change in barometric pressure; essentiallyBRF is a unction o time since the imposed load.

A good agreement is observed or all three water points.

In Dragons Breath lake e.g, there is a quick rise to 0.5and a longer term decay to a lower value (0.2 - 0.3 ater20 hrs), due to the slow passage o air through ractures.Te balance between external pressure and the aquieris reached at 0.1 value.Te shape o the three curves indicate an unconnedaquier with good hydraulic connections expeciallyb D B h d H ib l k hi l

the M2 component and this can be considered as an in-

dication o a higher trasmissivity zone (Merritt, 2004).Tis act is partly conrmed by the presence o a local

racture elongated ENE-WSW very close to Harasib lake.

Concluding remarks

Water levels fuctuations in aquiers are not only due torecharge variations. Barometric pressure and tides areamong the main concerns. Knowing barometric pres-

sure variation or a particular site, helps to validate apiezometric map or a pumping test. Modern pressuretransducers vented to the atmosphere are recognised tobe extremely useul when installed into boreholes. Re-cordings are dierent ollowing the type o aquier andthe graphs can be diagnostic o the degree o conne-ment or the monitored levels.Useul parameters that characterize this behaviour arethe barometric eciency (BE) and the barometric re-sponse unction (BRF). Te latter characterizes a deepunconned aquier when values are initially high andapproximate 0 on the long term response, converselythe aquier is conned/semiconned when values staycostant or approximate 1 on the long term response.Removing barometric eects is sometimes necessary to


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20. Harmonic components or the three water points


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Acknowledgements

Namgrows stands or Namibian Groundwater Systems,a project set up by the author and the colleague GraldFavre, with the partecipation o geologists and caversrom 4 dierent countries (Italy, Switzerland, Namibia,South Arica). Te project was supported in Namibia byeng. Sarel La Cante and his wie Leoni Pretorius (Ha-rasib arm).Te company SS-Italia sponsored us by providing the

water level sensors and its technical support.I also wish to thank pro. odd Rasmussen (Te Uni-versity o Georgia, Athens) or providing his valuableinsights into the data and particularly those regardingthe barometric eciency and earth tides.

Bibliography

Hare Paul Wm, Morse E. Ralph 1997. Water level fuc-tuations due to barometric pressure changes in an isolat-

ed portion o an unconned aquier. Ground Water n. 4

Merritt Michael L., 2004. Estimating Hydraulic Prop-erties o the Floridan Aquier System by Analysis oEarth-ide, Ocean-ide, and Barometric eects, Col-lier and Hendry Counties, FloridaRasmussen C. odd, Craword A. Leslie 1997. Identi-ying and removing barometric pressure eects in con-ned and unconned aquiers. Ground Water n. 3Spane F.A. 1999. Eects o Barometric fuctuations on

well water-level measurements and aquier test data.U.S. Dept. o Energy, Pacic North West National Lab-oratoryoll J. Natahnial, Rasmussen C. odd 2007. Removalo barometric pressure eects and earth tides rom ob-served water levels. Ground Water n. 1

Weeks P. Edwin 1979. Barometric fuctuations in wellstapping deep unconned aquiers. Water resources re-

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correcting gw levels using be

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