Extraction of soil solution with suction cups
Extraction of soil solution with
suction cups
Manual for glass suction cups (12.52) and plastic suction
cups (12.53)
P.O. Box 4, 6987 ZG Giesbeek
Nijverheidsstraat 30,
6987 EM Giesbeek,
The Netherlands
T +31 313 880200
F +31 313 880299
E [email protected]
I http://www.eijkelkamp.com
M2.12.52.E
1
Extraction of soil solution with suction cups
Extraction of soil solution with suction cups
Contents
1
Introduction
1.1 Construction of suction cups
1.1.1
Suction cups without shaft collection
1.1.2
Suction cups with shaft collection
1.2 Which cup for what purpose?
2
Preparations and installation
2.1 Cleaning of suction cups
2.1.1
Cleaning of plastic cups
2.1.2
Cleaning of glass cups
2.2 Installation
3
2.2.1
Installation without slurry
2.2.2
Installation with slurry
2.2.3
Installation of plastic cups
Connection and sampling
3.1 Application of single suction cups
3.2 Application of suction cup systems
3.3 Removal of samples
3.3.1
Cups without shaft collection
3.3.2
Cups with shaft collection
4
Possible sources of error and correction
5
Literature
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Extraction of soil solution with suction cups
Extraction of soil solution with suction cups
1
Introduction
Suction cups are used for the extraction of soil solution and thus to study solute concentrations
and solute fluxes in soils. They allow sampling of soil solution at the same site as often as required. Thus, the suction cup method is basically different from the destructive removal and
extraction of soil samples.
1.1 Construction of suction cups
1.1.1
Suction cups without shaft collection
In principle all suction cups (glass, plastic or ceramic cups) are constructed equally (Fig. 1, next
page). They only differ by the properties of their special filter element which must be chosen
carefully for the intended study (see ch. 1.2).
Fig. 1:
Construction of suction cups
The top of the filter element is connected to a waterproof
without shaft collection
shaft which encloses two tubes. One of those tubes is fixed to
a vacuum control chamber and ends at the top edge of the
cup’s cavity. Thus, the control chamber corresponds with the
Vacuum
control
chamber
pressure inside of the cup, and this pressure can be
Sampling tube
measured by a pressure gauge. For this purpose the septum
on the pressure chamber is pricked with a needle connected
to the pressure gauge, and the pressure in the cavity of the
cup is displayed. In this way the pressure in each cup can be
Vacuum control
controlled, and leaks can be detected easily.
and ventilation
tube
The other tube (sampling tube) conducts to the bottom of the
cup’s cavity, and its open end has to be connected to a
sampling bottle. Thus, when water is sucked out of the soil
the cavity of the cup will never be filled with soil solution, but
the water is directly sucked into the tube and stored in the
bottle.
Suction cup
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Extraction of soil solution with suction cups
The top of the filter element is closed, i.e. its cavity is not connected to the shaft’s cavity, and
therefore all suction cups are available without any shaft as well. Applications of this shaftless
version do make sense if there is a risk of soil maceration and shrinking, and, as a result, development of caverns along the shaft with the risk of fast water fluxes into greater depths. In this
case, and if intensive rainfall occurs, water may flow directly from the soil surface along the
shaft into the depth of the filter element and affect your measurements.
1.1.2
Suction cups with shaft collection
If the sum of installation depth (plus height of the sampling
equipment above the ground) and required suction in the
cup exceeds 800 cm water column or hPa, a vacuum level
of 800 up to 900 cm water column may not be sufficient to
get water out of the soil and into the sampling bottle. In
this case suction cups with shaft collection should be
applied. Within this form of collection system the sucked
soil solution is not conveyed to the soil surface or into the
sampling flask but sucked into the shaft and stored there
(Fig. 2). A second advantage of this construction is that
the collected soil solution is stored at low (soil)
temperatures which minimizes the risk of chemical
changes due to high or very low temperatures.
To suck soil solution into the shaft there has to be a
pressure difference between the cup and the soil matric
potential. Moreover, the additional height of the sampling
tube above the cup has to be exceeded.
Fig. 2:
Construction of suction cups with shaft
collection
Sampling tube
Vacuum tube
Shaft cavity for
water storage
Sampling tube
Example: If the soil matric potential around the suction
Suction cup
cup is about 200 hPa, soil solution can be sucked out of
the soil with an underpressure of 250 hPa (difference
50 hPa). If the height of the sampling tube above the cup is 80 cm, the pressure difference of
50 hPa is not sufficient to convey the solution out of the cup into the shaft. Thus, the vacuum
value in the cup has to be at least 330 hPa to get the water out of the soil and into the shaft.
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Extraction of soil solution with suction cups
After water has been collected in the shaft, it can be removed either with pressure or with underpressure (see 3.3.2). With this technique the sampling of soil solution is devided into two
steps which allow sampling depths of 10 m and even more.
1.2 Which cup for what purpose?
A very important point when using suction cups is the choice of the cup material. In contact with
different suction cup materials many solutes in soils show similar specific sorption properties as
with soil substances like humus or clay minerals. Depending on the investigated substance the
material for suction cups should therefore be chosen carefully. The following table represents
an overview and assigns the different solutes to the recommended cup material.
Table 1:
Suitability of suction cup materials for the examination of different solutes in
soils
Solutes
Pesticides, DOC
Phosphate
Nitrate, chloride, bromide, sulfate, sodium
Heavy metals
Ceramic
-++
--
Glass
++
++
++
-
Plastics
++
++
++
These recommendations result of many papers of scientific literature and of a laboratory study
conducted by a.o. the Institute of Soil Science of the University of Bonn (literature see at the
end of this manual). Special conditions (extreme pH-values or high concentrations of other solutes like DOC) may lead to another weighting than listed in table 1, but for a bigger part of studies these assessments should be valid.
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Extraction of soil solution with suction cups
2
Preparations and installation
2.1 Cleaning of suction cups
Before installation into the soil new suction cups should be cleaned to remove contaminants
due to manufacturing processes. For this procedure you need 0,1 molar HCl (hydrochloric acid),
deionised water, a vacuum stable flask (e.g. a sampling bottle) and a vacuum pump. Each cup
is plunged into the diluted acid, and an amount of about 1 L should be sucked into the bottle.
Afterwards the cup should be dipped into deionised water and percolated with at least 1 L water
to remove the acid. Please note the following two chapters when cleaning suction cups made of
plastics and glass.
2.1.1
Cleaning of plastic cups
The polyamide membrane of plastic suction cups is not completely acid proof, and therefore the acid solution has to be removed after the cleaning procedure immediately. After
leaching the cups with hydrochloric acid (as described in 2.1) they have to be washed with several litres of deionised water or plunged into a water bath until the washing procedure can be
conducted. Apart from that go on as described above.
2.1.2
Cleaning of glass cups
Glass suction cups are often used for the investigation of organic compounds in the soil solution. We therefore recommend – apart from the leaching procedure with HCl – an additional
cleaning with 0,1 molar sodium hydroxide solution. Between these steps of the cleaning procedure we recommend to leach it with deionised water.
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Extraction of soil solution with suction cups
2.2 Installation
2.2.1
Installation without slurry
It is possible to insert suction cups directly and without slurry into the soil. This kind of
installation requires a stone-free soil and a tool which has got the same (or slightly less)
diameter as the porous cup. For installation in greater depths we recommend to make a
hole with a tool (soil corer) which has got a greater diameter than the suction cup near to
the installation depth of the porous cup. After that the last centimeters of the hole can be
excelled by a metal pipe in the same (or slightly smaller) diameter like the porous cup.
Then the cup has to be pushed into the hole very carefully. Since glass and ceramic
cups are fragile to some extend, one has to work with adequate cautiousness.
After installation the cavity around the shaft should be refilled, and a cuff has to be put
around the shaft to avoid water fluxes along into greater depths.
2.2.2
Installation with slurry
In stony soils and in greater depths an installation with slurry is recommended. The hole has got
a greater diameter than the suction cup, but should be as small as possible to avoid needless
disturbance of the natural soil structure. Moreover, the material for refilling the borehole should
not be an artificial substrate, but soil material taken from the installation site and depth.
After drilling the hole into the right depth, some slurry made of the natural soil material has to be
poured into the bottom of it. Then the porous cup has to be pushed down immediately into this
slurry. Afterwards refill the remaining cavity around the shaft with the same soil slurry as mentioned above up to the surface. The last step is to place the cuff on the shaft to avoid water
fluxes along into greater depths.
7
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Extraction of soil solution with suction cups
2.2.3
Installation of plastic cups
The basic filter element of plastic suction cups is a membrane filter made of polyamide which is
fixed to a supporting base. The membrane is sensitive to mechanical damage and therefore has
to be protected to avoid damage during the installation procedure. Thus, the membrane is covered by a protection shell with extremely great pores.
The pores in the protection shell of plastic suction cups have to be filled with slurry during or
before starting the installation procedure (at least if you are going to install the cups horizontally). In the following the different steps are described; please take these tools with you into the
field:
- 1 stirring tool to make a suspension of water and soil (cordless drill driver with a tool for stirring wall paint)
- 1 bucket
- 1 can with water
- 1 sampling bottle
- 1 vacuum pump
Please go on as follows:
1. Make a fluid slurry out of water and your soil material.
2. Connect the suction cup with the sampling bottle and the vacuum pump.
3. Dig the cup into the slurry.
4. The suspension is sucked into the protection shell and thus the pores in the protection shell
are filled with soil material.
5. Go on with this procedure until all pores of the protection shell are filled with soil. This will
last about several minutes at least. The performance can be controled visually by wiping of
the soil material on the protection shell.
During this filling procedure the slurry suspension is filtered, the pores of the protection shell are
filled with solid soil material and thus a connection between the soil matric and the membrane is
build up. If the pores are filled sufficiently, please go on with installation like described in
ch. 2.2.2.
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Extraction of soil solution with suction cups
Suggestion for soils with high amounts of coarse sand or clay: In soils with a high amount of
coarse sand filling of the shell pores might be difficult and incomplete (pore size of the protection shell is about 0,5 mm, texture of the sand fraction up to 2 mm); in soils with a high amount
of clay the hydraulic conductivity may be reduced significantly by blocking the membrane holes
through clay particles. In both cases the addition of quartz flour to the original soil/water suspension may improve the hydraulic connection of the suction cup to the soil matric.
After installation
We recommend to take the suction cups into operation immediately after installation. This will
take most of the added water out of the system and thus will help to get back to normal soil
conditions again. The installation procedure has brought amounts of water and oxygene into the
soil, and the structure has been damaged around the cup. As a consequence, many chemical
and biological processes take place which cannot be avoided but which affect the concentrations of many solutes considerable (“first flush effect”). So one has to wait after installation until
the first data can be taken as representative. This period of time depends on the soil, the environmental conditions, the monitored solute etc., and cannot be determined in general. In any
case, sampling of soil water should be started immediately after installation. These samples can
either be casted away, or (even better) the solute concentrations should be measured to take
the data of these first samples into account to get an impression when the concentration values
go back to a normal level.
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Extraction of soil solution with suction cups
3
Connection and sampling
3.1 Application of single suction cups
The construction of the suction cups and the function of distinct parts has been explained in
ch. 1.1. In this chapter only the sampling procedure will be presented.
Please go on as follows: Take care that all connections are fixed properly and connect the end of the
sampling tube with the sampling bottle (Fig. 3).
Then connect the vacuum chamber with a manometer by pricking the needle through the septum.
If you use a vacuum pump with integrated manometer, another manometer is not necessary.
Then connect the vacuum pump with the sampling
bottle, remove the clamp on the soft silicone tube of
the evacuation connector and apply the pump until
the vacuum level is reached. Afterwards the clamp
is put back onto the silicone tube, the pump is removed and water will be sucked into the sampling
bottle.
Fig. 3:
Application of single suction cups
Vacuum
control
chamber
Sampling tube
Evacuation
connector
Vacuum control
and ventilation
tube
Sampling bottle
Suction cup
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Extraction of soil solution with suction cups
3.2 Application of suction cup systems
When using a system of several suction cups on the same site it is recommended to operate
these cups together in one tube system. The connection in principle is the same as described
above. The only difference is that the system is evacuated completely and not each sampling
bottle with its own evacuation connector (Fig. 4).
Capillaries to the suction cups
Vacuum tube
Vacuum control chamber
Vacuum tube with distributor
Field box
Sampling bottles
Fig. 4:
Vacuum system with 6 suction cups and 6 sampling bottles in a field box. Vacuum is applied through a common vacuum tube, which can be connected to a
mechanical or an electronical vacuum pump
The tube system is evacuated as a whole. A mechanical or electronical vacuum controller can
be employed to apply a controlled vacuum with individual settings.
We recommend the following three types of sampling bottles:
•
Brown glass bottles for the investigation of organic compounds (protection against photochemical reactions)
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Extraction of soil solution with suction cups
•
Special plastic (polypropylene) bottles with high wall thickness for the investigation of heavy
metals and other solutes which may adsorb to the walls of glass bottles
•
White glass bottles for all solutes which are neither expected to adsorb to bottle surfaces
nor are subjected to photochemical reactions (NO3, Cl etc.)
Our glass bottles are closed with a silicone plug which has got an evacuation connector. The
plug with connector can be taken off the bottle and put on again very easily. In comparison to
bottles with a screw top the sampling procedure (pouring the samples out of the bottles) is much
easier and therefore faster. The silicone plug has got a second borehole to insert the sampling
tube of the suction cup. When connecting the sampling tube to the plug, both parts should be
moistened to push the end of the tube at least 1 cm through the plug hole into the bottle. This
prevents any contact of solutes with the silicone plug.
Attention:
Do not handle glass bottles under vacuum! Danger of implosion! Ventilate
all bottles before removing the water samples! Replace damaged bottles
immediately (scratches, cracks etc.)
sampling bottles made of plastic have got a stable screw cap with two screw connectors. One
of these connectors is equipped with a evacuation tube, the other one is to fix the sampling tube
of the suction cup and has to be tightened with a jaw spanner.
3.3 Removal of samples
3.3.1
Cups without a collection shaft
After the sampling period soil water has been collected into the flask. Moreover, the sampling
tube is more or less filled with soil water, too. Without any mechanism to empty the sampling
tube this water would remain in the system until the next sampling takes place. This would lead
to several problems:
•
The soil water in the sampling tube will grow older and alter its composition
•
Microorganism growth can take place and affect the whole extraction system
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Extraction of soil solution with suction cups
•
At the following sampling date the old soil water in the sampling tube will be extracted into
the sampling bottle and either has to be removed first or will be mixed with the new solution
and thus affect the results
To avoid these disadvantages the sampling system is completely ventilated after the sampling
period. For this purpose the septum plugs have to be removed from the vacuum control chambers of the suction cups. While the vacuum pump is working on, air gets into the ventilation tube
and all remaining soil solution will be extracted out of the sampling tube into the bottle. After this
procedure the septum plugs are put back, and the sampling system is ready for operation again.
3.3.2
Cups with a collection shaft
When using suction cups with collection shaft the sampled soil water is either located directly in
the shaft (plastic suction cups) or in a special container which is integrated into the shaft (glass
suction cups; see Fig. 2, P. 5). There are two possibilities to remove the soil water:
Removal with vacuum: Normally the removal of the water sample is easier to do negative pressure because a vacuum pump is existing anyway. Thus, the sampling tube is connected to a
sampling bottle (see Fig. 2, P. 5) and the clamps on the sampling and the vacuum tube is removed, so air gets into the shaft until atmospheric pressure is reached. Then the vacuum pump
is connected to the vacuum connector of the sampling bottle and operated until the water sample is completely sucked into the bottle.
Removal with pressure: If the soil water cannot be removed with vacuum (great depths), the use
of (over)pressure is recommended. The pump is connected to the vacuum tube of the suction
cup, the sampling tube to a sampling bottle, and all clamps have to be taken off. By operating
the pump the water sample is pressed into the bottle. This procedure has got the advantage
that degasing of the water sample is avoided, and its pH value is less affected than with the
application of vacuum.
After removal of the water sample the clamps have to be put back onto the silicone tubes, and
the system is ready for operation again.
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Extraction of soil solution with suction cups
4
Possible sources of error and correction
If the suction cup is installed properly and the soil moisture is sufficient to keep the pores of the
suction cup filled with water (i.e. air tight), the applied vacuum value will go down very slowly
and the cup will work for several days or at least several hours, depending on the moisture conditions of the soil. Especially under dry conditions the hydraulic conductivity can be reduced
very much, and the flow velocity of soil water is very small. Thus, it may last days or even
weeks until an sample volume of e.g. 150 ml has been collected, and the vacuum value has to
decrease very slowly. Nevertheless you should note that the applied underpressure has to decrease when water is collected into the sampling system without any leaks. The more water is
sucked, the more the vacuum value will decrease. This loss of underpressure is proportional to
the infiltrating water volume and depends on the water content (or matric potential) and the
hydraulic conductivity. After a few days the vacuum value should be appreciable when there
hasn’t been a high amount of soil solution collected already. Otherwise you have to control your
system as described below (all fault descriptions are related to one suction cup with a sampling
bottle of 1 L volume):
a) Vacuum level is constant, but no soil water is collected
Possible source of error
Relief
The applied vacuum is lower than the soil matric Raise the vacuum level up to a value above the
potential. The capillary forces in soil are stronger as actual matric potential. For comparison purposes a
the forces generated by the vacuum in the sampling simple tensiometer can be used.
system.
The capillary between the cup and the sampling Clean the capillaries. Control it by checking the vacbottle or the hole in the flask stopper is blocked.
uum level by the vacuum control chamber.
b) Vacuum level is falling down immediately or cannot be established at all
Possible source of error
Relief
Capillaries are not connected properly and/or the Control all tube connectors (shaft, bottle, vacuum
closing device of the sampling bottle is not tight
chamber and pump)
Fasten the closing device
Check the clamps on the silicone tubes
When capillaries were extended, the junctions have
to be controlled
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Extraction of soil solution with suction cups
The vacuum gauge is not connected properly; wrong Fasten the syringe on the vacuum gauge
handling of the vacuum gauge may lead to pressure
Change the syringe
losses
Take care that the syringe is placed straight in the
septum on the vacuum chamber since shear forces
on the syringe may lead to small openings next to it
The valve in the vacuum pump has become leaky Clean the valve (compressed air) or replace it
and air is sucked back into the bottle
c) Vacuum decreases over several hours, but no soil water is collected
Possible source of error
Relief
Soil has run dry, suction cup is sucking air
When single suction cups are used, vacuum can be
established only when the soil has been wetted
again. In systems with suction cups in different soil
depths or levels (with different matric potentials) the
cutoff of the dry level is recommended. Thus, it is still
possible to get water out of the other levels.
Septum of the vacuum control chamber has become Replace the septum
leaky through frequent pricking
If questions remain, please contact us directly.
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Extraction of soil solution with suction cups
5
Literature (choice concerning suction cups and the adsorption of solutes)
Andersen, M.K., K. Raulund-Rasmussen, B.W. Strobel & H.C.B. Hansen (2002): Adsorption of cadmium, copper,
nickel, and zinc to a poly(tetraflourethene) porous soil solution sampler; J. Environ. Qual., 31, 168-175.
Creasey, C.L. & S.J. Dreiss (1988): Porous cup samplers: cleaning procedures and potential sample bias from
trace element contamination; Soil Science, 145 Nr. 2, 93-101.
Dehner, U., T. Bausinger & J. Preuß: Adsorption von Nitroaromaten an unterschiedlichen Saugkerzenmaterialien;
altlasten spektrum, 6/2003, 295-299.
DVWK (1990): Gewinnung von Bodenwasserproben mit Hilfe der Saugkerzen-Methode; Berlin., Bd. Merkblatt 217
Grossmann, J. (1988): Physikalische und chemische Prozesse bei der Probenahme von Sickerwasser mittels
Saugsonden; Diss. Institut für Wasserchemie und Chemische Balneologie der Technischen Universität
München, 147 S.
Guggenberger, G. und W. Zech (1992): Sorption of dissolved organic carbon by ceramic P 80 cups. Z.
Pflanzenernähr. Bodenkd. 155, 151-155.
Haberhauer, G. (1997): Adsorptionsverhalten von Saugkerzenmaterialien. 7. Gumpensteiner Lysimetertagung.
BAL Gumpenstein, 27-31.
Hansen, E.A. und A.R. Harris (1975): Validity of soil-water samples collected with porous ceramic cups. Soil Sci.
Soc. Am. J. 39, 528-536.
Hetsch, W., F. Beese & B. Ulrich (1979): Die Beeinflussung der Bodenlösung durch Saugkerzen aus NiSintermetall und Keramik; Z. Pflanzenernähr. Bodenk., 142, 29-38.
Koch, D. und M. Grupe (1993): Schwermetall-Sorptionsverhalten einer Saugkerze aus porösem Borosilikatglas. Z.
Pflanzenernähr. Bodenkd. 156, 95-96.
Spangenberg, A., G. Cecchini und N. Lamersdorf (1997): Analysing the performance of a micro soil solution sampling device in a laboratory examination and a field experiment. Plant and Soil 196, 59-70.
Tischner, T., G. Nutzmann & R. Pothig (1998): Determination of soil water phosphorus with a new nylon suction
cup; Bulletin of freshwater ecology and inland fisheries, 61 Nr. 3, 325-332.
Wenzel, W.W., R.S. Sletten, A. Brandstetter, G. Wieshammer und G. Stingeder (1997): Adsorption of trace metals
by tension lysimeters: nylon membrane vs. porous ceramic cups. J. Environ. Qual. 26, 1430-1434.
Wessel-Bothe, S., S. Pätzold, C. Klein, G. Behre und G. Welp (2000): Adsorption von Pflanzenschutzmitteln und
DOC an Saugkerzen aus Glas und Keramik; J. Plant Nutr. Soil Sci, 163.
Wessel-Bothe, S. (2002): Simultaner Transport von Ionen unterschiedlicher Matrixaffinität in Böden aus Löss unter
Freilandbedingungen – Messung und Simulation; Bonner Bodenkundl. Abh.; Bd. 38; 218 S.
16