Environ. Sci. Techno/. 1995,29,356-362
Introduction
MIN-JIAN WANG,+ STEVE P . M C G R A T H , ' A N D KEVIN C. JONES*'+ Institute of Environmental and Biological Sciences, and Lancaster University, Lancaster, LA1 4YQ, U.K., Soils and Crop Sciences Division, AFRC Institute of Arable Crops Research, Rothamsted Experimental Station, Harpenden, Hertfordshire, AL5 2JQ, U.K.
The chlorobenzene (CB) content of soil from a longterm agricultural experiment that received 25 separate sewage sludge applications from 1942 to 1961 is reported, together with data from an untreated control plot. Archived plough layer (0-23 cm) soil samples were collected, stored, and processed in a similar manner between 1942 and 1991 (Le., before, during, and after sludge applications), and samples of the applied sludges were available for analysis. The total concentration of CB compounds (CCBs) in soil increased in the sludge-amended plot. About 10% of the applied CCBs became recalcitrant and remained in the soil up to the present time, while most of the CCBs disappeared very rapidly following the amendment. Volatilization is regarded as the main loss mechanism for CBs from the soils. Hexachlorobenzene (HCB) was the most persistent CB: the loss of this compound from sludge-amended soil continued for over a decade longer than the other CBs. The 1,4-dichIorobenzene (DCB) content in both the sludge-amended and the control soils increased remarkably during the 1960s; trace level impurities in pesticides and/or atmospheric deposition are possible sources.
Application of sewage sludge to agricultural land has been practiced as an economic and efficient means of sludge disposal. About 50% of the sewage sludge produced in Britain (1.2 million tlyear, dry weight) is applied to agricultural land, with 1.3% of all U.K. agricultural land receiving sludge. Land disposal accounts for more than 30% of the sludge produced in the other European Community (EC) countries. In the United States, 23.3%of the wastewater sludge is applied to agricultural land. In Europe in recent years, the amount of sewage sludge produced has increased, but the options for disposal have become more restricted. The introduction of the EC wastewater directive requires that all sewage disposed of to sea must receive at least primary treatment, so that sewage sludge production in the U.K. will increase by-50%. The new EC directive will also stop dumping of sludge at sea by 1998,whichwill lead to major changes in the disposal outlets available for 30% of sludge discharged by the U.K. It is likely in the future that the agricultural use of sludge will increase. Regulations have been established to restrict the amount of some metals (e.g., Cr, Cd, Cu, Zn, Pb, and Nil added to agriculturalsoils by sludge application. Efforts have been made to set limits to restrict the amount of toxic organic chemicals entering soil through sludge (1-3). However,the case for organic compounds is more complex than that for heavy metals since the behavior and fate of organics introduced to soil by sludge are not yet clearly understood. This problem is compounded because many different organic contaminants can exist in sewage sludge. Chlorobenzenes (CBs), a major group of substituted monocyclic aromatics, are ubiquitous in sewage sludges (4-8). The CCB concentrations in U.K. sewage sludges have been reported to be between 0.795 and 193 mg kg-' (7). Dichlorobenzenes (DCBs), 1,2,4-trichlorobenzenes (1,2,4-TCB), and hexachlorobenzene (HCB) have been classified as priority pollutants by the United States Environmental Protection Agency (U.S. EPA) and by the EC. Some CBs (e.g.,HCB) are known human carcinogens (9, 10).
The U.S. EPA proposed new legislation on sludge application to land to set certain maximum loading rates of some chemicals, including HCB ( 1 ) . They were not well received, and a re-submission of the regulations was required (2). One of the specific recommendations was that the EPA should use field studies with municipal sludge instead of non-field studies with pure organic compounds. There is therefore a need to investigate the fate and behavior of CBs following sewage sludge application to enable an assessment of the effects of these contaminants to be made. The behavior and fate of CBs in a controlled experiment conducted in a glasshouse have been studied (11). This showed that losses of CBs from soil occurred in two phases, with a certain proportion of the CBs remaining in soil for a relatively long period, following a more rapid loss of a large proportion of the compound after sewage sludge application. However, if multiple applications of sewage sludge are made, some CBs may accumulate in field soil. +
Institute of Environmental and Biological Sciences.
* AFRC Institute of Arable Crops Research.
356 ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 29, NO. 2,1995
0013-936X/95/0929-0356$09.00/0
D 1995 American Chemical Society
soil was classified as a sandy loam with 9% clay, and its pH
TABLE 1
Chlorobenzene Concentrations and Total Weights in Sewage Sludges Applied at Woburn from 1942 to 19618 compound 1,3-DC6 1,CDCB 1,P-DCB 1,3,5-TCB 1,2,4-TCB 1,2,3-TCB 1,2,4,5-TeCB 1,2,3,4-TeCB PeCB HCB CCBs
concentrations (pg kg-l) range median mean N Db- 101 7.76-7.18 ND-126 ND-3.91 ND-14.4 ND-1.29 ND-0.97 ND-7.28 0.10-2.83 0.74-5.50 10.9-327
2.96 25.5 6.60 0.81 1.94 ND 0.25 0.26 0.39 2.53 38.9
10.7 29.8 17.4 1.22 2.63 0.21 0.33 1.89 0.69 2.51 67.4
SD
total wt applied (g ha-’)
21.6 17.6 27.9 1-09 3.32 0.36 0.33 2.56 0.57 1.27 65.4
8.93 21.9 13.4 0.86 2.15 0.18 0.27 1.71 0.55 2.03 52.0
a MCBand 1,2,3,5-TeCB were not detected. All thevalues have been corrected by the recoveries after a sample drying procedure (CAO/ CAS, see the text and ref 7). *Not detected.
To investigate the long-term behavior and potential accumulation of CBs in soil, experiments carried out under field conditions are needed. This paper therefore reports results of the analysis of soil samples from a long-term field experiment called the Woburn Market Garden Experiment, managed by Rothamsted Experimental Station, Harpenden, U.K., where multiple sludge applications were made.
Experimental Section The Market Garden Experiment at Woburn Experimental Station began in 1942. The original aim was to investigate the manurial value of different bulky organic manures on a sandy loam soil (12). Many different rates and types of organic and inorganic fertilizers were applied. In the present study, a plot was chosen which had received 25 separate applications of sewage sludge from 1942 to 1961. Sewage sludge was applied once each year during the period from 1942 to 1950,while from 1951 to 1961 the sludge was added before each crop in a rotation of three crops in 2 years. In 1942, sludge was applied at a rate of 20 t ha-’ (partially dried weight), whereas from 1943 to 1950 the application rate was 75 t ha-’, and from 1951to 1961a rate of 50 t ha-l was adopted. By the end of the applications, the total sludge addition had reached 1420 t ha-’ (partially dried weight), equivalent to 790 t ha-’ dry weight. All the sewage sludges were produced at the West Middlesex Drainage Works at Isleworth, West London. Both primary and activated sludges were anaerobically digested for 3 weeks at 30 “C and piped to nearby Perry Oaks, located near Heathrow Airport, where it was lagoon-stored and dried. The partially dried sludge was then transported to Woburn for application. Details about the CB content of the individual sewage sludges can been found in a previous paper (81, which reported 40 archived sludges but did not mention which of them were used in this specific study. The summary of the CB concentrations in the sludges used in this study are listed in Table 1. Following each sludge application, the soil was ploughed to a depth of 23 cm. The plot has only been treated with inorganic fertilizers and some pesticides since the sludge applications ceased in 1961. A control plot, which had never been treated with sewage sludges or other organic manures, was also selected. The plots were cropped and ploughed most years, but between 1974 and 1982 they were put down to grass. The
was maintained close to 6.5 by small additions of lime. Soil samples were collected from the sludge-amended and control plots in 1942, 1951, 1960, 1967, 1972, 1980, 1984, and 1991 (i.e.,before, during, and after the amendments). The samples were taken from the plough layer (0-23 cm) with a semicylinderauger. Nine cores were bulked for each sample, air-dried (except the one collected in 1991, which was analyzed in field moist condition), sieved to 1 2 mm, and stored in sealed glass bottles. No samples were taken within 1 m of the plot boundaries in order to avoid the effects of soil movement due to cultivation. A total of 20-25 g of archived soils was Soxhlet extracted with hexane. Activated copper powder was put into the flask of the Soxhlet system to desulfurize the extract. The extract was then concentrated to -2 mL in a Buchi RE 121 rotovapor (Switzerland)without using the vacuum system. This extract was cleaned up by eluting the concentrated extract with hexane from a Sep-Pak cartridge containing 1 g of Florisil and collecting the first 10- 15 mL in a glass vial. The clean extract was evaporated to 500 p L under a gentle flow of nitrogen, and 1 pL of 88 pg mL-’ of 1,3,5tribromobenzene (1,3,5-TBB) in hexane was added to act as an internal standard for the correction of retention times. The extractswere analyzed by gas chromatography (GC) with an electron capture detector (ECD) on DB Wax and Ultra-2 columns. Details of the GC analysis have been reported previously ( 1 1 ) . Recoveries of the CBs from the soil by this method were tested by analysis of soil spiked with CB standards. Recovery of monochlorobenzene (MCB) was about 38% on average; MCB also gave a poor response on the ECD so that the detection limit for MCB (500 pg kg-l) was much higher than the other CBs and not sensitive enough for this study. Recoveries of the other CBs were 82-97%, with coefficients ofvariation (% CV) less than 10%. The detection limits (3 x standard deviation of blanks) of the method for the soil samples were estimated as 0.005 pglkg (HCB) to 0.4 pg kg-’ (1,4-DCB) (see Table 2).
Results and Discussion CBs Applied to the Soil in Sewage Sludges. Twenty-five sewage sludges were applied to the selected Woburn plot. The range, median, mean, and standard deviation of the concentrations of CBs in the sludges are given in Table 1. All the data have been corrected for the recoveries of CBs after the sample drying procedure [centrifuged, air-, and oven-dried (CAO, simulating the archived sludges) over centrifuged,air-, and sodium sulfate-dried (CAS,simulating the sludges applied into the plot), i.e., CAO/CAS, see ref 81. MCB and 1,2,3,5-TeCB were not detected in the sewage sludges. The concentrations of CBs in the applied sewage sludges varied by more than 1 order of magnitude over the 20 years (1942-1961). The total CBs put into the soil each year were calculated, based on the concentrations of CBs in the sludge and the amount applied, and are shown in Figure 1. Duringthe 20-year application period, there were six years (1942,1947,1948,1952,1954, and 1959)in which the CB addition was less than 1 g ha-’, while the input of CBs reached more than 6 g ha-’ in another two years (1943 and 1953). The average load was 2.60 g ha-’ year-’. Table 1also gives the total application of CBs to the Woburn plot. From 1942 to 1961, about 52 g of CBs would have been applied per hectare in the sewage sludges. Of the CBs, the largest application was of DCBs, and 1,2,3-TCBwas smallest. VOL. 29, NO. 2. 1995 / ENVIRONMENTAL SCIENCE & TECHNOLOGY 1357
TABLE 2
Chlorobenzene Concentrations in Soil of Control Plot (ug kg-') 1W
compound 1.3-DCB 1.4-DCB 1.2-DCB 1.3.5-TCB 1.2.4-TCB 1.2.3-TCB 1.2.3.5-TeCB 1.2.4.5-TeCB 1.2.3.4-TeCB PeCB HCB XES XES1.4-DCB
1951
SD
mean
O.lEb 0.002 2.17 0.35 O.lEb 0.02 0.12 0.001 0.07 0.003 0.08 0.01 O.Olb 0.001 0.04 0.004 0.01 0.001 0.028 0.0004 0.13 0.001 3.01 0.36 0.85
1957
1960
SD
mean
0.15b 0.01 0.22 0.75 0.01b 0.02 0.06 0.01 0.06 0.01 0.03 0.003 0.004b 0.001 0.01b 0.01 0.01 0.002 0.014 0.001 0.067 0.0004 1.16 0.28 0.42
SD
mean
1972
SD
mean
mean
19M
mean
SD
0.06 0.31 0.02 0.36 0.01 0.30 0.09 9.82 0.97 3.90 0.58 3.06 0.16b0.01 0.12b0.01 0.07b 0.01 0.13 0.01 0.12 0.01 0.12 0.08 0.01 0.14 0.01 0.13 0.02 0.17 0.09 0.004 0.07 0.0040.12 0.0020.08 0.06 0.005 0.02 0.002 0.02 0.002 0.02 0.01b 0,001 O.Olb 0.02 0.03 0.001 0.04 0.01b 0.004 0.0046 0.0003 0.01 0.00 0.01 0.002 0.005b 0.035 0.008 0.022 0.002 0.028 0.025 0.001 0.13 0.007 0.18 0.018 0.15 0.012 0.15 2.43 0.16 10.9 0.97 4.97 0.62 4.06 1.07 1.07 1.00 0.70
0.20 1.73 0.096
1991
19114
SD
mean
SD
0.01 0.1W 0.02 0.14 1.40 0.08 0.01 0.10b0.02 0.01 0.10 0.02 0.01 0.15 0.01 0.0050.07 0.01 0.002 0.03 0.001 0.002 0.03 0.01 0.01 0.01 0.003 0.001 0.029 0.002 0.002 0.095 0.004 0.10 2.19 0.01 0.79
mean
SD
O.lOb
0.03
0.40 NDC 0.04 0.10 0.04 0.02
0.06
DL*
0.2 0.4 0.2 0.0050.03 0.0020.04 0.0010.02 0.001 0.02 O.Olb 0.005 0.02 0.01 0.01 0.01 0.041 0.002 0.008 0.11 0.005 0.005 0.88 0.10 0.48
.Detection limit. a B ~ l a w detection limit.
