On the sources of PBDEs in coastal marine sediments off Baja California, Mexico

Science of the Total Environment 571 (2016) 59–66

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On the sources of PBDEs in coastal marine sediments off Baja California, Mexico J.V. Macías-Zamora ⁎, N. Ramírez-Álvarez, F.A. Hernández-Guzmán, A. Mejía-Trejo Instituto de Investigaciones Oceanológicas, UABC, Carretera Tijuana-Ensenada No. 3917, Fraccionamiento Playitas, Ensenada CP 22860, Baja California, Mexico

H I G H L I G H T S

G R A P H I C A L

A B S T R A C T

• PBDEs were reported as very high just north of our study area in California • Consequently, PBDEs were measured for the first time on these coastal sediments. • PBDE concentrations are two orders of magnitude smaller than those reported in sediments up north. • Their distribution was found not close to the coast but rather in deeper sediments. • Only within Todos Santos Bay and Estuary, PBDEs distribution followed grain size. • Deca-BDE mixture is largely predominant followed by the Penta-BDE mixture in most samples.

a r t i c l e

i n f o

Article history: Received 29 April 2016 Received in revised form 19 July 2016 Accepted 20 July 2016 Available online xxxx Keywords: PBDEs Marine sediments Southern California Bight Mass balance Pacific Mexican Coastal area GRULAC region

a b s t r a c t Polybrominated diphenyl ethers (PBDEs) are widely distributed compounds in all types of matrices. In the northern portion of the Southern California Bight (SCB), there were reports of some of the largest PBDE concentrations in marine mammals and mussels. Because of this, we decided to analyze the status of PBDEs in the southern part of the SCB. An analysis of 91 samples of marine surface sediment was carried out. All of the 91 samples contained measurable amounts of PBDEs, which is a manifestation of the widespread distribution of these chemical substances. However, the levels detected are between one and two orders of magnitude smaller than those reported in southern California. Currents appear to control the distribution of PBDEs along the coast and the sedimentation sites with largest concentrations are favored by local bathymetry. Maximum concentrations were located in the middle and deeper platforms ranging from 0.02 to 5.90 (with a median 0.71) ng·g−1 d.w. Deca-BDE mixture is largely predominant in the sediments followed by the penta-BDE mixture. The mass balance for the latitudinal strata shows the largest concentrations in the north where the largest population centers are present and with a very clear southward gradient. The mass balance calculation values showed about 36 kg of PBDEs for the north, 22 kg for the center, and 10 kg for the south strata. In terms of depth, the PBDEs are mainly located on the middle and deep platforms rather than near point discharges, which is different than that reported by other authors. © 2016 Elsevier B.V. All rights reserved.

1. Introduction ⁎ Corresponding author. E-mail addresses: [email protected] (J.V. Macías-Zamora), [email protected] (N. Ramírez-Álvarez).

http://dx.doi.org/10.1016/j.scitotenv.2016.07.142 0048-9697/© 2016 Elsevier B.V. All rights reserved.

Polybrominated diphenyl ethers (PBDEs) are a group of substances that are chemically similar to PCBs except that chlorine atoms have

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been replaced by bromine atoms and both benzene rings are linked through an ether functional group. These chemicals have been extensively used as fire retardants since the 1960′s and 70′s (Horton et al., 2013; Vonderheidea et al., 2008). As explained by others, the source of these chemicals reach the marine environment as the result of their use in everyday products such as electronic equipment and furniture, television sets, computers, cell phones, sofas, clothing, mattresses, and other foam-based materials (Lam et al., 2009; Stapleton et al., 2012; Salvadó et al., 2016). These substances are also used in automobiles and airplanes (Hale et al., 2003; Vonderheidea et al., 2008). In North America, it is thought that PBDEs are less extensively used in Mexico than in Canada and the USA (Hale et al., 2003). There is consequently a paucity of information for part of the North American continent (Hale et al., 2003). However, many of the listed articles containing PBDEs are imported into Mexico from the USA, either as new products or even as used ones. There is a pattern of consumption on the Mexican side that is quite similar to that on the USA side (Sierra-López and Serrano-Contreras, 2002) being furniture and electronics the third and fourth items in importance. Moreover, furniture sold in California usually contained between 3 and 5% by weight PBDEs to comply with flammability standards (Zota et al., 2008). It is partially the result of having California one of the most rigorous fire retardant standards in the USA (North, 2004). In addition, there is a large production of automobiles in Mexico and other products containing PBDEs. A particularly interesting and worrisome mechanism of transportation of used furniture and other household products from the USA into Mexico has been carried out legally for many years by individuals that load small trucks with discarded carpets, mattresses, curtains, and furniture from residential units being renovated near the international border on the USA. Most of these products will eventually end up in municipal waste sites. From these sites, rain, wind, and heat or even fire will release PBDEs either to the atmosphere or they will be washed out by slightly acidic rain. Under these conditions of the slow transport of used and new products containing PBDEs, the concentration of these compounds in the local marine environment is expected to increase. We must point out that the lack of information on the amount of these substances in different Mexican coastal environments is troublesome for global estimations and modeling studies aimed at understanding chemical movements across international borders. Thus, we decided to study the behavior of PBDEs in this coastal region as it is an important component of the Southern California Bight (SCB) and is also part of the Group of Latin America and Caribbean Countries (GRULAC) to the UN region. In addition, to the best of our knowledge, this is the first report of PBDEs in marine environments in Mexican coastal waters. Due to their physicochemical characteristics, PBDEs belong to the group of persistent organic pollutants listed in the new POPs of the Stockholm Convention (http://chm.pops.int/TheConvention/ThePOPs/ TheNewPOPs/tabid/2511/Default.aspx). These manmade substances were used and sold as commercial mixtures for different applications. These mixtures include the Penta-BDE mixture, mostly containing penta and hexabrominated compounds (BDE-47; BDE-99; BDE-100; BDE-153); the Octa-BDE mixture, which consists of octa and nona brominated compounds (BDE-183; BDE-153; BDE-154); and the so-called Deca-BDE mixture, which contains almost pure, but not exclusively deca-brominated compound (BDE-209) (Alaee et al., 2003; Hale et al., 2003; Salvadó et al., 2016). The presence of PBDEs in the marine environment has been well reported and is widespread in many parts of the ecosystem. There are at least three important mechanisms by which these substances are introduced in marine sediments. In the SCB, at least from the north from Point Conception to the border between the USA and Mexico, it has also been reported that one of the mechanisms of introduction, and frequently the most important for POPs, has been wastewater discharged into the ocean. The second mechanism of introduction, and the most important for PBDEs (Dodder et al., 2012), is constituted by the runoff after storms, and the third mechanism is atmospheric

deposition, although this mode of transport for long-range transport (LRT) remains somewhat controversial, particularly for heavy PBDEs such as BDE-209 (Hale et al., 2003; Gouin and Harner, 2003; Jaward et al., 2004; Möller et al., 2010). The SCB is a very important coastal corridor both over the land as well as on the sea. On the land, it is host to many metropolitan areas including Santa Barbara, Los Angeles, and San Diego counties. In particular, our study area (Fig. 1) represents one of the largest metropolitan areas including the Tijuana-San Diego region with an estimated 1.7 million habitants in Tijuana (Copladem, 2013) and San Diego with an estimated population of 3.26 million. On the other hand, on the sea, this area belongs to an important marine corridor that includes the blue and gray whales transit-reproduction migratory route area (http:// www.nmfs.noaa.gov/pr/pdfs/rangemaps/graywhale.pdf). It also plays an important role in the location and distribution of the red spiny lobster (Panulirus interruptus), which is also one of the largest commercial fisheries for both Mexico and California, USA (Koslow et al., 2012), as well as sardines (Félix-Uraga et al., 2005), and consequently, many other predatory species in the region. In addition, this area represents an important corridor known as the Pacific Flyway for many birds (http://www.pacificflyway.gov/About.asp). Unfortunately, circulation near the coast in our study area is complicated and scarcely studied. In general, the California Current (CC) is the main factor affecting largescale circulation. This shallow current (0–100 m in depth and 200– 400 km in width) runs parallel to the coast and flows southward most of the year, with an average value of 25 cm/s (Durazo et al., 2010). Only limited studies have been carried out in the area on an inter-medium scale; the Cal-COFI program and recently the IMECOCAL program

Fig. 1. Sediment sampling sites and surface distributions for Σ16PBDEs for the coastal region of Baja California, Mexico. Main wastewater treatment plants locations are shown as red triangles. In the inset, we show as a pie chart the proportional volume discharges on average (million gallons per day-MGD) for the most important nearby wastewater plants. The enlargement of the Punta Banda estuary located inside the black lined box is shown in the bottom left side of the Figure. TBS: Todos Santos Bay. Dash line: isobath of 500 m.

J.V. Macías-Zamora et al. / Science of the Total Environment 571 (2016) 59–66

have realized a large-scale systematic hydrographic sampling of the CC system in this area. Closer to the coast and near Punta Bandera outfall (Fig. 1), the CC shows a seasonal flow inversion. The flow runs southward during the winter (20 cm/s) and then grows weaker (15 cm/s) before reversing northward during the summer (Alvarez-Sanchez et al., 1990; Marván and Navarro-Olache, 2014). Figueroa González (2006) analyzed surface current data for more than a year (September 2002– November 2003) using HF in the San Diego-Rosarito region. He found that 35% of the variability is related to the tides and the synoptic and diurnal winds. He also found that the mean flow was parallel to the coast,

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with a magnitude of 12 cm/s in the southward direction and important low-frequency (9 and 18 days) meridional variations. As mentioned before, PBDEs may be introduced via all (or some) of the mechanisms into the study area that may be active at one point or another during a particular year. The main motivation of this study was the realization that some of the largest concentrations were being reported for these chemicals in the north of the SCB, both in marine organisms (marine mammals) and marine sediments. Dodder et al. (2012) and references therein) stated that Southern California showed some of the largest concentration values for PBDEs, for example, in

Fig. 2. Concentrations of BDEs as Technical Flame-retardant Mixtures in surface sediments. a) Penta-BDE, b) Octa-BDEs and c) Deca-BDEs.

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Mytilus sp., California sea lions, and harbor seal tissues, and even relatively large concentrations in marine sediments. Given that the SCB is a shared feature of the coast (extending from Point Conception, northwest of Los Angeles and as much as 190 km south of the International Border). We wanted to investigate if our coastal environment had concentrations similar to those reported at the north of the SCB. We were also interested in studying if the particular congener composition of the PBDEs showed the same characteristic distributions as those reported in the north or if, on the contrary, there were particular differences that might have been the result of differences in the use and composition of PBDE isomer mixtures. We have included the following 16 PBDEs in this study: BDE-17, BDE-28, BDE-33, BDE-47, BDE-66, BDE71/49, BDE-77, BDE-99, BDE-100, BDE-138, BDE-153, BDE-154, BDE181, BDE-183, BDE-190, and BDE-209. However, only 14 PBDEs were detected in the samples. As suggested by Dodder et al. (2012), we expect that the local distribution of these chemicals in marine coastal sediments may be more strongly influenced by their closeness to river runoff sources. We also suspect that the main mechanisms introducing these substances into the sediments are water runoff after rainy seasons, wastewater discharges, and atmospheric deposition, most probably in that order of importance. We further speculate that the atmospheric mechanisms may be faster than marine current transport; however, given that some of the PBDEs are not very prone to LRT as BDE-209 (Breivick et al., 2006), the lighter congeners are susceptible to decomposition and the heavier ones are susceptible to attachment to particles and deposited at a shorter distance from the source, we hypothesize that atmospheric introduction may be the least important mechanism in this area. Compared to their equivalent PCB congeners, the difference in atomic weight between the chlorine and bromine atoms results in PBDEs becoming proportionally less amenable to undergoing long-range atmospheric transport (Wania and Dugani, 2003). 2. Materials and methods 2.1. Sample collection

then Soxhlet extracted for 16 h with dichloromethane (DCM, J.T. Baker, HPLC grade). Elemental sulfur interferences were removed by adding activated copper wire balls to each collecting flask. The cleanup procedure was carried out using liquid chromatography in a 1 × 30 cm glass column packed from bottom to top with 12 cm of silica (Sigma-Aldrich, 60–200 mesh, 150 Å, 3% deactivated) and 6 cm of alumina (Sigma-Aldrich, ~150 mesh, 58 Å, 3% deactivated). The elution sequence consisted of 15 ml of hexane (F1) and 60 ml of hexane/DCM (40 ml: 70/30 and 20 ml: 60/40, v/v, in sequence-F2). The fraction F2 was concentrated by rotary evaporation to about 2 ml and further reduced to 0.5 ml by drying with a gentle N2 stream. Prior to GC analysis, the volumes were adjusted to 0.5 ml with isooctane and an internal standard (FBDE-6001S, AccuStandard) was added. All GC–MS experimental measurements were performed using a gas chromatograph Agilent 7890A GC (Agilent Technologies, Santa Clara, CA, USA) coupled to a triple quadrupole mass spectrometer Agilent 7000 MS (Agilent Technologies) operated in EI mode. The GC system was also equipped with an autosampler Agilent model 7693A (Agilent Technologies), an air cooled multimode inlet (MMI), and a pneumatics control module (PCM). The column used for the separation of these compounds an Agilent DB-XLB (a length of 15 m, a diameter of 0.250 mm, and a film thickness of 0.10 μm). We used, with only minor modifications, the method proposed by Agilent (Kalachova et al., 2013) that consists of an injection of 2 μl of sample in a pulsed splitless mode using the multimode inlet (MMI). We used a compressed aircooled pulsed splitless injection at 88 °C during 0.2 min and then a ramp of 600 °C/min to 285 °C. The transfer line was kept at 300 °C, and all other parameters were equal to those reported by Kalachova et al. (2013). In general, we also used the same table of precursor ions and product ions as those suggested by Kalachova et al. (2013); we did however, change the gain for most of the BDEs, except for BDE-209. Also, the oven temperature program was the same as that suggested, with the exception that we extended the final temperature for 4.0 min. Elemental Carbon analysis to determine %TOC for all sediment samples was carried out using a LECO automated equipment (model CHNS932 microanalyzer). All samples were first treated with acid (0.1 M HCl) to remove carbonates.

As part of the Bight 2013 Mexican sector, we collected samples in the Southern part of the SCB. A standard procedure of collection using a Van Veen grab sampler was used (Macías-Zamora et al., 2014). Briefly, sediment samples were collected using a Van Veen grab sampler (top two centimeters). (Dodder et al., 2012) to measure the 16 mentioned congeners. In brief, the method is based on a randomly stratified design (Stevens and Olsen, 2003; Stevens and Olsen, 2004; Dodder et al., 2012) as is customary for the SCB projects. The collected samples were kept cold while they were transported to the laboratory, where they were maintained at −20 °C until analysis. We sampled a total of 91 stations; 41 were obtained from the northern stratum, 19 from the central stratum, 19 from the southern stratum, and 12 from additional stations located at the estuary within the southern stratum. From a vertical stratification, 29 samples were collected at each one of three shelf depths. The three depth-wise stratifications are as follows; the inner continental shelf (5–30 m water depth) with 29 samples, the mid-shelf (30– 120 m) with 29 samples, and the outer shelf (120–200 m) with the same number of samples. We also collected 12 samples from the only estuary in the whole area. The only difference with the rest of the samples was that the samples in the shallow estuary were collected from a smaller boat, and because it was not possible to use the larger Van Veen grab sampler, a ponar sampler of 0.02 m2 was used instead. As always, the top 2 cm of sediments were collected for the analysis in each case. 2.2. Sample extraction and instrument analysis The method used for PBDEs analysis is a modification of the method proposed by Zeng and Vista (1997). Briefly, a 40 g sample of dried sediment was spiked with a surrogate (FBDE-4001S, AccuStandard) and

Fig. 3. Box and whisker plots for the Σ16PBDEs measured at the different latitudinal (top) and depth strata (bottom). Values below the detection limit (D.L.) and undetected (N.D.) were replaced by LOD/√2for calculations of statistical descriptors.

J.V. Macías-Zamora et al. / Science of the Total Environment 571 (2016) 59–66

2.3. Measured concentrations at each stratum All of these were calculated using the following equation (Dodder et al., 2012). Mass ¼ m  ρ  a  d; Where Mass represents the estimated mass in the surface marine sediments; m is the sum of the concentration of the PBDEs (Σ16PBDEs) considered in the calculation; a represents the area for the

North

M1592 M1593 M1596 M1600 M1601 M1604 M1608 M1612 M1616 M1617 M1620

Central

M1594 M1597 M1602 M1605 M1606 M1609 M1610 M1613 M1614 M1618

50%

100%

M1065 M1067 M1069 M1070 M1071 M1073 M1074 M1075 M1063 M1079 M1081 M1082 M1083 M1085 M1086 M1087 M1089 M1090

South

South 100%

0%

50%

100%

M0001 M0002 M0003

North

50%

M1068 M1076 M1080 M1084

d) Estuary

c) Inner Shelf 0%

Central

M1077 M1061 M1062 M1064 M1066

M1591 M1595 M1599 M1603 M1607 M1611 M1612

M0532 M0535 M0536 M0539 M0540 M0543 M0549 M0551 M0552 M0555 M0556 M0559

0%

100%

North

50%

whole stratum under consideration (it can be calculated latitudinal by strata (North, Center, South), or vertically (by depth), Ro (ρ) represents the density of marine sediments, frequently taken (in dry weight) as 1.5 g/cm3, and d is the sampling depth in the sediment (in this particular case, it is equal to 2 cm). For the data analysis, and in particular for statistical calculations, all data below the detection limit (D.L.) were substituted by LOD/√2 (Croghan and Egeghy, 2003). For the congeners BDE 33, BDE-77 and BDE-181, those were not detected in enough sites. In fact they were purposely included to make sure of their absence for later use as possible internal standards.

b) Mid Shelf

a) Outer Shelf 0%

63

M0004 M0005 M0006 M0007

M0009 M0010 M0011 M0012

South

M0553 M0554 M0534 M0537 M0538 M0545 M0546 M0550

Central

M0008 M0541 M0544 M0557 M0560

TriBDE(17,28,33) TetraBDE ( 71/49, 47,66, 77) PentaBDE (99, 100) HexaBDE (153,154, 138) HeptaBDE (181, 183, 190) DecaBDE (209)

Fig. 4. Distribution frequency of tri-BDE, Tetra-BDE, Penta-BDE, Hexa-BDE, Hepta-BDE, and Deca-BDE in samples a) outer shelf, b) mid shelf, c) inner shelf, and d) estuary.

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3. Results and discussion PBDEs concentrations in marine sediments were detected at 91 of the 91 sampling sites, showing that they were widely distributed in the south part of the SCB sediments. From the 16 PBDEs studied, the total concentration was dominated by BDE-209, which accounted for 78% of the total (Fig. 2), followed by BDE-99 (4%), BDE-47 (3.9%), BDE-49/71 (2.5%), BDE-154 (2.1%), BDE-153 (1.6%), BDE-100 (1.6%), and BDE-17, 66, 183, and 28 (each at 1.0%). BDE-190 was found in only 16 out of 91 samples and BDE-138 was found in 27 out of 91 samples detected at near its DL, representing b1.0% of the total PBDE measured. In particular cases, such as those of BDE-77 and BDE-181, these two congeners were not detected at any of the sampled sites. This percentage distribution of BDE congeners is close to that previously reported by Dodder et al. (2012). Their three most abundant congeners in surface sediments in the north of SCB were BDE-209 (80%); BDE-47 (7.5%); BDE-99 (4.9%). It is somewhat similar to that reported for PBDEs in effluents further north in Palo Alto, CA where the largest concentration was for the BDE-99 followed by BDE-47 and in third place BDE-209 (North, 2004). From Fig. 2, it can be deduced that the two most frequently used mixtures at or near these sites were the Deca-BDE mixture, which consists mainly of BDE-209 with only a minor contribution from other PBDEs, and the Penta-BDE mixture (composed mainly of the Tetra, Penta, and Hexa-BDE), although we have to take into account that not all the congeners were included for the Octa–BDE mixture (see also Figs. 3 and 4). The strong predominance of BDE-209, which is sold and used as the Deca-BDE mixture, is clearly shown in Fig. 4 for the whole sampled area. The same figure also shows that the south stratum inner shelf shows smaller contribution from other than the BDE-209 congeners. The contribution of other congeners appears to grow towards the north coinciding with the largest population centers (Tijuana-San Diego region). The presence of PBDEs in coastal areas has been attributed by some to transport via mouths of rivers (Dodder et al., 2012), while others have explained the important role of wastewater discharges (Martellini et al., 2012). Given our special circumstances of scarce rain events (average 259.3 mm for the period of 1984–2013; INEGI, 2014), and, in particular, the surface concentration found in our area (see Figs. 2 and 6), we hypothesize that the main source to our site is wastewater discharges from treatment plants located in our study area. The PBDEs concentration has also been frequently associated with urban development; this would explain the clear north to south gradient in median concentration shown in Fig. 3. It is very likely that the circulation resulting from the CSC main southward direction given by the CC is transporting particles carrying the PBDEs and other pollutants, which may accumulate, near the isobaths of the 500 m. The accumulation spot located in the north of the central stratum has been a frequent feature of other

2.5 Todos Santos Bay

d.w.)

Method blanks (quartz sand), fortified blanks, and SRM 1944 organics in sediment (National Institute for Standards and Technology, Gaithersburg, MD, USA) were analyzed with each batch of 10 samples. Target analytes were not detected in blanks. The detection limit (DL) was estimated by injecting low concentrations of the target analytes (7 times). Three standard deviations were taken to make an estimate of the D.L. DLs are expressed in ng·g−1 d.w., considering a sample size of 40 g and a final volume of 0.5 ml. These values ranged from 0.004 to 0.02 ng·g−1 d.w. for individual BDEs. The recovery percentages for PBDE in SRM were 74.0 ± 6.4 for BDE-47, 91.9 ± 6.3 for BDE-99, 112.8 ± 12.6 for BDE-100, 109.8 ± 16 for BDE-154, 116.6 ± 6.2 for BDE-153, and 133.5 ± 10.7 for BDE-209. Surrogate standards were added to each of the samples to monitor procedural performance and matrix effects. Results for PBDEs were not corrected for the surrogate recoveries or blanks.

pollutants and has been attributed by us to an effect of the local topography. It is the largest concentration site determined for the area but its concentration is only 5.9 ng·g−1 d.w. At the north stratum, the main signal found (Fig. 1), is most probably attributable to the large discharge of particles from the Point Loma Wastewater Treatment Plant (PLWTP) and/or from the International Wastewater Treatment Plant (IWTP), which started its operations in the year 2000. This WTP process wastewater both from Mexico and from the US and discharges at b 30 m depth, right at the international border (http://www.swrcb.ca.gov/ sandiego/water_issues/programs/iwtp/index.shtml). Another possible explanation for the gradient observed in Fig. 1 at depths of around 250 m may be the flow of mostly untreated sewage down the Tijuana River (TR) Basin from Tijuana Mexico or a combination of these three sources (PLWTP, IWTP and TR). The presence of the discharge from Punta Banderas Wastewater Treatment Plant can be observed near the coast with concentrations close to 4 ng·g−1 d.w. The comparison between sediments reported for the north part of SCB, which ranged from below the detection limit to 560 ng·g−1 dry weight, is two orders of magnitude larger than those found in this work. In our case, the range extends from 0.02 to 5.90 (with a median 0.71) ng·g−1 d.w. This range in concentration is similar to that reported by Voorspoels et al. (2004) for a smaller number of congeners in the Belgian North Sea ranging from below the LOQ to 24 ng·g−1. In addition, we tested the often-reported association between PBDEs and normalizing parameters such as grain size and %TOC. We found that the best association between the Σ16PBDEs and %TOC was found in the southern strata within the Todos Santos Bay, including the Punta Banda's estuary data (Fig. 5). In the rest of the study area, there was no significant correlation between %TOC and the concentration of PBDEs. We attributed this lack of correlation to a more exposed environment outside the Bay. From the bay and up to the international border, the coast is more exposed to currents and waves making sedimentation more difficult. In addition, the marine platform is shorter and the sand grains are generally larger at the north (annex, Fig. 1). More wave energy usually means larger grain size and less surface are and consequently less ability to adsorb organic compounds and pollutants. Similar findings have been reported for the relationship between TOC and PBDEs elsewhere (Li et al., 2012). This suggests that the distribution is not being controlled by %TOC as a proxy of grain size except within the Bay. Finally, the measured concentrations at each stratum result in an estimated average total mass of about 36 kg of PBDEs for the north, 22 kg for the central, and 10 kg for the south stratum. The small estuary in Todos Santos Bay only contributes about 0.2 kg.

-1

2.4. Quality assurance and quality control

PBDEs Total (ng.g

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2.0

Plot 1 Regr Punta Banda Estuary

r²=0.78

1.5 1.0 Estuary inner

0.5 Estuary channel

0.0 Estuary mouth

0.0

0.5

1.0

1.5

2.0

2.5

TOC (%) Fig. 5. Linear regression for PBDEs Total concentration (Σ16PBDEs) in ng·g−1 dry weight versus the percent of organic content (%TOC) in marine surface sediments at Todos Santos Bay (yellow circles) and the Punta Banda Estuary (red circles). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

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marine sediment samples. We would also like to thank the Autonomous University of Baja California for partially financing this work with the internal project from 17th call contract #_629. We finally would also like to thank CONACyT for providing a doctoral scholarship for F. A. Hernández-Guzmán. Appendix A. Supplementary data Supplementary data to this article can be found online at http://dx. doi.org/10.1016/j.scitotenv.2016.07.142. References

Fig. 6. Proposed model of distribution of PBDEs from point sources attached to particles and deposited in marine surface sediments.

4. Conclusions The predominance of the BDE-209 is quite clear over the whole area. However, the total concentration for the sum of the 16 congeners is still about one to two orders of magnitude below that reported in the north of the SCB. This is most probably due to a larger population and the use of these compounds north of the international border. The fact that within our study area the normalizing relationship between the PBDEs and %TOC only exists within the Ensenada Bay and the Estuary (Fig. 5) located in the Bay suggests that the Bay may have a different behavior for marine particles and sedimentation patterns. Contrary to the finding in the north, within SCB we did not find most PBDEs near the coast. Instead, our data indicate that the largest concentrations and masses of PBDEs are located in the middle platform and the deeper sampled area. This may be due to two main factors: a prevailing north to south current flow and a very dynamic shallow platform exposed to high energy waves that do not allow the settling of small size particles (this is usually associated with larger surface area and greater capacity to adsorb organic compounds, including PBDEs). These particles settle further out and away from the coast. In addition, the maximum located near the coast just south of Rosarito is most probably due to the north-south transport and the particular topography, the place is a deep sink showing depths of about 100 m near the coast. We summarize the possible routes of these (and other) families of substances in the idealized flows of chemicals and/or particles from point sources to their maxima in Fig. 6. Our findings also indicate that the mixtures present in the Mexican part of the SCB are predominantly from the Deca-BDE and penta-BDE mixtures. The Octa-BDE mixture, taking into account that we did not measure all the congeners reportedly present in the mixture, was almost not detected, showing only a minor contribution to the Σ16PBDEs. Acknowledgments We would like to acknowledge the help of the “Algalita” catamaran and Captain Charles Moore for his help during the collection of the

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