ABSTRAK
PENGENALAN EMPAT TEKNIK PENGAMBILAN SAMPEL AIR POROS SEDIMEN YANG UMUM DIGUNAKAN. Air poros sedimen menyimpan informasi penting tentang status geokimia dan ekologi sedimen sehingga analisis air poros sedimen sering dilibatkan dalam studi ilmu lingkungan. Secara umum pengambilan air poros sedimen dapat dilakukan dengan dua metode yaitu ex-situ dan in-situ. Metode ex-situ dilakukan dengan mengambil sedimen dari lapangan kemudian air poros diekstraksi di laboratorium menggunakan teknik peras (squeeze) dan juga teknik sentrifugasi. Sedangkan metode in-situ dilakukan dengan mengambil air poros langsung dilokasi menggunakan teknik hisap (suction) dan juga teknik dialysis. Artikel ini akan mengulas tentang ke empat teknik pengambilan sampel air poros sedimen yang umum digunakan beserta kelebihan dan kekurangan masing-masing teknik. Dari keempat teknik tersebut, tidak terdapat teknik yang paling unggul dan paling dianjurkan penggunaannya dibandingkan dengan teknik yang lainnya. Pemilihan teknik yang tepat harus disesuaikan dengan tujuan dari pengambilan sampel itu sendiri.
1)Pusat Penelitian Oseanografi - LIPI INTRODUCTION
Pore water or interstitial water is defined as water that fills the spaces between mineral grains in sediments. Chemical composition contained in pore water can be used to assess the geochemical and ecological status of sediment (Ramírez-Pérez et al., 2015). Therefore, chemical analysis of pore water is commonly involved in many environmental science studies (Seeberg-Everfeldt et al., 2005). For instance, Gruca-Rokosz & Tomaszek (2015) and Rigaud
et al. (2013) estimated the sediment-water flux of gasses, nutrients and trace elements by analyzing pore water. Pore water was also used to quantify the rates of organic matter remineralization and soil methane production (Jahnke et al., 2005; Tong et al., 2015). Moreover, Doyle et al. (2003) used pore water to assess sediment contamination and its toxicity.
Oseana, Volume XLI, Nomor 2, Tahun 2016 : 21- 31 ISSN 0216-1877
Table 2. Typical Volume of Pore Water Obtained Through Various Types of Sampling Technique.
et al., 2012; Bertolin et al., 1995). The bigger the chamber size, the larger the volume of pore water extracted. However, increasing the chamber size can result in decreasing the spatial resolution.
Centrifuging and squeezing provide a moderate volume of pore water, depending on the size of the centrifuge tube and the squeezer compartment. Additionally, the volume also depends on the amount and moisture content of the sediment being used. For instance, 6 to 10 ml of pore water was obtained from 120 g of sediment using a 200 ml centrifuge tube (Ronday, 1997), while 49 ml pore water was obtained from 660 g of sediment using a 250 ml
centrifuge tube (Lopes & Ribeiro, 2005). With the same amount of sediment, Lopes and Ribeiro (2005) extracted the same volume of pore water within 18 minutes, using a squeezer that was 22.5 cm in length and 9 cm in diameter.
Larger volumes of pore water were extracted by Sasseville et al. (1974) using a squeezer with a 10 cm internal diameter. The squeezer could extract 75 ml of pore water from 200 ml of lake sediment within 15 minutes.
Interestingly, it could extract up to 800 ml of pore water. Table 2 shows the typical volume of pore water that can be extracted using various types of sampling technique.
Technique Substrate type
Obtained pore water
(ml)
Time required Source
Squeezing
Lake 75 15 minutes (mins) Sasseville et al. (1974) Lake 20 10–20mins Robbins & Gustinis (1976) Acid mine 49 18mins Lopes & Ribeiro (2005) Centrifuging
Marine 5–10 60mins De Lange et al. (1992) Grassland 6–10 50mins Ronday (1997) Acid mine 49 45mins Lopes & Ribeiro (2005) Dialysis
Bog 5 7 days Thomas & Arthur (2010)
Salt marsh 20 2 weeks Ugo et al. (1999) Lake 0.014 24–72 hours Xu et al. (2012) Mud flat 30 2 weeks Bertolin et al. (1995)
Suction filtration
Lake 5–10 30mins Shotbolt (2010)
Lake 0.01–0.05 < 0.5 Torres et al. (2013) Sand Unlimited Not informed Beck et al. (2007) Sand Unlimited Not informed Martin et al. (2003)
CONCLUSION
There are at least four common techniques used to extract pore water. Each pore water sampling technique has benefits and limitations. None of these techniques is widely accepted or is the most recommended for sampling pore water. Therefore, selecting a suitable technique for pore water sampling can
be problematic. Selecting a technique should be based on the purpose of pore water sampling.
For instance, a sampling technique that can collect a large volume of pore water should be chosen if a multi-parameter analysis is required.
However, a multi-parameter analysis does not always require a great sample volume. It depends on the methods and instruments used, and the number of measurement replication.
Dialysis technique has great potential to avoid temperature, pressure and oxidation artefacts (Jahnke, 1988) and provide a good resolution in the depth profile (Gao et al., 2012).
However, Gao et al. (2012) listed some disadvantages of the dialysis technique including the long equilibration times, the inability to observe temporal variations at high frequencies, the sediment disturbance for multiple deployments and the high cost, especially compared to other techniques. Some improvement has been conducted to deal with the limitation of dialyis technique. For instance, Jacobs (2002) developed a rechargeable dialysis pore water sampler that can be used for repeated sampling, enabling spatial and temporal resolution without destroying sediment-cap
stratification. In addition, Krom et al. (1994) and Torres et al. (2013) have also developed gel samplers that they claim are faster than a ‘peeper’
in achieving equilibration, taking hours rather than days.
Every sampling technique has its own advantages and disadvantages as shown in Table 1. Preference to a particular technique is usually based on the purpose of the sampling.
Sometimes additional handling is applied to a certain technique in order to deal with the disadvantages. For instance, a large nitrogen (N2) gas filled-glove box is used when sampling and manipulating samples (Shotbolt, 2010; Chen et al., 2015) through ex-situ method in order to minimize oxidation.
Table 1. Comparison of Common Pore Water Sampling Techniques (Bufflap & Allen, 1995a).
Technique Advantages Disadvantages
Squeezing Simple, inexpensive, immediate filtration
Potential for oxidation and temperature artefacts Centrifugation Simple, rapid Potential for oxidation and
temperature artefacts Suction filtration Limited potential for artefacts,
continuous monitoring Expensive, depth limitations Dialysis Limited potential for artefacts Equilibration time, placement
and retrieval
EXTRACTION CAPACITY OF VARIOUS PORE WATER SAMPLERS
Among the pore water sampling techniques mentioned above, suction filtration technique may collect the largest volume of pore water. Suction filtration samplers can extract pore water from as little as 10 ml (Torres et al., 2013) to an unlimited volume (Martin et al., 2003; Beck et al., 2007). The volume depends on the size of the sampler and the tool used for suction. For instance, Torres et al. (2013) used a 2 cm micro rhizon sampler tube with 1 mm internal diameter.
As a suction tool, a 1 ml syringe was used so
that 10 to 50 ml of pore water could be extracted from the sediment in less than 30 seconds each time. Conversely, Martin et al. (2003) used 3 m of a sampler tube with a 3.8 cm internal diameter.
A peristaltic pump with a suction speed of 1 ml per second was used as a suction tool instead of a syringe. As a result, unlimited pore water can be extracted from sediments as long as the water is available.
In comparison, the dialysis sampling technique may provide the lowest volume pore water samples. Typically, a dialysis sampler collects pore water in a range of 14 ml to 30 ml per chamber, depending on the chamber size (Xu There are two methods commonly used
when collecting pore water. The first one involves sediment coring from the field followed by core sample sectioning in the laboratory and subsequently isolating the pore water either by high-pressure squeezing or centrifugation technique. This method is well-known as an ex-situ method as it requires sediment removal from the environment. The other method—which is called in-situ—is conducted by extracting pore water from particular depths of sediment directly in the field. This method is usually performed using either suction filtration or dialysis technique.
EX-SITU PORE WATER SAMPLING As ex-situ method involves sediment removal from the environment and sample manipulation in the laboratory. Some literature refers this method as the destructive or the laboratory method (Fares et al., 2009). Pore water sampling using ex-situ method is cost efficient and easily handled so that it is the most widely-used method at present (Losso et al., 2009;
Torres et al., 2013). However, it gives a poor spatial resolution (Xu et al., 2012). In addition, chemical composition of the pore water sample obtained by this method is susceptible to change due to oxygen exposure, temperature variation and pressure change during transportation and extraction process (Delange et al. in Xu et al., 2012; Beck et al., 2007).
Figure 1. Nylon Squeezer Schematic (Reeburg, 1967). Key: 1. nylon gas inlet tube, 2. O-ring seal male plug, 3. Delrin cap, 4. dental dam rubber diaphragm, 5. nylon sample retainer with O-rings, 6. Filter, 7. nylon screens, 8. Delrin base, 9. nylon male plug, 10. nylon sample drain tube, 11. rubber or cork pad, and 12. modified c-clamp.
Figure 2. Modified Whole Core Squeezer Schematic (Jahnke, 1988). Key: A. unistrut, B. angle braces, C. threaded rod, D. bottom piston, E. nut, F. acrylic core barrel, G. sediment core, H.
overlying water, I. nylon screw with small O-ring, J. top piston, K. nut, L. threaded rod, M.
threaded end of the fitting, N. female luer fitting, O. filter, and P. plastic syringe.
A. Squeezing
The basic principle of squeezing technique is expelling pore water from a sediment core by introducing high pressure to the core (Beck et al., 2007). Therefore, pressure-related artefacts1 are the main weakness of high-pressure squeezing techniques (Toole et al., 1984). For instance, Fares et al. (2009) mentioned that high pressure can have a profound effect on mineral solubility. Additionally, Bufflap &
Allen (1995b) claimed that this technique is not very precise, as it may allow unfiltered particles or colloids to enter the collection vials due to poor seals around filters in the squeezer. Two types of squeezing technique are core section squeezing and whole core squeezing (Beck et al., 2007). In core section squeezing, a sediment
sample is sectioned and then compressed to obtain the pore water. In contrast, whole core squeezing is conducted by pressurizing a sediment core and expelling the resulting pore water through several ports along the core liner, which indicate a specific sampling depth (Beck et al., 2007).
Jahnke (1988) reported some benefits from using a modified whole core squeezing technique. He argued that the device is very simple and easy to operate. It does not need core sectioning; neither does it need to work in an inert atmosphere. Additionally, it can be performed quickly. An early simple squeezer used to extract pore water is the nylon squeezer designed by Reeburg (1967).
1 Undesired alteration to an observed object in a scientific investigation or experiment introduced during preparative or investigative procedure.
Frickers (1990) also reported that the suction sampler had some weaknesses, including low depth resolution (> 1 cm) and poor particle separation.
Therefore, they developed a multi-level sampler designed to extract pore water at five successive intervals, with 1 cm depth resolution.
Improvement to the suction sampler in term of sampling resolution was also conducted by Berg
& McGlathery (2001) by developing a suction sampler that can be applied to various sediments, ranging from coarse-grained carbonate to fine-grained sandy sediment with a depth resolution as fine as 1 cm. The sampler is relatively small so that it can minimise sediment disturbance, ensuring that the acquired pore water is truly representative.
B. Dialysis
The dialysis technique of pore water extraction is based on the diffusive equilibration of dissolved compounds between two aquatic environments, separated by a semi-permeable membrane (Teasdale et al., 1995; Jacobs, 2002).
The dialysis sampler was initially developed by Hesslein (1976) to observe methane and phosphate depth profiles in Hudson Estuary sediments. The sampler’s (which is also known as a ‘peeper’) main body is made from clear acrylic plastic with a vertical array of compartments filled with distilled water and covered by a dialysis membrane. As the sampler is deployed into the sediment, solutes in the pore water will diffuse into compartments through the membrane until equilibrium is reached. Subsequently, the sampler is retrieved and the water in the compartments is sampled for chemical analysis.
Figure 5. Common Peeper Designs (Teasdale et al., 1995). Key: (a) the bottle-insert peeper; (b) the original Hessleinpeeper; (c) the double-sided peeper; (d) plan view of peepers (b) and (c).
B. Centrifugation
Centrifugation is the process where pore water is separated from sediment through spinning by a centrifuge. Isolating pore water from a sediment core using centrifugation technique is a laborious procedure. The core must be sectioned properly according to the intended interval before extraction, and this should be conducted under in-situ temperatures and an inert atmosphere, to avoid altering pore water composition (Jahnke, 1988). Additionally, centrifugation requires filtration after the pore water has been completely extracted, to remove remaining suspended particles (Saager et al., 1990). The remaining particles may come from undisturbed precipitation during decantation of the extracted water, or they may still be suspended in pore water due to an insufficient centrifugation speed.Pore water acquired from centrifugation is susceptible to oxidation during the filtration process. Therefore, modification of the centrifugation technique is necessary to deal with filtration issues. The most common measure taken is to use centrifuge tubes that have a built-in filter, known as basal cups (Saager et al., 1990). As an alternative, inert solvents that are denser than water—such as
fluorocarbon (FC-78)—can be used to replace the filter function (Batley & Giles, 1979). The notion behind this technique is that less dense fluid will always overlay more dense fluid. As pore water is extracted from sediment during centrifugation, a solvent that is denser than pore water will force water to the top and take a position between the sediment precipitation and pore water. Thus, pore water can be separated easily from sediment by decantation. However, it must be noted that the selected solvent not only must be inert to water, but also must be inert to the target compounds to be quantified.
Centrifugation is not only laborious but may be expensive, as it involves the use of an anoxic chamber when placing sediment samples into bottles or tubes to prevent oxidation (Moncur et al., 2013). Basically, centrifugation is a rapid technique that can be undertaken completely in 30 minutes or less, depending on the sediment characteristics (Bufflap & Allen, 1995a). However, sometimes centrifugation requires a more extensive processing time, as it must be undertaken more than once to obtain a sufficient volume of pore water if the sediment sample is not moist enough (Koehler et al., 2000).
Figure 3. Basal Cup (Saager et al., 1990).
IN-SITU PORE WATER SAMPLING The in-situ pore water sampling, using either suction filtration or dialysis technique, is believed to have less potential for artefacts than the ex-situ method (Beck et al., 2007). This is the recommended method for pore water collection, as it can minimize temperature changes, diffusion, mixing, outgassing and redox changes (Torres et al., 2013), as well as storage effects (Ho & Lane, 1973). In addition to its potential for fewer artefacts, the in-situ method is very valuable and is used mostly if a high spatial resolution is required when extracting pore water from the top several centimetres (cm) of sediment (Beck et al., 2007).
A. Suction Filtration
Currently, various types of suction samplers are available, but they all operate through a similar procedure. In the experimental site, a porous tube connected to a pressure regulator is driven into the sediment. Pore water in the sediment will get into a sample chamber inside the tube through the pore. The water will
then be withdrawn if negative pressure is applied to the tube (Gao et al., 2012). The suction sampler was initially developed based on the principle of the porous cup lysimeter used in soil chemistry, and was believed to offer more benefits than its predecessors (Watson &
Fricker, 1990).Gao et al. (2012) mentioned that suction sampler can evaluate temporal variation of chemical composition, as it can be applied to extract pore water repeatedly from the same place. This is corroborated by Fares et al. (2009), who argue that porous suction is the best technique for long-term monitoring.
Additionally, current improvements and advanced technology have produced suction samplers with better and more reliable performance, such as the rhizon sampler (Seeberg-Elverfeldt et al., 2005; Shotbolt, 2010;
Torres et al., 2013).
Interestingly, Bufflap & Allen (1995b) do not recommend suction filtration, as it is imprecise, with poor accuracy and physical difficulties. Similar to the squeezing technique, the low precision from suction filtration results from poor seals around the filters. Watson &
Figure 4. Suction Filtration Sampler (Berg & McGlathery, 2001).
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· Pramudji dan L.H. Purnomo. 2003. Mangrove sebagai tanaman penghijauan pantai. Pusat Penelitian Oseanografi-LIPI, Jakarta: 30 hlm. (Contoh buku).
· Rositasari, R. 2006. Komunitas foraminifera di perairan Laut Arafura. Oseanologi dan Limnologi di Indonesia 40: 15-27. (Contoh jurnal/majalah).
· Ward, R. D., T. S. Zemlak, B. H. Innes, P. R. Last and P. D. Hebert. 2005. DNA barcoding Australia’s fish species. Philosophical Transactions of the Royal Society B:Biological Sciences 360(1462) : 1847-1857. (Contoh penulisan jurnal untuk author yang lebih dari satu).
· Aziz, A. dan H. Sugiarto. 2007. Status ekhinodermata di Teluk Gilimanuk, Taman Nasional Bali Barat. Dalam: Ruyitno, A. Syahailatua, M. Muchtar, Pramudji, Sulistijo dan T. Susana (eds.).
Status Sumberdaya Laut Teluk Gilimanuk, Taman Nasional Bali Barat. LIPI Press, Jakarta: 46-55.
(Contoh artikel dalam buku).
· Froese, R. and D. Pauly. 2015. FishBase [online]. http://www.fishbase.org. Diakses pada tanggal 13 Mei 2015. (Contoh artikel dari website).
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i. Komposisi referensi yang digunakan dalam terbitan Oseana: Referensi sepuluh tahun terakhir minimal 25 %, referensi online yang berasal dari artikel yang jelas sumbernya maksimal 10%, dan untuk