Science of the Total Environment 472 (2014) 888–900

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Science of the Total Environment

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The lead (Pb) isotope signature, behaviour and fate of traffic-related leadpollution in roadside soils in The Netherlands

N. Walraven a,⁎, B.J.H. van Os b, G.Th. Klaver c, J.J. Middelburg d, G.R. Davies e

a GeoConnect, Meester Dekkerstraat 4, 1901 PV Castricum, The Netherlandsb Rijksdienst voor Archeologie, Cultuurlandschap en Monumenten, P.O. Box 1600, 3800 BP Amersfoort, The Netherlandsc BRGM, 3 avenue Claude-Guillemin, BP 36009, 45060 Orléans Cedex 2, Franced University Utrecht, Faculty of Geosciences, P.O. Box 80021, 3508 TA Utrecht, The Netherlandse VU University Amsterdam, Faculty of Earth and Life Sciences, Petrology, De Boelelaan 1085, 1081 HV Amsterdam, The Netherlands

H I G H L I G H T S

• Lead isotope composition of litter and topsoil differs from the deeper soil samples.• Litter and topsoil contain anthropogenic Pb, with gasoline Pb as main source.• Anthropogenic Pb is strongly associated with organic matter in litter and topsoil.• Approximately 35–90% of atmospherically derived Pb migrated to groundwater.• Migration is caused by low soil pH and lack of reactive phases in subsoil.

⁎ Corresponding author.E-mail addresses: n.walraven@geoconnect.nl (N. Walr

(B.J.H. van Os), g.klaver@brgm.nl (G.T. Klaver), j.b.m.middg.r.davies@vu.nl (G.R. Davies).

0048-9697/$ – see front matter © 2013 Elsevier B.V. All rihttp://dx.doi.org/10.1016/j.scitotenv.2013.11.110

a b s t r a c t

a r t i c l e i n f o

Article history:Received 12 September 2013Received in revised form 18 November 2013Accepted 22 November 2013Available online 15 December 2013

Keywords:LeadIsotopesHighwaysSoilsGroundwaterTransport

In this study the origin, behaviour and fate of anthropogenic Pb in sandy roadside soils were assessed bymeasur-ing soil characteristics, Pb isotope composition and content. In 1991 and 2003 samples were taken at differentdepth intervals at approximately 8 and 75 m from two highways in The Netherlands. The Pb isotope compositionof the litter layer (206Pb/207Pb = 1.12–1.14) differs from the deeper soil samples (206Pb/207Pb = 1.20–1.21).Based on a mixingmodel it is concluded that the samples contain two Pb sources: natural Pb and anthropogenicPb, the latter mainly derived from gasoline.206Pb/207Pb ratios demonstrate that the roadside soils were polluted to a depth of ~15 cm. Within this depthinterval, anthropogenic Pb content is associated with organic matter. Although Pb pollution only reacheda depth of ~15 cm, this does not mean that the topsoils retain all anthropogenic Pb. Due to the low pH andnegligible binding capacity of soils at depths N15 cm, anthropogenic Pb migrated towards groundwaterafter reaching depths of N15 cm. The Pb isotope composition of the groundwater (206Pb/207Pb = 1.135–1.185)establishes that groundwater is polluted with anthropogenic Pb. The contribution of anthropogenic Pb to thegroundwater varies between ~30 and 100%.Based on the difference in soil Pb content and Pb isotope compositions over a period of 12 years, downward Pbmi-gration is calculated to vary from72 ± 95 to 324 ± 279 mg m−2 y−1. Assuming that the downward Pbflux is con-stant over time, it is calculated that 35–90% of the atmospherically delivered Pb has migrated to the groundwater.

© 2013 Elsevier B.V. All rights reserved.

1. Introduction

In 1921 the anti-knock properties of tetraethyl lead (TEL) werediscovered and were introduced to the market as leaded gasoline in1923. The environmental effects are evident over the entire globe.The highest Pb contents – due to the combustion of leaded gasoline –

aven), b.vanos@rce.nlelburg@uu.nl (J.J. Middelburg),

ghts reserved.

have been measured in roadside soils (up to 3000 mg/kg: e.g. Chow,1970; Milberg et al., 1980; Banin et al., 1987; Goyer, 1988; Groot andvan Swinderen, 1993; Othman et al., 1997; Emmanuel and Erel, 2002;Fakayode and Olu-Owolabi, 2003; Bakirdere and Yaman, 2008; Chenet al., 2010; Khan et al., 2011). Elevated Pb contents have also beenmeasured in remote parts of the globe, including Greenland snow(Boutron et al., 1991; Rosman et al., 1993, 1994) and the remote icecap of Antarctica (e.g. Wolff and Suttie, 1994; Barbante et al., 1997;Vallelonga et al., 2002).

For years it was thought that the topsoil horizon was an almostpermanent sink for anthropogenic Pb (Siccama and Smith, 1978).

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Early reports suggested that the residence time of Pb in forest floorscould be in the order of several hundred years (Benninger et al., 1975;Tyler, 1978; Friedland and Johnson, 1985; Turner et al., 1985). If theseestimates are correct, then despite the reduction in Pb deposition dueto legislative measures starting in the 1970s to curtail gasoline Pb, thePb content in soils should continually increase to potentially toxic levels(Watmough et al., 2004). However, it has been observed that Pb levelsin soil profiles have reacted to thedecline in Pb emission and deposition.This suggests that the residence time of Pb in these reservoirs is shorterthan previously estimated (Herrick and Friedland, 1990; Miller andFriedland, 1994; Steinmann and Stille 1997; Erel, 1998; Teutsch et al.,2001; Klaminder et al., 2006). Erel (1998) calculated that the Pb infiltra-tion velocity in Israeli soils is 0.5 cm/y, implying residence times of 100to 200 years for 50 to 100 cm deep soils. Not all soils are permanentsinks for Pb, potentially leading to groundwater pollution.

Previous workers demonstrated that stable Pb isotopes can be usedto 1) distinguish natural and anthropogenic Pb sources and 2) deter-mine transport rates of gasoline Pb (e.g. Petit et al., 1984; Facchetti,1989; Krause et al., 1993; Erel, 1998). Regional variations in Pb isotoperatios in gasoline, however, imply that Pb isotope measurementscannot simply be used to estimate the contribution of gasoline Pb inthe environment. In Belgium, for example, automobile exhaust hada 206Pb/207Pb ratio of 1.12–1.14 (Petit et al., 1984) while in northernItaly leaded gasoline had a 206Pb/207Pb ratio of around 1.19 (Facchetti,1989). Lead pollution in roadside soils in TheNetherlands has been littlestudied. In 1991 Pb contents in several roadside soils in TheNetherlandswere determined (Groot and van Swinderen, 1993). These sampleswere stored at the Dutch National Institute of Public Health andthe Environment (RIVM) and were available for Pb isotope analysis.Combined with samples collected in 2003, these samples offer a uniquepossibility to study changes in Pb isotope composition and content inroadside soils over a 12-year period. If the isotopic composition of road-side soils polluted with gasoline Pb differs measurably from naturalbackground Pb, it can be used to determine 1) transport rates of anthro-pogenic Pb through the soil and 2) if groundwater near the selectedhighways is (already) polluted with gasoline Pb.

2. Background information

Most of the sediments found close to the surface of The Netherlandswere deposited during the Quaternary period, and are of Pleistocene orHolocene age. They consist of fluviatile, marine and glacigenic sedi-ments, as well as sediments with a more local genesis, such as aeoliandeposits and peat. Soil formation in The Netherlands essentially startedafter the last glacial period (Weichselien) when temperature rose andvegetation started to stabilize theunconsolidated sediments. For greaterdetail see Van der Veer (2006) and Walraven et al. (2013a).

2.1. Sampling site description

In 1991 five roadside locations in The Netherlands were sampled tostudy traffic-related soil pollution (Groot and van Swinderen, 1993).In 2003 two of these locations were revisited and sampled to studychanges in anthropogenic Pb isotope composition and content over aperiod of 12 years. The selected sites are located along highway A28nearNunspeet and highwayA58 nearMoergestel. The sample locations,highway opening dates, sampling dates and traffic densities are shownin Fig. 1. At both locations samples of forest litter, soil and groundwaterwere taken.

2.1.1. Sampling site near NunspeetThis site is situated in a forested area of aeolian periglacial deposits.

In the forest, Scots pine (Pinus sylvestris), silver birch (Betula pendula),oak (Quercus sp.) and black cherry (Prunus serotina) are the mostdominant species (Groot and van Swinderen, 1993). The low cover con-sists of grasses (a.o. Molinea caerulea), mosses, lichens (o.a. Cladonia

rangiferina) and heather (o.a. Calluna vulgaris) (Groot and vanSwinderen, 1993). The site is located in an infiltration areawith averagehighest groundwater levels at depths N80 cm-g.l. and average lowestgroundwater levels at depth N160 cm-g.l.

Samples were taken at 8 m and 74 m from the highway A28(Appendix 1). Eight metres was the closest point to the highwaywhere the soil was undisturbed and situated within the forest.Seventy-four metres from the highway was the farthest accessiblepoint from the highwaywhere the soil was undisturbed and still situat-ed in the forest. The soil at 8 m from the highway is a humic podzolicsoil with hydromorphic characteristics (suborder xeropodzol soil),consisting of loam-poor fine sand with a low pH (pH-H2O ≈3.6;Groot and van Swinderen, 1993). The soil at 74 m from the highwayis a vague soil with hydromorphic characteristics (suborder xerovaguesoils), consisting of coarse sand with a low pH (pH-H2O ≈3.6;Groot and van Swinderen, 1993). The soil contains hardly any visiblepedogenesis.

2.1.2. Sampling site near MoergestelThis site is also situated in a forested area of aeolian periglacial

deposits. The forest consists predominantly of Scots pine, silver birchand oak (Groot and van Swinderen, 1993). The low cover consists ofgrasses (a.o.Molinea caerulea), mosses and ferns (o.a. Osmunda regalis)(Groot and van Swinderen, 1993). The site is located in an infiltrationarea where high and low groundwater levels are typically ~80 cmand ~160 cm below ground level respectively.

Samples are again taken from undisturbed soils and situatedwithin the forest at two distances from the highway A58: 10 m and75 m (Appendix 1).

The soil at 10 m from the highway is a humic podzolic soil withhydromorphic characteristics (suborder xeropodzol soil), consisting ofslightly loamy fine sand with a low pH (pH-H2O ≈4.4; Groot and vanSwinderen, 1993). The soil at 75 m from the highway is a humic podzolicsoil with hydromorphic characteristics (suborder hydropodzol soils),consisting of loamy fine sand with a low pH (pH-H2O ≈3.8; Groot andvan Swinderen, 1993).

3. Materials and methods

3.1. Sample collection

In October and November 1991, forest litter, soil and groundwatersamples were collected near highway A28 (Nunspeet) and A58(Moergestel) (Groot and van Swinderen, 1993). Soil and forest litterwere sampled every 50 cm along 80 m transects perpendicular to thehighway. This sampling strategy resulted in 160 forest litter samplesand 160 soil samples for each sampling distance from the highway.Every forty samples were put into a sample container, resulting in 8containers for each sampling distance. The sample containers werebrought to the laboratory for further analysis.

Groundwater was sampled (in monitoring wells) every 4 m along100 m transects perpendicular to the highway at two distances fromthe highway. Contamination of groundwater with lead fromthe topsoil was prevented by removing the litter and topsoil layer(~20 cm) before drilling thewell with a hand auger. To prevent verticalmovement of water, the groundwater well was sealed using granularbentonite. After placement of the monitoring well (casing), possiblecontaminated groundwater was removed from the well by pumping.Groundwater samples were taken a week after drilling the monitoringwell. Before sampling, the monitoring well was thoroughly pumpedand samples were taken when the water reached a constant tempera-ture and electrical conductivity. A total of 24 groundwater sampleswere taken for each sample distance. The groundwater samples forthe analysis of the heavy metals were filtered (0.45 micron pore size)and acidified with HNO3 (pro analysis) in the field. In the laboratory

Sample site Highway Highway

brought into use

(year)

Traffic

density#1

(cars/day)

Sampling

(year)

Nunspeet A28 ± 1962 50282 1991,

2003

Moergestel A58 ± 1963 63182 1991,

2003

Fig. 1.Map showing the locations and general information (sampling date, highway construction date, traffic density) of the two studied roadside soils. Nunspeet (N) is located 2 km ofhighway A28 and Moergestel (M) is located 1.5 km of the highway A58. #1 Rijkswaterstaat (2012).

890 N. Walraven et al. / Science of the Total Environment 472 (2014) 888–900

the 24 groundwater samples were combined to a total of six samplesbefore analysis.

In May and June 2003 the sample sites near Nunspeet andMoergestel were revisited and sampled. The remnants of the ground-water well from 1991 were still present so it was possible to resamplewithin 1 m of the 1991 sample sites. The 1991 procedure of Groot andvan Swinderen (1993) for soil samplingwas repeated. In 1991 a distinctlitter layer was present at 8 m from the highway A28 near Nunspeet.However, in 2003 this litter layer had partly disappeared (most likelybroken down). For consistency sufficient litter was collected to performthe planned analyses.

In addition to the sampling procedure followed in 1991, in 2003samples from the depth interval 45–55 cm and 90–100 cm weretaken with an Edelman hand auger and soil profiles were sampledby digging a pit with a depth of 60 cm. The soil profiles weresampled at 8 and 10 m from the A28 and A58 respectively. High-

resolution sampling was performed from the bottom to the top ofpit walls.

The procedure of groundwater sampling of Groot and van Swinderen(1993) was repeated. However, instead of 24 groundwater wells, 4groundwater wells were placed at each distance from the highway.The distance between each well was about 40 m. Again special carewas taken to prevent contamination of the groundwater with leadfrom the topsoil.

3.2. Preparation and chemical analyses of the litter and soil samples

Forest litter and soil samples from 1991 were stored in glasscontainers at 4 °C in the sample depot at RIVM. Only two of the foursoil samples from every distance and depth from the highway werepresent. The samples from 1991were analysed in 1996 and the samples

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from 2003 were analysed in 2003. Due to this time difference, someanalytical procedures differ slightly.

3.2.1. Sample pre-treatmentAfter determination of bulk density (based on 25 cm3 sample), all

soil samples were dried at 105 °C. The soil samples were disintegratedand sieved over a 2 mm sieve to remove the gravel fraction. Soil andforest litter samples were ground with an automated tungsten carbideswing-mill (Herzog HSM-HTP) to a grain size b20 μm, homogenisedfor 5 min in a Turbula T2C and stored in glass containers.

3.2.2. X-ray fluorescence (XRF)Aluminium (Al) and iron (Fe) have beenmeasured by X-ray fluores-

cence spectroscopy (XRF)with a precision (1 RSD) of 0.5–1.0% based onreplicate analysis. For details see Spijker (2005) and Walraven et al.(2013a). One reference sample (ISE 921) was added to each batchof 20 samples to determine accuracy. The certified Al and Fe contentof ISE 921 was 5.7 wt.% and 3.2 wt.% respectively. The accuracy for Aland Fe was 4–6% relative bias.

3.2.3. Thermo gravimetric analyses (organic matter and carbonate)Organic matter (OM) and carbonate (CO2carb) contents were

analysed with a LECO TGA 601. Temperatures were varied from 105 °Cto 550 °C for organic matter content determination, 550 °C to 800 °Cfor carbonate content (mainly CaCO3) and from 800 °C to 1000 °C forclay bound water content. The relative precision (2 RSD) for OM andCO2carb based on sample duplicates is 3.8% and 4.5% respectively. Onereference soil sample (ISE 921) was added to each batch of 20 samplesto determine accuracy. The accuracy for OM and CO2carb determinedon ISE 921 was 0.2% and 13% respectively (relative bias).

3.2.4. Inductively coupled plasma-mass spectroscopy (ICP-MS)Lead and Pb isotopes were analysed after HF-based sample destruc-

tion. Details of the methodology of the Pb content analysis for the 1991and 2003 samples can be found in Walraven et al. (1997) and Van derVeer (2006) respectively. The relative precision (2 RSD) for Pb basedon sample duplicates is 3.6%. The accuracy for Pb as determined bysoil standard ISE 921 (Pb = 167 mg/kg) was 0.7% (relative bias).

The methodology of the Pb isotope analysis is described in detail inWalraven et al. (2013a,b). All 206Pb/207Pb ratios of the 1991 sampleswere determined with a precision of 2RSD b1%. The 206Pb/207Pb ratiosof the soil and groundwater samples collected in 2003were determinedwith a precision of 2RSD b0.3% and 2RSD b0.8% respectively. The aver-age measured value and absolute precision (2 SD) of ISE 921 samplewas 1.167 ± 0.008 for 206Pb/207Pb. Average, precision and accuracyare based on the entire analytical procedure starting with the samplesplits. Blanks indicate that reagents contain negligible amounts of Pb(b20 ng/kg).

3.3. Preparation and chemical analyses of groundwater samples

The Pb content of the groundwater sampled in 1991wasdeterminedby means of AAS (details in Groot and van Swinderen, 1993). Thegroundwater samples from 2003 were analysed by ICP-MS (Agilent7500i) for 66 elements (including Pb) according to NEN 6427. Leadisotope analysis was performed according to the method describedin Section 3.2, with the difference that groundwater samples were notdiluted prior to introduction into the mass spectrometer.

4. Results and discussion

4.1. Natural and anthropogenic lead in roadside soils

The Pb content of polluted soil samples consists of natural Pb andanthropogenic Pb. Lead inherited from the underlying sediment litholo-gy and added in the formof dust, that is the natural fallout from regional

geological processes, will be referred to as natural Pb. Lead inheritedfrom anthropogenic activity, in the case of The Netherlands, pre-dominantly from the combustion of leaded gasoline (Walraven et al.,2013a,b, submitted for publication) is referred to as anthropogenic Pb(see discussion below). If the natural Pb content is known, the anthro-pogenic Pb content can be calculated. The natural Pb content (Pbn)in sediments and soils can be calculated based on the common relation-ship between Pb and Al in unpolluted soils and sediments (o.a. Huisman,1998; Van der Veer, 2006; Walraven et al., 2013a,b,submitted forpublication). Because roadside soils are seldom polluted with Al, andthe studied roadside soils are not located close to industrial sites, themeasured Al content is used as a lithogenic proxy for Pbn. Walravenet al. (submitted for publication) established the following relationshipfor unpolluted sedimentary soils in The Netherlands:

Pbn ¼ 3:69� Alþ 1:75 ð1Þ

where:

Pbn = calculated ‘natural’ Pb content (mg/kg).Al = measured Al content in wt.%.(n = 303; R2 = 0.89; Standard error of estimate is 2.6 mg/kg)

Based on Eq. (1), it is possible to calculate the anthropogenic Pbcontent in polluted soil samples according to Eq. (2):

Pba ¼ Pbt−Pbn ð2Þ

where:

Pbn = calculated ‘natural’ Pb content (mg/kg).Pbt = measured total Pb content (mg/kg).Pba = calculated anthropogenic Pb content (mg/kg).

The calculated natural Pb content in the roadside soils fromNunspeet andMoergestel varies from3.1 to 10.3 mg/kgwith an averageof 6.3 mg/kg (Appendix 2 to 4). The calculated natural Pb content agreeswell with the total Pb content of the soil samples from the depth-intervals of 45–55 cm-g.l. and 90–100 cm-g.l. (4.5 to 7.0 mg/kg withan average of 5.8 mg/kg; Appendix 3–4). This implies that these deepersoil samples do not contain excess Pb.

In Fig. 2 the Al contents of the soil and litter samples, gathered in2003, are plotted versus the total Pb content. The Pb contents ofunpolluted Dutch soils are also presented in this figure (Walravenet al., 2013a,b). All litter samples are strongly enriched in Pb with thesamples from Moergestel having the highest content. The enrichmentfactors, defined as Pbt/Pbn, are as high as 35 for Nunspeet and 115 forMoergestel.

The anthropogenic Pb content in the roadside litter and topsoils ishigher than the average anthropogenic Pb content in rural sandy top-soils in Dutch forests (12 mg/kg (n = 55); Walraven et al., 2013b).The highest anthropogenic Pb contents in the roadside soils fromMoergestel and Nunspeet are 645 mg/kg and 191 mg/kg respectively.The Pb contents in the Dutch roadside soils are comparable to roadsidesoils reported from Vancouver and Auckland (Pb contents up to1500 mg/kg; Schmitt et al., 1979; Ward et al., 1977). Extremely highPb levels of over 6000 mg/kg have been recorded in individual roadsidesoil samples from Venezuela (Garcia-Miragaya et al., 1981) and UnitedKingdom (Harrison et al., 1981).

The Pb content of the soils from Nunspeet is considerably lowerthan the soils from Moergestel (Fig. 2). The extent of Pb pollution inroadside soils is often strongly (linearly) correlated with traffic density(e.g. Kloke and Riebartsch, 1964; Ward et al., 1977; Ho and Tai, 1988;Fakayode and Olu-Owolabi, 2003). However, the traffic density nearMoergestel is only 20% higher than near Nunspeet (based on figuresfrom 2000 to 2003: no data available earlier than 2000), while the Pbcontent in the litter samples from Moergestel is 2 to 4 times higher.The difference between the two studied sites cannot be explained by

Fig. 2. Al content (wt.%) of the litter and soil samples from Nunspeet and Moergestel,sampled in 2003, are plotted vs. Pb content (mg/kg). The line indicates the averagerelationship between the Al and Pb content of unpolluted Dutch sedimentary soils,based on 303 samples (●). The dashed lines are the 95% confidence intervals of therelationship between Al and Pb.

892 N. Walraven et al. / Science of the Total Environment 472 (2014) 888–900

traffic density, however, prevailing wind direction in The Netherlandsis southwest. Moergestel is situated downwind of the highwayand Nunspeet to windward of the highway. In addition, Moergestelis located 30–40 km from several Zn smelters. This might explain thehigher Pb content in the soils from Moergestel compared to Nunspeet(see Section 4.2).

4.2. Sources of Pb

The measured Pb isotope ratios of the litter and soil samples repre-sent a mixture of natural and anthropogenic Pb in which the natural Pbcontent in the studied soils varies only slightly (3–10 mg/kg; Appendix2–4). The individual 206Pb/207Pb ratios of a mixture of two Pb sourcescan be determined with a hyperbolic mixing equation under the condi-tion that the 206Pb/207Pb ratios of the two Pb sources differ substantially(Faure, 1986). The equation for a two component mixture is given inEqs. (3a), (3b) and (4):

206Pb=207Pb� �

m¼ Pba � 206Pb=207Pb

� �aþ Pbn � 206Pb=207Pb

� �n

h i=Pbt

ð3aÞ

hence

206Pb=207Pb� �

m¼ 206Pb=207Pb

� �a

þ Pbn � 206Pb=207Pb� �

n−206Pb=207Pb

h �a

h i�=Pbt

ð3bÞ

where:

Pbn = calculated ‘natural’ Pb content (mg/kg).Pbt = measured total Pb content (mg/kg).Pba = calculated anthropogenic Pb content (mg/kg).(206Pb/207Pb)m = measured isotope ratio.(206Pb/207Pb)a = isotope ratio of anthropogenic Pb.(206Pb/207Pb)n = isotope ratio of ‘natural’ Pb.

This is the equation of a hyperbola in co-ordinates of (206Pb/207Pb)mand Pbt:

206Pb=207Pb� �

m¼ a=Pbt þ b ð4Þ

where: a = Pbn × [(206Pb/207Pb)n − (206Pb/207Pb)a] and b =(206Pb/207Pb)a.

Themixinghyperbola can be transformed into a straight line by plot-ting (206Pb/207Pb)m versus the reciprocal of Pbt. The intercept (b) repre-sents the 206Pb/207Pb ratio of anthropogenic Pb (end-member). In Fig. 3Pbt and 1/Pbt are plotted against (206Pb/207Pb)m. The 1991 and 2003data for both Nunspeet and Moergestel show mixing hyperbolae fortwo Pb sources. No difference is observed between the 1991 and 2003data indicating that the 206Pb/207Pb ratios of the two Pb sourceswere constant in time. The average 206Pb/207Pb ratios of natural Pb(end-member) for Nunspeet and Moergestel are well defined at 1.21and 1.20, respectively (average 206Pb/207Pb values of the soil samplesfrom the depth interval of 45 to 55 and 90 to 100 cm-g.l., sampled in2003). These values are in accordance with the Pb isotope compositionof unpolluted soils in The Netherlands (206Pb/207Pb = 1.18–1.22,Walraven et al., 2013a). The mixing relationships record significantscatter in the 206Pb/207Pb ratios making the anthropogenic Pb sourcefor Nunspeet and Moergestel less well defined, with 1.122 and 1.139respectively (Fig. 3). These values resemble the values for gasoline Pbobserved in Belgium (206Pb/207Pb = 1.12–1.14, Petit et al., 1984) andGermany (206Pb/207Pb = 1.10–1.11, Krause et al., 1993). According toPetit et al. (1984), Pb added in gasoline in Belgium and neighbouringcountries comes mainly from Canadian and Australian Pb ores of pre-Cambrian origin having characteristic low 206Pb/207Pb ratios of 1.04–1.07 (Table 1).

Fig. 3(D) shows that 206Pb/207Pb ratios versus the reciprocal ofPb content of the samples from Moergestel do not fit a straight line, incontrast to samples from Nunspeet. The Moergestel 206Pb/207Pb ratiosare slightly more radiogenic (1.139 vs. 1.122). This may point to anadditional anthropogenic Pb source with a 206Pb/207Pb ratio N1.12.The Moergestel sample site lies 30 to 40 km downwind of Belgian Znsmelters in Balen, Lommel, Neerpelt and Overpelt (the Zn smelters inBudel, Balen and Overpelt are still in operation). The processed Znores mainly originate from the former BelgiumCongo (nowDemocraticRepublic of Congo). The average Pb isotope composition of these oresis 1.135, 2.400 and 0.473 for 206Pb/207Pb, 208Pb/207Pb and 206Pb/208Pbrespectively (Doe andRohrbough, 1977; Sonke et al., 2002). The smelterscould represent the ‘extra’ Pb source in Moergestel. This is supported bythe higher Zn contents in the litter layer in Moergestel (67–190 mg/kgwith a median of 82 mg/kg) than in Nunspeet (28–78 mg/kg with amedian of 48 mg/kg). Accordingly, the Moergestel sample site mightalso have been influenced by atmospheric deposition from Belgian Znsmelters.

Besides gasoline Pb and smelter Pb, other atmospherically derivedanthropogenic Pb sources could potentially be present in the roadsidesoils (e.g. fly ash and coal ashes). Atmospheric deposition also tookplace before construction of the highways. However both roadsidesample locations are located in rural forested areas and the input ofother anthropogenic Pb sources is expected to be negligible. The aver-age anthropogenic Pb content in rural sandy topsoils in forests in TheNetherlands is 12 mg/kg (n = 55;Walraven et al., 2013b) This is negli-gible compared to the input measured in the roadside soils of Nunspeetand Moergestel. Walraven et al. (2013b) also showed that even inthese rural forested topsoils (far away from highways) gasoline Pb isthe major anthropogenic Pb source. In addition, a clear relationshipbetween the anthropogenic Pb content and distance to the highwaysis observed in this study (see Section 4.3.1). The Pb content in the litterand topsoils close to the highways (proximal sites) is substantiallyhigher (factor 2–4) than further away (distal sites). We thereforeconclude that Pb in the roadside soils near Moergestel and Nunspeet ispredominantly a mixture of two sources: natural Pb and anthropogenicPb, the latter mainly derived from gasoline.

4.3. Behaviour of Pb

4.3.1. Lead content in relation to distance from roadIt is generally considered that Pb content decreases rapidly and

exponentially with distance from the road (a.o. Page and Ganje, 1970;

Fig. 3. A: Hyperbolic mixing curve defined by 206Pb/207Pb and Pb content (mg/kg) of litter and soil samples from roadside soils near Nunspeet. B: Plot of 206Pb/207Pb and reciprocalPb content of litter and soil samples from roadside soils near Nunspeet which fit on a straight line (R2 = 0.97). C: Hyperbolic mixing curve defined by 206Pb/207Pb and Pb content(mg/kg) of litter and soil samples from roadside soils near Moergestel. D: Plot of 206Pb/207Pb and reciprocal Pb content of litter and soil samples from roadside soils near Moergestelwhich fit on a straight line (R2 = 0.96). E: Hyperbolic mixing curve defined by 206Pb/207Pb and Pb concentrations (μg/l) of groundwater samples collected near two highways(A58 near Moergestel and A28 near Nunspeet). F: Plot of 206Pb/207Pb and reciprocal Pb concentrations of groundwater samples collected near two highways (A58 near Moergesteland A28 near Nunspeet). 1 = Petit et al. (1984); 2 = Krause et al. (1993); 3 = this study.

893N. Walraven et al. / Science of the Total Environment 472 (2014) 888–900

Impens et al., 1973; Fakayode and Olu-Owolabi, 2003). In Nunspeetand Moergestel, the soil Pb content also decreases rapidly withdistance from the highway (Fig. 4). However, since samples wereonly taken at two distances from the highway (approximately 8 mand 75 m), it was impossible to determine the exact spatial controlon the decrease.

Anthropogenic Pb in the forest litter, sampled in 1991, decreaseswith distance from the highway by 74% and 46% for Nunspeet andMoergestel respectively. The decrease in litter Pb content with distancefrom the highway in 2003 is comparable, 69% for Nunspeet and 41%

for Moergestel. The same pattern is observed for the Pb content in thetopsoil (0–10 cm).

4.3.2. Depth profilesPb contents in soils close to the highways (8–10 m) decrease with

depth (Fig. 5). There is no resolvable enrichment of Pb at depths greaterthan ~15 cm below soil surface. Fig. 5 demonstrates that high organicmatter content is associated with high Pb content. According to Nriagu(1978) significant elevation of Pb above the background level is notcommon below ±10 cm from the soil surface. Evidence for deeper

Table 1Pb isotope composition (206Pb/207Pb) of somemajorworldwide Pb ore deposits and the Pbisotope composition (206Pb/207Pb) of leaded gasoline used in The Netherlands andneighbouring countries.

Location Sample type 206Pb/207Pb Reference

Broken Hill Australia Pb ore (galena) 1.04 Facchetti (1989)Canada Pb ore (galena) 1.06 Hurst et al. (1996)Mexico Pb ore (galena) 1.19 Hurst et al. (1996)United States,Mississippi Valley

Pb ore (galena) 1.31 Hurst et al. (1996)

United States, Idaho Pb ore (galena) 1.06 Hurst et al. (1996)The Netherlands Leaded gasoline 1.062 ± 0.019 Hopper et al. (1991)Belgium Automobile exhaust 1.12–1.14 Petit et al. (1984)Germany Leaded gasoline 1.10–1.11 Krause et al. (1993)United Kingdom Leaded gasoline 1.06–1.07 Krause et al. (1993)Great Britain Leaded gasoline 1.059–1.079 Monna et al. (1997)France Leaded gasoline 1.069–1.094 Monna et al. (1997)

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penetration of gasoline Pb have been encountered in soils wherethe combined effect of low temperature and the presence of sodiumchloride diminish the ability of the topmost layer to retain Pb(Vandenabeele andWood, 1972). However, a low Pb content at greaterdepths (depth below 10–15 cm) does not necessarily mean that Pbis completely retained in the topsoil. If the soil pH and binding capacityof the soil at greater depths are extremely low, dissolved Pb simplymigrates to the groundwater without being absorbed. The bindingcapacity of a soil is predominantly determined by the amountof clay, calcium carbonate (CaCO3), organic matter (OM) and reac-tive iron (Fereact). The content of Fereact was calculated accordingto Fereact(wt.%) = (Fe2O3(wt.%) − (Al2O3(wt.%)/4) × 1.4297 (Huismanand Kiden, 1997). The soils deeper than 15 cm contain minimal amountsof these reactive phases (Fig. 5). In Nunspeet, CaCO3, OM and Fereactcontent below 15 cm are ≤0.1 wt.%, ≤0.6 wt.% and not present (n.p.)respectively. Comparable values are found in Moergestel, CaCO3,OM and Fereact ≤0.3 wt.%, ≤2.7 wt.% and n.p. respectively. Althoughthe clay content was not determined specifically, the low Al valuesand field observations suggest that clay contents are negligible (Appen-dix 1). In Dutch soils, Al content shows a positive correlation with claycontent (Huisman, 1998). However, in general soils with Al contentsbelow2.0 wt.% donot contain clayminerals andAl ismost often presentin feldspars (Huisman, 1998 and Mol et al., 2003).

A low soil pH in combination with a low binding capacity of thesoil could result in percolation of anthropogenic Pb towards thegroundwater. In addition, the soils close to the highway receivelarge amounts of sodium chloride, which is used to de-ice the high-ways during winter. Groot and van Swinderen (1993) showed thatthe chloride concentrations in groundwater close to the highways

Fig. 4. Sampling distance from the highway vs. Pb content (mg/kg)

(80–120 mg/l) are significantly higher than distal soils (15–30 mg/l).Since chloride ions can participate directly in anodic dissolution reac-tions of heavy metals (a.o. Pb and Zn), they are expected to enhancedownward transport of Pb in polluted soils (a.o. Vandenabeele andWood, 1972; Norrström and Jacks, 1998 and Lu, 2005).

4.3.3. Lead concentrations and Pb isotope composition of groundwaterBy determining Pb isotope composition and concentration of the

groundwater it is possible to verify if anthropogenic Pb reached thegroundwater. Lead concentrations in groundwater at the studied sitesvary from 0.22 to 15 μg/l (Table 2). The background concentration ofPb in shallow groundwater (b5 m g.l.) in The Netherlands is 6.2 μg/l(Fraters et al., 2001). With the exception of three samples (M2, M3and N5), Pb concentrations in groundwater from the studied sites arelower than theDutch background concentration for Pb (Fig. 3). However,based on the 206Pb/207Pb ratios of thegroundwater samples– analysed inthis study – it can be concluded that Pb in all groundwater samplesin this study has an anthropogenic origin. The 206Pb/207Pb ratios of thegroundwater samples (206Pb/207Pb = 1.135–1.176) are significantlylower than the 206Pb/207Pb composition of unpolluted soils in the studyarea (average (206Pb/207Pb)n values of the soil samples from thedepth interval of 45 to 55 and 90 to 100 cm-g.l., sampled in 2003are 206Pb/207Pb = 1.20–1.21). Table 2 demonstrates that there isno clear trend between the distance from the highway and the Pbisotope composition and concentration of the groundwater.

In Fig. 3 the Pb concentrations and the reciprocal of the Pb concen-trations are plotted versus the 206Pb/207Pb ratios of the groundwatersamples. These data show that in general the 206Pb/207Pb ratios becomemore radiogenic with decreasing Pb concentrations. In addition, a clearmixing hyperbola between two sources (natural Pb and anthropogenicPb) is observed. In Fig. 3 the y-axis intercept is 1.149 suggesting thatgasoline Pb is an important anthropogenic Pb source in groundwater(Fig. 3). However, 206Pb/207Pb ratios of groundwater vary between1.135 and 1.176 suggesting that natural Pb is also mobilized. Mol et al.(2003) showed that in acidic sandy soils, such as at Moergestel andNunspeet, feldspars tend to dissolve and release Pb to groundwater.

These data can be used to determine the contribution of anthropo-genic Pb to the total groundwater Pb. The average 206Pb/207Pb ratio ofbackground Pb forNunspeet andMoergestel is 1.21 and 1.20 respectively(Section 4.2). The average 206Pb/207Pb ratio of anthropogenic Pb in soilsfromNunspeet andMoergestel is 1.122 and 1.139 respectively (Fig. 3) in-dicating that 30–70% and 40–100% of groundwater Pb in Nunspeet andMoergestel respectively, originates from anthropogenic Pb.

4.3.4. Changes in soil Pb between 1991 and 2003Variation in Pb content and 206Pb/207Pb ratios of the roadside soils

between 1991 and 2003 are visualized in Fig. 6. The average Pb content

of the litter and topsoils (0–10 cm) sampled in 1991 and 2003.

Fig. 5. A: Pb content (mg/kg), 206Pb/207Pb ratio and content of OM (wt.%), Fereact (wt.%), and CaCO3 (wt.%) for the litter and soil samples, collected from the soilpit 8 m from the highwaynear Nunspeet, are plotted vs. sampling depth (cm-g.l.). The error bars indicate the depth interval at which the samples are taken. B: Pb content (mg/kg), 206Pb/207Pb ratio and contentof OM (wt.%), Fereact (wt.%), and CaCO3 (wt.%) for the litter and soil samples, collected from the soilpit 10 m from the highway near Moergestel, are plotted vs. sampling depth (cm-g.l.).The error bars indicate the depth interval at which the samples are taken.

895N. Walraven et al. / Science of the Total Environment 472 (2014) 888–900

in roadside litter and soil near Nunspeet were higher in 2003 than in1991. In contrast, the average Pb content in roadside litter and soilnear Moergestel were lower in 2003 than in 1991. However, these

Table 2Acidity (pH), Pb content and Pb isotope composition (206Pb/207Pb) of roadsidegroundwater samples (#1 Average relative error ± 1RSD: 206Pb/207Pb = 0.28 ± 0.13%.This error is higher than observed with the Pb isotope measurements of the soilsamples, because Pb concentrations of the groundwater samples were lower than50 μg/l (increased mass fractionation).

Location Distance fromhighway (m)

Groundwaterdepth (cm-g.l.)

pH Pb (μg/l) 206Pb/207Pb#1

N 1 8 282 4.56 0.65 1.158N 2 8 302 4.53 0.30 1.167N 3 8 323 4.42 0.87 1.156N 4 8 323 4.45 0.75 1.168N 5 80 367 4.76 10 1.148N 6 80 304 4.56 0.76 1.161N 7 80 294 4.51 0.22 1.185N 8 80 293 4.57 0.49 1.174M 1 8 258 4.52 1.23 1.160M 2 8 275 4.08 8.58 1.135M 3 8 288 3.97 15 1.143M 4 8 269 5.26 0.93 1.155M 5 80 220 3.86 2.10 1.176M 6 80 227 5.26 0.23 1.172M 7 80 250 4.35 1.62 1.153M 8 80 221 5.29 1.14 1.145

differences in Pb content are not significant because of variability. Inboth locations there are, however, significant changes in the averagePb isotope compositions in most roadside litter and soil samplesbetween 1991 and 2003. These data imply that there is transport ofanthropogenic Pb in the studied soils (Fig. 6). The only exception isthe Nunspeet litter sampled 8 m from the highway, which shows nosignificant change in the 206Pb/207Pb ratios between 1991 and 2003.The 206Pb/207Pb ratios of roadside litter from Moergestel increasedsignificantly between 1991 and 2003, whereas the 206Pb/207Pb ratiosof litter 75 m from the highway decreased. The average 206Pb/207Pbratios of the topsoils fromMoergestel in 1991 and 2003 are within ana-lytical error. The changes in 206Pb/207Pb ratios between 1991 and 2003appear to be caused by a temporal change in the isotopic compositionof anthropogenic Pb. If the atmospheric Pb flux between 1991 and2003 and its Pb isotope composition are known, it is possible to calculate1) the downward Pb flux from the litter layer into the topsoil (0–10 cm)and 2) the Pb isotope composition of the mobile Pb fraction.

4.4. Fluxes of Pb

The downward Pb flux can be calculated from the changes in Pbinventories using the following mass-balance equation:

Fdown L→T ¼ Fatmos− CL−2003 � ρ2003 � z2003 � 10ð Þ− CL−1991 � ρ1991 � z1991 � 10ð Þð Þ=Δtð5Þ

Fig. 6. Sampling date (1991 and 2003) vs. Pb content (mg/kg) and 206Pb/207Pb ratios of the litter and topsoils (0–10 cm). Error bars indicate the variation in Pb content and 206Pb/207Pbratios of the replicates. A and B = Nunspeet; C and D = Moergestel; 1 = Petit et al. (1984); 2 = Krause et al. (1993); 3 = this study.

896 N. Walraven et al. / Science of the Total Environment 472 (2014) 888–900

Where:

CL = Pb content (mg/kg) of the litter layer in 1991 and 2003.ρ = bulk density (g/cm3) of the litter in 1991 and 2003.z = thickness (cm) of the litter layer in 1991 and 2003.Fatmos = average atmospheric Pbflux (mg m−2 y−1) between1991and 2003.Fdown L → T = average downward Pb flux (mg m−2 y−1) fromthe litter layer towards the topsoil (0–10 cm) between 1991 and2003.

Eq. (5) is based on the assumption that water movement throughthe soil profile was largely vertical and bulk density and layer thicknessare assumed constant (ρ1991 = ρ2003; z1991 = z2003). The latterassumptions had to be made since there are no data regarding thedensity and exact thickness of the litter layer and topsoil sampled in1991. If Fdown L → T is known, it is also possible to calculate the down-ward Pb flux from the topsoil (0–10 cm) to the subsoil (N10 cm),expressed as Fdown T → S, by replacing Fatmos with Fdown L → T in Eq. (5)and using the Pb content, bulk density and thickness of the topsoil inplace of the litter layer.

The results of the downward Pb fluxes can be validated by calculatingthe 206Pb/207Pb ratios of themobile Pb fraction according to Eq. (6). If the

calculated 206Pb/207Pb ratios of the mobile Pb fractions agree with themeasured 206Pb/207Pb ratios of the soils and groundwater samples(206Pb/207Pb =1.11–1.21) this means that the calculated Pb fluxes areof the correct order of magnitude.

206Pb=207Pb� �

Fdown L→T¼ ð 206Pb=207Pb

� �Fatmos

� Fatmos−ðð 206Pb=207Pb� �

Cl−2003

�CL−2003 � ρ2003 � z2003 � 10Þ−ð 206Pb=207Pb� �

Cl−1991

�CL−1991 � ρ1991 � z1991 � 10ÞÞ=ΔtÞ=Fdown L→T

ð6Þ

Where:

(206Pb/207Pb)Cl = 206Pb/207Pb ratio of the litter layer in 1991 and2003.(206Pb/207Pb)Fatmos = average 206Pb/207Pb ratio of atmospheric Pbbetween 1991 and 2003.(206Pb/207Pb)Fdown L → T = average 206Pb/207Pb ratio of Pb movingfrom the litter layer towards the topsoil (0–10 cm) between 1991and 2003.

The Pb isotope composition of themobile fraction in the topsoil wascalculated in an analogous way. The calculated downward Pb fluxes aresomewhat uncertain, since atmospheric Pb deposition, Pb content,

897N. Walraven et al. / Science of the Total Environment 472 (2014) 888–900

thickness and density of the litter and topsoil layers are subject to tem-poral and spatial variation. Estimations of the uncertainties were calcu-lated using the Gaussian law for propagation of error, taking all errors(as 1 SD) of the quantified variables in Eqs. (5) and (6) into account.

4.4.1. Atmospheric Pb deposition and its Pb isotope composition(1991–2003)

Downward Pb transport can only be calculated if atmospheric Pb de-position between 1991 and 2003 is known (Eq. (5)). Data on atmo-spheric Pb deposition in The Netherlands and its isotopic compositionare summarised in Table 3 and include lake deposits, snail shells andaerosols as proxies of atmospheric Pb deposition (Walraven et al.,submitted for publication; Notten et al., 2008; Rosman et al., 1998).Clearly some of these data will include input from natural Pb de-rived from the underlying sediments. Walraven et al. (submittedfor publication) reported atmospheric deposition based on thechemical composition of lake sediments from Lake Vechten, whichis located only 75 m from a national highway (A12). AtmosphericPb deposition in the roadside soils near Nunspeet and Moergestelis assumed to be similar to that deposited in Lake Vechten. The atmo-spheric Pb deposition between 1991 and 2002 in Lake Vechten variesfrom 121 to 173 mg m−2 y−1 with an average of 142 mg m−2 y−1.These values are in the same order of magnitude as determined byothers in peat and lake sediments (Table 3). However, atmospheric Pbdeposition in The Netherlands is higher than in most other westernEuropean countries (2–50 mg m−2 y−1), except for Lake Geneva(Table 3). The Netherlands is a more densely populated countrycompared to the other countries and the studied lake sediments inThe Netherlands are from an urban area (city of Utrecht) and hencePb deposition is predicted to be among the highest recorded in Europe.

Table 3Atmospheric Pb flux (mg m−2 y−1) and 206Pb/207Pb of atmospheric Pb between 1991 and 200

Location Samplingmedium

Y

The Netherlands and neighbouring countriesThe Netherlands (Utrecht) Lake sediments 1

The Netherlands (Petrusplaat, Lelystad) Aerosols 2Rainwater 2

The Netherlands (Groningen, Emmen) Aerosols 1Belgium (Kempen) Peat bog sediments 1Germany (Mainz, Nurnberg) Aerosols 1Germany (Constance) Aerosols 1Germany (Plön) Aerosols 1

Other Western European countriesUnited Kingdom, South Drumboy Hill Ombotrophic peat 1United Kingdom, Oxford Aerosols 1United Kingdom, Loch Lomond (Scotland) Lake sediments 1United Kingdom, Perth (Scotland) Aerosols 1United Kingdom (Scotland) Moss 1

2United Kingdom (Scotland) Rainwater (open gauge) 1

Rainwater (filter gauge) 1Ireland (Dublin) Aerosols 1Switzerland (5 peat bog lakes) Peat 1Switzerland (Lake Constance) Lake sediment 1Switzerland–France (Lake Geneva) Lake sediment N

Chateau of Versailles, 15 km from Paris, France Soil 112

France (Montpellier, Grenoble) Aerosols 1France (Mont Blanc) Ice 1France (Lille) Aerosols 1France (Grenoble) Aerosols 1France (Avignon) Aerosols 1Straits of Dover Aerosols from North Western Europe 1Straits of Dover Aerosols from Western Europe 1Straits of Dover Aerosols from Eastern Europe 1

The 206Pb/207Pb ratios of anthropogenic Pb, deposited between 1991and 2003 in Lake Vechten, vary from 1.162 to 1.169 with an average of1.166 (Table 3). These values are in agreement with the data fromNotten et al. (2008) (206Pb/207Pb = 1.151–1.172), but deviate fromthe values of Rosman et al. (1998)(206Pb/207Pb = 1.133–1.137). Thevalues of Walraven et al. (submitted for publication) and Notten et al.(2008) also deviate from the Pb isotope composition of aerosols fromNorth Western Europe (206Pb/207Pb = 1.145 ± 0.003) as determinedby Deboudt et al. (1999). However, the database reported by Deboudtet al. (1999) includes nodata fromBelgiumand TheNetherlands. Signif-icant spatial and temporal variations in the Pb isotope composition ofaerosols are observed in Europe (Table 3). In France and Switzerland,for example, the 206Pb/207Pb ratios varied from 1.110 to 1.162 and1.114 to 1.155 respectively over the last 10 years (Table 3). These differ-ences are a.o. caused by seasonal variations in the origin of aerosols andlocal factors, such as the presence of Pb working industries and theorigin of the Pb that is being processed. In this study the atmosphericPb flux and 206Pb/207Pb ratios of atmospheric Pb deposition based onthe lake sediments from Lake Vechten are used to calculate Pb fluxesfrom Dutch topsoil.

4.4.2. Pb fluxes in the roadside soilsThe calculated values for Fdown L → T, (206Pb/207Pb)Fdown L → T, Fdown

T → S and (206Pb/207Pb)Fdown T → S and uncertainties (2σ) for Nunspeetand Moergestel are presented in Table 4.

4.4.2.1. Pb flux from litter to topsoil to subsoil. The downward flux of Pbfrom the litter layer to the topsoil (Fdown L → T) varies from 117 ± 70to 261 ± 211 mg m−2 y−1. The downward flux of Pb from thetopsoil to the subsoil (Fdown T → S) – consisting of fine sand witha negligible amount of reactive phases – varies from 72 ± 95 to

3 in The Netherlands and other western European countries.

ear Pb (mg m−2 y−1)(n)

206Pb/207Pb(n)

Reference

991–2002 121–173 (10) 1.162–1.169 (10) Walraven et al.(submitted for publication)

002–2003 – 1.155–1.172 (3) Notten et al. (2008)003 1.151–1.152 (2)994 – 1.133–1.137 (2) Rosman et al. (1998)991–1995 446–563 (2) 1.130–1.132 (2) Sonke et al. (2002)994 – 1.133–1.145 (3) Rosman et al. (1998)997–1998 – 1.138–1.148 (6) Bollhöfer and Rosman (2002)997–1998 – 1.147–1.165 (6) Bollhöfer and Rosman (2002)

990 ±8 (1) ±1.13 (1) MacKenzie et al. (1998)997–1998 – 1.118–1.123 (6) Bollhöfer and Rosman (2002)995 6–50 1.125–1.145 Eades et al. (2002)994 – 1.101 (1) Rosman et al. (1998)990–1999 – 1.125–1.163 (16) Farmer et al. (2002)000 1.137–1.165 (7)997–1998 – 1.112–1.121 (68) Farmer et al. (2000)997–1998 1.118–1.161 (30)994 – 1.101 Rosman et al. (1998)990–1993 4.5–37 (3) 1.131–1.153 (3) Weiss et al. (1999)994 12 (1) 1.114 (1) Kober et al. (1999)1991 25–150 1.13–1.155 Monna et al. (1999)991 29 ± 14 (1) – Semlali et al. (2004)994 19 ± 12 (1)002 2.2 (1)994 – 1.130–1.142 (3) Rosman et al. (1998)990–1991 – 1.155–1.162 (3) Rosman et al. (2000)995 – 1.158 Deboudt et al. (1999)996–1998 – 1.110–1.134 (10) Bollhöfer and Rosman (2002)997–1998 – 1.124–1.134 (6) Bollhöfer and Rosman (2002)995 – 1.145 + −0.003 Deboudt et al. (1999)995 - 1.111–1.142 Deboudt et al. (1999)995 – 1.145–1.169 Deboudt et al. (1999)

Table 4Pb fluxes (mg m−2 y−1) through the soil and 206Pb/207Pb ratios of the migrating Pb fraction.

Location Distance from highway (m) Pb flux from litter to topsoil Pb flux from topsoil to subsoil (mg m−2 y−1)

Fdown L → T (mg m−2 y−1) (206Pb/207Pb)Fdown L → T Fdown T→S (mg m−2 y−1) (206Pb/207Pb)Fdown L → T

Nunspeet 8 117 ± 70 ≈1.16 73 ± 58 ≈1.21Nunspeet 74 133 ± 114 ≈1.17 72 ± 95 ≈1.19Moergestel 10 261 ± 211 ≈1.10 324 ± 279 ≈1.10Moergestel 75 149 ± 120 ≈1.19 174 ± 159 ≈1.18

898 N. Walraven et al. / Science of the Total Environment 472 (2014) 888–900

324 ± 279 mg m−2 y−1. Fdown L → T and Fdown T → S near Moergestelare higher close (10 m) to the highways than further away (75 m).This is most likely caused by the larger amount of anthropogenic Pbpresent in the soils close to the highways (Fig. 4). In addition, the useof sodium chloride to de-ice the highways duringwinter will contributeto the higher downward flux close to the highways. The nature indownward Pb transport differs at Nunspeet and Moergestel in that thedownward Pb movement near Nunspeet is higher from the litter layertowards the topsoil than from the topsoil to the subsoil, whereas nearMoergestel the opposite relationship is observed. The reason for thisvariation is unknown.

The calculated 206Pb/207Pb ratios of themobile Pb fraction vary from1.10 to 1.21. These values agreewith the valuesmeasured in the soil andgroundwater samples (206Pb/207Pb = 1.11–1.21), which establishesthat the calculated Pb fluxes are of the right order of magnitude.Although the 206Pb/207Pb ratio measured close to the highway nearMoergestel is low (206Pb/207Pb = 1.10), the value falls within therange of Pb isotope compositions of gasoline Pb known to have beenused in The Netherlands and neighbouring countries (Table 1). The cal-culated 206Pb/207Pb ratios indicate that both anthropogenic and naturalPb is transported downwards, which is confirmed by the 206Pb/207Pbratios in the groundwater (Table 2). We conclude that the low pH ofthe soils caused Pb mobilization of both anthropogenic and natural Pb,the latter most likely released due to dissolution of feldspars (Molet al., 2003). Due to the large errors in the flux calculations it is impos-sible to establish if the isotopic composition of the mobile Pb fractionclose and distal to the highway is identical.

4.4.2.2. Pb transport towards groundwater.At depths N15 cm g.l., Pb is nolonger retained due to the low pHand lowbinding capacity of the sandysoils. This means that most Pb that migrated through the topsoil, even-tually ends up in the groundwater. It is possible to estimate the totalamount of anthropogenic Pb thatmigrated to the groundwater between1962/1963 (construction date of the highways) and 2003, assumingthat Fdown T → −S was constant during this time. The total amount ofPb that migrated towards the groundwater at the studied sites variesfrom 2.95 to 12.96 g/m2 (Table 5). By comparing the sum of migratedPb and anthropogenic Pb still present in the soils with the total amountof anthropogenic Pb present in the lake sediment of Lake Vechten, itis possible to check if the downward Pb flux was constant over time,assuming that 1) Pb in the lake sediments is immobile and 2) anthropo-genic Pb deposition in Lake Vechten was similar to that at the roadsidesoil locations. Since the lake is located about 75 m from a national high-way, the comparison is best made with the sampling sites 75 m from

Table 5Anthropogenic Pb (in g/m2)migrated towards groundwater and still present in the soil/sedimen

Location Distance fromhighway (m)

Pb migrated towardsgroundwater (g/m2)

Lake Vechten 75 –

Nunspeet 8 2.99Nunspeet 74 2.95Moergestel 10 12.96Moergestel 75 6.96

the national highway near Moergestel and Nunspeet (Fig. 4). Between1960 and 2003, a total of 12.65 g/m2 anthropogenic Pb was depositedatmospherically in the lake (Table 5). The total amount of anthropogen-ic Pb (still present andmigrated to groundwater) that entered the soil ata distance of 75 m from the highway nearMoergestel (15.05 g/m2) is ingeneral agreement with the amount deposited in Lake Vechten duringthe same time period (12.65 g/m2). However, this is not the case forNunspeet (74 m from the highway). The total amount of Pb migratedtowards the groundwater and still present in the soil near Nunspeet(4.38 g/m2) is lower than deposited in Lake Vechten. Although thetraffic density is ~ a factor of 2 lower near Nunspeet (~50,000 cars/day:Fig. 1) than near Lake Vechten (~90,000 cars/day: based on data from2000 to 2003), this difference is not sufficiently large to explain theobserved difference. It is most likely that the downward transport ofanthropogenic Pb before 1991 was considerably higher between 1991and 2003 due to the higher input of gasoline Pb before 1991 (Walravenet al., submitted for publication) and the higher mobility of gasoline Pbthat has just (freshly) entered the soil compared to aged gasoline Pb(Laxen and Harrison, 1997; and references therein). By comparing thetotal amount of anthropogenic Pb still present in the roadside soils withthe total amount of anthropogenic Pb deposited in Lake Vechten, it isestimated that about 90% of the deposited Pb in the studied soils nearNunspeet, ended up in the groundwater compared to 35% in Moergestel.This difference can be explained by the difference in soil thickness, organ-ic matter content and stability of the litter layer for the two locations. Thelitter layer near Nunspeet is thinner and contains less organicmatter thanat Moergestel. In addition, a net breakdown in litter between 1991 and2003 is observed near Nunpseet. A decrease of 40–90% in the Pb burdenof soils is high. Friedland et al. (1992) and Miller and Friedland (1994),for example, found a decrease of the Pb burden in forest floors at severalsites in eastern North America in the order of 20–40%. Zhang (2003)calculated that about 56% of the atmospherically derived Pb hasmigratedinto the lower A horizon of agricultural soils in central Illinois in 50 years.Marsh and Siccama (1997) reported that about 65% of atmospherically-derived Pb on long abandoned, ploughed, soils in New England hadmigrated into mineral soils as of 1990.

This study demonstrates that anthropogenic Pb in the studied road-side soils ismobile. In addition, thework establishes that the groundwa-ter is already pollutedwith anthropogenic Pb. However, it remains to beestablished why Pb fluxes in the studied soils are so high compared tosoils studied by others. Lead fluxes in the studied soils might be highdue to the low soil pH, negligible calcium carbonate and reactive ironcontent, poor soil development and thin litter thickness compared tosoils in the other studies. It would be worthwhile to determine and

t. Anthropogenic Pb (%) still present in roadside soils compared to data from Lake Vechten.

Total anthropogenicPb still present (g/m2)

Anthropogenic Pb stillpresent compared to Lake Vechten (%)

12.65 100%3.55 –

1.43 ±10%18.43 –

8.09 ±65%

899N. Walraven et al. / Science of the Total Environment 472 (2014) 888–900

quantify the factors that determine fluxes of anthropogenic Pb in soilsin more detail.

5. Conclusions

Sandy roadside soils near Nunspeet and Moergestel (NL) arepolluted with Pb. The highest Pb content (up to 652 mg/kg) hasbeen measured in the litter layer. The Pb isotope composition ofthe polluted top soils (206Pb/207Pb = 1.12–1.14) differs from thedeeper soils (206Pb/207Pb = 1.20–1.21). Based on the characteristic206Pb/207Pb ratios of the polluted soils it was concluded that gaso-line Pb was the dominant anthropogenic source responsible for theelevated Pb content in the roadside soils.

Since the Pb isotope composition of the anthropogenic componentdiffers from natural background Pb, the behaviour and fate of theanthropogenic Pb in the sandy roadside soils could be calculated. Themain findings were:

1. Only the litter layer and the topsoil retain anthropogenic Pb. Anthro-pogenic Pb is strongly associated with organic matter in the topsoil.The reason for the absence of anthropogenic Pb at depths N15 cmis the low soil pH (pH-H20 = 4.3–5.0) and the lack of reactive phases(e.g. organic matter, clay, reactive iron and calcium carbonate), towhich anthropogenic Pb can bind.

2. Pb isotope measurements of the groundwater (206Pb/207Pb = 1.135–1.176) prove the presence of anthropogenic Pb. Due to the absenceof reactive phases in the deeper soils and the low soil pH, anthropo-genic Pb migrated rapidly towards the groundwater. The contribu-tion of anthropogenic Pb to groundwater Pb concentrations variesbetween ~30 and 100%.

3. Downward Pb transport was calculated to vary from 72 ± 95 to324 ± 279 mg m−2 y−1, depending on distance from the highwayand type of soil horizon.

4. Based on estimates of total atmospheric Pb deposition from 1962/1963 to 2003, it was calculated that 35–90% of the atmosphericallyderived Pb migrated to the groundwater.

5. This study supports the recent findings of others that anthropogenicPb can be highly mobile, resulting in groundwater pollution.

Supplementary data to this article can be found online at http://dx.doi.org/10.1016/j.scitotenv.2013.11.110.

Conflict of interests

All authors hereby state that there are no actual or potential conflictsof interest including any financial, personal or other relationships withother people or organizations (within three years of beginning thesubmitted work) that could inappropriately influence, or be perceivedto influence, their work.

Acknowledgements

This workwas supported financially by TNO and Deltares.We thankJaap Willemse and Hans Reinders of the Dutch National Institute ofPublic Health and the Environment (RIVM) for collaborating in thisresearch (providing the roadside samples from1991). Erik vanVilsterenand Rob van Galen are acknowledged for performing the chemical andisotopic analysis. Special thanks toMenno van der Heiden for providingthe map. We thank the four anonymous reviewers for their thoroughand constructive reviews of the manuscript.

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