Environ. Sci. Technol. 1999, 33, 1137-1140
Aquatic Sediment Pre-Extraction Preparations and the Effects on Aroclor 1248 Concentrations DAVID J. PRICE* AND WESLEY J. BIRGE School Biological Sciences, 101 Morgan Building, University of Kentucky, Lexington, KY 40506-0225
Although there is extensive information regarding analysis of polychlorinated biphenyls (PCBs) in sediments, a standardized procedure for preparation of stream sediments prior to extractions has not been clearly established. Objectives of this study were to determine effects of wet versus dry sediment extractions on final PCB concentrations and to evaluate PCB relationships with sediment organic carbon (OC) and sediment particle size. Sediment samples were collected from the Mud River in southwestern Kentucky and analyzed by gas chromatography. Comparative treatments included wet samples; air-dried unsieved samples; and sieved samples, including gravel-coarse sand, medium sand, fine sand, and clay-silt fractions. Results among treatments did not differ significantly, but, compared with dry sediment assays, Aroclor 1248 concentrations were lower by 52-83% when wet sediments were extracted and expressed on a dry weight basis. Aroclor 1248 values also did not differ significantly among the four sieved sediments fractions, and no distinct trend was observed. Concentrations of Aroclor 1248 and sediment OC in the gravel-coarse sand fraction were greater than expected. Aroclor 1248 dry weight concentrations (mg/kg) correlated with sediment OC when the ratio of Aroclor 1248 to sediment OC was less than 1:10 000. Results support basing final determination of PCBs on assays of dried and sieved sediments.
Introduction Polychlorinated biphenyls (PCBs) have long been recognized as worldwide environmental contaminants (1, 2). Most PCBs were produced in the United States by the Monsanto Chemical Company under the tradename Aroclor (3, 4). Aroclor formulations were assigned a four-digit numbering system, such as Aroclor 1248, where the 12 indicates the number of carbons in the biphenyl rings and the 48 indicates the weight percent of chlorine (1, 4). PCBs are hydrophobic and persistent compounds that bioaccumulate in fish and other aquatic organisms and may be detrimental to the health of fish consumers (4-6). Although procedures for the analysis of Aroclor mixtures in sediments have received considerable attention (4, 7, 8), a standardized technique for the preparation of stream sediments prior to PCB extractions has not been clearly established. Currently, the sample preparation indicated by U.S. EPA (7; Method 3540C) for Soxhlet extraction is the use * Present Address: Research Associate, 101 T.H. Morgan Bldg., School of Biological Sciences, University of Kentucky, Lexington, Kentucky 40506. Telephone (606) 257-5800; fax (606) 257-1717; e-mail address
[email protected]. 10.1021/es980959a CCC: $18.00 Published on Web 02/24/1999
1999 American Chemical Society
FIGURE 1. Location of sampling sites on the Mud River in southwestern Kentucky. This is a moderate to low gradient system with some variability in sediment composition. of 10-g whole wet sediment, from which water is decanted and large detritus physically removed. Percent dry weight is determined using a second oven-dried sample. On the other hand, Method 3541, Automated Soxhlet Extraction (7) recommends the use of 48 h air-dried sediments with grinding to obtain 1-mm sieve size for dried sediment/soil samples. The first objective of this study was to determine the effects of sample preparation prior to extraction on Aroclor 1248 concentrations. Other objectives were to determine Aroclor 1248 concentrations in the different sediment particle size fractions and to evaluate correlations between PCB concentrations and sediment organic carbon (OC).
Materials and Methods The Mud River, a PCB impacted river in southwestern Kentucky, was selected for this study. Of the original 25 monitoring sites described by Birge et al. (9) samples from 10 stations, representative of sediment composition, were included in this study (Figure 1). Station MR-2 was located 8.1 km upstream of the PCB source and was used as a reference site. The last station (MR-25) was located 108.5 km downstream, 0.2 km from the confluence with the Green River. Sediment samples were collected with stainless steel scoops and/or spoons near midstream and taken to a depth of 10 cm, including depositional areas when found. All sediment samples were collected in acetone-rinsed 0.47 L glass jars with Teflon or aluminum foil-lined lids. Stainless steel instruments used for collections were acetone-rinsed between sampling stations. Sediment samples were prepared prior to extractions by four different procedures. The first procedure involved the extraction of a wet subsample, with results expressed as wet weight. In the second procedure, Aroclor 1248 concentrations determined in wet samples were converted to dry weight after air-drying and corrections for moisture loss (10, 11). In the third procedure, unsieved subsamples were air-dried for 7 days on acetone-rinsed stainless steel pans, weighed, and extracted. In the fourth procedure, subsamples of 7-days VOL. 33, NO. 7, 1999 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 1. Variations in Aroclor 1248 Concentrations of Stream Sediments as Determined with Different Pre-Extraction Proceduresc Aroclor 1248 concentration expressed in dry wt (mg/kg)
mean organic carbon ( SD (g C/kg dry wt)
% moisture
4 6 8b 14 16 18 19 20 25
2.93 ( 2.28 20.91 ( 20.82 7.47 ( 6.87 0.97 ( 0.80 19.05 ( 2.78 36.88 ( 10.54 20.23 ( 2.77 17.69 ( 3.01 34.06 ( 7.83 24.99 ( 2.51
18.2 22.5 32.0 18.4 23.2 37.5 22.4 29.5 31.3 19.3
0.02 5.04 0.03 0.13 1.84 0.12 0.36 0.04 0.05 0.02
0.05 3.40 0.04 0.14 4.25 0.54 0.31 0.24 0.18 0.05
0.08 4.35 0.07 0.24 7.52 0.83 0.59 0.47 0.38 0.12
mean ( SD
18.52 ( 12.03
25.4 ( 6.7
0.76 ( 1.60
0.92 ( 1.55
1.47 ( 2.48
sampling station 2b
a
wet extraction unsieved soil
unsieved soil
b
dry extraction sieved soila
clay/silt only 0.09 4.16 0.05 0.26 5.96 0.73 0.53 0.43 0.38 0.09 1.27 ( 2.06 c
Averaged for all particle size classes. Sediment organic carbon was not determined for the gravel-coarse sand fraction. Aroclor 1248 values were higher in assays of sediments that were dried prior to extraction.
air-dried sediments were sieved to obtain different particle size fractions, and each fraction was analyzed individually for Aroclor 1248. Particle size fractions included gravel-coarse sand (500-µm mesh), medium sand (250-µm mesh), fine sand (180-µm mesh), and silt-clay (12, 13). All sieves were solventcleaned stainless steel. Each sample was weighed before sieving, and each sediment fraction was weighed prior to extraction. Sample weights prior to extraction averaged 27.71 ( 5.35 g for wet sediments and 26.27 ( 2.22 g for dried sediments, giving an average percent moisture content of 25.4 ( 6.7. Sieved fractions weighed prior to extraction averaged 9.51 ( 4.62, 12.62 ( 2.82, 12.59 ( 3.54, and 15.20 ( 4.28 g for gravel-coarse sand, medium sand, fine sand, and clay-silt, respectively. Determinations of sediment OC in the sieved sediment fractions were performed using the dichromate method for oxidizable carbon (14). All solvents used in PCB analysis were pesticide grade and were screened for organic contaminants prior to use. Weighed sediment subsamples were extracted with 300 mL of acetone/dichloromethane (1:1 v:v) in a 500-mL Soxhlet extractor for 15 h. The extract was concentrated to near dryness in a Rotoevaporator (Buchi Model RE121) and further concentrated with ultrapure nitrogen gas. The samples were then reconstituted to a volume of 10.0 mL in isooctane. Elemental sulfur was removed by shaking a mixture of 2-propanol (2 mL) and tetrabutylammonium sulfite (2 mL), followed by adding ultrapure water (8 mL) and reshaking. The extract was cleaned further with concentrated sulfuric acid (15). Further cleanup of interfering compounds was performed by eluting a 2.0-mL sample through a microcolumn of activated 100-200 mesh FLORISIL (100 °C/24 h) with 10.0 mL hexane and evaporating to 2.0 mL (4). A 4-µL subsample was analyzed by gas chromatography. Aroclor 1248 was analyzed using a Hewlett-Packard (HP) Model 5890A gas chromatograph equipped with an electron capture detector and an HP Model 7673A Automatic Sampler. Samples were analyzed with a 60 m × 0.53 mm ID SPB-5 (0.5-µm film) fused silica megabore column (Supelco, Inc.): temperature program, 160 °C (6 min)-10 °C/min-235 °C (0 min)-0.9 °C/min-260 °C (10 min); injector temperature, 280 °C; detector temperature, 300 °C; ultrahigh purity helium and nitrogen as carrier and makeup gases, respectively. Aroclor 1248 concentrations were based on 6-9 peaks. Peak heights were quantified using an HP Model 3396A integrator, and multiple-peak linear regression analysis was performed with LOTUS-123 software. Five external standards were used to establish calibration curves, and, during analysis, every tenth sample included either a solvent blank or a standard. Spiked sediment recoveries were also analyzed, and Aroclor 1248 recoveries averaged 97.75 ( 8.98%. Statistical procedures 1138
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included analysis of variance and Pearson’s correlation coefficients which were calculated using MINITAB statistical software and by methods described in Steel and Torrie (16). Evaluations of normality and variance homogeneity were performed using Shapiro-Wilk’s and the Bartlett’s tests with STATISTICA software.
Results and Discussion Previous studies by Birge et al. (9) indicated that Aroclor 1248 was the predominant PCB mixture detected throughout the Mud River system. The 10 stations included in this study are shown in Figure 1. Chemical and physical parameters of the sediments and Aroclor 1248 concentrations for the different sample preparations are given in Table 1 and Figure 2. The wet sediment values (mg PCB/kg wet wt) were also expressed as calculated dry concentrations (mg PCB/kg dry wt) after correcting for moisture loss (Table 1). Concentrations in the dried unsieved sediments and sieved sediment fractions also were expressed as mg PCB/kg dry wt. Average Aroclor 1248 concentrations taken collectively for combined sieved fractions were calculated as Σ(mg PCB in size fraction)/Σ(kg dry wt of all size fractions). In agreement with the study by Birge et al. (9), Aroclor 1248 concentrations generally decreased with distance downstream from the source (Figures 1 and 2). However, there was a localized decrease in Aroclor 1248 at stations 6 and 8, followed by an increase at station 14, and this variation was attributed to changes in stream morphology, substrate composition, and variation in sediment OC (Table 1). The stream bed in this area was comprised mainly of rocks and sandy clay loam. Downstream of station 8 the river increased in width, displayed greater discharge volume, and contained more depositional area. This area also included more claysilt and somewhat more sediment OC. Similar variability in Aroclor 1248 concentrations among these stations was observed by Birge et al. (9). Average wet weight and calculated dry weight concentrations of Aroclor 1248, determined with sediments extracted wet for the 10 stations, were 0.58 ( 1.22 and 0.76 ( 1.60 mg/kg, respectively. These values were less than those obtained with the unsieved 7-d dried sediments (0.92 ( 1.55 mg/kg) (Table 1 and Figure 2). Concentrations for Aroclor 1248 averaged for all sieved fractions (1.47 ( 2.48 mg/kg) and those expressed for the clay-silt fraction (1.27 ( 2.06 mg/kg) consistently were greater than observed for the other pre-extraction preparations. Although results did not differ significantly (i.e. one-way analysis of variance), there was a distinct trend in which wet extracted sediments consistently gave Aroclor 1248 values less than those determined in assays
FIGURE 2. Aroclor 1248 concentrations in stream sediments as determined with the different pre-extraction preparations, shown with mean sediment organic carbon values.
TABLE 2. Sediment Organic Carbon (g C/kg Dry wt) and Aroclor 1248 Concentrations (mg 1248/kg Dry wt) for Sieved Sediment Fractions sampling station soil fraction
2
4
6
8
14
16
18
19
20
25
gravel/coarse sand % soil fraction sediment OCa,b Aroclor 1248b medium sand % soil fraction sediment OCb Aroclor 1248c fine sand % Soil Fraction sediment OCb Aroclor 1248c clay/silt % soil fraction sediment OCb Aroclor 1248c
0.5 NA 0.66 30.8 0.77 0.06 48.5 2.70 0.05 20.2 5.31 0.09
9.7 51.66 12.33 30.6 15.36 3.94 26.2 6.49 2.13 33.5 10.12 4.16
11.1 15.21 0.17 15.9 11.17 0.13 28.6 0.68 0.04 44.5 2.81 0.05
0.4 NA 4.06 14.3 1.90 0.49 58.7 0.50 0.15 26.6 0.52 0.26
20.2 21.06 7.94 28.5 21.62 9.67 12.7 15.74 5.53 38.7 17.79 5.96
15.4 47.20 1.02 21.3 39.22 0.89 7.5 38.93 1.01 55.8 22.15 0.73
22.2 20.60 0.80 21.3 20.98 0.51 12.2 22.96 0.60 44.3 16.38 0.53
24.1 17.21 0.47 23.8 17.77 0.52 8.3 21.55 0.54 43.9 14.23 0.43
33.8 38.43 0.37 20.2 36.77 0.39 9.4 38.66 0.44 36.6 22.39 0.38
24.8 24.69 0.12 23.6 28.43 0.13 25.4 22.41 0.13 26.2 24.44 0.09
a
wt.
NA ) sediment OC was not available. b Sediment OC expressed in g C/kg dry wt. c Aroclor 1248 concentrations expressed in mg 1248/kg dry Aroclor 1248 concentrations in all four sediment fractions generally decreased at downstream Stations.
d
of unsieved 7-d dried and dry-sieved preparations. Extracts of wet sediments underestimated Aroclor 1248 concentrations by 52 to 83% when wet values were expressed in dry weight. Analyzing individual sieved sediment fractions (Table 2) demonstrated that Aroclor 1248 was present in all fractions and was not restricted to one particle size. These findings are in agreement with Boese et al. (8) and Pierard et al. (17) for specific PCB congeners. At upstream stations 2-8, the concentrations of Aroclor 1248 in the gravel-coarse sand fraction were greater than for other sediment particle sizes. This was attributed to greater amounts of detritus and higher sediment OC in the gravel-coarse sand fractions as observed at stations 4 and 6 (Table 2). However, analysis of variance
between individual sediment fractions indicated no significant differences (p
