Sources, Vertical Fluxes, and Accumulation of Aliphatic Hydrocarbons

Plata, Av. Calchaqui km 23500 (1888) Florencio Varela,. Buenos Aires, Argentina, Comisión de Investigaciones. Cientıficas, Provincia de Buenos Aires...
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Environ. Sci. Technol. 2005, 39, 8227-8234

Sources, Vertical Fluxes, and Accumulation of Aliphatic Hydrocarbons in Coastal Sediments of the Rı´o de la Plata Estuary, Argentina J . C . C O L O M B O , * ,†,‡ N . C A P P E L L E T T I , †,‡ J . L A S C I , † M . C . M I G O Y A , †,§ E . S P E R A N Z A , †,‡ A N D C . N . S K O R U P K A † Laboratorio de Quı´mica Ambiental y Biogeoquı´mica, Facultad de Ciencias Naturales y Museo, Universidad Nacional de La Plata, Av. Calchaqui km 23500 (1888) Florencio Varela, Buenos Aires, Argentina, Comisio´n de Investigaciones Cientı´ficas, Provincia de Buenos Aires, Buenos Aires, Argentina, and Consejo Nacional de Investigaciones Cientı´ficas y Te´cnicas, Rivadavia 1917, Buenos Aires

Settling particles and underlying sediments collected at 1, 2.5 and 4 km off the metropolitan Buenos Aires coast were analyzed to evaluate the sources and accumulation of resolved (RES), unresolved (UCM), and biomarker aliphatic hydrocarbons. Sedimentary aliphatic concentrations (RES 0.11-14 µg‚g-1; UCM 0.1-800 µg‚g-1) included variability associated with north-south gradients and an exponential offshore reduction. Highest concentrations were registered close to Buenos Aires port and sewer, compared to cleaner northern stations and southward sites affected by a seaward residue transport. Sediment traps deployed in the sewer area revealed large hydrocarbon (38 and 319 mg‚m-2‚day-1, RES and UCM) and total organic carbon fluxes (29 ( 26 g‚m-2‚day-1). The composition of RES and hopanes evaluated by principal component analysis indicated a consistent offshore gradient defined by the relative contribution of lower vs higher molecular weight components. Distant sites showed decreasing proportions of petrogenic n-C17-26 alkanes, isoprenoids, and C20-27 terpanes and relative enrichment of n-C27,29,31,33 terrestrial plant alkanes and C31-33 homohopanes. Sediment hydrocarbon profiles showed an average 2-fold reduction down to 20 cm depth with preferential removal of lower molecular weight components and enrichment of n-C23-35 alkanes and hopanes. Sediment inventories and trap depositional fluxes indicate the accumulation of 58009700 tons of aliphatic hydrocarbons in the top 0-5 cm sediments with a strong interfacial alteration and selective preservation of refractory components: n-C13-22 (1.0%) < isoprenoids (3.2%) < n-C23-35 (6.1%) < hopanes (47%) ∼ UCM (50%), compared to intermediate stability of organic * Corresponding author phone and fax: 54 11 4275-8266; e-mail: [email protected]. † Laboratorio de Quı ´mica Ambiental y Biogeoquı´mica, Facultad de Ciencias Naturales y Museo, Universidad Nacional de La Plata, Av. Calchaqui km 23500 (1888) Florencio Varela, Buenos Aires, Argentina ‡ Comisio ´ n de Investigaciones Cientı´ficas, Provincia de Buenos Aires. § Consejo Nacional de Investigaciones Cientı ´ficas y Te´cnicas 10.1021/es051205g CCC: $30.25 Published on Web 09/22/2005

 2005 American Chemical Society

carbon (12%) and quantitative preservation of polychlorinated biphenyls (PCBs) (91%).

Introduction Chronic spillages from land-based facilities, vessels, effluent discharges, and accidental spills introduce large amounts of petroleum hydrocarbons to urbanized coastal areas. Depending on the partitioning properties of hydrocarbons, a large fraction adsorbs to suspended particles and accumulates in underlying sediments, which constitute longterm reservoirs and secondary sources (1). Municipal waste discharges are also major sources of petroleum hydrocarbons in these environments, accounting roughly for 5% of the total global input, or 3 × 105 metric tons of hydrocarbons per year (2-4). The Rı´o de la Plata (RLP) is a turbid, coastal-plain estuary with a total surface area of 35 000 km2 and a huge particulate load of 90 millions tons of solids per year (5). The upper freshwater sector is dominated by a vast delta in front of Buenos Aires City, which concentrates one-third of the total Argentinean population. The main municipal sewer serves 5 million inhabitants and discharge 2.2 million m3 day-1 of untreated effluents 2.5 km offshore Berazategui (Figure 1). In addition, the port of Riachuelo, which includes a petrochemical complex, discharges an unknown amount of toxic chemicals to the coastal ecosystem. Earlier studies carried out in the RLP, 60 km south from Buenos Aires, indicated that sedimentary hydrocarbons showed distinct contributions of petrogenic compounds, vascular-plant n-alkanes, and pyrogenic aromatic hydrocarbons with distance offshore (6). Accidental oil spills also introduced a heavy load of hydrocarbons in the intermediate estuary, which lasted 3-4 years until recovery (7, 8). Although these studies provided the first hydrocarbon assessments in this estuary, they were concentrated in the southern sector, leaving uncovered the most critical metropolitan Buenos Aires area. In this paper we evaluate the vertical fluxes, sources, distribution, and alteration of aliphatic hydrocarbons in coastal sediments from the upper estuary.

Methods Sediment sampling was performed in April, August, and September 2002-2003 by use of a 14 m boat covering 50 km in the shallow (3-5 m depth) upper Rı´o de la Plata Estuary (Figure 1). Superficial sediments were collected with a stainless steel grab sampler along 13 transects at 1, 2.5, and 4 km offshore. Sediment cores were collected at the 1-km Riachuelo station and at the Berazategui sewer with a stainless steel Limnos Ltd. sediment sampler. Four sequential sediment slices were separated on board down to 15-25 cm depth. Sediment samples were stored in organic-free precleaned glass jars maintained in portable coolers until arrival to the laboratory. Settling particles were collected 1.5 m below the surface in the Central area in 13 trap deployments during spring, summer, autumn, and winter in 2002-2004 using a freedrifting 14.5-cm diameter bicylindrical trap (total surface area 330 cm2) and two fixed 10-cm diameter monocylindrical traps (total surface area 78.5 cm2) deployed upstream and downstream from the sewer area for 1.5-46 h. Sediment samples were split for grain size analysis (sieve and pipet method), for total organic carbon and nitrogen analysis (Thermo Finnigan, CE FlashEA 1112 elemental analyzer) and for the determination of trace organics (6-9). VOL. 39, NO. 21, 2005 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 1. Study area in the Rı´o de la Plata Estuary showing metropolitan Buenos Aires (based on a Landsat satellite image). Stations acronyms are defined in Table 1. Only the 2.5 km sampling sites are shown. The arrows indicate the Riachuelo Port and Berazategui sewer (note the plumes from both sources). Briefly, the analytical method included sediment drying at 30 °C in a clean oven, Soxhlet extraction with acetonedichloromethane-petroleum ether (1:2:2), nitrogen concentration, activated copper treatment, silica gel fractionation, and analysis by high-resolution gas chromatography (HRGC) with flame ionization detection (FID) (Agilent 6890N) and mass spectrometric detection (MSD) (Agilent 68505973N; electron ionization (EI) 70 eV, 2.94 scans/segment, 50-550 amu). Quantification was performed by an external standard of 31 individual aliphatic hydrocarbons (n-C10 to n-C38 plus pristane and phytane; AccuStandard Inc.). Linear responses were obtained with four-point calibration curves (1, 10, 50, and 100 ng‚µL-1). C29 Norhopane and C30 hopane identified at m/z 191 were quantified on the basis of the FID response factor of n-C31 eluting between them. The unresolved complex mixture (UCM) was quantified by planimetry. MSD data were used to confirm the aliphatic identities (m/z 57, 71, and 85), to correct the possible coelution of isoprenoids with linear alkyl benzenes (LABs, m/z 91 and 105; Figure 2), and to characterize the profile of tri- and pentacyclic terpane biomarkers (m/z 191, 177, and 205). Duplicate analysis of 19 sediment samples and of a reference sediment from La Plata harbor (2.8 ( 0.55 µg‚g-1, n ) 13) indicated an average method reproducibility of 16-20%.

Results and Discussion Total Aliphatic Hydrocarbons in Superficial Sediments. Rı´o de la Plata coastal sediments consist basically of sandy silts (57% ( 22% silt, 35% ( 25% sand) with low clay (7.5% ( 4.2%) and total organic carbon (TOC) contents (0.6% ( 0.5%). Sediment texture is relatively homogeneous as indicated by the uniform fine contents in north (63% ( 6.3%, 48% ( 30%, and 85% ( 6.6%), central (65% ( 22%, 73% ( 20%, and 70% ( 27%), and south stations (70% ( 4.0%, 55% ( 22%, and 72% ( 30%) at 1, 2.5, and 4 km offshore from metropolitan Buenos Aires, respectively. Table 1 presents total resolved (RES) and unresolved aliphatic (UCM) concentrations in sediments and average trap material. Total aliphatic concentrations are not homogeneous, extend over 2-3 orders of magnitude (RES 0.11-14 µg‚g-1, UCM 0.1-800 µg‚g-1) throughout the study area, and include variability associated with north-south and on-off-shore gradients. Aliphatic sedimentary levels in the Rı´o de la Plata are high, comparable to other environments polluted by 8228

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refinery, urban, and industrial effluents, for example, Coatzacoalcos River, Mexico [RES 0.9-7.4 µg‚g-1, UCM 4.5-41 µg‚g-1 (10)], Forth Estuary, U.K. [RES 7.7-21.9 µg‚g-1 (11)], GreenDuwamish River, U.S. [RES 6-230 µg‚g-1, UCM 3-160 µg‚g-1 (12)], and Southern California Bight, U.S. [RES 1.9-7.6 µg‚g-1, UCM 15-527 µg‚g-1 (13)]. All of the on-off-shore transects in the Rı´o de la Plata show higher concentrations at 1 km and decreasing offshore values. The central area presents the highest levels (1-km average 5.7 ( 3.6 and 454 ( 324 µg‚g-1, RES and UCM, respectively) reflecting the contribution from polluted streams, port, and the main sewer (Figure 1). The analysis of sediment samples from Riachuelo Port effectively confirmed very high aliphatic concentrations (155 and 6981 µg‚g-1, RES and UCM), comparable to the trap material from the sewer area (107 and 957 µg‚g-1). In contrast to the impacted central sector, northern stations show 5-40 times lower hydrocarbon levels (RES 1.0 ( 0.25 µg‚g-1, UCM 12 ( 4.9 µg‚g-1 at 1 km), indicating reduced anthropogenic inputs probably due to a rapid particle decantation downstream major sources. The southward transport of residues is indicated by the high and uniform concentrations at 1 km in PC and PL (RES 6.0 ( 0.02 µg‚g-1, UCM 260 ( 32 µg‚g-1), a vegetated zone without significant human influence that also showed an enrichment of PCBs (9). Following the general offshore decreasing pattern, at 2.5 km hydrocarbon concentrations are lower but maintain the spatial difference between cleaner northern sites (0.92 ( 1.4 and 3.1 (5.3 µg‚g-1), the polluted central area (4.3 ( 4.4 and 124 ( 132 µg‚g-1), and rapidly recovered southern stations (1.9 ( 0.7 and 33 ( 27 µg‚g-1 for RES and UCM, respectively). At 4 km from the coast, average hydrocarbon concentrations reflect lower, more homogeneous background levels, with still high UCM values in the central area (RES 1.5 ( 0.5, 1.2 ( 0.6, and 1.3 ( 0.4 µg‚g-1 and UCM 5.2 ( 2.5, 32 ( 65, and 9.8 ( 1.7 µg‚g-1 for the north, central, and south areas, respectively). Offshore Gradients. The reduction of hydrocarbon concentrations with distance offshore was modeled by exponential regressions fitted to each transect. The statistics are very significant, especially in central and southern transects and particularly for the UCM. For resolved hydrocarbons, in 7 out of 12 transects the regressions explain 67-100% (R2) of the total variability and present very consistent grand mean

FIGURE 2. GC traces of n-alkanes and terpanes (m/z 191) at 1, 2.5, and 4 km offshore at north, central, and south sites. statistics (RES0 12 ( 5.6 µg‚g-1, k ) 0.59 ( 0.13, R2 ) 0.94 ( 0.12). All are south and central sites, excepted QU (R2 0.45), and BO (R2 0.14). The regressions are not significant in north transects characterized by lower, background hydrocarbons levels. The exponential coefficients range from 0.44 to 0.81 and are highest at central sites, implying an 83-96% RES reduction from 0 to 4 km. The exponential regressions of the UCM are more significant and present a steeper reduction with distance offshore. In 10 from 13 transects, the regressions explain 69-100% of the total variability (UCM0 ) 1507 ( 1359 µg‚g-1, k ) 1.12 ( 0.48, R2 ) 0.88 ( 0.11). They are also not significant in two northern transects. The exponential coefficients (0.21-1.84) are highest in central sites, implying a 57-100% UCM reduction with distance offshore. To evaluate the effect of the association of hydrocarbons with the organic content of sediments along offshore gradients, Figure 3 shows the relationship of RES and UCM with TOC. The correlation is very significant for RES (R2 ) 0.87), but UCM shows a weaker general correlation with TOC (R2 ) 0.69) due to a divergent trend observed with distance offshore. At 1 and 2.5 km the UCM-TOC correlations are very significant (R2 ) 0.91) but with steeper slopes close to the shore, whereas at 4 km there is no relationship. These contrasting trends of RES and UCM with TOC reflect the different sources contributing resolved aliphatics, which are dominated by detrital plant lipids, and the UCM, originating

from petrogenic discharges from coastal outfalls. Thus, although RES displays a higher anthropic influence near the shore (see composition), the signal is superimposed on a predominant background of terrestrial plant n-alkanes, which co-varies with the carbon content of sediments. As indicated by the steeper offshore gradients, the UCM is a more reliable tracer of anthropogenic inputs and shows a significant correlation with TOC close to the shore but has no association at 4 km. Total Aliphatic Hydrocarbons in Settling Particles. Sediment traps installed in the central area collected 0.6-9 g of material in 1.5-45 h corresponding to mass fluxes of 137-618 g‚m-2‚day-1 (mean 361 ( 124 g‚m-2‚day-1) and very high sedimentation rates (5.0 ( 1.7 cm‚year-1). These large fluxes reflect the anthropogenic discharges of solids but basically the natural high turbidity of this deltaic environment (5). The concentration of aliphatic hydrocarbons in settling particles collected in the central area ranged from 16 to 428 µg‚g-1 for RES (mean 109 ( 116 µg‚g-1) and 235-2445 µg‚g-1 for the UCM (957 ( 733 µg‚g-1; Table 1). The sharp decline of resolved aliphatic hydrocarbons from settling particles to 1-km sediments (i.e., 20 times) suggests intense degradation at the sediment-water interface, as is also indicated by the rapid decay of TOC (8.9% vs 1.1%). Since the biodegradation of fossil hydrocarbons includes a rapid n-alkane removal and concurrent relative increase of the UCM (14), the shift VOL. 39, NO. 21, 2005 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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TABLE 1. Concentrations of Aliphatic Hydrocarbons in Rı´o de la Plata Sediments Collected at 1, 2.5, and 4 km from the Coasta 1 km RES Aeroparque Puerto Norte Canal Norte mean SD

AE PN CN

Riachuelo Santo Domingo Don Bosco Bernal Quilmes Berazatergui Platanos Hudson mean SD

RI DO BO BN QU BZ PA HU

Punta Colorada Punta Lara mean SD

PC PL

0.76 1.27 1.02 1.02 0.25

UCM 6.4 14.8 15.0 12.1 4.9

2.5 km total

RES

North 7.2 16 16 13 5.1

0.11 0.12 2.51 0.92 1.38

UCM 0.1 0.1 9.2 3.1 5.3

4 km total

RES

UCM

total

0.21 0.18 12 4.0 6.6

1.07 2.06 1.51 1.58 0.50

2.9 4.8 7.9 5.2 2.5

3.9 6.8 9.5 6.7 2.8

Central 11.7 2.7 1.9 8.2 2.4

801 104 76 613 160

813 107 79 621 162

6.3 6.8 5.7 3.6

732 690 454 324

739 697 459 329

6.0 6.0 6.0 0.02

238 283 260 32

244 289 266 32

5.98 1.19 2.48 3.37 3.18 14.4 2.50 1.12 4.28 4.37

160 43 84 93 126 428 47 11 124 132

166 44 87 96 129 443 50 12 128 136

2.43 0.60 1.65 1.00 1.48

179 9.0 12 4.2 9.4

181 9.6 14 5.2 11

1.19 0.69 1.20 0.68

8.5 2.8 32 65

9.6 3.4 33 65

2.44 1.38 1.91 0.75

52 14 33 27

54 15 35 28

1.04 1.68 1.36 0.56

11 8.6 9.8 1.7

12 10 11 1.3

South

mean (13) SD

109 116

BZ 0-3 cm BZ 4-5 cm BZ 8-10 cm BZ 14-20 cm Riachuelo Port RI 0-3 cm RI 7-10 cm RI 14-16 cm RI 21-24 cm

11.2 5.7 5.0 4.7 155 20 14 15 11

957 733

Sed. Trap 1066 849 Cores

a

433 291 223 202 6981 1118 834 729 556

444 297 228 206 7147 625 487 465 294

Concentrations are given in micrograms per gram of dry weight. RES, resolved; UCM, unresolved complex mixture.

of the UCM/RES ratio from settling particles to sediments (9 to 39-116), supports the early diagenetic alteration of hydrocarbons. The intense interfacial degradation of lipid fractions including hydrocarbons has been previously reported for Lake Michigan, Dobob Bay, and Saint Lawrence Estuary (15-18). The large mass sedimentation and high aliphatic concentrations in the trap material from the central area results in huge aliphatic fluxes ranging from 7.1 to 217 mg‚m-2‚day-1 RES (mean 38 ( 55 mg‚m-2‚day-1). UCM fluxes are almost 10 times higher (319 ( 290 mg‚m-2‚day-1), totaling an average aliphatic flux of ∼ 350 mg‚m-2‚day-1. These high values correlate with the large organic carbon flux (29 ( 26 g‚m-2‚day-1) and elevated sedimentary hydrocarbon levels in this coastal area. Comparison with other environments is limited by the availability of sediment trap data for freshwaterestuarine systems, but the combination of high turbidity and crude discharges guarantees that Rı´o de la Plata fluxes are probably among the highest reported (see Table 2 in Supporting Information). Rı´o de la Plata aliphatic fluxes are effectively 200-900 times higher than those reported for Lake Michigan [0.07 mg‚m-2‚day-1 RES (19)], Dabob Bay [0.05 and 0.33 mg‚m-2‚day-1 RES and UCM (15, 20)], Puget Sound [0.06 and 1.5 mg‚m-2‚day-1 RES and UCM (21)], and the Saint Lawrence Estuary [0.16 and 0.69 mg‚m-2‚day-1 RES and UCM (17)]. Rı´o de la Plata aliphatic fluxes are also 60-140 times higher than values reported for the coastal area off San Diego affected by sewage and river discharges [RES 0.27 g‚m-2‚day-1 (22)] and for the Santa Maria Basin off central California 8230

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FIGURE 3. Regression of resolved and UCM aliphatic hydrocarbons versus organic carbon content of Rı´o de la Plata sediments at 1, 2.5, and 4 km offshore.

FIGURE 4. Principal component analysis performed with the relative contribution of C13-35 n-alkanes plus isoprenoids in sediment samples and the trap material (TP, [). The sewer within the 2.5 km series is indicated by a shaded circle. The detailed load to principal component 1 and the C27+29+31/C26+28+30 CPI ratios are shown in the lower panel. affected by recurrent oil spills [0.59 and 2.4 mg‚m-2‚day-1 RES and UCM (23)]. The lower hydrocarbon fluxes in these deep freshwater-marine environments reflect the prevalence of biogenic inputs and the 50-700 times lower sediment and total organic carbon fluxes relative to the turbid and polluted Rı´o de la Plata coastal area. Resuspension of bottom material could be important in this shallow estuary, especially during storm events, which were specifically avoided in our sampling scheme. The fresher aliphatic signature of settling particles relative to bottom sediments suggests no significant capture of degraded resuspended bottom material in the traps (see composition). Hydrocarbon Composition. Overall, RLP sedimentary hydrocarbons present a marked predominance of oddnumbered carbon n-alkanes maximizing at C29 and C31, characteristic of terrestrial plant waxes (n-C23-35, 79% ( 9.5% total RES), followed by hopanes (11% ( 6.8%) and a small proportion of lower molecular weight n-alkanes and isoprenoids (n-C15-17, 1.6% ( 1.3%; C32). The triplet ratio (C26+C26/ C24) also shows lower values in the trap material and sewer sediments (1.94-1.99) relative to north (2.45 ( 0.55) and offshore sites (2.47( 0.8 at 4 km) but presents higher variability. The biodegradation pattern of hopane homologues has been a controversial issue concerning the lower vs higher molecular weight susceptibility against microbial attack (25). The examination of petroleum biomarkers in samples collected in the Strait of Magellan 24 years after the Metula oil spill revealed a degradation sequence analogous to our results: tricyclic degraded faster than pentacyclic terpanes, C30 hopane showed a lower stability relative to C31-32(S) homologues but similar to C31-32(R) epimers, and heavier homohopane homologues were more readily degraded in a sequence of C35 > C34 > C33 > C32 > C31. The C31(S)/C30 and C32(S)/C30 ratios increased from 0.69-0.39 to 0.73-0.54, respectively, from a reference oil to the most degraded sample (28). The offshore reduction of less stable fossil hydrocarbons and relative increase of recalcitrant petrogenic and biogenic compounds observed in Rı´o de la Plata sediments agrees with the reduction of lower chlorinated congeners observed for PCBs (9), reflecting the impact of fresher anthropic inputs close to the shore. Vertical Profiles of Aliphatic Hydrocarbons. The sediment cores collected in polluted stations Riachuelo and Berazategui contain high and uniform organic carbon contents (1.5-1.8% at BZ and 1.9-2.3% at RI) and decreasing hydrocarbon concentrations with depth at both sites: from 11 to 5 and from 430 to 200 µg‚g-1 RES and UCM, respectively, at Berazategui, and from 20 to 11 and from 1118 to 556 µg‚g-1 at Riachuelo (Figure 6). This hydrocarbon reduction with sediment depth continues the 20-fold decay observed for RES between settling particles and sediments. Following the reduction of hydrocarbons from the trap material to deeper sediment layers, the composition of RES shows a gradual depletion of lower molecular weight n-alkanes and isoprenoids from petrogenic sources and relative enrichment in hopanes and > n-C23 terrestrial plant n-alkanes. This pattern follows a clear alteration sequence from a fresh crude oil to settling particles and bottom sediments (Figure 6). Settling particles show an incipient alteration with still abundant 2.5 km offshore). The total hydrocarbon input calculated with the mean sediment trap flux (38 and 319 mg‚m-2‚year-1 for RES and UCM) and the surface of the 0-2.5 km central-south sector (84 km2) amount to 1160 and 9750 tons for RES and UCM, respectively, 20-2 times higher than sediment inventories for the area (58 and4888 tons, respectively). As observed

previously, the lower difference of the UCM suggests that this is related to the early diagenetic alteration of hydrocarbons at the sediment-water interface. The comparison of sediment accumulation rates with trap fluxes for different hydrocarbon components effectively indicates that the preservation efficiency follows the compounds’ intrinsic stability. The preservation increases from n-C13-22 (1.0%) to isoprenoids (3.2%) to n-C23-35 (6.3%) to hopanes (46%) to UCM (50%). A similar calculation for TOC indicates that it is more reactive (12%) than hopanes and UCM, whereas PCBs are quantitatively preserved in these coastal sediments [91% (9)]. A similar preferential loss of lower molecular weight n-alkanes and isoprenoids and higher preservation of >nC21 and UCM has been reported for Puget Sound affected by sewage effluents (4) and for the Saint Lawrence Estuary (18). The large variability of the trap fluxes and restricted temporal coverage (13 deployments) might introduce some incertitude in the calculations. However, in general, trap data are in reasonable agreement with sediment inventories and confirm the thousand tons magnitude of RES and UCM inputs in this coastal ecosystem. Considering the 2-fold reduction of aliphatic hydrocarbons with depth in the cores (Table 1), the total inventory buried down to 20 cm depth, which corresponds to the last 4-5 years, would be roughly 17 500 tons (RES + UCM). Although huge, these quantities are not unexpected considering that untreated effluents have been introduced in this environment for the last four decades. Taking into account the per capita discharge reported for Seattle and southern California treatment plants [3.1-4.9 g person-1 day-1 (2, 4)], a conservative estimation for the Berazategui sewer (5 million people) suggests an input of 15-25 tons of hydrocarbon per day or 5600-8940 tons per year-1, comparable to the inventory and trap estimations.

Acknowledgments We are indebted to Dr. C. Colombo and Mr. A. Pulcini for sampling support with the Boyero. This paper has benefited from the constructive criticism of two reviewers.

Supporting Information Available Two tables including comparison of sediment trap fluxes in different environments, and Rı´o de la Plata inventory calculations. This material is available free of charge via the Internet at http://pubs.acs.org.

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Received for review June 24, 2005. Revised manuscript received August 19, 2005. Accepted August 19, 2005. ES051205G