Environ. Sci. Technol. 2008, 42, 9086–9091
Cold-Trapping of Persistent Organic Pollutants in the Mountain Soils of Western Sichuan, China D A Z H O U C H E N , †,‡ W E N J I E L I U , † X I A N D E L I U , * ,† J O H N N . W E S T G A T E , § AND FRANK WANIA§ Chinese Research Academy of Environmental Sciences, Beijing 100012, China, National Research Center for Certified Reference Materials, Beijing 1000013, China, and Department of Physical and Environmental Sciences, University of Toronto Scarborough, 1265 Military Trail, Toronto, Ontario, Canada M1C 1A4
Received July 4, 2008. Revised manuscript received October 8, 2008. Accepted October 9, 2008.
In the Chinese province of Sichuan steep mountains rise from the very densely populated and intensely cultivated Chengdu basin more than 4000 elevational meters to the Tibetan Plateau. This steep physical gradient is exceptionally well suited to investigate the transport of persistent pesticides and other organic contaminants from low to high elevations. In spring and autumn 2006, 25 soil samples were taken at five elevations ranging from 2636 to 4479 m along the East-facing slope of Balang Mountain in Wolong Nature Reserve. Analysis of soil extracts was done by gas chromatography-high resolution mass spectrometry. Whereas hexachlorobenzene (HCB), hexachlorocyclohexanes (HCH), and dichlorodibenzotrichloroethane and its degradation products (DDTs) were present at levels of a few ng/g, only two light PCB congeners were detected at levels below 1 ng/g in soil. Soil concentration for all analytes increased significantly and exponentially with altitude. The rate of concentration increase, expressed quantitatively through the slope of the linear regression between the logarithm of the concentrations and altitude, increases along the sequence HCB < PCB < HCH e DDT. This trend is consistent with, and therefore lends additional observational support to, a mountain cold-trapping mechanism based on the temperature dependence of precipitation scavenging.
Introduction Organochlorine pesticides (OCPs) and polychlorinated biphenyls (PCBs) are persistent semivolatile compounds, which are ubiquitously detected around the globe. Because of longrange atmospheric transport, they can reach and affect remote ecosystems at high latitudes (1-3) and altitudes (4-7). Mountain cold-trapping refers to the relative enrichment of some semivolatile organic compounds at higher altitudes as a result of temperature controlled environmental partitioning processes (6, 8-10). Wet deposition processes are efficient in transferring organic contaminants from the atmosphere to the Earth’s surface, if they partition strongly to atmospheric particles, rain droplets, and snow flakes. Because the * Corresponding author e-mail:
[email protected]. † Chinese Research Academy of Environmental Sciences. ‡ National Research Center for Certified Reference Materials. § University of Toronto. 9086
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partitioning into these condensed phases increases at lower temperatures, wet deposition processes tend to be more efficient at high altitudes and in winter time. This has been hypothesized to be the main driving force for the contaminant enrichment with altitude in mountains (10, 11). Organic contaminants in mountain environments may influence the quality of water resources for drinking and agricultural use in much wider regions (9). In Sichuan province in Western China, a steep altitudinal transect separates the highly populated and intensively cultivated Chengdu plain from the Tibet-Qinghai Plateau. The transition area is of great ecological value and includes one of the major habitats for giant panda (Ailuropoda melanoleuca) in China. Despite being largely herbivorous, captive giant panda have been shown to bioaccumulate OCPs such p,p′-DDE and β-HCH (12). Whereas there have been a number of studies on OCP contamination of the TibetQinghai Plateau itself (e.g., refs 13 and 14,), the potential for contaminant transport and accumulation in this transition area has not previously been investigated (15). In this study, soil, representing the major environmental storage reservoir, was collected twice, in April and October 2006, along the East-facing slope of Balang mountain, Sichuan. The samples were analyzed for OCPs and PCBs using gas chromatography-high resolution mass spectrometry (GC-HRMS). The objective was to interpret the concentration patterns along the altitudinal gradient and its seasonal variations, with the aim to gain insight into the underlying environmental processes such as atmospheric transport, deposition, evaporation, and degradation.
Experimental Section Sampling Area. The sampling sites are on the East-facing slope of Balang Mountain in Wolong Nature Reserve (WNR), in Wenchuan county, Aba Tibet and Qiang Minority Autonomy, Sichuan (Figure 1). Provincial road 303 winds its way through WNR on the slope of Balang Mountain and reaches the divide at 4475 m above sea level (m). Chengdu plain to the East is the most frequent region of origin of air arriving at WNR, although on the synoptic scale the area is under the influence of westerlies during winter (from November to April) and a southeast monsoon during summer (from April to October). There are obvious vertical climatic variations. The temperature lapse rate is about 0.6 °C per 100 m and wind speed and insolation increase with elevation. Annual precipitation is approximately 800 mm and increases only very slightly (by approximately 10%) between 2700 and 3400 m asl (16). Approximately 70% of the annual precipitation falls in the summer months (May through September) (16). As a consequence of the meteorological variations, vegetation cover changes with elevation: deciduous woods dominate below 2000 m; deciduous and coniferous woods mix from 2000 to 2600 m; coniferous woods become dominant above 2600 m; from 3600 m upward alpine bush and meadow are the major biomes, whereas above 4000 m only alpine meadow can be found. Surface soils become thinner and patchy, and are rocky at these elevations. Some mountains have snow-covered peaks all year round. Soil Sampling and Characterization. Soil was sampled at five sites along the Balang Mountain transect in both spring and autumn 2006. At high altitudes, the sites were chosen based on reasonable accessibility and low heterogeneity of the mountainous terrain. At each site, organic-rich surface soil (about 5-10 cm depth) was taken at 5-6 spots within an area of 100-200 m2 using a stainless steel shovel and mixed on an enameled plate to form a homogeneous sample, 10.1021/es8018572 CCC: $40.75
2008 American Chemical Society
Published on Web 11/17/2008
FIGURE 1. Maps showing the location of the province of Sichuan within China, of the Wolong Nature Reserve within Sichuan, and of Balang Mountain within the Reserve. which was wrapped in baked aluminum foil, sealed in plastic bags, and stored frozen until analysis. For replication, this procedure was repeated two to three times within the same area. Geographical coordinates and sampling times of all 25 soil samples are given in Supporting Information (SI) Table S1. Organic carbon (%OC) in individual soil samples, measured using a CHN elemental analyzer (MT-5) as described in ref 17, ranged from 1 to 16% (SI Table S1). Organic carbon (%OC) was usually within a factor of 2 for samples taken at the same site. There is no strong relationship between %OC and altitude, although on average soils sampled below the tree line have approximately double the %OC of those above (SI Figure S1). Yak herding, which could be seen at sites above 3000 m, alters the natural seasonal vegetation cycle and may result in %OC in soil that is higher than would be expected for an undisturbed alpine meadow. Extraction and Quantification. After cold drying and sieving, 2,4,5,6-tetrachloro-m-xylene (TMX) and PCB209 were added to the soil samples as recovery surrogates (J&K Company). Extraction of soils with acetone and hexane (1:1) lasted for 16 h. Soil extracts were passed through a Florisil column and sulfonated. All samples were prepared for GCHRMS analysis in a final volume of 1.0 mL. OCPs and PCBs were separated on a 30 m, 0.25 mm i.d. VT-1 capillary column (film thickness 0.25 µm) and analyzed using a Finnigan MAT 900 XL gas chromatography-high resolution mass spectrometer (GC-HRMS) operated in the electron impact mode. The instrumental conditions were as follows: injector temperature 210 °C; ion source temperature 220 °C; interface temperature 250 °C; filament 0.55 mA and 42 eV; electron amplifier 1.75 kV; mass resolution 10 000; temperature program: 70 °C (1 min), 70-150 at 20 °C min-1, 150-250 at 2 °C min-1, 250 °C (20 min). The carrier gas was helium at a constant flow rate of 1.2 mL min-1. A 1 µL sample was injected in splitless mode,
and switched to split mode one minute later. The following 11 compounds were quantified: R-HCH, β-HCH, γ-HCH, δ-HCH, HCB, PCB-28, PCB-52, p,p′-DDE, p,p′-DDD, p,p′DDT, o,p′-DDT. Standard solutions BW3702 (15 OCPs in isooctane) and BW3706 (seven indicator PCB congeners in isooctane) were purchased from the China National Research Center for Certified Reference Materials in Beijing. δ-HCH was not detected in any sample. Prior to GC-HRMS analysis, a known mixture of 13C-labeled γ-HCH, HCB, PCB-28, PCB52, p,p′-DDE, and p,p′-DDT from Cambridge Isotope Laboratories (Andover, Massachusetts) was added as internal standards. The selected ions for target analytes and corresponding 13C-labeled compounds are 219/225 for HCHs, 286/ 292 for HCB, 318/330 for p,p′-DDE, 235/247 for p,p′-DDD, p,p′-DDT, o,p′-DDT, 258/270 for PCB-28 and other trichlorinated biphenyls; 292/304 for PCB-52 and other tetrachlorinated biphenyls. Mean surrogate recovery was 78% for TMX and 77% for PCB-209. Concentrations in solvent blanks were low. Analytical procedure blanks (in units of ng/g dry weight for 10 g soil samples) ranged from 0.003 for PCB-52 to 0.026 for γ-HCH and p,p′-DDT; method detection limits ranged from 0.002 for PCB-52 to 0.07 for HCB. More details are provided in the SI. The recoveries of the target analytes were fairly constant and averaged 80 ( 3%. Airshed Calculation. Five-day back trajectories arriving at the coordinates of the 95 km milestone site were calculated at 50, 100, and 200 m above ground level at 6 h intervals for the 6 months preceding soil sampling using the Canadian Meteorological Centre Trajectory Model. The more than 4000 trajectories for each of these three altitudes were compiled to produce back trajectory probability density maps, referred to as “airsheds”, which show Chengdu plain was the most frequent region of origin of air parcels arriving at the site (SI Figure S2). The easterlies from Chengdu plain prevail all year VOL. 42, NO. 24, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 1. Median and Range of the Concentrations of Organochlorine Compounds Measured in Soil Samples (in ng · g-1, Pooled for Spring and Autumn Samples) from the Wolong Nature Reserve in Sichuan, As Well As Parameters Describing the Exponential Regressions of the Organic Carbon Normalized Concentrations against Elevation (Eq 2) R-HCH β-HCH γ-HCH HCB PCB-28 PCB-52 p,p′-DDT p,p′-DDE p,p′-DDD Sum of pp′-DDX o,p′-DDT
median
range
r2
m · 104
c
significance
0.25 0.06 0.08 0.25 0.009 0.004 0.14 0.25 0.03 0.43 0.22
0.05-0.81 0.03-0.14 0.03-0.25 0.07-0.54 0.006-0.16 0.002-0.027 0.02-0.39 0.02-0.63 0-0.75 0.08-1.3 0.07-0.4
0.94 0.76 0.98 0.89 0.84 0.94 0.90 0.71 0.90 0.94 0.73
5.3 ( 0.8 5.5 ( 1.8 5.7 ( 0.4 3.2 ( 0.7 3.5 ( 0.9 4.8 ( 0.6 3.7 ( 0.7 1.9 ( 0.7 8.1 ( 1.6 5.5 ( 1.8 2.6 ( 0.9
-(1.5 ( 0.3) -(2.1 ( 0.7) -(2.0 ( 0.2) -(0.8 ( 0.2) -(2.1 ( 0.3) -(2.9 ( 0.2) -(1.2 ( 0.3) -(0.3 ( 0.3) -(3.5 ( 0.6) -(5.4 ( 1.5) -(0.5 ( 0.3)
p < 0.01 p < 0.10 p < 0.01 p < 0.05 p < 0.05 p < 0.01 p < 0.05 p < 0.10 p < 0.05 p < 0.01 p < 0.10
round. As for the earlier origin of the air mass, there were seasonal differences. In the summer half-year (April to October) it came from a wide coastal area ranging from the South China Sea, Guangdong and Guangxi provinces to Shangdong, Jiangsu, Henan, Anhui, Hubei provinces in the lower reaches of the Yellow River and the Yangzhi River, as well as North China including Shanxi, Gansu, Qinghai and Inner Mongolia; in the winter half-year (October to April next year) it came from provinces in the lower reaches of the Yellow River and the Yangzhi River, Yunnan to the south, Tibet-Qinghai Plateau to the west as well as Shanxi, Gansu and Xinjiang in northwestern China.
Results and Discussion Soil Concentrations. Blank corrected averages and ranges of the concentrations (ng g-1) in WNR soil samples are listed in Table 1 and compared with literature data in SI Table S2. Concentrations of HCHs, HCB, and DDTs in WNR soils were similar (
