byproducts of nuclear testing for the purpose of estimating the oceans' uptake of fossil fuel C0 2 . Their approach used the strong correlation between natural radiocarbon and dissolved silica to reconstruct the natural radiocarbon component. Their model provides an estimate of the global ocean inventory of bomb radiocarbon as of January 1, 1975. It also estimates the distribution of bomb radiocarbon in the oceans along four thermocline isopycnals per ocean. The authors demonstrated that the ratio of bomb tritium to 14C for waters entering the thermocline in the northern hemisphere is nine times greater than for waters entering the thermocline in the southern hemisphere. This contrast may help monitor interhemispheric transport of upper ocean waters. {Global Biogeochem. Cycles, 1995 9(2), 263-88)
DRINKING WATER Viral detection Accurate methods for detection and quantitation of Giardia cysts and Cryptosporidium oocysts in low-turbidity water samples are essential for evaluating the effectiveness of drinking water treatment processes. E. C. Nieminski, F. W. Schaefer III, and J. E. Ongerth compared viral recovery by the ASTM method and an alternative membrane filtration method; both use flotation and immunofluorescence staining. They tested raw water, water from a flocculation basin, and filtered water seeded with Giardia cysts and Cryptosporidium oocysts. The alternative membrane filter method had higher recovery rates, took less time to perform, and was less expensive. However, it may show false-positive results if crossreacting algae are present. Results indicate that the sampling step in both methods is the greatest source of recovery loss. (Appl. Environ. Microbiol. 1995, 6US), 1714-19)
MEASUREMENTS Herbicide metabolites Triazine herbicides are used widely and are found in many aquatic systems, but little is known about the occurrence and fate of their metabolites because of the difficulties in
Biodégradation half-lives figure into fate Because of the toxicity of polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/F), it is important to understand their environmental fate. P. Adriaens, Q. Fu, and D. Grbic-Galic reported that these compounds can be transformed by biologically mediated reactions in anaerobic sediments. They spiked laboratory microcosms with individual penta-, hexa-, and hepta-chlorinated congeners and inoculated the microcosms with anaerobic organisms. The data were compared with autoclaved biological and chemical controls. Results showed that reductive dechlorination can take place in the more chlorinated homologues, producing trace quantities of lesser chlorinated homologues. Half-lives based on the initial PCDD/F spike into sediment in the microcosm were between 1 and 4 years. However, actual environmental half-lives may be greater by orders of magnitude, as the data also showed increased sorption of PCDD/F, thus increasing the half-life for the residual contamination. {Environ. Sci. Technol., this issue, 2252-60)
determining trace-level metabolite concentrations. M. Berg, S. R. Millier, and R. R Schwarzenbach developed a method for simultaneously determining five triazine herbicides and four major metabolites in water samples. One-liter water samples were solid-phase extracted using a graphitized carbon black cartridge and eluted with a mixture of methanol and dichloromethane. The extract was concentrated to 250 pL under a gentle nitrogen stream and analyzed by HPLC with diode array detection. Tests with spiked lake water showed recoveries ranging from 88% to 108% with an RSD of less than 13% and a method detection limit ranging from 3 to 11 ng/L. The method was compared with the standard GC/MS method and, although both produced consistent results for parent compounds, the HPLC method also determined metabolites with greater precision. {Anal. Chem. 1995, 67(11), 1860-65)
SFE efficiencies Supercritical fluid extraction (SFE) has been limited to analysis of pollutants such as PAHs, PCBs, and dioxins in environmental matrices because of
difficulties with extraction efficiencies and differences in the extraction of spiked and native compounds. J. Pawliszyn and colleagues studied the kinetics of SFE extraction of these compounds from real-world environmental samples by using a model to describe this process. Pollutant molecules are present on the core of a particle that is surrounded by an organic layer. The model includes terms for compound transport through the organic layer, partitioning between the organic layer and the supercritical fluid, and bulk transport of the supercritical fluid. A native compound will undergo all three processes, a spiked compound only the latter two. The authors tested the model with dioxinand PAH-contaminated and spiked samples at varying temperatures. They found that the extraction process corresponded to a three-rate constant curve. (Anal. Chem. 1995, 67(10), 1727-36)
METHODS Preparing bioassay samples Preparation of environmental media samples for interface with bioassay techniques traditionally has been done through lyophilization, vacuum distillation, solid-phase or solvent extraction processes, or adsorption of resins and other solid phases. These techniques, however, may interfere with bioassay through incomplete recoveries, loss of alteration of chemical constituents, or a residual effect of the solvents on the bioassay organism. M. F. Wolfe, D. E. Hinton, and J. N. Seiber examined sample preparation with SFE as a means of avoiding these interferences. Analytes from aqueous sample extracts were eluted with supercritical C0 2 and redissolved in reconstituted water. They were evaluated for toxicity using a 96-h static toxicity bioassay. Analyte recoveries were high, and the bioassay results using the water derived from the SFE were comparable to those obtained by the use of spiked water. {Environ. Toxicol. Chem. 1995, 24(6), 1001-9)
POLICY Hazwaste transport T. S. Glickman and M. A. Sontag evaluated optimal routes for the in-
VOL. 29, NO. 9, 1995 / ENVIRONMENTAL SCIENCE & TECHNOLOGY • 3 9 3 A
