elevated temperature and pressure within an extraction cell. After extraction the solvent is collected using pressurized nitrogen to force the solvent from the extraction cell to the collection vial. Their procedure can be used on samples ranging from 1 to 30 g. The extraction is quantitative and comparable to Soxhlet extraction but takes only 15 min and requires an amount of solvent 1.2-1.5 times the size of the extraction cell. The researchers report results on the effects of temperature cind pressure with a variety of environmental matrices and analytes including several certified samples for PCBs and PAHs [Anal Chem 1996 68(6) 1033-39)
Easier separations
7 ml of "SO/SO hexane/methvlenp rhlnrirlp hirh removed thp nonnla i
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and PCDFs subsequently were eluted with 10 mL of xylene. The authors provide results for the test of the systern with human adipose tissue. (Anal. Ch
S. J. Goldstein and co-workers evaluated energy-dispersive X-ray fluorescence (EDXRF) as a field-based method for elemental analysis of soils. EDXRF measures total analyte amounts and generally gives greater results than the EPA standard nitric acid digestion protocol. It is well suited to soil studies because it requires minimal sample preparation and can simultaneously measure a wide range of elements. Field tests showed no significant spectral interferences but detection limits decreased as atomic number increased The method greatly exceeded field screening analytical requirements and proviripri quantita-
Analysis of PCDDs, PCDFs, and planar PCBs is complex because these compounds must be separated from nonplanar PCBs and pesticides that are extracted at the same time. These separations generally are done during cleanup procedures on carbon-based traps. B. van Bavel and colleagues developed a carbon trap that can be substituted for standard traps in commercial supercritical fluid extractors, making possible a single instrumental procedure for extraction and cleanup of PCDDs, PCDFs and planar PCB The extract can be analyzed by GC/MS without further cleanup The authors tested four possible traps and found that 0 5 e trap of carbon PX-21 dispersed on 5 4 ng/g octadecyl silica worked best Following suDercritical fluid extraction the traD was eluted with
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Elemental field analysis of soil samples
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REMEDIATION Rhizosphere effects Recent research indicates that the presence of plants in hydrocarboncontaminated soil can enhance contaminant degradation by increasing microbial populations in the rhizosphere. K. A. Reilley and colleagues studied in a greenhouse the effect growing plants had on the concen-
tiv/p data with a c n i r a r v hpttpr than + m 0 / [Environ
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"Probabilistic" risk EPA and Health Canada both measure health risks using deterministic risk assessment methods, which often are presented as precise derivations when in fact they are only approximations. A newer risk assessment method, called probabilistic or stochastic risk assessment, calculates a range of risks for an individual or a population, which is useful for making cost-benefit risk management decisions. Using 20 different organic and inorganic compounds, G. M. Richardson compared both methodologies and points out the strengths and weaknesses of each For 15 of the compounds the deterministic and probabilistic risk estimates were about the same For the other five the probabilistic method was more conservative (Hum Ecol Risk Assess 1199 2(1) 44-54)
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Dermal exposure trations of anthracene and pyrene in soil. After 24 weeks, the compounds' concentrations in soils with growing plants were 30-44% lower than in unvegetated soils. Microbial concentrations also were greater in vegetated soils. The authors concluded that vegetating contaminated surface soils may be a feasible remediation option, compared to the typical land farming approach of continuous tillage and fertilization without plants. (J. Environ. Qual. 1996, 25, 212-19)
RISK Cancer potency factor E.A.C. Crouch developed a method of incorporating epidemiological studies' results and laboratory bioassays into a single probability distribution for a human cancer potency factor. Crouch used the method to construct a human inhalation risk for acrylonitrile. First, he adjusted EPA's epidemiological data on acrylonitrile by calculating probability distributions for several uncertainties such as exposure. Updated epidemiological data were used to update the human risk estimate. He then calculated the probability density function for unit risk based on rat and human study data using Bayes' theorem (Hum Ecol Risk sess 1996 2(1) 130-49)
2 7 6 A • VOL. 30, NO. 7, 1996 / ENVIRONMENTAL SCIENCE & TECHNOLOGY / NEWS
Risk estimates from dermal exposure to carcinogens in soils determine cleanup goals at roughly one-sixth of Superfund sites. J. C. Kissel and coworkers evaluated current default values for soil adherence to skin across a range of indoor and outdoor activities. Vigorous soil contact from activities such as farming and playing rugby produced adherence values within the current default range. However, other activities produced values drastically higher or lower than the default values. Adherence was highest for hands in all activities Based on the results the thors suggest skin adherence values appropriate to different levels of soil-contact activity (Risk Analysis 1996 16(1) 115-25)
Population thresholds S.J.S. Baird and colleagues proposed an alternative to EPA's noncancer risk assessment guidelines. Rather than a single reference dose value based on point estimates of uncertainty, their approach expresses the human population threshold for adverse effects as a probability distribution taking the major sources of scientific uncertainty into account. This approach estimates a population threshold by modifying the noobserved-adverse-effect level with probabilistic adjustment factors such as animal-to-human and subchronic-to-chronic data conversions They consider this estimate to be a better
