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Sediments and water qualityWim Salomons aa Delft Hydraulics Laboratory, Haren Branch , c/o Institute for Soil Fertility , P.O. Box 30003,RA Haren, 9750, Gr, The NetherlandsPublished online: 17 Dec 2008.

To cite this article: Wim Salomons (1985) Sediments and water quality, Environmental Technology Letters, 6:1-11, 315-326

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Environmental Technology Letters, Vol. 6, pp. 315-326© Science and Technology Letters, 1985

SEDIMENTS AND WATER QUALITY

Wim Salomons

Delft Hydraulics Laboratory, Haren Branch, c/o Institute for Soil Fertility,P.O. Box 30003, 9750 RA Haren (Gr), The Netherlands

(Received 5 June 1985; accepted 28 June 1985)

Abstract

Sediments play a crucial role in water quality. On the one hand theyremove pollutants from the water column whereas on the other handpollutants accumulated in the sediments may provide the surface water(long after the source of pollution has ceased) with contaminants. Anoverview of the immobilization and remobilization processes affectingheavy metals and organic micro-pollutants is given.

Introduction

A rough description of the sediment-water system is presented in figure1. In the aquatic environment four abiotic reservoirs of micro-pollutants are distinguished: the suspended matter, the sediments, thesurface waters and the pore waters. These four reservoirs stronglyinteract with each other. Between the suspended matter and thepollutants in solution, adsorption/desorption and co-precipitationprocesses (for metals) take place. The suspended matter and thedeposited sediments are interlinked through sedimentation and erosionprocesses. Processes taking place after deposition provide theinterstitial waters with sometimes high concentrations of pollutants,which are able to influence concentrations in the surface watersthrough processes as diffusion, consolidation and bioturbation.

The role of sediments in an aquatic ecosystem is threefold:

- adsorption of micro-pollutants on the suspended matter and subsequentsedimentation remove pollutants from the water column. This is abeneficial effect.

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AQUATICSTUDIES

SUSPENDEDMATTER

(

SEDIMENT

yBIC TA

' " '

SURFACEWATER

-J

INTERSTITIALWATER

' . - " ' • . . • :

MAN

dGROUNDWATER

/-€••

-----

VEGETATIO^

- : ' . • * • •

i

SOIL , ^

_ ^ '

•":• C TERRESTRIAL. ; ^ STUOIES

Figure 1. Schematic presentation of contaminant reservoirs and theirinteractions in aquatic and terrestrial ecosystems(i).

- After sedimentation part of the accumulated pollutants may be releasedto the water column. If infiltration takes place the pollutants aretransported to the groundwater- contaminations in sediments may have a detrimental effect on benthicorganisms.

It is possible to define a fourth role:- accumulated sediments (whether polluted or not) are a nuisance if theyinterfere with ship movements in harbors and waterways

The aim of this symposium "CONTAMINATED SEDIMENTS IN AQUATIC SYSTEMS"was to present a summary of several approaches to study the influence ofsediments on aquatic systems. In this overview the beneficial anddetrimental effects of sediments will be discussed.The other four papers in this volume discuss specific or more generalareas of research.The cycling of copper and zinc in lakes is discussed by Baccini showingthe strong interaction between major element and metal cycles. Smith etal. shows how modeling can help to predict the environmental impact ofdisposal of dredged material. The paper by Marquenie gives an overviewon the use of bio-assays to predict the impact of contaminants insediments on biota. The important results obtained with mesocosms onsediment-water interchange of pollutants are discussed by Santschi.

Suspended matter and pollutant interactions

The mobility and the toxicity of trace metals and organic micro-pollutants are strongly dependent upon their physical form (dissolvedversus particulate). Understanding the processes which can transferpollutants from the particulate to the dissolved form and vice versa aretherefore extremely important for predicting their impact on theecosystem.Numerous studies have been conducted on adsorption processes of heavymetals on model substances like clay minerals, iron hydroxides andmanganese hydroxides (1i2). The more important findings are:

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- the adsorption of metals depends strongly on the pH. Over anarrow range of pH-values (typically 1 to 1.5 units) the adsorptioncurves present a sharp increase. This part of the curve is called pH-adsorption edge (3). The pH variations in nature (diurnal seasonal andspatial) therefore influence the distribution of trace metals over thesolid and dissolved phase. An example of seasonal variations in pH andthe resulting effect on dissolved zinc concentrations is shown in figure2.

9-

pH

8-

X X

Zn20-

(JJ9/I)

10-

X »observed••»calculated

J A J O

" •* * '

Figure 2. Seasonal cycles in pH and dissolved zinc concentrations in theIJsselmeer. The calculated values were obtained from a simple black boxcalculation with adsorption as the only process (37).

The location of the adsorption edge depends on amount of solids insuspension(3). Since the range of pH-values for most (non acidic)surface waters is between 7 and 9 inspection of several curves publishedin the literature shows that from the heavy metals cadmium is easilyaffected by (small) changes in pH.

Changes in suspended matter concentrations, as occurs in estuaries, alsohave a significant influence on metal removal(^3). In this case,however, there is strong link between hydrodynamic conditions andadsorption processes. The increased removal of metals due to an increasein suspended matter concentrations (turbidity maximum) is counteractedby an increase in chlorinity which, for a number of metals, has a stronginfluence on adsorption. The location of the turbidity maximum withrespect to chlorinity is critical.

- complexing agents prevent or in some cases have been found toenhance the adsorption. In the estuarine environment the influence ofchlorinity on adsorption of trace metals which form strong metal-chloride complexes is important. Small changes in chlorinity cause largechanges. Figure 3 shows the influence of chlorinity on the speciation(including adsorbed species) of cadmium.

Much information has been obtained on the influence of NTA on metaladsorption processes. However few studies have been carried out underrealistic low conditions for the NTA concentrations and/or suspendedmatter concentrations (1). Experiments with suspended matter from theRhine showed that NTA concentrations of 200 ug/1 are able to influenceadsorption and release processes. This is within the range predicted forthe river Rhine if all phosphates in detergents are replaced by NTA(1).

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1001t

•9000 mg/Isuspended

matt or• 1300

CdCl*

CHmg/l)

Figure 2. The influence of chlorinity and pH on adsorption of cadmium onsuspended matter and the speciation of cadmium as a function ofchlorinity (Cd-ads is the adsorbed species).

Natural processes also cause changes in the amount of complexingsubstances and their nature in surface waters (44). Both changes in thenature of waste loads and extracellular products cause changes in the"complexation capacity" of surface waters. These changes also affect thetoxicity of lake waters(45).

For organic micro-pollutants the important adsorbing phase in thesuspended matter/sediments is the organic matter. The adsorptionproperties of organic micro-pollutants can be related to the oetanol-water partition coefficient and the organic matter content of thesuspended matter/sediments(46). Although this simple picture gives agood description of the adsorption processes for organic micro-pollutants there is increasing evidence that also the dissolved organicmatter (DOM) has to be taken into account to predict the adsorption-desorption processes for this class of pollutants(11,12).

The solid phases interacting with dissolved constituents in naturalwaters consist of a variety of components including clay minerals,carbonates, quartz, feldspars and organic solids.Often not the bulk minerals determine adsorption phenomena but ratherthe surface coatings. It has been stressed (6) that particles immersedin seawater rapidly lose their surface charge properties and acquireweakly electronegative properties consistent with the formation of amacromolecular organic film (7,8).Dissolved organic matter from a Swiss lake with molecular weight greaterthan 1000 formed strong complexes with alumina surfaces, but lowmolecular weight compounds were weakly adsorbed(4). Complexation ofcopper with the functional groups of adsorbed organic matter was

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stronger than complexation with uncovered alumina surface hydroxyls.Cadmium adsorption was not significantly affected by the presence oforganic matter at the surface due to weak complex formation with theorganic ligands (5).Although lots of data have been gathered on adsorption phenomena both inthe laboratory and in the field very few investigators have addressedthe the reversibility of the adsorption process. For salt marshsediments i t was shown (8) that the pH-dependence of the cadmiumadsorption was reversible; lead however was only partly released. Alsofor organic micro-pollutants there is evidence that the adsorptionprocess is not completely reversible (47) Knowledge on reversibility isessential if laboratory data on adsorption are used to predict the fateof pollutants in the environment.As an example some results of the modeling of dissolved copper in atropical river system affected by mine tailings discharge are shown infigure 4. The pre-mine dissolved copper concentrations are shown by asolid line. Since no information was available on the amount ofexchangable copper on the pa r t i c l e s in th is 1000 km long r ivercalculations had to be performed for different amounts of exchangablecopper in order to get an order of magnitude estimate on dissolvedcopper concentrations.The results of the calculation show that a reduction in dissolved copperlevels is achieved in the mining area (this is simply due to increasedsuspended matter levels, offering more sites for adsorption). However, aprediction of dissolved copper levels further downstream depend stronglyon the amount of exchangable copper present on the particles.

Cu,ug/l

4

3

2

—> Exchangeable Cu 25%

0 1 2 3 4 5 6 7 8 9 . 10 11 12 13riversection

Figure 4. Predicted dissolved copper concentrations in a river affectedby mine tailings for different amount of exchangable copper present onthe suspended matter.

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Probably . i t will be possible to uset for metals, the selectiveextraction schemes (1) to determine the exchangable fraction. Thisexchangable fraction is not necessarily identical to the amountsreleased by the several methods used to determine the exchangablemetal fraction. These methods al l are derived from soil science andprobably (and even in this case not always) give the amount ofexchangable major cations like calcium or magnesium. In all probabilitythere will be "a sliding scale" of exchangability. Numerous studies areavailable on the influence of external factors like pH and complexingagents on adsorption processes. Equally important is an understanding ofthe processes which determine these external factors. Recognition ofthe intimate links between major elements cycles (C, N» S) and theminor element cycles is essential for a better understanding andprediction of pollutant behavior. An example of the interactionsbetween the carbon cycle (seasonal changes in pH due to eutrophication)and dissolved zinc concentrations was presented in figure 2.Baccini in this volume addresses this important aspect for tracemetal behavior. However, also for organic micro-pollutants there shouldbe a strong interaction between the carbon cycle and their behavior.Production or destruction of organic matter in sediment-water systemsis equivalent to increasing or decreasing the amount of adsorptionsites for organic micro-pollutants. In addition dissolved organic matteris able to adsorb organic micro-pollutants (9» 10, 11» 12).

Transport of particle bound pollutants

Suspended matter is a complex mixture of various particles. Each ofthese particles has i t own adsorption characteristics and thereforeits own pollutant content. During transport fractional sedimentationis possible as is well known for clay minerals in deltaic areas.The physical separation during transport in salt marsh areas causesthe fine grained sediments which accumulate close to the coast to bemore enriched in trace metals compared with the fine grainedsediments. To correct for these differences in grain size variousprocedures have been proposed (13, 14) but no standarization has beenachieved. Also during transport fractional sedimentation will affectthe composition of the suspended matter. Important results have beenobtained for a number of estuaries and for the North Sea(15). Theresults show that at low sediment concentrations, the more finelygrained fraction, "the continuously suspended fractional5) contains thelargest amounts of trace metals. This shows that the suspended matter iscomposed of two fractions which each their different composition andsedimentation behavior. According to Duinker the more coarse grainedfraction, with lower metal concentrations originates from resuspendedbottom sediments. The relative contribution of the permanently suspendedfraction to the total suspended matter is low at high suspended matterconcentrations but is high at low suspended matter concentrations. As aresult metal concentrations per unit weight for suspended matter fromthe open North Sea are high. More detailed results have been obtainedwith fractional filtration by Eisenreich and co-workers(i6).

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There is still no good correlation between erosion/transport/-sedimentation behavior and the composition of the particles. Duringtransport the participates (in the marine environment) are flocculatedpresumably involving organic matter. After deposition the original floc-structure will be destroyed. This destruction is caused by the breakdownof organic matter and sulfate reduction which results in a destructionof iron/manganese hydroxide coatings (formation of iron/manganesesulfides and/or carbonates).

After erosion new flocculation processes will take place resulting in awide spectrum of particles with different settling velocities anddifferent pollutant concentrations (not necessarily with the sameproperties as before deposition. In this way part of the pollutants willbe deposited once weather conditions become quiet again whereas thenewly formed finely grained particles (and probably with high pollutantloadings) may not settle at all and be transported far away from theerosion or dumping site. In addition those heavy metals present assulfides sediments are unstable in the surface waters. This results in arelease followed by a partial readsorption (39).Hardly any attention has been given to pollutant loading of particlesand their settling behavior before and after erosion. Therefore, it isnot possible to predict the fate of pollutants present in dredgedmaterial once it is dumped in a highly energetic environment like theNorth Sea.

Processes after sedimentation in the deposits:early diagenesls

After deposition conditions are drastically different from those in thesurface waters. The oxidation of the organic matter takes place in thewell known redox sequence. This sequence is schematically shown infigure 5. It should be emphasized that the deposited sediments are notsimply a stacked layer of redox sequences, and that the contact areabetween the sediments and the surface waters is not simply equal to the•surface area of the sediments. Bioturbation, like schematically shown infigure 5b results in an effective increase in the sediment-waterinterface.

NO;

Mn

Fe

C02

Figure 5. Sequence of redox reactions in an undisturbed and in abioturbated sediment(17).

I t is possible to schematize the deposited sediments by a simple two-boxmodel(i8, 19) consisting of an oxic and an anoxic layer (figure 6).

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INTERFACEBOUNOARYLAYER

Figure 6. Processes at the sediment-water interfaced 9)

Trace metals and organic micro-pollutants are adsorbed to the variouscomponents making up the suspended matter. After deposition and underanoxic conditions a number of carriers for trace metals are destroyed.The hydrous iron and manganese oxides are transformed intosulfides/carbonates and a decrease of the organic matter content takesplace, unlike organic micro-pollutant, the trace metals are able to formdiscrete compounds. Important for anoxic sulf idic conditions are theformation of sulfides(20»21). From an impact point of view i t i simportant to know whether the concentrations in the pore waters aredetermined by a d s o r p t i o n / d e s o r p t i o n p r o c e s s e s o r . byprecipitation/dissolution processes. If the latter i s the case theconcentrations in the pore waters of pollutants are independent of theconcentrations in the solid phase. The difference between these twoprocesses is schematically shown in figure 7.

Adsorption - Desorption

Meporewater

Precipitation- Dissolution

Meporewater

Me.sediment

Me,,sediment

Figure 7. Pore water concentrations for adsorpt ion-desorpt ion versusprecipitation/dissolution controlled processes.

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There is strong direct (22, 23» 24) and indirect evidence (20,21) thatthe concentrations of copper, cadmium and zinc in sulfidic pore watersare determined precipitation-dissolution processes. The concentrationsof arsenic and chromium are probably controlled by adsorption-desorptionprocesses. Their concentrations in the pore waters depend on theconcentrations in the solid phase. With selective extraction techniques,if suff icient precautions are taken, i t i s possible to obtaininformation of the speciation of trace metals in anoxic sediments(25).An example, and the result for freeze-dried and oxygenated sediment, isshown in figure 8.

100

80exchangeable

carbonate | | 2

easily reducible I 13 60

moderately reducible

organic /sulfidic

residual

40

20

3

J

2

3

\ |

\

\

3

Figure 8. Influence of drying on mode of occurrence of cadmium in ananoxic sediment sample. A. Extraction performed in a glove box with Aratmosphere. B. After aeration of the sample for 1 hour. C. After feeze-drying D. after oven-drying 60° C for 1 hour (25).

These results (Figure 8) indicate the transformation with respect tostability from solidly bound (sulfidic) to easily available forms duringaeration, i.e. resuspension, dredging, land disposal etc.

The active layer with regard to the quality of surface waters is thesediment-water interface. Without sulfide present the metalconcentrations will be controlled by adsorption-desorption reactions.The high DOM concentrations will solubilize trace metals due tocomplexation and possibly also cause an enhanced solubility of organicmicro-pollutants. Bioturbation enhances the diffusion of oxygen and inareas with high bioturbation rates i t is expected that the active layeris more important for water quality control than in less bioturbatedareas (26).

Sharp redox gradients are observed at the oxic/anoxic interface (figure6). The transport of redox sensitive species can be governed by these

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redox gradients. The classical examples are manganese and iron for whicha simple conceptual model can describe well the qualitative aspects oftransport phenomenology (27)For other trace metals (e.g. Cu, Zn, Cd, Pb) analogous models are s t i l llacking. However, there are several experimental evidences (28, 29, 30,31) that the newly formed iron (III) and manganese (IV) oxides offer newadsorbing surfaces to transfer trace metals from the dissolved to theparticulate phase. Upward diffusing trace metals encounter this freshlyprecipitated material and are able to adsorb. By this , the reflux ofthese metals is reduced (see iron oxide barrier in fig. 6.

The iron barrier moves upward with continued sedimentation(32). Thechemical nature of these newly formed inorganic polymers is not yetfully understood (33) The presence of dissolved organic matterinfluences strongly the size and the stability of the polymers. Moreoverthe speciation of the dissolved metals in the pore waters and hencetheir adsorbing properties will be influenced. It may be expected thatalso the organic micro-pollutant concentrations -in the pore waters ofthe oxidised layer are increased due to the presence of highconcentrations of dissolved organic matter. Field evidence for a release(other than methylated species) has been presented for copper from avariety of environments(34, 35t 36) and for arsenic in lakes (37).

Sediaents and organisas

The role of organisms in sediments is twofold:

1. a passive role: the accumulation of micro-pollutants by organisms(this part is dealt with by Marquenie in this symposium.

2. an active role: organisms are in fact an, often overlooked, importantgeochemical factor.

The active role is chemical and physical in nature. Examples of theactive chemical role is the methylation of mercury, arsenic and tin bybacteria. Sulfate reduction and the release of extra-cellular productsby algae as well changes in the pH of surface waters due. to algal bloomsare examples of an indirect chemical role of organisms. In these casesthe activity of the organisms indirectly influences metal behavior:- sulfate reduction results in the formation of hydrogen sulfide whichconverts certain trace metals to rather insoluble metal sulfides.- the extra-cellular products increase the complexing capacity ofsurface waters and may increase the concentration of metals in thefilterable ("dissolved11) fraction. In general an increase in thedissolved organic matter levels which is the result of bacterialbreakdown of organic matter will cause an increase in "dissolved"pollutant (metals and organic micro-pollutants) levels.

Bioturbation results in an increase in the penetration depth of oxygenand an increase in the oxidised surface layer. This results in anincrease in the layer in which metal concentrations are increased. Ingeneral sediment-water exchange processes are enhanced by biota.Karickhoff et al. (41) studied the influence of bioturbation on therelease of organic micro-pollutants. The release of micro-pollutant was

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increased 4-6 fold in bioturbated sediments compared with undisturbedsediments. In addition the digested sediments (pellets) made by theorganisms (tubificids) were able to bind the organic micro-pollutantsmore strongly than the sediment itself.Also for trace metals an increase in the release from the sediment hasbeen observed (42).These experiments show that organisms not only play a passive (bio-assays-accumulation) role in the sediments-water system but are also animportantt often overlooked, geochemical factor. More details on how tostudy the influence of sediments on water quality and the influence oforganisms on geochemical cycling is given by Santschi in this volume.

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