Environ. Sci. Technol. 1996, 30, 2567-2571
Persistence of PCDD/Fs in a Sludge-Amended Soil MICHAEL S. MCLACHLAN,† ANDREW P. SEWART,‡ JEFFREY R. BACON,§ AND K E V I N C . J O N E S * ,‡ Ecological Chemistry and Geochemistry, University of Bayreuth, 95440 Bayreuth, Germany, Environmental Science Division, Institute of Environmental and Biological Sciences, Lancaster University, Lancaster, LA1 4YQ, U.K., and Macaulay Land Use Research Institute, Craigiebuckler, Aberdeen, AB9 2QJ, U.K.
Data are presented on PCDD/F persistence in a sludgeamended soil sampled from a long-term field experiment started in 1968. Over 50% of the PCDD/ Fs present in the soil in 1972 were still present in 1990. The concentrations of all congeners were observed to decrease gradually and in the same manner over this time, indicating that either physical loss of material from the experimental plot had occurred or all congeners had undergone a uniform reduction in extractability over time. Half-lives for the disappearance of PCDD/Fs from the sludge-amended plot post-1972 were of the order of 20 years; however, the degradation/alteration of PCDD/Fs in soil may take much longer since these half-lives are believed to be principally affected by physical removal.
Introduction Polychlorinated dibenzo-p-dioxins and -furans (PCDD/Fs) are two groups of organic compounds that are ubiquitous in the environment at ultra-trace levels, but have attracted considerable concern among scientists and policy makers because of their environmental persistence, their tendency to bioaccumulate through the food chain, and their toxicity. PCDD/Fs are not produced intentionally but are released into the environment in ultra-trace amounts from various combustion processes and as a result of their occurrence as unwanted byproducts in various chlorinated chemical formulations [e.g., pentachlorophenol (PCP)]. The principle route of background human exposure to PCDD/Fs for the general population living in industrialized countries is via the agricultural food chain pasture/feedsgrazing livestocks meat/milk/dairy products (1-3). In turn, PCDD/Fs enter agricultural soils and crops via atmospheric deposition (46), and this can be supplemented where sewage sludges are applied as an amendment (7-9). Routine application of sludge to agricultural land will increase the burden of PCDD/Fs in the soil (4). Sludge itself receives a “baseline” * Corresponding author fax: +44 1 524-843854. † University of Bayreuth. ‡ Lancaster University. § Macaulay Land Use Research Institute.
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1996 American Chemical Society
input of PCDD/Fs from combustion-derived sources that may be supplemented by trace impurities released from the manufacture and use of various chloroaromatic compounds (4, 5, 9-15). PCP use in textiles has attracted attention as an important contributor of PCDD/Fs (12, 13), while impurities in the dyestuff chloroanil have also attracted attention (16). In addition, there is evidence that some PCDD/Fs may be formed from simpler molecules during wastewater treatment or in the sludge itself (17, 18). In the U.K. over 1 million t of sewage sludge is produced annually, of which about 50% is applied to agricultural land; only around 1% of all U.K. agricultural land receives sludge in any one year (4). A recent national inventory for the U.K. estimated that atmospheric deposition and sewage sludge currently supply roughly equal amounts of ∑PCDD/F to U.K. soils each year, although deposition will supply a greater total toxicity equivalent (∑TEQ) (4). Substantial quantities of sewage and sludges currently disposed of to sea will require an alternative method of disposal by 1998 due to changes being implemented under the U.K.’s Urban Waste Water Treatment Directive. In the U.K. and Germany, sludge application to agricultural land is controlled under legislation established by the European Commission (19), which sets limits for the amounts of various heavy metals that can be added to soils in sludge over a 30-year period (20). In practice in the U.K., there are also constraints on the amounts of sludge that can be applied in ‘nitratesensitive’ areas, because of the need to safeguard groundwater quality (4). A typical application rate of sludge in the U.K. is ∼10 t/ha to arable soils and ∼5 t/ha to pasture (4). No legislative limits have been applied in the U.K. for organic chemicals present in sewage sludge. However, in Germany strict guidelines for PCDD/F in soil have been imposed (21, 22). In July 1992, a limit value of 100 ng of TEQ/kg dry matter was established for PCDD/Fs in sludges used in agriculture, set in conjunction with a sludge application rate limit of 5 t of sludge (DW)/h over a 3-year period (22). This will restrict the options available for the disposal of sewage sludge in Germany. There are economic, political, and scientific implications arising from restricting the addition of sewage sludges to agricultural land on the basis of their PCDD/F content. Consequently, various attempts have been made to assess the potential impacts of sludge-derived PCDD/Fs entering the agricultural food chain on human exposure (4, 8, 9, 23, 24). One of the major uncertainties in these pathways analyses and risk assessment exercises concerns the persistence or half-lives of PCDD/Fs in soils (4, 23, 24). Very little data exist from which to make valued judgments. It is known that PCDD/Fs have ‘long’ half livessof the order of many yearssbut uncertainties remain over their longterm fate, behavior, and biological significance in soils (4, 9). Hagenmaier and co-workers (25) analyzed two soils over a period of 8 years and were unable to detect any changes in the PCDD/F concentrations in this time period. Immobility of PCDD/F in soil was also demonstrated by Young and co-workers (26). They buried Agent Orangecontaminated with tetrachlorinated dibenzo-p-dioxin (TCDD) in a sandy soil on a military base. After 12 years, the contaminant profile in the soil was measured. The amount of TCDD originally applied was completely re-
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covered at the same depth at which it had been buried. The only change detected was a small band broadening, which the authors attributed to gaseous diffusion. In this paper, we present data on PCDD/F persistence in a sludge-amended soil sampled from a long-term field experiment started in 1968 at Luddington (near Stratford on Avon) in the U.K. It should be noted that the experiment was originally envisaged as a study over ca. 5 years of the transfer of metals from sludge-amended soils to crop plants. Some aspects of the field design and sampling therefore preclude the possibility of completing a detailed mass balance of PCDD/Fs at this site; these factors are highlighted in the Materials and Methods section. Nonetheless, given the current widespread interest in the long-term fate and behavior of PCDD/Fs in soils, we present data from the study and are careful to discuss the limitations that apply to it.
Materials and Methods Details of the Field Experiment and Sampling. In 1968, the U.K. Agricultural Development Advisory Service (ADAS) and the Macaulay Institute for Soil Research (now the Macaulay Land Use Research Institute, MLURI) set up experiments at Luddington to investigate the uptake of selected metals by crops on sewage sludge-amended soils. Fuller details can be found in Berrow and Burridge (27). In brief, sewage sludges contaminated with either Cr, Cu, Ni, or Zn and a rural uncontaminated sludge were applied to different soil plots at different rates. The soils at Luddington are 79% sand, 6% silt, and 15% clay, with an organic matter content of 1.8% and a pH of 5.8. Each plot was ca. 4 m × 2 m in size, and initially asbestos boards were sunk around the plots to help enclose the large quantities of sludge as they were applied. Following application, the sludge was dug or rotavated in to a depth of 15 cm. The asbestos boards were subsequently removed to allow cultivation of the soils; 1-m borders were incorporated into the design of the experiment to restrict transboundary transfers between the ends of plots, although there was no border between plots along their lengths (i.e., the 4-m sides). Between 1968 and 1972, red beet and lettuce were grown and harvested for the metal uptake studies; the soils were therefore turned over by hand each year to minimize transboundary transfers. In 1972/1973, the plots were plowed and put under grass, with herbage sampled intermittently thereafter. The field experiment had a total of 64 plots because of the different sludge amendments and replication. For the purposes of this investigation, two plots were chosen, an untreated control plot and a plot treated with Zn-rich sludge that had received an application of 125 t (dry weight)/ha in 1968. Both the sludge-amended plot and the control plot selected for this study were corner plots and therefore adjacent to only two other sludge-amended plots. A sample of the sludge and the pre-sludged (control) soil were taken and stored in 1968. Since then, soil samples have been taken periodically (in 1972, 1976, 1981, 1985, and 1990) from both control and sludge-amended plots. Samples were taken to a depth of 15 cm using a pot auger with at least 25 cores bulked together to provide a representative sample. The 1985 sample was taken by ADAS staff, but no details are available as to how it was taken. The soil samples were air-dried at room temperature, disaggregated, and sieved (to
