A new method to study biogeochemical processes in sediments by a percolation technique

Estuarine, Coastal and Shelf Science (1991) 32,173-186

A New Method to Study Processes in Sediments Technique

Martin KerneF, Hans-Diethard

Hartmut Knauthb

Biogeochemical by a Percolation

Kauscha

“University of Hamburg, Institute for Hydrobiology 9, D-Hamburg 50 and bGKSS-Forschungszentrum, D-Geesthacht 2054, FRG

and and Fishery Science, Zeiseweg Max-Planck-Strasse,

Received 24 January 1989 and in revised form 13 August 1990

Keywords: conditions;

laboratory methods; sediment structure; porewaters; anaerobic humic substances; tidal flats; Elbe estuary; Germany coast

A device for analysis under simulated natural conditions is described. It continuously exchanges the porewater in undisturbed sediment zones under controlled laboratory conditions. Using this percolation technique, defined aerobic and anaerobic conditions in the sediment and the mean velocity of the porewater can be maintained or varied over extended periods of time. The concentrations of chloride and humic substances in the porewater of an intertidal mudflat sediment were determined continuously in the outflow of the analog model and compared with the results of in situ porewater profile analyses for the same chemicals in the same type of sediment. The pore volume affected by the percolation technique was measured with an amended fluorescent tracer together with chioride as a conservative tracer in the sediment. Vertical gradients of the chemicals in the porewater of an intertidal mudflat sediment were determined using both the percolation and the in situ porewater analysis techniques. Both chloride and humic substances decreased with increasing depth of the sediment. The results obtained using this device indicate differences in the distribution within a sediment layer due to such processes as the accumulation of humic substances in the micropores. In the surface layer, up to 11 times the concentrations of humic substances had accumulated compared to in situ porewater measurements. The application of the percolation technique for the investigation of different biogeochemical processes in sediments is discussed.

Introduction Sediment structure can be defined as the geometry of the solid and pore components sediment. It is generally accepted that the structure is important for understanding physical behaviour of sediments (Ringrose-Voase, 1987).

of the the

‘This article is based in part on a doctoral study by Martin Kemer in the Faculty of Biology, University of Hamburg. 0272-7714/91/020173+

14 $03.00/O

@ 1991 Academic Press Limited

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Although the processes and controls in the cycling of nutrients and heavy metals in sediments are studied using highly sophisticated chemical analysis techniques, the sediment structure is considered mainly as a factor in the sampling of undisturbed sediment cores. However, in an intertidal marsh sediment, it was shown that hydrological factors in the overlying water affect differences in the porewater chemistry (Gardner, 1973). Harvey et al. (1987) studied subsurface hydrology and found the discharge from a creek bank effectively removes porewater from the sediment. Reed (1987) suggested sampling sediments influenced by tidal action at intervals of no more than 30 min to monitor the discharge accurately. Furthermore, the effect of marshes on nutrient cycling has been shown to depend on their elevation (Jordan et al., 1983). Kerner et al. (1986) showed that the periodic submersion and exposure to air during each tidal cycle promoted alternations in the biological processes of nitrification and denitrification at the surface of freshwater tidal flat sediments in the Elbe estuary. Due to the discharge from these sediments, the porewater movement induced oxidation in the deeper layers. It was calculated that only one fifth of the oxygen entering the sediment system is consumed by nitrification processes that lead to the nitrate concentration measurable in the porewater of an intertidal mudflat sediment (Kerner et al., 1990). In subtidal or fully submerged sediments, the interchange of nutrients and heavy metals between the sediment and water is highly dependent on the microstructure of the sediment surface (Aller, 1982; Jorgensen & Revsbech, 1985; Riber & Wetzel, 1987). Furthermore, there is much evidence that physical disturbances to sediments with stratified redox gradients can cause large errors in the estimates of microbial metabolic activity (Findlay et al., 1985). Kaspar (1982) found high denitrification rates on a microscale at a few discrete sites within a matrix of undetectable or low activity. Denitrification capacity in estuarine sediments was shown to depend mostly on the autochthonous production of nitrate in the same sediment layer (Jenkins & Kemp, 1984). While determining sulfate reduction rates in sediments using the core injection technique, Jorgensen (1978) found that compared to this method the rates in sediment suspensions are underestimated by a factor of up to 30 times. Assuming that Fe reduction occurs if nitrate is depleted (Sorensen, 1982), the reduction of ferricoxyhydroxide must be a step in the major pathway for the decomposition of organic matter in sediment even if nitrate is still detectable (Lovley & Phillips, 1986; Jahnke, 1985). Thus, according to the findings of Tessier et al. (1985), the microstructure can also affect adsorption and mobilization of trace metals. The literature indicates that neglect or underestimation of the importance of the sediment structure in biogeochemical processes has resulted from the absence of appropriate methods of investigation. The purpose of this paper is to describe a simple and rapid in situ method for studying processes in the sediment under laboratory conditions. A percolation technique is described by which changes in the concentrations of chemicals in the porewater are detected after it has passed through 2 cm of an intertidal mudflat sediment core. The pore volume affected by the percolation is determined for each layer. The parameters selected tend to be specific for establishing the reliability of the percolation technique and evaluating its effectiveness for investigation of biogeochemical processes in sediments.

Biogeochemical processes in sediments

Figure 1. Sediment corer with stainless steel coring clyinder.

Materials

an inner

175

segmented

plexiglass

tube (a) and (b) an outer

and methods

Study site

Sediment sampleswere collected from a freshwater intertidal mud flat in the Elbe estuary in May 1988. The sedimentsat the surface of the sampling site have a grain size frequency distribution of 81% silt/clay (~63 pm), 14% fine sand (< 125 l.tm) and 5% sand (> 125 urn). Sediments at this site are generally 70% water (v/v), have a bulk density of about 1.33 g cm -3 and are 30% organic matter. During a tidal cycle, the mudflat sediment is covered with water for about 3 h and exposed to the air for about 9 h. Due to the discharge and recharge, the ground water table changesabout 12 cm during one tide and the percolation rate is 2.56 x 10e4ml s-l cmV2. Sampling

procedure

Undisturbed sediment cores, 14 cm long, were taken in a plexiglass tube (6.4 cm ID, 100 cm long) while the tidal flat sediment was exposed at low tide. The upper 20 cm of the plexiglass coring tube had been previously cut into 2-cm sections and rejoined with adhesive tape [Figure l(a)]. For the sampling, the plexiglass tube was tirrnly set in a stainlesssteel cylinder [Figure l(b)], and the cylinder with the tube was placed on the sediment surface at the sampling site. As the sharp edgeof the cylinder wasforced into the sediment, the core ascendedinto the tube. When the top of the core had reached the desired level in the tube, the upper end

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was closed with a rubber stopper to prevent the core from sliding out, and the tube was withdrawn from the cylinder. By this procedure, it is possibleto obtain undisturbed cores to a depth of about 60 cm, even from sedimentscontaining roots. The total compressionof the cores 15 cm long is only 2-5%. The sediment cores in the upper part of the plexiglass tubes were then removed, and the endsof the coreswere sealedwith parafilm. The samples were immediately placed in anaerobic boxes (Beckton Dickson). Within 1 h of sampling, the cores were transported under oxygen-free conditions at 4 “C to the laboratory for further treatment. The anaerobic boxes were placed in a glove box, opened in a pure nitrogen atmosphere (0, < O.OS%), and cut with a fine sawblade (D = 0.2 mm) into 2-cm layers at the precut spots in the plexiglass sections. The sediment layers still in the plexiglass rings were then placed in the percolation units. For the in situ porewater analysis, 2-cm sediment layers were homogenized and weighed in tubes for the centrifugation in an inert fluid (FC72, 3M-Deutschland) under a pure nitrogen atmosphere (Bately & Giles, 1980). The porewater was then removed in a syringe, filtered through a cellulose acetate filter (Minisart 0.45 urn), and kept under oxygen-free conditions for further analysis. Two porewater samples were obtained from the samesediment layer, one shortly after homogenization and the other after an additional 10 h of equilibration in a nitrogen atmosphere. The experiments described in this paper were conducted using sediment from the upper 2 cm, 2-4 cm, and 4-6 cm below the surface at room temperature (22 “C). The percolation

apparatus

The percolation unit consistsof top and bottom disks(Figure 2) madeof plexiglass. On the bottom disk there is a Teflon sinter plate with a 250~urn pore size on which the plexiglass ring with the sediment samplerests, its upper side down against the bottom disk. Before the unit was closed with the top disk, a depth filter of glassfibre (Sartorius, PS = 200 urn) was placed between the sediment and the water outlet. The system was sealedwith viton rings to prevent the loss of gasesor liquids. The water flowed into and out of the percolation unit containing the sediment layer through holes in the bottom and top disk, respectively. Plugs sealedthe entrances for the viton feed pipes (ID = O-8mm). A stockvessel (20 1) was used to regulate the velocity of flow and pressure of the artificial porewater running into the percolation unit. Pressure was maintained by pumping nitrogen into the vessel. The pressure in the vesselwas controlled proportional to the height of a water column, that the gasexiting the vesselhad to overcome (Figure 2). During the experiments described in this paper, the depth of this water column was increasedfrom 20-40 cm in lo-cm increments and finally to 60 cm. The trials began about 5 h after sampling. About 5 h was allowed for equilibration before the experiments were continued at a pressureof 30 cm of H,O. Each layer was tested without interruption. The percolation experiments were continued until the concentrations of humic substances, affected by the dynamics of the system, became stable with a steady reading ( f 1%) for about 30 min. The porewater that had percolated through the unit was drawn off with a peristaltic pump (Gilson) in such a way as to prevent changes of pressure within the percolation unit at the outlet of the top disk. The porewater flowing out was analysed. The flow rate wasmeasuredby a fraction samplerand reported asthe amount of water that had passedthrough the percolation unit during l- or 5-min intervals. The percolation apparatus was free of leaks. The possibility of oxygen diffusion was alsoinvestigated and was eliminated asa possible source of error.

Biogeochemicalprocesses

in sediments

x :: L, (II I F

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M. Kerner et al.

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Analytical procedures The results shown in this paper were obtained by analysis of chloride-free artificial porewater, that was buffered at pH 7 with Titrisol (Merck). A 600 ug 1-l sodium salt of fluorescein (Uranin, Merck C.I.45350) was added as a fluorescence tracer, and pure nitrogen gas (99*999o/o) was bubbled in to maintain anaerobic conditions in the sediment layer during the percolation processes. Humic substances were determined continuously in the outflow using a fluorescence spectrophotometer (Hitachi, FlOOO) at the wavelength of excitation, (Ex) 350 nm, and the wavelength of emission, (EM) 42Onm, while the water was still under anaerobic conditions to prevent the oxidation of Fe*+ and its precipitation with the humic substances (Orem et al., 1987). The instrument was calibrated with 200 mg 1-l of salicylic acid. Therefore, the concentrations of the humic substances given in this paper are equivalent to the concentrations of salicylic acid. The fluorescence of Uranin does not falsify the measurements of humic substances. At the same time, the concentration of the fluorescent tracer in the outflow was determined with a fluorescence spectrophotometer at EX 350 nm and EM 520 nm. A calibration line was plotted shortly before the percolation experiments from a series of solutions prepared by diluting the artificial porewater solutions to known concentrations of Uranin. The humic substances do not have a distinctly defined fluorescence spectrum and for the type of sediment used, the emission at 520 nm was 2 1o/oof that at 420 nm. Therefore, the measurements of Uranin at EM 520 nm had to be corrected by the fluorescence of the humic substances determined at EM 420 nm. The adsorption of the fluorescence tracer was found to be undetectable at the concentrations used in the experiments. The porewater for the spectrometric measurements was collected at l- or 5-min intervals and analysed for chloride with an ion-sensitive electrode (Orion 96-17-00). Processing the data The mass eluted (E) during the percolation of humic substances, calculated using the equation. E = c [(lC(i)

- fl

x

V(i)

+ IC(i + 1) - fl

chloride, and Uranin was

x V(i + 1))/2] x (t (i + 1) - t(i)) (1)

i=l

where C(z) is the concentration of the chemicals in the outflow (ug ml- ‘) for the increment of measurement i = 1,2. . .n at the time t(z) in minutes for the volumetric flow rate V(z) in ml min- ‘; H(ug ml-‘) is the concentration of the chemical at the inflow. The total emission of the chemicals (E-tot) was calculated using equation (1) having assumed the boundary condition that the deviation of the concentration in the outflow from its minimum concentration in the experiment (i= n) was less than 1%. The volume of the pores in the sediment, Vol(ml), that were affected during the percolation was computed with equation (2) from the total emission of each chemical. Vol = E/C(O)

(2)

where C(0) for Uranin is its concentration at the inflow, and for chloride, its concentration measured in the in situ porewater. The concentrations of the humic substances in the porewater that was exchanged during the percolation were calculated for each level of pressure as the total amount of emission for the humic substances (E-tot) divided by the pore volume (Vol-tot) calculated for the emission of chloride and for Uranin.

Biogeochemicalprocesses

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in sediments

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A

0 t

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0 0.00

0.00

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24.00 32.00 Tome (mln)

40.00

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16.00 24.00 Time (mln)

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40.00

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*3 8.00 16.00 Time hn)

0.0

E a, z ; 2 LL

Figure 3. Emission of chemicals during percolation through the 4-6 cm layer of an intertidal mudflat sediment under different pressures: (a) 20 cm H,O, (b) 30 cm H,O, (c) 40 cm H,O, (d) 60 cm H,O. 0, Flow rate; -, humic substances; A, tracer; x , chloride.

Results Percolation

characteristics

The establishment of a stability in the emission of the chemicals during the continuous percolation at a defined pressure was demonstrated in different layers of a mudflat sediment core. The data from the chemical analysesshown in Figures 3,4 and 5 are normalized to the highest value for the chemical measured in one sediment layer in the in situ porewater and during the percolation experiments. Generally, the humic substancesand chloride concentrations are high at the beginning of the experiments [Figures 3(a), 4(a), 5(a)]. There is a nearly linear decreasein the concentrations during the first few minutes followed by a reduced emission. The concentration curve asymtotically reachesa minimum (lag-phase). However, if the time intervals between the measurementsare too long, the linear emissionphaseis poorly documented [Figure 4(a)]. The concentration curves for Uranin at the beginning of the experiments are similar but opposite to those for the humic substancesand chloride. In the experiments employing an increaseof pressureafter equilibration (30 cm H,O), the plots of the humic substancesand chloride have the samecharacteristic emission phasesdescribed for the lower pressure. The curve for the Uranin tracer indicates clearly that the chemical was diluted by

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et al.

0.0 0.00 Time

(min)

’

’ ’ ’ 8.00 16.00 Time (min)

T .c E .o 3 Q, z .O 2 Time

(min)

s

0.2 0.0 0.00

3.0 0.0 8.00 Time

16.00

t aJ -6 ; z

(min)

Figure 4. Emission of chemicals during percolation through the 2-4 cm layer of an intertidal mudflat sediment under different pressures: (a) 20 cm H,O, (b) 30 cm H,O, (c) 40 cm H,O, (d) 60 cm H,O. 0, Flow rate; -, humic substances; A, tracer; x , chloride.

porewater. However, the measurementswere not done in time intervals short enough to monitor this accurately. Therefore, the Uranin tracer analyseswere not included in the calculation of the pore volumes affected in the percolation experiments with increased pressure. The initial concentrations of the humic substancesare equal to or exceed those found in the experiments with lower pressure. With a further increasein the pressure, changesin the emission remain measurable. At the beginning of these experiments, the concentrations of humic substancesand chloride always increase compared to the stable minimum concentrations measuredat the end of the experiment with a lower pressure[Figures

3@),(c)>(d),4(b)>(c)>(4, 5(b)>(c),(41. The flow rates tend to increase shortly after the beginning of the experiments, and within minutes reach values that show no significant changesfor the further duration of the percolation experiments at a given pressure. In every sediment layer a linear correlation between the pressure and the flow rate was found, with correlation factors above 0.98 (Table 2).

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181

,.p ) A- A-A

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24.00 Time

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(min)

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(min)

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(min)

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Figure 5. Emission of chemicals during percolation through the O-2 cm layer of an intertidal mudflat sediment under different pressures: (a) 20cm H,O, (b) 30cm H,O, (c) 40 cm H,O, (d) 60 cm H,O. 0, Flow rate; -, humic substances; A, tracer; x , chloride.

1 .Chloride and humic substance concentrations in pore water an intertidal mudflat sediment before and after* equilibration

TABLE

Depth

(cm)

Chloride

o-2

175 175* 115 115* 100 105*

2-l 4-6

Porewater

(mg I ‘)

Humic

substances

at various

depths in

(mg 1~ ‘)

62.1 93.6* 52.3 71.5* 28.4 46,5*

data

Table 1 shows the in situ porewater data for chloride and humic substances.The arrows in the Figures 3(a), 4(a) and 5(a) also show the in situ porewater concentrations for the humic substancesbefore ( A ) and after (7) the equilibration of the sediment in relation to the porewater data from the percolation experiments. There is no difference in the

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TABLE 2. Data on the effect of percolation in specific layers of an intertidal mudflat sediment core under different pressures

Flow rate (ml min-‘) Layer (-1

Pore volume affected (ml) measured with the data of

maximal

minimal

Chloride

Uranin

2.90 3.36 4.20 4.80

2.26 3.04 3.90 4.10

4.4 2.1 0.8 0.5

5.3 -

2.58 3.56 4.5 6.36

2.24 3.24 4.2 6.27

6.6 18.3 5.8 1.2

7.0 -

2.20 1.05 2.02 2.92

1.50 0.96 1.82 2.72

9.8 5.8 1.6 3.6

12.0 -

O-2 cm layer Pressure (cm H,O) 20 30 40 60 2-4 cm layer Pressure (cm H,O) 20 30 40 60

4-6 cm layer Pressure (cm H,O) 20 30 40 60

concentration of chloride before and after the equilibration of the sediment. The concentrations of humic substancesbefore the equilibration of the sediment layer are 26.8 389% lessthan the concentrations measured thereafter. The concentrations of chloride as well as humic substancesin the in situ porewater decreasewith the depth of the sediment layers. The concentrations of humic substancesin the porewater were determined at the beginning of the percolation experiments at a pressureof 20 cm H,O. They were always found to be above the concentration of the in situ porewater in the corresponding sediment layer before an equilibrium was reached and below the concentration thereafter [Figures 3(a), 4(a), and 5(a)]. Because the concentrations of chloride were determined at specific time intervals during the percolation, the data for the outflow are not directly comparable with those for the porewater in situ. However, data for samples taken shortly after the initiation of the experiment are comparable with those for the porewater in situ [Figures 4(b), 5(a)]. Sensitivity of the percolation method The percolation experiments were conducted at pressuresthat produced flow rates (Table 2) through the sediment layers 2-13 times higher than those determined for the samekind of sediment under natural conditions of 256 x lo-* ml s-l cmm2(Kemer et al., 1990). As the pressuresused in the experiments were varied in a small range, a disturbance effect on the sediment layers due to the changes in pressure can be excluded. Furthermore, the pressuresused are in the samescaleof the height of the water column of about 80 cm the sediments are covered with at high tide (Kerner et al., 1986). Therefore, the results obtained in the percolations are alsotrue for in situ sediments.

Biogeochemical processes in sediments

183

The pore volumes exposed to the percolation processesare shown in Table 2. The results of the emission of chloride show that 9.8%, 14.7% and 21.7% of the total porewater in the different sediment layers are exchanged at the percolation during the beginning of the experiments at the pressure of 20 cm H,O. This porewater derives from the pore volume affected by the percolation. The pore volumes calculated from the concentrations of Uranin are 20-4, 6.1 and 22.4% higher than the values obtained from data on the emissionof chloride. Although it wasnot found in pre-experiments, sorption of Uranin cannot be completely ruled out and could be a reason for the differences in the calculation of the pore volume. However, it could be more likely explained by measurablediffusion processesas it can be seenin the outflow concentrations of chloride that never decreasedto zero. The massflux due to the diffusion processesdepends on concentration gradients. Therefore, an increasing amount of chemicals was transported out of the sediment due to diffusion as the emission curves reached minimum concentrations. Futhermore, the emission of the chemicals calculated at increased pressuresis not related to the pore volume but rather to the occurrence of percolation through the pores and to dispersion and diffusion processes. The exchanges of humic substances between porewater and sediment definitely influence the amount of emissionduring percolation. The error resulting from this cannot be quantified from the data shown in this paper. As the kinetics of desorption and adsorption generally depend on the concentration of a chemical in the liquid phase, the increase of the concentration in the percolation experiments when the pressureis increasedleadsto decreaseddesorption. Therefore, the amount of emission under increasing pressure is probably underestimated in our calculations. However, stable minimum concentrations are reached within minutes during all percolation experiments. This indicates that the effect of desorption on the emissionof humic substancesis small compared with that from the convection due to percolation processes. The concentrations of humic substances, calculated from the amount of porewater exchanged during percolation and the amount of humic substancesemitted with this porewater, are shown in Table 3. In the surface sediment layer during low pressure experiments, the concentrations of humic substancesare 2.3 times higher than in porewater obtained by the centrifugation of in situ sediment. During experiments using higher pressure a concentration up to 10.9 times higher than the one of porewater in situ has been found. In the deeper sediment layer, the difference between the porewater concentrations in situ and those in the percolation water is insignificant at the 95% level (Weir-test). However, at a depth of 4-6 cm, there is a significant increase in the concentration of humic substanceswith increasing pressure, and up to 11.9 times the concentrations in the porewater in situ were recorded. Discussion There are difficulties in determining biological or chemical processesin sediments by recording the concentration of chemicals in the porewater. The main problem is the destruction of the sediment structure in order to obtain porewater samples.To minimize the effect of the destruction, the sediments were studied in specific layers, even though earlier studies indicated that chemical concentrations may differ even within a single sediment layer (Jahnke, 1985; Jenkins & Kemp, 1984; Jorgensen 1978).

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TABLE 3. Data measured or calculated* for the pore water in percolation experiments at specific layers of an intertidal mudflat sediment under different pressures Humic substances concentrations (mg 1-Y

Chloride concentrations (mg 1-Y maximal

minimal

maximal

minimal

Chloride*

Uranin*

O-2 cm layer Pressure (cm H,O) 20 30 40 60

52.9 34.8 6.6 5.2

4.4 3.9 3.4 3.0

63.1 78.9 7.9 8.9

5.5 5.0 4.5 4.2

148.3 435.6 147.3 679.9

121.9 -

2-4 cm layer Pressure (cm H,O) 20 30 40 60

22.5 111.2 16.8 13.8

4.3 10.3 7.4 7.8

64.2 63.1 10.8 15.2

5.8 6.0 6.0 4.7

67.5 56.8 98.6 66.1

63.3 -

4-6 cm layer Pressure (cm H,O) 20 30 40 60

88.9 34.3 19.3 12.1

17.1 17.8 7.9 6.3

37.8 78.9 34.4 24.7

13.4 33.9 15.5 14.5

38.4 135.7 338.2 225.2

31.3 -

In the present woik, either lo,15 or 22% of the porewater in the O-2,2-4 and 4-6 cm sediment layers of an intertidal mudflat sediment were exchanged during the initial percolation under controlled laboratory conditions at a pressureof 20 cm of water. The pores in the sediment layers affected by the percolation at the given pressurewere flushed continuously with artificial, oxygen-free porewater. Stable conditions in the percolated pores were maintained. It is commonly accepted that the strength of the capillarity within the pores is proportional to their diameter (Scheffer & Schachtschabel, 1982). An increasein the pressure forcing the water out of the pores always produced an increasedemissionof chemicalsfrom the sediment layers. This suggeststhat a rise in pressuredrives water out of smallerpores, which have enough capillarity to resist flow at lower pressure. The percolation technique should therefore be employed to study the following characteristics of sediments: (a) the two-dimensional spatial distribution of dissolved chemicals (b) the coupling and capacity of biological reduction processes (c) the cycling of nutrients and heavy metals under defined redox conditions. In the work described here, the percolation technique was employed to study the distribution of the humic substancesin an intertidal mudflat sediment. In both, the in situ porewater determinations and the percolation experiments, a vertical decrease of the concentrations of humic substancesfrom the sediment surface to the deeper layers was found. However, at the sediment surface, in situ porewater analysesyielded only about half the values determined by the percolation technique. The percolation experiments indicate that the humic substancesare accumulated in sediment pores with smaller diameters. At the sediment surface, this accumulation was

Biogeochemicalprocesses

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found to be as much as 11 times that in the porewater in situ. Krom and Sholkowitz (1977) in their study of interstitial waters of Loch Duich found an increase in the concentrations of humic substances with depth. Furthermore, they showed that organic matter of high molecular weight accumulates in porewater even in oxic sediments as condensation products of organic substances of low molecular weight. Thus, in conjunction with our findings it can be concluded that humic substances may accumulate in areas within a sediment where more reduced redox conditions exist. References Aller,

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