Environ. Sci. Technol. 1999, 33, 4310-4317
Land Treatment of PAH-Contaminated Soil: Performance Measured by Chemical and Toxicity Assays G R E G O R Y D . S A Y L E S , * ,† CAROLYN M. ACHESON,† MARGARET J. KUPFERLE,‡ YONGGUI SHAN,‡ QIN ZHOU,‡ JOHN R. MEIER,§ LINA CHANG,§ AND RICHARD C. BRENNER† National Risk Management Research Laboratory and National Exposure Research Laboratory, U.S. Environmental Protection Agency, Cincinnati, Ohio 45268, and Department of Civil and Environmental Engineering, University of Cincinnati, Cincinnati, Ohio 45221-0071
The performance of a soil remediation process can be determined by measuring the reduction in target soil contaminant concentrations and by assessing the treatment’s ability to lower soil toxicity. Land treatment of polycyclic aromatic hydrocarbon (PAH)-contaminated soil from a former wood-treating site was simulated at pilot scale in temperature-controlled soil pans. Nineteen two- through six-ring PAHs were monitored with time (initial total PAHs ) 2800 mg/kg). Twenty-five weeks of treatment yielded a final total PAH level of 1160 mg/kg. Statistically significant decreases in concentrations were seen in total, two-, three-, and four-ring PAHs. Carcinogenic and fiveand six-ring PAHs showed no significant change in concentration. Land treatment resulted in significant toxicity reduction based on root elongation, Allium chromosomal aberration, and solid-phase Microtox bioassays. Acute toxicity, as measured by the earthworm survival assay, was significantly reduced and completely removed. The Ames spiral plate mutagenicity assay revealed that the untreated soil was slightly mutagenic and that treatment may have reduced mutagenicity. The variety of results generated from the chemical and toxicity assays emphasize the need for conducting a battery of such tests to fully understand soil remediation processes.
Introduction The objective of this study was to evaluate the performance of land treatment of creosote-contaminated soil using standard chemical analysis for the target contaminants and bioassays for soil toxicity. Land treatment of site-contaminated soil is usually defined as the tilling of an 8-12-in. deep layer of soil to promote aerobic biodegradation of organic contaminants. Typically, full-scale land treatment would be conducted in a prepared-bed land treatment unit, an open shallow reactor with an impermeable lining on the bottom * Corresponding author phone: (513)569-7607; fax: (513)-5697105; e-mail:
[email protected]. † National Risk Management Research Laboratory. ‡ University of Cincinnati. § National Exposure Research Laboratory. 4310
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ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 33, NO. 23, 1999
and sides to contain leachate, control runoff, and minimize erosion, and with a leachate collection system under the soil layer (1). Tilling is usually conducted weekly. Although there are only a few published studies of land treatment of sitecontaminated soils, as early as 1993, land treatment was used in 61% of all soil bioremediation projects (2). Treatment of soils contaminated with polycyclic or polynuclear aromatic hydrocarbons (PAHs) by land treatment has been studied at bench scale (3) and pilot scale (4-7) and applied at full scale (8-10). Typically, land treatment of PAHcontaminated soil produces a relatively rapid drop in two-, three-, and four-ring PAH concentrations in the first 1-3 months of operation followed by a relatively slow rate of removal for the remainder of the operation. Five- and sixring PAHs are rarely affected by the treatment. Final removals based on the sum of the two- to six-ring PAHs (“total” PAHs) are often observed in the range of 50-70%. However, few studies have evaluated whether the changes in PAH levels due to treatment are reflected in reduced soil toxicity. Traditionally, remedial technologies such as land treatment, developed to reduce the risk associated with hazardous soils, have been evaluated using assays for specific chemicals. Including toxicity bioassays can strengthen these technology evaluations in several ways. First, bioassay testing evaluates the aggregate effect of the soil on the reporting organisms and, thus, includes aspects such as soil matrix effects, sorption/desorption behavior, bioavailability, and chemical mixture interactions. As a result, bioassay testing can demonstrate changes in toxicity rather than inferring risk reduction based on chemical concentrations. In addition, by using bioassays to evaluate soil samples following remedial technology processing, increased responses due to incomplete treatment or toxicity introduced by process amendments can be identified. For example, in bioremediation processes, incomplete degradation of target contaminants is a common concern. Toxicity, measured before and after treatment, provides a rational basis for evaluating whether a treatment is effective. It would be difficult to monitor biodegradation products in any other way due to the large number of these compounds that can potentially be formed. Several assay characteristics were considered in the process of selecting bioassays for this study including test organism, exposure period, and assay endpoint. In general, assays were selected to cover a range of species, exposure periods, and assay endpoints. For example, the assays selected include plants, animals, and bacteria as test organisms. In addition, both acute (exposing the test organism for a relatively short period of time) and chronic (exposing the test organism for a substantial portion of the organism’s life cycle) assays were selected. Finally, assays may address a variety of endpoints such as mortality, mutagenicity, growth, or endocrine system disruption. A variety of endpoints were included in order to develop a useful characterization of the toxicity. The assays selected for this study are listed in Table 1. These assays were generally selected based on their relevance to soil toxicity (9, 11-18). The list is far from exhaustive and would be strengthened by the addition of chronic sublethal animal assays such as earthworm growth and reproduction. The Ames assay was added to the list to more fully characterize changes in the mutagenicity of soil due to land treatment. Because it is relatively inexpensive and rapid, the solid-phase Microtox assay was also included. The current assay list provides a substantial toxicological evaluation; testing costs prevented using a larger collection of assays. 10.1021/es9810181 CCC: $18.00
1999 American Chemical Society Published on Web 10/22/1999
TABLE 1. Toxicity Bioassays Used To Evaluate Land Treatment exposure period assay and species used
cepaa
acute
Allium mitotic aberrationsAllium AmessSalmonella typhimuriuma 14-day earthworm survival sEisenia fetida and Lumbricus terrestrius 5-day root elongationsAvena sativa and Lactuca sativa 5-day seed germinationsAvena sativa and Lactuca sativa solid-phase MicrotoxsVibrio fischeri a
chronic
x x x x
x x
endpoint lethal
x x
sublethal
refs
mitotic aberration mutagenicity growth metabolism
11, 13 9, 11-18 9, 11-17 11-17 16-18
Test uses a soil elutriate or extract of soil.
Materials and Methods Pilot Scale Land Treatment Reactors. The soil pan reactors are described in detail elsewhere (5, 6). In brief, four 30 cm × 30 cm × 45 cm (w × l × d) stainless steel pan reactors were suspended in a water bath and housed in a custombuilt plexiglass and steel glovebox. Air entered each box through a high efficiency particulate arresting filter and was vented through a common ventilation duct. Six such boxes were built, holding a total of 24 soil reactors, and operated indoors in U.S. Environmental Protection Agency’s Test and Evaluation (T&E) Facility in Cincinnati, OH. The experiment described here used one reactor per box or a total of six reactors. The water baths for temperature control of each box were connected in series such that the water reservoir containing the heater and chiller fed water to box 1, which fed water to box 2, etc., until the water leaving box 6 was returned to the reservoir. The reactors were filled, from the bottom up, with 3 in. of clean, graded gravel underdrain, a stainless steel screen, and 8 in. of test soil. Each pan held 28.8 kg of dry soil. The soil was tilled weekly by hand using a pitchfork. The temperature was controlled at 20 °C, and soil moisture was maintained in the range of 12-14% (moisture mass relative to wet soil) by weekly soil moisture measurements and a corresponding weekly addition of a calculated amount of distilled water. Experimental Design. An experiment run in parallel to the test described here showed that all 24 reactors were virtually equivalent as experimental units. Thus, the six reactors used for this test were treated as replicate reactors. One reactor was chosen randomly from each box for use in this experiment. Killed control reactors were not operated due to the large amount of soil needed for each soil pan and because the preliminary bench-scale screening test operated for a comparable time period showed
