Tracing Natural and AnthropogenicPb in Sediments along theMediterranean Coast of Israel UsingPb IsotopesY E H U D I T H A R L A V A N , * , †

A H U V A A L M O G I - L A B I N , † A N DB A R A K H E R U T ‡

Israel Geological Survey, Jerusalem 95501, Israel, and IsraelOceanographic and Limnological Researches, Haifa, Israel

Received December 23, 2009. Revised manuscript receivedJuly 13, 2010. Accepted July 14, 2010.

The natural and anthropogenic sources of Pb in surfacesediments offshore the Israeli Mediterranean coast were studiedusing the isotopic composition of Pb in diluted acid sedimentextracts. Surface sediments were collected at the lower reachesof coastal streams, along a south-north offshore transectand at selected monitoring stations of the Dan Region WastewaterPlant (DRWP) outfall pipe. The background values of the Pbisotopic composition were determined from the deepest part oftwo representative cores collected offshore and were foundto have a narrow range dominated mainly by clays derived fromboth inland soils and the Nilotic cell and to a lesser extentfrom the Saharan dust. The impact of the DRWP activated sludgecan be traced to a distance of ca. 2 km from the outfallpipe. Enrichment factors of Zn, Cu, and Pb were up to 25 andare strongly correlated with each other and with the Pbisotopic composition, thus demonstrating the sludge to betheir common source. The isotopic compositions of Pb in streamsediments have the widest range of values and indicate astrong anthropogenic contribution, probably from both post-1992 aerosols and point sources. However the impact of streamsediments on marine sediments could not be clearly detected.

IntroductionWhen considering metal pollution, it is important to locateand estimate the potential sources for pollution. Lead, whichis one of the most widely used metals in industry, is presentnaturally in rocks, soils, and in the hydrosphere. However,up until the industrial revolution, Pb was not present in theatmosphere except as part of natural airborne particles. Theintroduction of anthropogenic Pb to the atmosphere is dueto the extensive use of Pb in industry, predominantly as anadditive to gasoline in the form of tetra-ethyl lead (TEL;(CH3CH2)4Pb). As TEL burns, elemental Pb, PbS, PbSO4, andPbO are emitted to the atmosphere. Lead particles adhereto aerosols typically smaller than 10 µm which have long-range transport. The isotopic composition of Pb is utilizedin many cases to trace Pb in the environment. Lead has fourstable isotopes: 204Pb, 206Pb, 207Pb, and 208Pb, and whereasthe first is a primordial nuclide, the others are decay products

of 238U, 235U, and 232Th, respectively. Thus the isotopiccomposition of Pb in any given material (i.e., natural oranthropogenic) depends on the U/Pb and Th/Pb ratios andtime. Indeed, the isotopic composition of Pb in gasolinearound the world is shown to reflect the source of TEL usedin the refinery at that time. In many studies the distinct Pbisotopic composition of Pb in ores facilitate its use todistinguish the natural versus anthropogenic componentsand also to track its source, route, and fluxes (e.g. refs 1-9).

The growing urban population around the MediterraneanSea leads to increasing inputs of Pb to the marine environ-ment via sewage, solid waste, industrial, and air pollution.It is estimated that about half of the total external input ofPb to the Mediterranean Sea was deposited between 1950and 1992 (10). In contrast to several other ocean waters, sometrace metals in the Mediterranean Sea have high concentra-tions in surface water and are low and constant in their verticalprofile (e.g. refs 11 and 12). It was suggested that thisnonsteady-state is due to dissolved atmospheric and ter-restrial inputs to surface water. For example, it was calculatedthat from 1960 to 1985 concentrations of Zn, Pb, Co, and Cdin surface water increased annually by 6, 2, 2, and 2%,respectively (13). Nonetheless, since then, Pb concentrationshave decreased in the Western Basin due to Pb consumptionregulations regarding the western European countries.Indeed, a decrease in Pb concentrations is observed in thenorthwestern and western parts of the Mediterranean in bothsurface seawater (i.e., dissolved Pb) and in the atmosphere(i.e., aerosol and rainwater) from ca. 1986 to 1995 (14, 15).All the same, it was postulated that if Pb consumption is notlimited in the Eastern Mediterranean, Pb concentrations willincrease and reach critical concentration for marine organ-isms by the middle of the 21st century (10). Thus theMediterranean Sea, which had acted as a sink for Pb priorto 1980, has become a source of Pb for the Atlantic Ocean.

In Israel, as a result of growing development pressures,the sea is subjected to contamination via atmospheric fallout,direct discharge, or via streams. Since natural Pb concentra-tions in coastal seawater are low (0.05-0.25 ppb (16)) anyatmospheric addition should significantly influence thecoastal environment. The major source of atmospheric Pbin Israel is derived from gasoline combustion, which increasedfrom ca. 0.5 in 1970 to over 2 million tons in 1997 ((17) andreferences therein). The use of unleaded petrol in Israel beganin 1991, and by the end of 2005 it comprised ca. 100% of thepetrol used. Overall, the calculated mean annual atmosphericmass flux for the eastern Mediterranean basin is 36-72 g m2

y-1 (18). As a consequence, offshore sediments consist of upto 80% of eolian deposition which in turn is responsible formore than half of the Pb input to the Mediterranean (19). Itis estimated that along the Israeli coast, the dry atmosphericinput of Pb to marine sediments decreases from south tonorth from ca. 2.5 to 1.1 g m-2 y-1 (20), and it contributesabout twice the amount of Pb compared to that of the riverine(1600 vs 850 tones y-1, 9). The aerosol solubility in seawaterdepends on whether the aerosol is of urban- or crustal-richorigin. This is because ca. 20 to 80% of the Pb in urban-richaerosols (anthropogenic Pb) resides in the exchangeable sitescompared to 30% in crustal-rich (19, 21).

An additional source of anthropogenic metals to sedi-ments off the Israeli shore is the direct discharge of sewagesludge by the Dan Region Wastewater Project (DRWP), aplant which treats the sewage of ca. 1.5 million inhabitantsof the Tel Aviv metropolitan area since 1987 (22). The activatedsludge is discharged through a single outfall, 5 km offshore.The percent of particulate matter, bacteria and heavy metals

* Corresponding author phone: (972)2-3514295; fax: (972)2-314330;e-mail: y.harlavan@gsi.gov.il.

† Geological Survey of Israel.‡ Israel Oceanographic and Limnological Researches.

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in the sludge is ca. 1%. The impact of the DRWP sludge onthe sediments in its vicinity has been monitored since 1992(22, 23). The other potential sources for contaminants arethe coastal streams (24), which, except for a few, have nonatural flow to the sea except during winter storm/floodevents. Stream sediments are affected by point sources thatare situated along their course. Shallow sediments along thecoastline were found to be enriched in Hg, Cu, Zn, and Cdat point sources (1988 to 1992) (25, 26). Nevertheless, thelack of Pb enrichment close to these point sources suggeststhat most of the Pb in marine sediments originates from theatmosphere (26).

In most cases, Pb levels in marine sediments offshoreIsrael are below the threshold of a contaminate guideline(375-1000 ppm) (24-27). Thus in this study, we tested theconcept that Pb isotopes are a more sensitive tool than Pbconcentration in distinguishing anthropogenic from naturalsources in Mediterranean coastal sediments offshore Israel.In order to study the potential sources of Pb, sediments werecollected from stream reaches, the DRWP monitoring stationsand coastal sediments along a south-north (S-N) transectas well as reference samples taken from deep parts of twomarine cores.

MethodsSampling and Sample Preparation. In order to study thedifferent sources of Pb in the marine sediments offshoreIsrael’s coast, four sets of sediment samples were collected(Table S1, Figure 1).

(1) Reference cores: samples were taken from two shortcores located ∼27 km apart, offshore the Israeli coast (PL-29,AS-1). Cores were subsampled from a box corer (BX700 ALOcean Instruments) using Perspex pipes. The three deepestsamples (28-30 cm) from each core were analyzed.

(2) DRWP monitoring stations: surface sediments (1 cmthick) were collected from 10 monitoring stations which aresituated along the 38 m water depth to the north and southof the outfall station (Figure 1). It should be noted that in thisstudy, stations PL-29 and AS-1 were studied for their referencevalues (deepest samples) as well as two of the surface samplemonitoring stations. In addition, in stations AS-1, PL-29, andPL-3 surface sediments to a 4 cm depth were also studied.

(3) Fifteen sediment samples were collected at the mouthsof various coastal streams, at distances of 50 and 150 mupstream. Since point sources of pollution can vary along astream, samples were taken at the reaches and thus reflectan integrated contribution to the marine sediments. Thesesediments were previously studied for their metal concen-trations (Figure 1b 24, 25).

(4) Surface sediments collected along a south-northtransect in the shallow continental shelf (Figure 1, Table S1).These sediments were previously studied for their metalconcentrations (26).

Samples were lyophilized, and for the most part the <250µm size fraction was used for analysis. This fraction waschosen to avoid heterogeneity due to the presence of largeparticles, mainly fauna fragments. In addition, in selectedsamples other size fractions were also analyzed.

Sediment Extract. In order to better distinguish betweenthe anthropogenic Pb and the fraction incorporated inminerals (residue Pb) a diluted acid extract was carried out.One gram of sample (bulk and/or <250, <125, >125, and >125µm size fraction) was leached using 10 mL of 0.6 M HCl atroom temperature for 24 h. The extract was then separatedby centrifuge and analyzed for element concentrations andPb isotopic composition. Major and minor elements weremeasured using ICP-OES optima 3300 and trace elementusing ICP-MS Elan 6000 at the Geological Survey of Israel.The isotopic composition of Pb was measured by MC-ICP-MS along with repeated measurements of the SRM 981standard for precision (e.g. ref 28). Because of the presenceof trace amounts of Hg, the ion beam of 202Hg was monitored,and the 204Pb ion beam was corrected for isobaric interferencefrom 204Hg, using a 204Hg/202Hg ratio of 0.2299. Typically, thiscorrection was less than 0.0001 V on the 204Pb measurements.The 1σ precision of the 207Pb/206Pb and 208Pb/206Pb measure-ments is <50 ppm and less than 200 ppm for the 206Pb/204Pb,207Pb/204Pb, and 208Pb/204Pb ratios.

Results and DiscussionThe Role of Sediment Extract. As noted above, rather thanstudying Pb in the bulk sediments, Pb was studied in thesediment extract, which should presumably reflect theadsorbed and easily removed Pb in the sediments. This isespecially important when considering the anthropogeniccontribution which is likely to adsorb onto particles and insome cases may comprise only a small percentage of thebulk Pb. In general, the bulk sediments consist mainly ofquartz grains (up to 70%), feldspars, Ca-carbonates (up to∼10%), clays, Fe oxides and hydroxides, and Mn oxides. Thusthe minerals that probably retain Pb in the sediment and arelikely to release it under diluted acid conditions are CaCO3,clays, and Fe/Mn phases. Indeed, the calculated percentextract (%Ext.) for major elements (%Ext.element ) 100*[Cextract/Ctotal]element), where C is the element concentration,is high for CaO and MgO while low for Al2O3 and SiO2 (TableS2). The %Ext. of Ba is only 2%, thus feldspars probably werenot significantly affected by the weak acid. In addition, the%Ext. for MnO is high while that for Fe2O3 is significantlylower (ca. 50 vs 10%, respectively, Table S2) which indicatesthat Mn-oxides are more dominant in the sediments thanFe-oxides. In addition, this method was found to generallyreproduce the sum of several steps of sequential extractionprocedures (29). To summarize, the weak acid extract couldbe considered as representing the adsorbed fraction of themarine sediment and thus may be affected by anthropogeniccontribution.

Natural Pb in Coastal Shelf Sediments. In general, thenatural phases that may contribute Pb to the sedimentextracts are aerosol, biogenic fragments, Mn-Fe oxides, andclay minerals originating from both, the Israeli inland (viastreams) and the Nilotic cell. These various components can

FIGURE 1. Sample location map. A. Sampling map showinglocations of sediments taken from the lower reaches of thecoastal streams (full dots), north-south transect (gray dots). B.The relevant Dan Region Wastewater Project (DRWP)monitoring stations at which samples were taken (PL).

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be either dissolved by diluted acid or release Pb adsorbedonto their surfaces. The potential release of Pb by eachcomponent is discussed independently and then combined.

Inland Clay. The Israeli inland streams drain the exposedMesozoic carbonate bedrocks which are covered in mostplaces by soil. Thus, clays and to a much lesser extent rockfragments (Ca-carbonate) originating in the Israeli inlandmay be present in the marine shelf sediments. Indeed, it isestimated that ca. 50% of the shallow marine clay fractionis contributed by Israeli inland streams (<2 µm (30)). The Pbconcentration and isotopic composition of clays and othersoil components originating from the Israeli inland can bededuced and projected from soil studies (9, 17) and arepresented in Figure S1 and Table 1.

Nilotic Clay Fraction. The Nile sediments are derived fromthe Ethiopian highland, which consists of Neogene basalts,and from the White Nile which drains the amphibolite-faciesbasement rocks of East Africa. At present, “virtually all of thedetritus carried by the Nile is produced in the Ethiopianhighland” (31). Lead concentrations in the Ethiopian basaltsare 1-5 ppm, and the average isotopic composition is 206Pb/207Pb ) 1.193 ( 0.17, 208Pb/206Pb ) 2.064 ( 0.02 ((22); EV,Figure S1). Note that these values are within the range of thesoil residual fraction and soil extract values (Table 1, FigureS1). Nonetheless, the shallow marine silt fraction (<63 µm)consists of 18-25 ppm of Pb (26).

Natural Pb in Aerosol. Natural Pb in aerosol is a mixtureof local (soil dust) and remote sources (Saharan dust). Aswith other metals, Pb concentrations in aerosol range widely.While the isotopic composition of the local dust shouldresemble that of the local soil (Table 1), the Saharan dustsitself is a mixture of a wide variety of North African sources.No direct data regarding the natural component of Pb forthe Saharan dust over the Eastern Mediterranean is available.Nevertheless the lower end-member of the trend lineobserved for aerosols collected in Israel (32) may representthe Saharan dust (SD; 206Pb/207Pb)1.162, 208Pb/206Pb)2.093;Table 1, Figure S1). As expected, this value is different fromthat observed for the Saharan dust over the northeasternAtlantic Ocean (206Pb/207Pb ) 1.193, 208Pb/206Pb ) 2.080 (6))or in studies from the Western Mediterranean Sea (e.g. refs8, 33, and 34).

Carbonate Bedrock. Since no carbonate rock fragmentswere identified in the sediments, their contribution isprobably insignificant. Moreover, in spite of its high Pbisotopic ratios, it has low Pb concentrations (ca. 0.3 ppm;

Table 1). Thus the Mesozoic carbonate bedrock is found tohave no significant contribution to the whole soil andconsequently to the marine sediments.

Biogenic Pb. Most biogenic fragments in sediments of theIsraeli marine shelf are found in the 150 to 2000 µm sizefraction (35) and thus in the current study were largelyexcluded by sieving. Moreover, when comparing the isotopiccomposition of Pb in two size fractions above and below 125µm from station PL-29 (2-3 cm depth), the coarser fraction,which probably contains more foraminifera and mollusks,has a lower 206Pb/207Pb ratio but a higher 208Pb/206Pb ratiothan the finer fraction (Table S3), however, differences aretoo small to draw any clear conclusion. Likewise, the Pbisotopic composition of surface sediments from variousmonitoring stations was analyzed in extracts of both bulkand <250 µm sediment fractions and found, if not identical,to be similar within 2σ. Finally the insignificant contributionof biogenic Pb to the sediment extracts could also be inferredfrom the lack of correlation between the %Ext. of Ca and thatof Pb (not shown here).

Fe-Mn Oxides. Since Pb is largely adsorbed onto Fe-Mnoxides, its isotopic composition should resemble that of theseawater. Lead concentration in the southeast Mediterraneancoastal seawater is (0.05-0.25 ppb) (16); however, there areno data regarding its isotopic composition. Thus, for thecurrent study the isotopic composition of Pb in the equatorialAtlantic is used (SW, 206Pb/207Pb ) 1.213, 208Pb/206Pb ) 2.055(36)).

Considering the above relevant data, the possible naturalPb sources for the marine sediments offshore Israel areplotted in Figure 2. In addition, the average isotopiccompositions of the extracts of the deepest part of bothreference cores, which are assumed to represent the naturalbackground values, are also plotted.

The data points form a linear array connecting the Saharandust value and both the background and the seawater values.The soil residue (bulk Al silicates) and extract values also lieon this trend line. Since the isotopic compositions of bothreference cores (deepest section) lie below the potentialsources (i.e., Ethiopian basalts and inland soils), an additionalend-member is required. Two possible additional sourcesare (1) Pb in seawater and (2) a different clay assemblage.Indeed, the average value of PL-29 is similar to the seawatervalue, while that of AS-1 is slightly lower (Table S3). Thisdifference may reflect two different sedimentation periodsin the east Mediterranean Sea where the White Nile con-

TABLE 1. Compiled Data of Pb Isotopic Composition of Feasible Sources

typea 206Pb/207Pb 208Pb/206Pb Pb (ppm) reference

Inland rocks:whole rock (n ) 22) 1.23-2.55 0.91-2.00 0.3 9, 17, 38carbonate fraction (n ) 3) 1.67 ( 0.08 2.43 ( 0.01 1 17residual fraction (n ) 3) 1.31 ( 0.02 1.87 ( 0.02 5 17Inland soils:whole soil (WS, n ) 5) 1.21 ( 0.04 2.056 ( 0.004 11-26 17soil residue (SR, n ) 9) 1.20 ( 0.01 2.062 ( 0.01 12-13 17, 9soil extract (SE, n ) 7) 1.18 ( 0.02 2.08 ( 0.02 9Saharan dust (SD) 1.162 2.093 9Sea water (SW) 1.213 2.055 36Ethiopian volcanic (EV) 1.19 ( 0.2 2.064 ( 0.02 1-5 39Reference coresPL-29 (n ) 3) 1.2106 2.0517 current studyAS-1 (n ) 3) 1.2117 2.0466Emissions:pre-1992 1.190 2.066 9, 17, 40post-1992 1.115 2.145 9, 17Cairo 1.153 2.108 41Turkey 1.123 2.141 41

a n ) number of samples other than 1.

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tributed more clay to the Nilotic cell, thus suggesting adifferent clay assemblage. This subject should be investigatedfurther.

The Pb isotopic composition also reveals that the marinesediment value is more affected by the residual fraction ofthe inland soils than by the Saharan dust (Figure 2). Toconclude, the expected natural end-member for Pb in themarine sediments probably resembles that of the deepestpart of the reference cores PL-29 and AS-1.

Sources of Anthropogenic Pb in the Marine Sediments.In general, the anthropogenic Pb in Israel is mainly associatedwith fuel combustion and to a lesser extent with coal burningand industrial use. The three probable sources of anthro-pogenic Pb in coastal sediments offshore Israel are (1)atmospheric deposition; (2) DRWP sludge or other minormarine outfalls; and (3) stream sediments.

Anthropogenic Pb in Aerosol. The major sources foratmospheric Pb are both local and remote. The isotopiccomposition of Pb emitted in Israel has two distinct valueswhich correspond to two different periods, pre- and post-1992 (Table 1). The isotopic composition of Pb emitted pre-1992 is very similar to the soil residue values (Table 1,Figure 2). The post-1992 value was derived from soil samplesnear major roads (9). It should be noted that the use of Pbas an additive to gasoline is relevant to sediments sampledbefore 2005, the year its use was generally ended. In additionto these two sources, Pb emitted in Europe and in Cairo wasalso observed to reach the Israeli inland and thus probablythe coastal sediments as well (9, 37). Since these latter sourcesare dependent upon the seasonal meteorological conditions,it was shown that aerosol values form an array extendingbetween the post-1992 value and most probably that of theSaharan dust (9, 38).

DRWP Sewage Sludge. Since 1987, the DRWP plant hasdischarged some 16,000 m3 day-1 of sludge through a singleoutfall situated 5 km offshore. Though the sludge consistsmainly of an organic biomass, it may contain up to 1700kg/yr-1 Pb, beside other metals (22). Plotted in Figure 2a isthe isotopic composition of surface sediments at 100 m to5.5 km away from the sludge outfall, with possible Pb end-members. Sediments at the DRWP outfall area fall along amixing line where the natural marine end-members, i.e., PL-29, AS-1, soil residue, and seawater form the lower end-member and the post-1992 emission value forms the upperend-member. Three of the monitoring stations serve asreference stations south (PL-29, AS-1) and north (PL-58) ofthe outfall station (PL-0). These stations, which are locatedmore than 4.5 km away from the DRWP outfall pipeline,yielded values of Pb isotope ratios similar to those of thenatural Pb end-members but have significantly different206Pb/207Pb and 208Pb/206Pb ratios than those of stationslocated closer to the outfall (Figure 2a). This confirms thatthese stations are free of significant anthropogenic Pb. Thismay also imply that other heavy metals which accompanyPb are not contributed by the sludge to these stations. Indeedvarious aspects studied in sediments from the same moni-toring stations, excluding the above station, indicated thatelevated concentrations of Corg, Hg, Cd, Cu, Zn, and Pb weredetected mostly north of the outfall in the direction of theprevalent longshore current, up to ca. 4 km (22).

Monitoring stations up to 1.5 km north and south of theoutfall have Pb isotopic values that depart from that of thenatural end-member and are closer to the post-1992 Pbvalues. Surprisingly, on this mixing line the most contami-nated station is not the outfall station (PL-0) but station PL-5, which is situated 1 km further north of the discharge pipe.This indicates removal of Pb and other contaminants bycurrents immediately after discharge.

Examining the change of Pb isotopic composition withdepth in three monitoring stations (AS-1, PL-29, and PL-3)revealed that the isotopic composition of PL-3. which is 200 mnorth of the outfall, changes significantly toward naturalvalues with depth while that of the other two stations variesslightly or remains constant with depth (PL-29 and AS-1,respectively; Figure S2), thus confirming the observationabove that while PL-3 is contaminated, the other two arefree of contamination.

Studies on the DRWP sludge impact on sediment qualityand benthic assemblages concluded that the sludge ac-cumulates seasonally during a period when the water columnis stratified, whereas during winter storms the fine particlesof the DRWP sludge are suspended and dispersed (22). Thisimplies that pollutants are not deposited and buried on sitebut are transported elsewhere. Indeed, the low concentrationof Pb found in the surface sediments (Table S2) suggests thatmost of the Pb is removed by winter storms along with other

FIGURE 2. The 208Pb/206Pb vs 206Pb/207Pb ratios in sedimentextracts of (a) DRWP samples (0-2 cm) along with possible Pbsources and (b) the 206Pb/207Pb ratio taken on different dates(0-2 cm), (M/Y) ) month and year. SD ) Saharan dust, EV )Ethiopian volcanics, SW ) sea water, SR ) soil residue, SE )soil extract, pre- and post-1992 ) the isotopic composition ofPb emission in Israel pre- and post-1992. Shaded area ) lowernatural end member.

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organic matter and heavy metals. Moreover, comparing thePb isotopic values of the surface sediments of three stations(AS-1, PL-29, and PL-3) with time (Figure 2b) reveals thatwhile the two assumed clean sites (PL-29 and AS-1) show aconstant composition with time, the third station, PL-3, variesseasonally, with high 206Pb/207Pb ratios during March-Apriland September and lower values in July and January,suggesting low and high contribution of sludge to the marinesediments, respectively. Thus although most of the Pb isremoved from the sediments after winter storms, the Pbremaining in the sediments, which might mistakenly beconsidered as natural Pb, is in fact derived from the DRWPsludge. The mixing trend between natural and anthropogenicPb in the surface of marine sediments is also shown usingother Pb isotope ratios vs 1/Pb (Figure 3a). On this plot, allstations but PL-29 and AS-1 form a linear trend where themost polluted samples are enriched in Pb (low 1/Pb values)and have low 206Pb/207Pb values. The distribution of thepolluted stations along the array agrees with previousobservations where PL-5 is the most polluted station and theoutfall station (PL-0) is less polluted. Moreover, stations northof the outfall are more polluted than those souths of it.Furthermore, station PL-3 yields contaminated values all yeararound, as observed in Figure 2b. Stations PL-29 and AS-1though yielded different values, showing constant 206Pb/207Pbvalues with 1/Pb and low Pb content.

The enrichment factor (EF) for the DRWP sediments wascalculated using Al as a conservative element and the extractvalues of AS-1 and PL-29 (28-30 cm depth) as backgroundvalues. The enrichment factors were found to be in thefollowing order: Zn.Cu > Pb. The EF of Cu and Pb correlateswell against that of Zn (Figure 3b), thus confirming theircommon anthropogenic origin, most likely the sludge. Mostprominent is the good correlation between the isotopiccomposition of Pb and the enrichment factors of theseelements (e.g., 208Pb/206Pb ratio, Figure 3c). This is validationthat the isotopic composition of Pb is a powerful tool intracing pollutants other than Pb. All suspected pollutantsare more enriched in stations located to the north of theoutfall station than stations to the south of it, probablybecause the longshore current transports the sludgenorthward.

Lead in the sludge may originate from the atmosphericfallout (washout) or from industrial use (sewage). Since Pbisotopic composition in the sludge approaches the post-1992 values, it is suggested that urban washout dominatesthe sludge value which in turn probably originated fromatmospheric fallout. Because of the lack of knowledgeregarding the isotopic composition of Pb used in industries(e.g., point sources), we cannot distinguish it from theatmospheric Pb.

Stream Sediments. In this study all stream sedimentextracts fall along a mixing line between natural Pb and post-1992 emissions (Figure 4, Table S4). In addition, all samplesbut one (Lachish) lie between the pre-1992 and the post-1992 values, and only one sample lies above the Saharandust value. The conclusion that all but one sample (Lachish)should be considered polluted is 2-fold: (1) samples extendwithin the pre- and post-1992; and (2) the contribution ofSaharan dust should not be larger in stream sediments thanthat from inland soils (Figure 4). The widespread of valuessuggests either additional point sources for each stream ordifferent relative mixtures between natural and post-1992aerosols. The latter option is favored because different pointsources would have resulted in a more scattered pattern thanthe one observed (Figure 4). Among the streams, the Polegstream is the most polluted, while the Lachish stream isuncontaminated and its Pb isotopic value is similar to thatof the corresponding offshore marine station (AS-1, Figure4). In all five cases, in which samples were taken from both

FIGURE 3. The isotopic composition of Pb in DRWP sedimentextracts: (a) 206Pb/207Pb vs 1/Pb; (b) a strong correlation betweenPb, Cu, and Zn enrichment factors (EF); (c) Zn, Pb, and Cuenrichment factors vs 208Pb/206Pb. The vertical line connects differentEF of each monitoring station. Trend lines are shown (not calculated).

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up (bridge) and down stream (mouth), the 206Pb/207Pb ratioincreases down stream, suggesting a greater contribution ofanthropogenic Pb closer to the stream mouth.

Offshore Sediments. This set of samples taken along anorth-south transect further enlightens the contribution ofpollutant brought by the streams to the marine environment.In Figure 5 the isotopic composition of Pb in offshoresediment extracts is presented along with sediment extractsof the streams which are likely to contribute to the marinesediments. The offshore samples are distributed along a trendline on a plot of 206Pb/207Pb versus 208Pb/206Pb from north tosouth; excluding sample C9, which is located offshore Tel-Aviv (Table S4, Figure 5). In addition, no correlation isobserved between the stream value and its relevant marinesample value, suggesting that sediments contributed via

streams are transported northward with the longshorecurrent. All samples extend between the Saharan dust andthe lower natural end-members. Moreover, there is no clearcorrelation between 1/Pb to Pb isotopic composition (TableS4). Thus in contrast to the DRWP sediments, in this case theobserved trend may also reflect the decrease of clay contentand increase of CaCO3 northward. Indeed, a dominant sourceof Nile-derived material trending from south to north wasobserved in the same set of offshore sediments (26), andthus the argument draw for the stream sediments samplescould not be applied in this case.

Environmental Implications1. The natural end-members shown to contribute Pb to themarine sediments offshore the Israeli coast are clays andaerosols from both the mainland and the Nile (dominatedby Ethiopian basalts) and, to a lesser extent, the Saharandust. The biogenic fauna and the Cenozoic bedrock areinsignificant sources for Pb in the marine sediments.

2. Atmospheric deposition with post-1992 Pb isotopiccomposition is a major contributor to marine sedimentsoffshore Israel (including DRWP monitoring stations) as wellas to stream sediments.

3. The DRWP sludge contributes Pb, Cu, and Zn to themarine sediments up to ca. 2 km away from the outfallpipeline. Enrichment factors of Zn, Cu, and Pb were up to25 and significantly correlated with each other, indicatingtheir common source. The sludge is transported northwardsfrom the outfall by the longshore current.

4. The stream sediments yielded anthropogenic Pb valueswhich are probably due to post-1992 aerosol fallout and localpoint sources. Yet the impact of stream sediments on marinesediment offshore could not be clearly detected.

AcknowledgmentsThe authors thank N. Tepliakov, O. Yofe, and I. Segal fortechnical assistance. They wish to thank A. Ayalon and N.Teutsch for useful comments. Two anonymous reviewersare acknowledged for providing useful comments whichgreatly improved the manuscript.

Supporting Information AvailableSampling data, chemical composition of bulk sediment andextracts and the isotopic composition of Pb in sedimentextracts of reference cores, DRWP, stream and S-N transectsamples. This material is available free of charge via theInternet at http://pubs.acs.org.

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FIGURE 4. The 208Pb/206Pb vs 206Pb/207Pb ratios of streamsediments along with possible Pb sources. SD ) Saharan dust,EV ) Ethiopian volcanics, SW ) sea water, SR ) soil residue,pre- and post-1992 ) the isotopic composition of Pb emissionin Israel pre- and post-1992.

FIGURE 5. The 208Pb/206Pb vs 206Pb/207Pb ratios of sedimentsalong a north-south transect. Also shown are streamsediments which are likely to contribute sediments to themarine shelf at that location.

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