Distribution of organic pollutants and natural organic matter in urban storm water sediments as a function of grain size

SC IE N CE OF T H E TOT AL E N V I RO N ME N T 4 0 3 ( 2 00 8 ) 1 7 8–1 87

a v a i l a b l e a t w w w. s c i e n c e d i r e c t . c o m

w w w. e l s e v i e r. c o m / l o c a t e / s c i t o t e n v

Distribution of organic pollutants and natural organic matter in urban storm water sediments as a function of grain size Anne-Laure Badin a,⁎, Pierre Faure b , Jean-Philippe Bedell a , Cécile Delolme a a

Université de Lyon, Lyon, F-69003, France; ENTPE, Vaulx en Velin, F-69518, France; Laboratoire des Sciences de l'Environnement, Vaulx en Velin, F-69518, France b G2R UMR 7566, Nancy-Université, CNRS, BP 239, 54506, Vandoeuvre-les-Nancy, France

AR TIC LE I N FO

ABS TR ACT

Article history:

The sealing of surfaces in urban areas makes storm water management compulsory.

Received 17 September 2007

Contaminated particles carried from urban surfaces are deposited in infiltration ponds. This

Received in revised form 5 May 2008

gives rise to a highly organic (11% DW) contaminated sedimentary layer (Zn:1.2 mg/g,

Accepted 17 May 2008

Cd:15 mg/kg) that could threaten groundwater quality.

Available online 24 June 2008

During infiltration, particle arrangement impacts infiltrating water and sediment exchanges. In this context, understanding particle arrangement and leachable

Keywords:

components is essential. This study investigates Organic Matter (OM) not only as a

Aggregation

pollutant but also as a substrate and a structuring element. The leachable fraction was

DCM extraction

collected and grain size fractionation was performed. OM of sediments and isolated

Hydrocarbons

fractions were characterized by measuring organic carbon content, isolating aromatic

Leaching

hydrocarbons, saturated hydrocarbons and polar compounds after dichloromethane

PAH

extraction, and by gas chromatography–mass spectrometry (GC–MS) molecular analyses.

Soil structure

The organic compounds observed were petroleum byproducts (steranes and terpanes,

Storm water sediment

unresolved complex mixture (UCM) and polycyclic aromatic hydrocarbons (PAH)), but plant and bacteria biomarkers were also found (phytol and derivatives, sterols). Leachable OM consisted of 6% of sediment OM (associated with particles N 0.45 µm). This leachable OM is easily extractable by dichloromethane (96%) and contains fewer macromolecules than other fractions. Isolated grain size fractions showed dissimilarities (total organic carbon from 3.5 mg/g to 88.6 mg/g, extraction rate from 24 to 96%, aromatic hydrocarbon distribution) and similarities (proportions of aromatic and saturated hydrocarbons and polar compounds, molecular distribution of saturated hydrocarbons and polar compounds). The results suggest that organic macromolecules take part in the aggregation of sediments and prevent fine particles (b 10 µm) from being leached. On the other hand, leachable particles (20 µm grain size mode) could carry low molecular weight organic molecules. The physical structure of the sediments and the leaching of particles containing contaminants are considerably affected by the presence of OM. © 2008 Elsevier B.V. All rights reserved.

1.

Introduction

Surface sealing in urban areas is an important aspect of storm water management. In areas with highly permeable soils such

as those found in alluvial plains, the storm waters collected can be allowed to infiltrate into the groundwater. In east Lyon, France, this practice has been implemented for decades in the form of infiltration ponds. However, studies of storm water

⁎ Corresponding author. LSE, ENTPE, 2 rue Maurice Audin, 69518 Vaulx en Velin cedex, France. Tel.: +33 472047057; fax: +33 472047743. E-mail addresses: [email protected], [email protected] (A.-L. Badin). 0048-9697/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.scitotenv.2008.05.022

S CIE N CE OF T H E TOT AL E N V I RO N ME N T 4 0 3 ( 2 00 8 ) 1 7 8–1 87

quality (Legret and Pagotto, 1999; Göbel et al., 2006; Göbel et al., 2007) have raised questions about this practice. When storm water runs off roads and buildings, it can carry particles commonly called ‘road sediments’ or ‘road dusts’ deposited on surfaces. They consist of a wide range of materials with both natural and anthropogenic sources. In the infiltration pond system, storm water loaded with particles settles on the bottom, giving rise to a sedimentary layer. This layer is highly contaminated with Zn, Pb, Cu, Ni, Cd, Cr, hydrocarbons and PAH (polycyclic aromatic hydrocarbons) (Pitt et al., 1999; Dechesne et al., 2004; Durand et al., 2005; Clozel et al., 2006). Moreover, it contains large amounts of organic matter (from 15 to 22% dry weight, Durand et al., 2005). As a consequence, it could be a potential source of contamination for groundwater (Pitt et al., 1999; Datry et al., 2003, 2004). Quantities of DOC (dissolved organic carbon) have been shown to reach groundwater under storm water infiltration ponds, whereas no heavy metals nor hydrocarbons have been measured (Datry et al., 2004). Thus it is necessary to study the potential mobility of organic matter (OM) in these deposited sediments. This mobility is largely dependant on water and sediment exchange. Water-soil exchanges are largely dependent on structure, i.e. the assembly of particles into aggregates and pores (Duchaufour, 1994). The water retention properties of porous sediments are greatly influenced by porosity which depends on the matrix structure. Consequently, this has an impact on the kinetics of chemical and microbiological reactions in the soil. The sedimentary layer of stormwater infiltration ponds has specific characteristics: on the one hand it results from the deposition of transported particles; on the other hand, it is subject to dry and wet cycles and sustains plant growth. As we are interested in structure, part of our bibliographic study calls on soil science, despite the fact that the sediment layer cannot be described as a soil. Regarding contamination (soil and sediments), much use has been made of textural approaches (solid particles dispersion before fractionation) (Ducaroir and Lamy, 1995; Stone and Droppo, 1996; Manno et al., 2006). Soil particle structure i.e. aggregation, has often been ignored in this context, but the topic of aggregation has been dealt with extensively by soil sciences. Organic matter has been shown to be involved in aggregate constitution (Tisdall and Oades, 1982) and thus contributes to the formation of soil structure. OM influences soil structure (particles layout in soil) which in turn influences chemical and biological reactivities. For example, aggregation has been shown to influence carbon mineralization in soil (Oades, 1988; Baldock and Skjemstad, 2000; Six et al., 2004; Yoo et al., 2006). For example, if linked with clay, OM is less accessible to microorganisms and less degradable. Indeed, in soil, molecules of high molecular weight (highly hydrophobic) tend to associate with soil particles (Duchaufour, 1994). Among approaches used to study soil structure, Hattori (1988) stated that structured soils are composed of two parts: the inner part related to microporosity (mostly in aggregate form), and the outer part related to macroporosity (surface aggregates, i.e. pores from 2.5 to 6 µm). He defined these parts as two different microhabitats for microorganisms. During water flow, the particles and solutes (as soluble pollutants) surrounding the macropores (preferential flow pores) can be removed and

179

transported to the subsoil. As Ranjard et al. (1997), have shown, this approach can be used to describe the relationship between bacteria, contaminants and soil constituents. Besides biological approaches, physicochemical characterizations can be performed for the inner and outer parts to improve knowledge of sediment composition and structure. As the outer part results from macroporosity, it could be assumed that it is the most leachable, or leachable part of the sediment. What is more it can provide information about potential contaminant mobility. Heavy metal mobility in storm water sediments has been the subject of many studies (columns and sequential extractions (Clozel et al., 2006)) though the mobility of organic matter has as yet been given little attention. Traditionally, in soil sciences, OM extraction is performed on the basis of the solubility of humic matter in dilute alkaline solution. In petroleum organic geochemistry, organic solvents are used to extract organic matter while organic contamination studies use both techniques. Studies of storm water OM of whole sediments have already been carried out by using the humic–fulvic approach formulated by Durand et al. (2005). Also, the technical competences of petroleum organic geochemistry have been used to study environmental organic contaminants in the environment (Faure et al., 2000; Faure et al., 2004; Jeanneau et al., 2006). In this paper we focus on the relation between the structure and organic matter of sediments deposited on storm water infiltration ponds, and the organic content of urban storm water sediments, its distribution and its mobility. The main aim of this research is to consider the link between storm water sediment structure and organic matter. It can be divided into two parts: (i) the characterization of how OM spreads into sediments (grain size fractionation); and (ii) the estimation and characterization of the amount of OM that can be leached by a solution representing storm water (the leachable part). OM was studied using bulk (total organic carbon: TOC) and molecular parameters (liquid chromatography and GC–MS analysis).

2.

Materials and methods

2.1.

Sampling and sample conservation

The pond studied is located at Chassieu, NE of Lyon, France in an urban area. The 1 ha infiltration pond receives storm waters from an urban and industrialized watershed of 185 ha. The pond is divided into two parts: the first, impervious, in which larger particles settle, and a second, where the infiltration of water occurs. Despite the settling pond, a large amount of suspended matter is deposited on the surface of the infiltration pond. Infiltration occurs through a carbonate fluvioglacial deposit. The most recent dredging was done two and half years ago. The studied materials were collected in the infiltration part of the pond and sampling was performed at the beginning of May 2006. The infiltration pond surface was not totally flat and water remained at low points. The sampling area chosen was 2 m away from the edge of remaining water. No plants were growing in the sampling area chosen. To prevent high spatial variability of the chemical characteristics of the surface layer, the decision was taken to sample a 2 m2 area on the surface of the infiltration pond.

180

SC IE N CE OF T H E TOT AL E N V I RO N ME N T 4 0 3 ( 2 00 8 ) 1 7 8–1 87

The surface layer was sampled (about the first five cm). The boundary between the sediment layer and the natural carbonate subsoil was evaluated visually (color and texture). Sampling was done with a clean shovel. Five samples of 1 kg each were taken from the 2 m2 area. They were kept at 4 °C for 5 weeks before fractionation experiments, aliquots were frozen at −18 °C. Sampling and conservation were performed in glass bottles.

2.2.

Fractionation procedure

The leachable part of sediments were recovered, it fits the fraction that could be mobilized with water. The remaining sediments called inner part were subdivided into consistent particle size fractions (adapted from Ranjard et al., 1997) (Fig. 1). Fraction threshold cuts were determined by a preliminary study of sediment particle size distribution. Five fractions were chosen for segregation: N1000, 160–1000, 10–160, b10 µm and the leachable part. To perform this fractionation, 200 mL of NaCl 0.8% solution (in ultrapure water) added to 40 g of equivalent dried sediment in a beaker. Care was taken to avoid serious disturbance of sediment structure. The sediments were shaken (100 r/min) for 1 min and then kept still to settle for 1 min after which the supernatant was removed. This was repeated 15 times and the supernatants were pooled (the leachable fraction of sediments). The inner part, consisting of the remaining sediments, was fractionated by wet sieving (1000 and 160 µm) and sedimentation (10–160 and b10 µm). Once the repeatability of this fractionation was checked for the 5 sediment samples (with regard to weight assessment and grain size analyses), fractionation was performed on only one of the 5 samples. It was performed on the sample chosen kept at 4 °C and also on the same sample kept at − 18 °C. This was intended to avoid both artifacts due to freezing effects on structure and artifacts due to changes on OM at 4 °C for 5 weeks. The reproducibility of the fractionation was checked by a laser diffraction grain size analyzer and by weight assessment of the 5 sediment samples.

2.3.

Grain size analyses

Grain size analyses were performed in a Malvern Mastersizer 2000G laser diffraction grain size analyzer for the non fractionated sediments and for the fractions, except the N1000 µm fraction (too large). Prior to analysis, the Non Fractionated (NF) sediment sample was sieved at 1000 µm. Two series of three measurements were performed on every sample analyzed. The 1st series was performed for a gently stirred sample. Prior to the 2nd series, ultrasounds were applied for 1 min to the sample to eliminate the aggregated part. The grain size analyzer used light diffraction to estimate the solid volume represented by each of its constitutive particle grain sizes (particles were assimilated with spheres).

2.4.

Total Organic Carbon analyses

Total Organic Carbon (TOC) analyses were performed over 3 out of 5 sediment samples. The internal variability of these samples was checked by analyzing one of them in triplicate. TOC was analyzed in each non-extracted fraction (raw fraction) and previously extracted fraction (extraction residues) with dichloromethane (DCM extraction presented below). Raw sediment samples, larger fractions (N1000 µm and 160– 1000 µm) and extraction residues were solids, whereas finer fractions (b10 µm, 10–160 µm and leachate) were suspensions. The latter were filtered (0.45 µm membrane filters, type HA Millipore) and filter cakes and filtrates were recovered. The solid samples and filter cakes were dried and finely ground in a clean mortar. For the solid fractions, TOC was measured in 50 mg of sample and H3PO4 5% was added to remove non organic C. The sample was dried for one night. Following catalytic combustion, an analysis was performed using a non dispersive infrared cell according to the NF ISO 10694 standard. Dissolved Carbon Organic (DOC) was measured in filtrates and H3PO4 5% was added to remove non organic C.

Fig. 1 – Recovery of the leachable part (supernatant collection after 1 min agitation followed by 1 min settling, repeated 15 times) and fractionation of the storm water sediments into grain size fractions (wet sieving and sedimentation). On the left: the experimental design, on the right the conceptual scheme.

S CIE N CE OF T H E TOT AL E N V I RO N ME N T 4 0 3 ( 2 00 8 ) 1 7 8–1 87

181

The TOC of suspended fractions was obtained by combining the organic C contents of mixed filtrates and filter cakes.

2.5.

DCM extraction of OM

All the materials used for isolation and handling were carefully cleaned: once with soapy water, once with ethanol and once with dichloromethane (DCM). Sediments and the products resulting from fractionation were kept by freeze-drying (Lauda, Christ Alpha 1–4) until no apparent cohesive block remained (at least 24 h). The freeze-dried samples were subjected to OM extraction with an accelerated solvent extractor (ASE 200 — Dionex®). In order to remove molecular sulfur and remaining water, activated copper and Na2SO4 were placed between the filters (GF/F Whatman filters) at the bottom of the cell used for extraction. Before each sample extraction, these cells were cleaned as per the following conditions: 130 bar, 130 °C, 150% flush. Sample extraction consisted of two dichloromethane (DCM) runs collected separately (130 bar, 100 °C, 150% flush). Seven minutes were needed to heat the cell and temperature was kept constant for 5 min (static phase). Once extraction had been performed, the solids were recovered and stored. DCM was evaporated from organic extracts under a low nitrogen flow after which the extracts were recovered and weighed.

2.6.

Fig. 2 – 160–1000 µm fraction particle grain size distribution. The black line matches the measurement made without preliminary ultra-sonication, the gray one matches the measurement made after sonication. It shows the aggregated part of the 160–1000 µm fraction. at the top of the SiO2 columns. Then, the saturated hydrocarbons were collected at the bottom of the column by pentane elution. Then a 65:35 (v:v) mix of pentane/DCM was percolated through the SiO2 to obtain the aromatic hydrocarbons. Finally, 50:50 (v:v) methanol/DCM was percolated and the remaining polar molecules trapped in the SiO2 were collected and pooled with the polar compounds recovered beforehand (Al2O3 column).

Preparative liquid chromatography 2.7.

The OM extract was fractionated by liquid chromatography successively over SiO2 and Al2O3 columns. Thus the aromatic hydrocarbons, saturated hydrocarbons and polar molecules were separated. The SiO2 and Al2O3 columns were cleaned by successive cyclohexane, methanol and DCM baths and activated prior to being placed inside the microcolumns (Pasteur pipette). Extracts were dissolved in DCM and injected into the top of the Al2O3 microcolumns. A portion of the polar molecules trapped by the Al2O3, hydrocarbons was collected at the bottom of the microcolumns. Polar molecules were removed by a mix 50:50 (v:v) DCM/methanol. Prior to complete fractionation, the samples were dried and weighed. The hydrocarbons were passed through the columns to complete fractionation i.e. they were dissolved in pentane and injected

Table 1 – Weight proportions (n = 15) and TOC (n = 3 for finer fractions, 6 for larger) of size fractions in storm water infiltration sediments Leachable Sediments (Inner part) part b10 10– 160– N1000 160 1000 Weight proportions (%) TOC (mg/g) Percent of TOC mass in 1 g of dry sediments (%) Percent of TOC in sediment fractions that is extracted by DCM

2±1 81.4 6 96

3 ± 1 11 ± 4 14 ± 3 88.6 81.8 41.4 13 37 22 71

38

24

70 ± 9 3.5 22 –

Gas chromatography–mass spectrometry (GC–MS)

Aliphatic and aromatic hydrocarbons as well as polar compounds were analyzed by gas chromatography–mass spectrometry (HP 5890 Serie II GC coupled to an HP 5971 MS), using a split–splitless injector, a 60 m DB-5 J&W, 0.25 mm i.d, 0.1 μm film fused silica column. The temperature program was 70 to 130 °C at 15 °C/min, then 130 °C to 315 °C at 3 °C/min followed by an isothermal stage at 315 °C for 15 min (constant helium flow of 1.4 ml/min). Because of the presence of alkanoic acids and sterols in the polar fractions, silylation using BSTFA + TMCS (99/1) was carried out in order to improve chromatographic resolution (Wenclawiak et al., 1993; Kokinos et al., 1998). A small aliquot of polar compounds was dissolved with the derivative solution at 8 mg/ml concentration and treated for 15 min at 60 °C. Then, 1 µl of the solution was injected directly into the gas chromatograph. Compounds were identified according to their mass spectra and retention time with reference to the computerized mass spectral libraries of Wiley and the U.S. National Bureau of Standards.

3.

Results

3.1.

Fractionation

The weight proportions of each fraction in the sediments were determined (Table 1). 70% of the sediment mass consisted of N1000 µm particles. The leachable part consisting of fine particles (mode at 20 µm) represented 2% of sediment weight.

182

SC IE N CE OF T H E TOT AL E N V I RO N ME N T 4 0 3 ( 2 00 8 ) 1 7 8–1 87

Table 2 – Resulting fractions from liquid chromatography on OM extracted in DCM Leach. b10 10– 160– N1000 NF 160 1000 a. Percent of the DCM extract weight

Loss in alumina column Saturated hydrocarbons Aromatic hydrocarbons Polar compounds b. Percent of Saturated recovered hydrocarbons matter: loss Aromatic in alumina hydrocarbons column not Polar taken into compounds account

11

29

27

23

33

27

36

35

32

33

30

32

11

8

9

10

8

8

43

28

34

35

29

33

40

50

43

43

45

43

12

11

12

12

12

11

48

39

46

45

43

45

The proportions of the resulting fractions are stated a) relative to the total OM extracted by the DCM, b) relative to the total recovered OM after liquid chromatography (loss in columns were not taken into account). Leach.= leachable part. NF= non fractionated sediments.

3.2.

TOC content is a bulk parameter, it gives information on the overall organic matter content (Table 1). TOC was 30.7±0.9 mg g− 1 in the NF sediment. The higher TOC content is in the b10 µm fraction (88.6 mg g− 1). Leachate and 10–160 µm TOC values are quite similar to the former, respectively 81.4 mg g− 1 and 81.8 mg g− 1. The 160– 1000 µm TOC value is half as much (41.4 mg g− 1). The TOC value of N1000 µm seems to be far lower (3.5 mg g− 1). The latter must be considered carefully because of the high variability of its large particle grain size. Sufficient quantities of the N1000 µm fraction were not available to perform a valid analysis. The percent of TOC in sediment fractions that is extracted by DCM (Table 1) results from calculations combining size fraction weight proportions with the TOC content in each fraction. The 10–160 µm is the fraction which contains the largest share of TOC (37% of total TOC). About 6% of sediment TOC is leached by a 0.8% NaCl solution (leachable part). Both N1000 µm and 160–1000 µm fractions contain 22% of sediment TOC respectively. The sum of the TOC calculated in each fraction is 24.5 mg g− 1; this value is close to the 30.7 mg g− 1 found in the NF sediment.

3.3. The 160–1000 µm fraction (14% of sediment weight) was the only separated fraction in which aggregation predominated (Fig. 2). The volume proportions of the mode represented by the 160–1000 µm fraction decreased from 7.5% to 3% of sediment volume (Fig. 2). Ultrasound led to aggregate fragmentation. Some entities measuring between 160– 1000 µm in dimension were broken down into finer particles (b100 µm). This was performed on every fraction in triplicates. The other fractions did not appear to have aggregated (no mode modification) in the way as the 160–1000 µm fraction.

TOC in fractions (not extracted)

DCM extraction efficiency

DCM extraction is a preparative step used to recover the lipid fraction of sediment OM, and it also provides information about the overall OM of the sediment (non extracted/extracted OM ratio). Analyzing TOC in DCM extraction residues allows evaluating the TOC in DCM extract by difference with TOC in non extracted samples. The TOC in DCM extraction residue of the N1000 µm fraction was lacking so there was insufficient matter available to perform a reliable measurement. Proportions of extracted TOC are not similar in all fractions (Table 1). The proportions decrease as particle grain size

Fig. 3 – (a) Saturated hydrocarbons (fullscan) and (b) pentacyclic triterpanes (m/z = 191) chromatograms of the 160–1000 µm fraction (see Table 3 for identifications).

S CIE N CE OF T H E TOT AL E N V I RO N ME N T 4 0 3 ( 2 00 8 ) 1 7 8–1 87

Table 3 – Pentacyclic triterpane identification in the chromatogram of Fig. 3b Number 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17

C34

with high molecular mass) which may be related to asphaltene, was trapped in the Al2O3 column. It represents at least from 10% (in the leachable part) to more than 30% (in N1000 µm) (Table 2a). Aromatic hydrocarbons represent less than 12% of the extracted OM; they vary from 7.7% in the b10 µm fraction up to 11.1% in the leachable fraction. The proportions of saturated hydrocarbons vary within a narrow range from 30.4% (in the N1000 µm fraction) up to 35.6 (in the leachable part). Proportions of polar compounds vary from 27.7% (in the b10 µm fraction) up to 42.7% (in the leachable part). The loss of organic molecules definitively trapped in the Al2O3 column (asphaltenes) is lower in the leachable part compared to other grain size fractions. In order to compare proportions of polar compounds, saturated and aromatic hydrocarbon proportions were calculated without taking into account asphaltenes (Table 2b). It should be noted that all the fractions have fairly similar proportions of aromatic hydrocarbons (11% to 12%), saturated hydrocarbons (40 to 50%) and polar compounds (39 to 48%).

C34

3.5.

C35

Firstly, it must be underlined that whereas many successive steps are necessary to perform GC–MS analysis, the experiment replicates are very similar. Saturated hydrocarbon and polar compound chromatograms (Figs. 3 and 4) are very similar whatever the grain size, whereas differences were observed in the aromatic hydrocarbon chromatograms. Saturated hydrocarbon chromatograms (Fig. 3a) are dominated by a large unresolved complex mixture (UCM) generally associated with a mix of iso and cyclo alkanes (Gough and Rowland, 1990). Sterane and terpane are biomarkers which are relatively resistant to biodegradation and generally studied by petroleum geochemists as thermal maturity indicators (Tissot

Name

Carbon number

17α(H)-22,29,30-trisnorhopane 17β(H)-22,29,30-trisnorhopane 17α(H), 21β(H)-30-norhopane 18α(H)-30-norneohopane 17α(H), 21β(H)-Hopane 17α(H)-30-nor-29-homohopane 22S-17α(H), 21β(H)-30 homohopane 22R-17α(H), 21β(H)-30 homohopane Gammacerane 22S-17α(H), 21β(H)-30 bishomohopane 22R-17α(H), 21β(H)-30 bishomohopane 22S-17α(H), 21β(H)-30 trishomohopane 22R-17α(H), 21β(H)-30 trishomohopane 22S-17α(H), 21β(H)tétrakishomohopane 22R-17α(H), 21β(H)tétrakishomohopane 22S-17α(H), 21β(H)pentakishomohopane 22R-17α(H), 21β(H)pentakishomohopane

C27 C27 C29 C29 C30 C30 C31 C31 C31 C32 C32 C33 C33

C35

increases. In the leachable part, 96% of the TOC present is extracted whereas only 24% is in the 160–1000 µm fraction.

3.4.

183

Preparative liquid chromatography

As the quantity of OM extracted is low, the asphaltene precipitation frequently carried out in petroleum geochemistry was not performed. A molecular fraction (probably molecules

Molecular analyses

Fig. 4 – GC–MS chromatograms of polar compounds isolated from OM extracted from the 160–1000 µm fraction.

184

SC IE N CE OF T H E TOT AL E N V I RO N ME N T 4 0 3 ( 2 00 8 ) 1 7 8–1 87

Fig. 5 – Chromatograms of aromatic compounds extracted from the 160–1000 µm fraction (a) and from the leachable fraction (b). (ACY: acenaphthylene, FLU: fluorene, PHE: phenanthrene, ANT: anthracene, Ph-NAP: phenylnaphthalene, FLA: fluoranthene, PYR: pyrene, BaA: benzo[a]anthracene, CHR:chrysene, TRI: triphenylene, B[b]FLA: benzo[b]fluoranthene, B[k]FLA: benzo[k] fluoranthene, B[a]PYR: Benzo[a]pyrene, I[123cd]PYR: Indeno(1,2,3-c,d)pyrene, B[ghi]PER: benzo[g,h,i]perylene, COR: coronene, m: methyl, dim: dimethyl, trim: trimethyl).

and Welte, 1984; Peters and Moldowan, 1993). In our samples, their distributions, and especially hopane distribution (Fig. 3b), are characteristic of thermally mature OM (petroleum, coal) (Faure et al., 2004; Jeanneau et al., 2006), in agreement with other work on storm water sediment (Durand et al., 2004). As with the saturated hydrocarbon chromatograms, the polar compound chromatograms (Fig. 4) are similar whatever the grain size. Natural markers occur among the molecules identified. n-Alkanols in the range of C22 to C32 with an even over-odd predominance, phytol (Ph2) and phytol by-products (Neo, Ph1) and specific sterols (stigmasterol, sitosterol, campesterol) underline higher plant inputs (Huang and Meinschein, 1979; Killops and Killops, 2004). n-Alkanoic acids in the range of C10 to C20 with an even over-odd predominance can be related to bacterial contributions (Huang and Meinschein, 1979; Killops and Killops, 2004). The occurrence of waste water markers (coprostanol, coprostanone, 24-ethyl-coprostanol) and phthalate (plastic by-products) was observed to be associated with these natural inputs (Jarde et al., 2007). Two different groups of grain size fractions can be distinguished based on aromatic hydrocarbon distribution (Fig. 5): on the one hand, the leachable part, 10–160 µm and N1000 µm fractions and, on the other hand, the b10 µm and 160–1000 µm fractions. Polycyclic aromatic hydrocarbons (PAH) are sorted into two groups: those containing 3 or less benzene rings, generically called “light” aromatic hydrocarbons (acenaphthylene, fluorene, phenanthrene, anthracene, fluoranthene, pyrene) and those containing more than 3

benzene rings, generically called “heavy” aromatic hydrocarbons (benzo[a]anthracene, chrysene, triphenylene, benzo[k] fluoranthene, benzo[b]fluoranthene, benzo[e]pyrene, benzo[a] pyrene, perylene, indeno[1,2,3-c,d]pyrene, dibenzo[a,h] anthracene, benzo[g,h,i]perylene, coronene). The pyrene peak is one of the major peaks in both chromatograms. Fluoranthene and phenanthrene largely dominate the leachable part

Fig. 6 – N3 rings PAH/b3 rings PAH ratio versus alkyl Phe/Phe (Phenanthrene) diagram comparing polycyclic aromatic hydrocarbons distribution among sediment fractions.

S CIE N CE OF T H E TOT AL E N V I RO N ME N T 4 0 3 ( 2 00 8 ) 1 7 8–1 87

chromatogram whereas they do not in the 160–1000 µm chromatogram. Comparison of PAH and the occurrence of their alkyle forms and relative quantities underlined considerable differences between two groups of fractions: the b10 µm and 160–1000 µm fractions and the NF sediment on the one hand, and the leachable part, 10–160 µm and N1000 µm fractions on the other hand. The b10 µm and 160–1000 µm fractions and the NF samples contain over 10 times more heavy PAH than light ones, whereas the leachable part, 10–160 µm and N1000 µm contain only 1 to 3 times more heavy PAH than light ones (Fig. 6). Moreover, the b10 µm and 160–1000 µm fractions and the NF samples have a higher alkylated phenanthrene over phenanthrene ratio (ratio N2) than the other fractions (ratio b0.5) (Fig. 6).

4.

Discussion

Urban storm water sediments are quite unusual matrices with intermediate characteristics between soil and sediments. As a soil, they constitute a life support for plants; they stem from the parallel evolution of parent rock, OM deposits and exogenous particles. On the other hand, as sediments, they result from particle transport due to water flow and deposition over long periods. Contrary to most river or marine sediments, they are exposed to dry and wet cycles that inevitably participate in sediment structuring and changes in OM content. Part of the OM flux in the pond originates from human activities and they can be deposited over road surfaces and leached by rain waters (soot particles emitted by wood or coal heating, vehicle combustion by-products, oil leakage, etc.) and/or derived from direct industrial and domestic effluent inputs. But part of the OM in the pond also originates from plants and microflora growing on the surface of the pond or is leached from road surfaces. Thus the OM studied is a complex mix of natural and anthropogenic OM. The study's first objective was to understand how widespread OM is in urban storm water sediments at millimetric scale. This was done using two different scales of analysis: bulk (TOC and compound type proportions), and molecular (GC–MS analyses). Each scale of OM analysis gives information about the distribution of OM in sediments, but they do not discriminate samples in the same way. The combination of these results with each other and with grain size fraction proportions is a powerful tool that avoids simplistic conclusions. Global parameters such as TOC values and DCM extraction efficiency seem to be largely correlated with particle grain size. They also provide information on non extractible OM, with higher quantities in the larger fractions. Almost all the OM of the leachable part was analyzed by GC–MS (extraction efficiency: 96%). It is surprising to notice that the 10–160 µm fraction exhibits a TOC value as high as the b10 µm fraction. According to liquid chromatography fractionation, saturated and aromatic hydrocarbons, polar compounds and asphaltene proportions do not exhibit noticeable differences between grain size fractions except for the leachable fraction. At molecular level, the results confirm what Durand et al. (2004) have shown, i.e. that contaminants are mainly generated from anthropogenic petroleum sources. However, the

185

occurrence of natural biomarkers confirms the dual natural and anthropogenic contribution in storm water sediment. Saturated hydrocarbon and polar compound GC–MS analyses do not rank samples in the same way as aromatic hydrocarbon GC–MS. Two groups of samples could be distinguished using the GC–MS data: one group includes the aggregated fraction, the b10 µm fraction and the non-fractionated (NF) sediment; and the other group consists of the leachate, 10–160 and N1000 µm fractions. We have shown a differential OM distribution in storm water sediment structure. As in soil, differential OM distribution could be due to diverse factors. In this subsection, we will examine mechanisms that could explain differential OM distribution after which we will focus on what the results appear to explain about OM distribution in storm water sediments. Firstly, in soil, more degraded organic fractions are found in the finer fractions. When incorporated into soil, plant and animal debris are first found in larger grain size fractions prior to gradual degradation and reduction in size. This could explain why DCM extraction efficiency decreases as grain size increases. Secondly, OM distribution could result from differences between organic sources. We can assume that soot particles (from combustion) constituted part of the b10 µm particles, whereas oil type contaminants were found throughout the sediment fractions. Thirdly, OM distribution could also result from the differential evolution of organic molecules (association with soil constituents and (bio)degradation). Moreover, the association of organic molecules with soil constituents makes them less accessible to (bio)degradation (Richnow et al., 1997; Amellal et al., 2001). Regarding biodegradation, molecule structural differences impact molecule biodegradation; Richnow et al. (1997) and Ahmed et al. (1999) have shown that the susceptibility of aromatic hydrocarbons to biodegradation decreases as the number of aromatic rings and number of alkyl substituents increase. Here we discuss the results of the organic matter analysis of 3 grain size fractions regarding the factors mentioned above and the method of obtaining these fractions.

4.1.

The aggregated fraction

Both intermediate TOC content and heavy metal content (data not shown) suggest that aggregates come from the combined evolution of storm water material input and the fluvio-glacial substratum. Grain size analyses show that aggregate disintegration gives rise to particles from 10 µm to 160 µm. In soil science, macroaggregates are considered to be larger than 250 µm and they could be produced by the assembly of microaggregates (20 to 250 µm) (Oades and Waters, 1991). In this study, we assimilate aggregates from 160 to 1000 µm with macroaggregates and elements from 10 to 160 µm with microaggregates. The 10–160 µm fraction analyzed is certainly composed of not only primary particles but also microaggregates (too stable to be destroyed by applied ultrasound). Regarding the PAH proportions, macroaggregate OM differs from microaggregate OM. Macroaggregates (160–1000 µm) exhibit lower DCM extractible OM and higher proportions of large PAHs than other fractions. It can be assumed that aggregation is the result of an evolutionary process: microaggregates and

186

SC IE N CE OF T H E TOT AL E N V I RO N ME N T 4 0 3 ( 2 00 8 ) 1 7 8–1 87

elementary particles were embedded in sticky (very condensed) organic matter in which “heavy” and substituted PAH are present. Amellal et al. (2001) have underlined the major role of soil structure and aggregate nature on pollutant (i.e. PAHs) immobilization in controlled aging experiments. They observed that only PAHs of lower-molecular weight (i.e. 3 or 4 rings) appear to be easily available for bacterial degradation. Macroaggregates seem to result from what Richnow et al. (1997) have suggested i.e. the agglomeration of larger PAHs (due to their low water solubility). As Amellal et al. (2001) have suggested, large aggregates are not optimal spots for PAH biodegradation. Small aggregates could represent more favourable environments for microbial metabolism: nutrients are nearer and embedding is lower.

4.2.

The b 10 µm fraction

As grain size analyses show that the leachate and b10 µm fractions have relatively similar particle grain size distributions, it could have been assumed that b10 µm particles were similar to the particles eroded when obtaining the leachate fraction. However, DCM extraction efficiencies and the proportions of PAH of these fractions are different. It is necessary to recall how these fractions were obtained: the leachate fraction, i.e. the easily extractable one, was obtained first, while the b10 µm fraction, i.e. everything that must have been removed from the surface, was obtained last. The b10 µm fraction could be composed of particles stuck at the sediment surfaces, thus more energy was necessary to remove them than that necessary for the particles in the leachate. As the proportions of large aromatic hydrocarbons are high, it could be assumed that soot particles are part of the b10 µm particle fraction (urban air particles have been shown to exhibit higher proportions of large PAHs (Tasdemir and Esen, 2007)). Usually, combustion byproducts, such as soot, do not exhibit good DCM extraction efficiency. The DCM extraction efficiency of the b10 µm fraction is good, thus the previous assumption was false, unless the sample grain size is small enough to optimise extraction.

4.3.

The leachable fraction

Our study second objective was to evaluate the leachable fraction which represents a threat to groundwater quality. The leachable fraction results from the removal of particles that were formerly on the surfaces around the macropores. Almost 6% of sediment TOC (i.e. almost 2 mg g− 1 of dried sediments) was leached. This TOC was almost all linked to the particles, and only 4% of TOC was dissolved in the NaCl 0.8% solution. Almost all the TOC is extractible (96%), and there is almost no OM that is not DCM extractible. As might have been assumed, due to differences in solubility, the amount of very large lipids (asphaltenes) and the proportion of large PAHs are lower than in other fractions. It seems that OM forms bonds between sediment particles. The NaCl solution might have leached particles that were bonded to surfaces more weakly. Weaker bonds in sediments might be due to fewer hydrophobic molecules (in the case of an aqueous solution). The particles at the surface of sediment macropores and formerly attached to surfaces by “not over-hydrophobic” organic molecules, could have been leached by the flow of NaCl solution.

5.

Conclusion

Urban storm water sediments exhibit specificities: they are neither really soils nor sediments, while the OM present originates both from anthropogenic and natural systems. This required adapting technical knowledge from soil sciences. Information was obtained from simple tests and multiscale OM analyses, making it possible to make assumptions about: – formation by aggregation – particle origin – the leachable part of sediments in this state. Contrary to the study on metals, the study of OM requires taking into account not only the differential affinities of molecules but also their degradation and kinetics. Storm water sediments undergo dry and wet conditions, suggesting that sediment structure changes with potential energy of water. These structural modifications may lead to changes in contamination potential, thereby threatening the groundwater in different degrees. From the point of view of management, it would be interesting to identify the best and worst key conditions regarding potential risks of contamination. Controlled experiments should be performed to improve understanding of the impact of organic matter on the structure of contaminated soil and sediments.

Acknowledgements The work presented was funded in part by the EMMAUS (“Etude et Modélisation du transfert des Métaux lourds issus de l'Assainissement pluvial Urbain dans les Sols”) project backed by the ANR-ECCO research program. It was also carried out in the framework of the regional observatory on urban hydrology (OTHU).

REFERENCES Ahmed M, Smith JW, George SC. Effects of biodegradation on Australian Permian coals. Org Geochem 1999;30:1311–22. Amellal N, Portal JM, Berthelin J. Effect of soil structure on the bioavailability of polycyclic aromatic hydrocarbons within aggregates of a contaminated soil. Appl Geochem 2001;16:1611–9. Baldock JA, Skjemstad JO. Role of the soil matrix and minerals in protecting natural organic materials against biological attack. Org Geochem 2000;31:697–710. Clozel B, Ruban V, Durand C, Conil P. Origin and mobility of heavy metals in contaminated sediments from retention and infiltration ponds. Appl Geochem 2006;21:1781–98. Datry T, Malard F, Vitry L, Hervant F, Gibert J. Solute dynamics in bed sediments of a stormwater infiltration basin. J Hydrol 2003;273:217–33. Datry T, Malard F, Gibert J. Dynamics of soluted and dissolved oxygen in shallow urban groundwater below a stormwater infiltration basin. Sci Total Environ 2004;329:215–29. Dechesne M, Barraud S, Bardin JP. Spatial distribution of pollution in an urban stormwater infiltration basin. J Contam Hydrol 2004;72:189–205.

S CIE N CE OF T H E TOT AL E N V I RO N ME N T 4 0 3 ( 2 00 8 ) 1 7 8–1 87

Ducaroir J, Lamy I. Evidence of trace metal association with soil organic matter using particles size fractionation after physical dispersion treatment. Analyst 1995;120:741–5. Duchaufour P. Pédologie. Sol, végétation, environnement. 4ème édition. Paris: Masson; 1994. pp. Durand C, Ruban V, Ambles A, Oudot J. Characterization of the organic matter of sludge: determination of lipids, hydrocarbons and PAHs from road retention/infiltration ponds in France. Environ Pollut 2004;132:375–84. Durand C, Ruban V, Amblès A. Characterisation of complex organic matter present in contaminated sediments from water retention ponds. J Anal Appl Pyrolysis 2005;73:17–28. Faure P, Landais P, Schlepp L, Michels R. Evidence for diffuse contamination of river sediments by road asphalt particles. Environ Sci Technol 2000;34:1174–81. Faure P, Elie M, Mansuy L, Michels R, Landais P, Babut M. Molecular studies of insoluble organic matter in river sediments from Alsace-Lorraine (France). Org Geochem 2004;35:109–22. Göbel P, Dierkes C, Coldewey WG. Storm water runoff concentration matrix for urban areas. J Contam Hydrol 2006. Göbel P, Dierkes C, Coldewey WG. Storm water runoff concentration matrix for urban areas. J Contam Hydrol 2007;91:26–42. Gough MA, Rowland SJ. Characterization of unresolved complex mixtures of hydrocarbons in petroleum. Nature 1990;344:648–50. Hattori T. soil aggregates as microhabitats of microorganisms. Rep Inst Agr Res Tohoku Univ 1988;37:23–36. Huang WY, Meinschein WG. Sterols as ecological indicators. Geochim Cosmochim Acta 1979;43:739–45. Jarde E, Gruau G, Mansuy-Huault L, Peu P, Martinez J. Using sterols to detect pig slurry contribution to soil organic matter. Water, Air, and Soil Pollution 2007;178:169–78. Jeanneau L, Faure P, Montarges-Pelletier E, Ramelli M. Impact of a highly contaminated river on a more important hydrologic system: changes in organic markers. Sci Total Environ 2006;372:183–92. Killops SD, Killops VJ. An introduction to organic geochemistry. Second edition. Oxford: Blackwell publishing; 2004. 408 pp. Kokinos JP, Eglinton TI, Goni MA, Boon JJ, Martoglio PA, Anderson DM. Characterization of a highly resistant biomacromolecular material in the cell wall of a marine dinoflagellate resting cyst. Org Geochem 1998;28:265–88.

187

Legret M, Pagotto C. Evaluation of pollutant loadings in the runoff waters from a major rural highway. Sci Total Environ 1999;235:143–50. Manno E, Varrica D, Dongarra G. Metal distribution on road dust samples collected in an urban area close to a petrochemical plant at Gela, Sicily. Atmos Environ 2006;40:5929–41. Oades JM. The retention of organic matter in soils. Biogeochemistry 1988;5:35–70. Oades JM, Waters AG. Aggregate hierachy in soils. Aust J Soil Res 1991;29:815–28. Peters KE, Moldowan JM. The biomarker guide. Interpreting molecular fossils in petroleum and ancient sediments. Peters and Moldowan; 1993. 363 pp. Pitt R, Clark S, Field R. Groundwater contamination potential from stormwater infiltration practices. Urban Water 1999;1:217–36. Ranjard L, Richaume A, Jocteur Monrozier L, Nazaret S. Response of soil bacteria to Hg(II) in relation to soil characteristics and cell location. FEMS Microbiol Ecol 1997;24:321–31. Richnow HH, Seifert R, Hefter J, Link M, Francke W, Schaefer G, et al. Organic pollutants associated with macromolecular soil organic matter: mode of binding. Org Geochem 1997;26:745–58. Six J, Bossuyt H, Degryze S, Denef K. A history of research on the link between (micro)aggregates, soil biota, and soil organic matter dynamics. Soil Tillage Res 2004;79:7–31. Stone M, Droppo IG. Distribution of lead, copper and zinc in size-fractionated river bed sediment in two agricultural catchments of Southern Ontario, Canada. Environ Pollut 1996;93:353–62. Tasdemir Y, Esen F. Urban air PAHs: concentrations, temporal changes and gas/particle partitioning at a traffic site in Turkey. Atmos Res 2007;84:1–12. Tisdall JM, Oades JM. Organic matter and water-stable aggregates in soils. J Soil Sci 1982;33:141–63. Tissot BP, Welte DH. Petroleum formation and occurrence. Berlin: Springer; 1984. 294 pp. Wenclawiak BW, Jensen TE, Richert JFO. GC/MS-FID analysis of BSTFA derivatized polar components of diesel particulate matter (NBS SRM-1650) extract. Fresenius J Anal Chem 1993;346:808–12. Yoo G, Spomer LA, Wander MM. Regulation of carbon mineralization rates by soil structure and water in an agricultural field and a prairie-like soil. Geoderma 2006;135:16–25.