Journal of Environmental Quality 31:896-903 (2002)
© 2002 American Society of Agronomy, Crop Science Society of America, and Soil Science Society of America
TECHNICAL REPORTS
Organic Compounds in the Environment
Modeling the Fate of Benzo[a]pyrene in the Wastewater-Irrigated Areas of Tianjin with a Fugacity Model
X. L. Wang,
S. Tao*,
F. L. Xu,
R. W. Dawson,
J. Cao,
B. G. Li and
J. Y. Fang
Department of Urban and Environmental Sciences, Lab. for Earth Surface Processes, Peking Univ., Beijing 100871, China
* Corresponding author (taos{at}urban.pku.edu.cn)
Received for publication August 10, 2001.
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ABSTRACT
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A Level III fugacity model was applied to characterize the transfer processes and environmental fate of benzo[a]pyrene in wastewater-irrigated areas of Tianjin, China. The physical–chemical properties and transfer parameters of benzo[a]pyrene were used in the model and the concentration distribution of benzo[a]pyrene in sediment, soil, water, air, fish, and crop compartments, as well as transfer fluxes across the compartments, were then derived under steady-state assumptions. The calculated results were compared with monitoring data for air, soil, water, and sediment collected from the literature. The results indicate that there was generally good agreement and the differences were within an order of magnitude for air, soil, and sediment. The concentration of benzo[a]pyrene in the ambient air in the area was very low with a majority present sorbed to aerosol. In the water compartment, approximately 70% of benzo[a]pyrene dissolved in water phase. Relatively high concentrations of the compound were found in the soil and sediment, with the soil serving as the dominant sink in the area. Benzo[a]pyrene, with a slow metabolic rate, was found to accumulate in fish in the area.
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INTRODUCTION
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POLYCYCLIC AROMATIC HYDROCARBONS (PAHs) are by-products of the incomplete combustion or pyrolysis of organic materials, residential heating, incineration, internal combustion engines, and industrial activities (International Agency for Research on Cancer, 1983). There is increasing concern about the presence of PAHs in the environment, because the biotransformation of these chemicals in animals may produce metabolites with mutagenic and carcinogenic properties (Harvey, 1991). They are also one of the more significant contaminants in China's deteriorating environment. Of the various PAHs, benzo[a]pyrene is one of the compounds of greatest concern.
Predicting the risk of a chemical to an ecosystem or to human health requires a better understanding of the fate and accumulation levels of the chemical in various compartments of the environment. A number of models have been proposed for prediction of the fate of chemicals in the environment (Schnoor and Mcavoy, 1981; Yoshida et al., 1987). Among these, the fugacity model has seen wide application due to its simple structure and minimal parameter requirements (Mackay, 1979). The model, which is divided into four levels based on complexity, was often used to predict both relative and absolute amounts of organic chemicals emitted into the environment. Degradation reactions and advective flows, as well as transfers between compartments, can be included in the model (Mackay, 1979). Mackay et al. (1983) used a quantitative interaction fugacity model (QWASI) to predict the fate of chemicals in water, air, and sediment. The model has been applied to predictions of the fate of organic chemicals on a regional scale (Holysh et al., 1986; Mackay et al., 1992; Ling et al., 1993), as well as to investigations into the kinetic processes of both organic and inorganic chemicals (Mackay and Diamond, 1989). Another important application of the model is to estimate accumulations of organic chemicals in aquatic organisms and the transport processes of pesticide in the environment (Connolly and Pedersen, 1988; Brooke and Matthiessen, 1991; Ballschmiter, 1992).
The primary objective of this study was to simulate the transfer processes and fate of benzo[a]pyrene in the Tianjin area using a Level III fugacity model. Distributions of benzo[a]pyrene concentrations in air, water, soil, and sediment and the transfer fluxes of benzo[a]pyrene across these compartments were then estimated. Equilibrium concentrations of benzo[a]pyrene in fish, crops, and vegetables were also estimated.
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METHODOLOGY
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Study Site
Tianjin is located in northern China near Beijing. It is the third largest industrial center in China. A majority of the rivers in the area are filled with wastewater from the Tianjin and Beijing urban areas (Tianjin Environmental Protection Bureau, 1996). As with other large or medium size cities in northern China, agricultural production in the area is increasingly confronted with a major water shortage. Both wastewater irrigation and sludge application have been common practices in this area for more than 40 years. Tianjin's suburban area, covering approximately 3500 km2, is dominated by farmlands irrigated with wastewater (Tianjin Environmental Protection Bureau, 1991, 1996). These wastewater-irrigated areas were used in this study (Fig. 1)
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Rice (Oryza sativa L.) and wheat (Triticum aestivum L.) constitute the primary crops grown in the area, although a large portion of the farmland is also used for vegetables (Tianjin Environmental Protection Bureau, 1991). The wastewater used for irrigation is often a mixture of domestic sewage and industrial discharge. Sludge from wastewater treatment plants has occasionally been applied to the fields (Tianjin Environmental Protection Bureau, 1991). Furthermore, atmospheric precipitation of dust from the burning of coal, especially during the heating season, has added additional pollutants to agricultural land. Consequently, the area is severely contaminated (Tianjin Environmental Protection Bureau, 1996). Because of the water shortage in the area and a lack of relevant legislation, wastewater is also being used for pisciculture. The total area of wastewater-treated fishpond reaches approximately 4500 hectares, with an annual production of fish close to 10000 Mg (Tianjin Environmental Protection Bureau, 1996). Fish from these pisciculture ponds are taken directly to the market for human consumption.
The study area was defined as four bulk compartments: air, water, soil, and sediment. The relationships between compartments are illustrated in Fig. 2
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Fig. 2. The main compartments and the transfer processes between compartments. The terms DAW1, DAS1, and DSW1 represent diffusion rate coefficients across air–water, air–soil, and water–sediment interfaces respectively; DAW2, DAW3 and DAS2, DAS3 refer to dry and wet deposition rate coefficients from air to water and soil respectively; DSW2 is the deposition rate coefficient from water to sediment; DWR and DSR represent transfer rate coefficients from soil to water via runoff of either water or solids; I1 and I2 are inflow fluxes via air and water; D1a and D2w refer to output rate coefficients via air and water; D2f and D3c are output rate coefficients with consumption of fish, crops, and vegetables; and D3d and D4d are biodegradation rate coefficients in soil and sediment, respectively.
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For modeling purposes, the dimensions, volumes, and organic carbon fractions (if applicable) of the four compartments were defined. Within the 3500-km2 study area, surface water bodies (mostly filled with wastewater) cover approximately 1.3% of the total area (Tianjin Environmental Protection Bureau, 1996) with a mean depth of 1.7 m (Jiang, 1992). The height of the air compartment adopted in the study was 2000 m above ground (Mackay and Paterson, 1991). The thickness of surface soil (mostly used for growing wheat, rice, and vegetables) and sediments were estimated as 20 and 2 cm, respectively (National Soil Census Office, 1998). The organic carbon contents of the soil and suspended solids in water were both 5.5% and that in the sediment was 10% (National Soil Census Office, 1998). Dissolved organic carbon in the water phase was taken to be 8.0 mg C/L (Tao et al., 1988).
Each of the four compartments is composed of both pure and particle phases as subcompartments. For instance, bulk air comprises pure air plus aerosol particulates, and bulk water comprises pure water plus any suspended solids. Sediment is a mixture of water and solids. Other subcompartments include biota, namely fish for water and vegetables and crops for soil. Although they only account for a small fraction of the total volume (with little significant influence on the model output), their importance lies in the fact that humans directly consume them. Fish were estimated as 1.15 x 10-4 of the total volume of water while vegetables and crops were estimated as 2.50 x 10-3 of the total volume of the soil. They were derived from the total production (Tianjin Environmental Protection Bureau, 1996; Editorial Committee on China Statistical Yearbook, 1990–1995) and mean densities of fish (Jiang, 1992) and crops and vegetables (Paterson et al., 1991) from the area. Biota in air and sediment were omitted. The mean value of particles in the air was recorded as 0.288 mg/m3, equivalent to approximately 2.90 x 10-10 in the volume fraction (Tianjin Environmental Protection Bureau, 1996). The average of suspended solids in water was estimated as 9.23 x 10-5 in the volume fraction according to monitoring data provided by Tianjin Environmental Protection Bureau (1996). The adopted fractions for air and water in the soil were 20 and 25%, respectively (National Soil Census Office, 1998). The water content of the bottom sediment was taken as 70% (Mackay and Paterson, 1991). These data are presented in Table 1, where the volume fractions of the subcompartments are designated as Fij, where i represents the ith bulk compartment and j indicates the jth subcompartment.
Model Description
Fugacity Model
A Level III fugacity model was applied to describe the partitioning and transfer of chemicals between air, water, soil, and sediment and their subcompartments, especially biota (Mackay and Paterson, 1991). The input profiles were mainly air and water inflow into the area from the outside and locally generated wastewater and gas, while the output profiles included degradation and agricultural products in addition to the air and water flowing out of the area. Transfer between adjacent compartments consists of both diffusive and nondiffusive processes. These processes of input, output, and transfer are shown in Fig. 2.
The concentration distribution of benzo[a]pyrene in the compartments and subcompartments as well as the transfer fluxes between adjacent compartments were then modeled. Assuming the system to be at steady state, a set of steady-state mass balance equations were established in terms of fugacity (f) and transfer rate coefficients (D) among the bulk compartments. Transfer rates between adjacent compartments are expressed as transfer fluxes equal to the products of f and D in mol/h. For the bulk compartments of air, water, soil, and sediment, the respective mass balance equations are as follows:
where the terms on the left and the right hand sides represent the input and output fluxes to and from individual compartments, respectively. The products of the fugacity and transfer rate coefficient (fiDij) on the left hand side are inputs from the other bulk compartments, while those on right hand side are outputs to the other bulk compartments. The other terms are either inputs or outputs to and from the entire area. For example, I1 and I2 represent the outside fluxes to bulk air and bulk water, respectively. The first terms on the right hand side of the equations represent outputs from the air, water, soil, and sediment, respectively. The transfer rate coefficients for output include those being brought to areas outside by airflow (D1a) and water flow (D2w), losses do to the harvest of agricultural products of fish (D2f), crops and vegetables (D3c), and loss through degradation of the soil (D3d) and sediment (D4d), respectively.
The values of input terms (I) and transfer rate coefficients (Dij) were derived from the relevant data collected from the literature. The set of the mass balance equations was solved using MATLAB (The Mathworks, 1999) to derive fugacity values, which were then used to calculate the concentrations of benzo[a]pyrene in the various compartments and subcompartments. Consequently, transfer fluxes among compartments could then be calculated using the derived fugacity values. Measured concentrations of benzo[a]pyrene in air, water, soil, and sediment were collected from the literature and applied to the model for validation purposes.
Thermodynamic and Biodegradation Properties of Benzo[a]pyrene
The thermodynamic properties of benzo[a]pyrene used in the model included molecular weight (MWt = 252.0), Henry's constant (H = 0.05 Pa m3/mol; Sheng and Xu, 1988), octanol–water partition coefficient (Kow = 1.15 x 106; Sheng and Xu, 1988), biological concentration factor of fish (log BCF = 3.6 L/kg; Hu et al., 2000), organic carbon standardized adsorption coefficient (KOC = 1.07 x 104 L/kg; Sheng and Xu, 1988), and vapor pressure (PS = 7.46 x 10-7 Pa; Sheng and Xu, 1988). The biodegradation rate constant (KR = 2.30 x 10-5 L/yr) followed after Martin et al. (1998).
Fugacity Capacity
In order to calculate transfer coefficients, the fugacity capacity for each compartment is required. The fugacity capacity of the ith bulk compartment is designated as Zi in mol/m3 Pa. The Z value for each bulk compartment is, therefore, a sum of the contributions from all subcompartments; the latter being equal to the product of fugacity capacity and the volume fraction (Table 1) of a particular subcompartment, expressed as FijZij. The subscripts i and j represent the ith bulk compartment (1–4 for the air, water, soil and sediment, respectively) and the jth subcompartment (1–4 for air, water, solids and biota, respectively). The fugacity capacities for the four bulk compartments are as follows:
Z11 and Z31, air subcompartments in the bulk air and soil, were calculated as:
where R is the gas constant having a value of 8.314 Pa m3/mol K and T is the absolute temperature (298 K) (Lun et al., 1998). For the water subcompartments in the bulk water, soil, and sediment, Z22, Z32, and Z42 are identical and equal to:
where H is the Henry's constant (Lun et al., 1998). For the aerosol particulate in the air, Z13 is inversely proportional to the vapor pressure as:
where PS is the vapor pressure in Pa (Lun et al., 1998; Mackay and Paterson, 1986). The fugacity capacities of the other three solids' subcompartments in bulk water, soil, and sediment (Z23, Z33, and Z43) were calculated based on Henry's constant, the organic carbon–water partition coefficient (Koc) along with the organic carbon content in solids (C23, C33, and C43, the sixth column in Table 1), and the solid density (
23, 33, and 43) of each subcompartment:
A value of 9.23 x 10-5 was adopted for 23 (Tianjin Environmental Protection Bureau, 1996). Values of 1.0 and 1.4 kg/L were cited for 33 and 43 from National Soil Census Office (1998). The term Z24 describes fish–water partition and was estimated from the biological concentration factor (BCF), Henry's constant, and fish density (24), which was estimated as 1.2 x 10-4 (Tianjin Environmental Protection Bureau, 1996; Jiang, 1992).
For biota in the soil compartment, Z34 was estimated based on a modified Paterson approach (Paterson et al., 1991), which separated the crops (wheat and rice mainly) and vegetables into root and leaf areas (1:1):
where the first two terms in the parentheses represent Z values for root and stem, the sum of the last three terms is the Z value for the leaf, and ZA, ZW, and Zx represent the fugacity capacities of air, water, and xylem:
The term RCF refers to the equilibrium ratio of chemical concentration in the root to that in water and SXCF is the equilibrium ratio of the chemical concentration in the stem to that in the xylem's sap:
The terms R and S are the density of root and stem with a same value of 0.83 kg/L (Paterson et al., 1991) and W represents density of water.
Input from Outside of the Area
The primary inputs of benzo[a]pyrene into the area were from upstream wastewater (I2U), local wastewater discharge (I2L), long-range air transport (I1U), and local gas emissions (I1L). The input fluxes from these sources were products of flow rate (Q) and benzo[a]pyrene concentrations in the carrying media (C):
The flow rate of long-range air input from outside of the area was estimated according to mean wind velocity of the area and the cross section area of the air compartment. Data for local emissions of gases into the air were taken from the literature. Upstream wastewater was carried into the area via three main sewers; the North, South, and Beijing Sewer. The amount of wastewater discharged locally was routinely recorded by Tianjin Environmental Protection Bureau (1991)( 1996) and an average of that data from 1986 to 1995 applied. Flow rates for the main inputs into the area are listed in Table 2.
Previously measured concentrations of benzo[a]pyrene in wastewater, ambient air in Tianjin and the surrounding area, and local gas emissions were collected from the literature. Data for wastewater came primarily from Wang et al. (1986), Xue (1987), and Chen et al. (1988). Data measured by Zhong et al. (1983) were used for estimation of the benzo[a]pyrene concentration of air blown in by the wind. Benzo[a]pyrene concentrations in emissions were those measured by Cao et al. (1996). Since waters from both upstream and local discharge were all heavily contaminated, all wastewater data were grouped together. The data are summarized and tabulated in Table 3 and all concentrations are shown at the 10th, 50th (median), and 90th percentiles.
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Table 3. Selected percentiles of benzo[a]pyrene concentrations in wastewater and emission. The numbers in the parentheses are sample numbers.
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Output to the Outside of the Area
Main outputs from the area are outflows of air and water as well as degradation. Fish, crops, and vegetables produced from the area were also considered to be part of the output in emphasizing the potential hazards to consumers. From a mass-balance point of view, the absolute amount of benzo[a]pyrene in these products can be ignored. However, concentrations of benzo[a]pyrene in these products are of interest due to their direct contribution to human consumption. It was possible to calculate a number of D values for these outputs as defined in the mass balance equations describing these outputs:
where Q1O and Q2O are flow rates of air and water leaving the area. The former equals the flow rate of air into the area (Q1U = 8.87 x 1011 m3/h) while the latter was estimated as 2.33 x 105 m3/h (Tianjin Environmental Protection Bureau, 1996). The term KRi is the biodegradation rate of chemicals (1/h) with identical values taken for soil and sediment. The term Vi is the volume of soil or sediment and YF is fish production (Mg/h) based on an annual amount landed of 8850 Mg/yr (Tianjin Environmental Protection Bureau, 1996). The terms YC and YV are annual yields of crop stalks, derived from the annual yields of crops (754000 Mg/yr) and vegetables (5460000 Mg/yr) from the area (Editorial Committee on China Statistical Yearbook, 1990–1995), and the ratio of the weight of seeds to that of stalks is 0.44 (Xu et al., 1998). The terms 24* and 34 are mean densities of fish (1 kg/L; Jiang, 1992) and crops or vegetables (0.83 kg/L; Paterson et al., 1991), respectively.
Transfer Parameters of Benzo[a]pyrene
Transfer processes of benzo[a]pyrene between air and water, air and soil, and soil and water, as well as those between water and sediment (Fig. 2), were taken into consideration in the modeling. Bulk transfer can be expressed in terms of D values as described by Mackay and Paterson (1991). Transfer between air and water, air and soil, and water and sediment are similar to each other and include two types of processes, either diffusive or nondiffusive. The transfer processes from air to water, air to soil, and water to sediment are designated as D12, D13, and D24, while the transfer processes in the reverse directions are D21, D31, and D42, respectively:
where the first terms in each of the equations, DAWI, DASI, and DSWI, are diffusive transfer processes including volatilization and adsorption:
and where A12, A13, and A24 are the interfacial areas of air–water, air–soil, and water–sediment (m2), kAW and kAS are air-side mass transfer coefficients (MTC) over water or soil (m/h), kWA and kWS are water-side mass transfer coefficients over air and sediment (m/h), L3 and L4 are diffusion path lengths in soil and sediment (m), and BA, BW, and BS represent molecular diffusivity in air, water, and sediment, respectively.
For nondiffusive processes from the air to water or soil, dry and wet depositions were considered (Mackay and Hickie, 2000; Lun et al., 1998), with DAW2 and DAW3 representing conditions for water, DAS2 and DAS3 representing those for soil, and DSW2 describing the conditions for sediment deposition:
where kP and kR represent the dry deposition velocity and rain rate (wet deposition) respectively, SC is the scavenging rate of the precipitation, and kSD is the sediment deposition rate. Because the terrain of the study area is essentially flat, the resuspension term was omitted during the modeling. There is a one-way transfer from soil to water via runoff of either water (DWR) or solids (DSR) carried by water. Therefore, D32 is the sum of the two terms:
where UWR and USR are water and solid runoff rates, respectively.
In additional to the dimensional variables, the relevant transfer parameters used for calculating D values and their sources are presented in Table 4.
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RESULTS AND DISCUSSION
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Calculated Concentrations of Benzo[a]pyrene in the Bulk Compartments
Concentrations of benzo[a]pyrene in the four bulk compartments were calculated based on fugacity values derived by solving the set of mass balance equations. The results are presented in Table 5 as the 10th, 50th, and 90th percentiles of the log-transformed concentrations.
In general, the concentration of benzo[a]pyrene in ambient air in the area was very low, with relatively higher concentrations in the water, soil, and aquatic bottom sediments. Concentration in sediment was slightly higher than that in soil, possibly due to the relatively high organic carbon content of the sediment along with input through sedimentation of suspended solids in the water column. The total amounts of benzo[a]pyrene accumulation in the four bulk compartments under steady-state assumption were calculated using the estimated median concentrations and respective volumes of the bulk compartments. Results of the estimations were 140, 1.9 x 103, 5.1 x 106, and 1.1 x 104 mol for air, surface water, soil, and bottom sediment, respectively. Given the large total volume of soil, more than 99.4% of the total benzo[a]pyrene found in the area accumulated in the soil compartment. Soils, then, serve as the dominant sink for benzo[a]pyrene in the area. The validation applied only to benzo[a]pyrene.
Calculated Concentrations of Benzo[a]pyrene in the Subcompartments
A large portion of the benzo[a]pyrene found in the air was sorbed on particles. The mean concentrations estimated in the gas and solid phases were 8.52 x 10-15 and 6.85 x 10-2 mol/m3, respectively. Almost all of the benzo[a]pyrene found in the air was bound to the solid phase (1.39 x 102 mol, 99.3%) with only a negligible amount in the gas phase (5.95 x 10-2 mol, 0.7%).
In water compartment, although the concentration in the suspended solids (5.27 x 10-2 mol/m3) is three orders of magnitude higher than that of the water phase (2.46 x 10-5 mol/m3), the absolute amount of benzo[a]pyrene dissolved in water accounted for more than 70% of the total. For the soil compartment, approximately 99.6% of benzo[a]pyrene was distributed in the solid phase, while that for sediment was 98.2%.
The mean concentration of benzo[a]pyrene in fish was 9.78 x 10-2 mol/m3, while that for crops and vegetables was 1.04 x 10-2 mol/m3. The concentration factor of fish to water was close to 4000, indicating a tendency for bioaccumulation. A similar factor calculated for crops and vegetables over soil was 1.4. For factors less than 600, the benzo[a]pyrene concentration in the soil solution is used instead of the concentration of the soil matrix, showing a relatively weak accumulation tendency in plants.
Transfer Fluxes
As a steady-state model, the mass inputs into and outputs from each compartment are balanced. This was also true for the system as a whole. Figure 3
presents the major input and output fluxes for the four bulk compartments. The components shown in the stacked bars are only those that contribute significantly to the total flux. The fluxes identified in Fig. 3 account for 99.14% of the total.
The greatest input of benzo[a]pyrene to the area is from air (209 kmol/yr, Component B in Fig. 3). Of this, only 0.9 kmol/yr (around 0.4% of the total) is from local emissions. Diffusion of benzo[a]pyrene from soil to the air (A, A') represents only a very small portion of total input to the ambient atmosphere. Approximately 74% of the total air input left the area simply through airflow (C). The other 26% was deposited on the ground (D, D'), serving as the sole input to soil compartment.
Interestingly, the largest input of benzo[a]pyrene to the water compartment was not from wastewater (either upstream or local discharge). Instead, nonpoint sources (G, G') contributed about half of the total input (55%) to water bodies in the area. Another 38% came from bottom sediments (F, F'), which almost balanced with the deposition from the water column to the sediment (J, J'). A relatively large amount of benzo[a]pyrene left the area as surface water runoff (I), the majority of which entered Bohai Bay.
Taking all transfer processes into consideration, the net output of benzo[a]pyrene from the area was 2224 mol/yr; compared with 859 mol/yr from gas emissions and 1365 mol/yr from wastewater discharge, both of which were taken as input terms in the modeling.
Among those transfer processes omitted in Fig. 3 are diffusive and nondiffusive exchange processes between the air and water, diffusion between the air and soil, degradation of benzo[a]pyrene in soil and sediment, and output via fish. Degradation of benzo[a]pyrene in soil and sediment is an extremely slow process, accounting for less than 0.06% of the total flux through the system, further demonstrating the high persistence of the compound.
Model Validation
The independent observed benzo[a]pyrene concentrations in the compartments collected from the literature were also summarized in Table 5 at the 10th, 50th, and 90th percentiles (Dong et al., 1989; Hong et al., 1986; Zhu et al., 1993; Tu et al., 1986; Liu et al., 1984). These data were not used in the modeling process, but were used for comparison with the estimated values for model validation. It is evident that for three of the bulk compartments, air, soil, and sediment, differences between the estimated and observed median values were less than one log-unit, indicating a good agreement between observed and estimated benzo[a]pyrene concentrations. Unfortunately, detailed observations of benzo[a]pyrene concentrations in the surface waters of the area were not available. The only data that could be found were listed as a range (maximum and minimum) (Shen et al., 1988), and the maximum value was lower than the calculated mean (Table 5). More than likely, there is some slight overestimation of benzo[a]pyrene in surface water, because the estimated concentration based on the fugacity modeling was 2.50 x 10-5 mol/m3, which is even higher than that for wastewater (1.31 x 10-6 mol/m3). The reasons behind the overestimation should be further investigated once more data is available.
Also used for model validation were the measured benzo[a]pyrene concentrations in crops and vegetables in the studied wastewater-irrigated areas. Based on an investigation of benzo[a]pyrene in the agricultural farmlands in the area, the mean concentrations found in vegetables and crops were 2.56 x 10-3 and 1.68 x 10-3 mol/m3 (Shen et al., 1988), respectively, while the estimated concentrations were 1.04 x 10-2 mol/m3. Although there was no distinction between vegetables and crops based on the modeling, they are generally in a good agreement, although the means from the observations and the median from the estimation cannot be exactly compared.
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CONCLUSIONS
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The mean concentration of benzo[a]pyrene in the ambient atmosphere of the study area was found to be approximately 1.99 x 10-11 mol/m3, the majority of which (99.3%) was bound to particles. For surface waters in the area, the mean concentration was 2.46 x 10-5 mol/m3. Although the level of benzo[a]pyrene in suspended solids in the water was some three orders of magnitude higher than that in the pure water phase, approximately 70% remains dissolved. The benzo[a]pyrene concentrations in soil and sediment reached 7.41 x 10-3 and 1.18 x 10-2 mol/m3, respectively. Soil serves as the dominant sink for benzo[a]pyrene in the area, with more than 99.4% of the area's total accumulations for all compartments. The tendency of benzo[a]pyrene to bioaccumulate in fish, rather than crops or vegetables, was obvious. The largest fluxes of the chemical into and out of area were through airflows. Nonpoint sources provided the largest input to the water bodies of the area. The high persistence of the compound in the environment was demonstrated by the very slow rate of degradation in either sediment or soil.
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NOTES
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Funding was provided by The National Scientific Foundation of China (40031010, 40024101).
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ABSTRACT
INTRODUCTION
