The studies described above, many of which were published over 20 years ago, set the stage for the continued use of combined radiocarbon and stable carbon to examine the physical, geochemical, and biological transformations of contaminants in the environment. Most of the previously described studies focused on natural carbon turnover, in other words groundwater DIC and DOC as traditionally done in the fields of geology and geochemistry. It is not surprising that the use of radiocarbon in pollution studies originated from these fields. The next logical step was to identify sources of pollutant in environmental media, and production of biodegradation and end products that has expanded interest in the use of combined 14C and 13C to the fields of biogeochemistry, environmental chemistry, biology, and microbiology.
Although tremendous potential exists because of its innate sensitivity and comprehensive mineralization assessment, naturally occurring radiocarbon has not been as well developed as stable carbon isotopes for application to contaminant biodegradation at a field or laboratory scale. With the development of AMS, small amounts of CO2 and methane can be analyzed and provide information about the origin of the carbon used to generate gaseous end products such as CO2 and methane. However part of the limitation of the radiocarbon analysis is precisely due to the sophistication of the instrumentation and its associated expense. Radiocarbon analyses by AMS currently cost approximately 10 times those of stable carbon. Regardless, radiocarbon data overcome several of the limitations of 13C applications and interpretation of the data as are illustrated in the following section.
The combined use of 14C and 13C can be applied to the study of gaseous end products of degradation in contaminated environments, the investigation of the specific contaminants and their transformation products, and finally to the biological components of organisms in uncontaminated and contaminated environments (see Section 11.5). Addressing each of these areas is not feasible in one chapter. In the following sections, the application of combined 14C and 13C will focus primarily on the measurement of end products of contaminant degradation, CO2 and methane, and to a smaller degree on the compound specific application to contaminants.
11.6.1 Use of Combined 14C and 13C Analysis for Gaseous End Products
Contaminant degradation can cause stable isotopic shifts in metabolic end products including CO2 to lower S13C values than the parent compound. However, that shift alone has not been adequate to definitively estimate biodegradation because shifts are generally small for aerobic processes ( < 6%c), and there can be significant overlap in stable isotope ratios of plant root respired CO2 (see Chapter 5). Anaerobic processes that generate methane cause larger shifts in S13C (see Chapter 6). Thus the DIC produced by the acetate reduction pathway will be enriched in 13C compared to the substrate, and the S13C of the methane generated will be depleted compared to the substrate (Figure 11.9; Clark and Fritz 1997) resulting in limited overlap with root-respired CO2. In field studies in which multiple contaminants and microbial degradation processes are possible and in which plants may contribute to CO2 and DIC through root-respiration, the combined use of naturally occurring radiocarbon and stable carbon isotopes may elucidate microbial degradation processes better than either measure individually.
Over 40 years ago, Rosen and Rubin (1965) described the use of radiocarbon to distinguish between industrial pollution and natural organic matter, primarily readily oxidized organics in animal and vegetable matter in garbage and wastewater. Their objective was to develop a method to differentiate between organic wastes such as municipal wastewater that carries a high biological oxygen demand (BOD), and
CO2 reduction □ Aravena et al. 1995
Pathway not identified a Aravena et al. 1993 O Lansdown et al. 1992 ♦ Grossman et al. 1989 A Gelwicks 1989 "Barker and Fritz 1981
* Conrad et al. 1999
FIGuRE 11.9 Enrichment of 13CO2 and depletion of 13CH4 due to CO2 reduction and acetate fermentation processes. (Modified from Clark, I. D., and P. Fritz, Environmental isotopes in hydrogeology, 1st ed., CRC Press, Boca Raton, FL, 1997. With permission.)
industrial pollution that has limited BOD but may be highly toxic to aquatic organisms. The samples in their study were collected from surface water supplies that were impacted by different land uses and their respective surface water discharges. These surface water discharges ranged from entirely industrial, to mixed municipal and petroleum refinery, to predominantly municipal. Thus some of the study sites contained natural organic materials that were contemporary or modern in radiocarbon, and others contained fossil carbon materials such as petroleum, coal, and natural gas that were depleted in radiocarbon. From the 14C data, Rosen and Rubin (1965) qualitatively differentiated between recent and ancient carbon sources to surface water, and identified the distribution of carbon of several different organic pollutants and natural organic matter in surface water (Figure 11.10). In order to measure the carbon in these different surface water systems, an exhaustive extraction of organic adsorbate from an activated carbon filter was used. The extracted samples were converted to acetylene and analyzed using the acetylene 14C dating method of the U.S. Geological Survey (Suess 1954). The authors noted that possible errors were associated with the adsorption and desorption of the extraction technique, the 14C sample treatment and counting technique, the assumed contemporary activity baseline, as well as the unknown contribution of photosynthesis to the total organic content of the stream. Despite these unknowns and limited analytical capabilities, Rosen and Rubin (1965) surmised that as sampling proficiency increased and size of sample needed decreased, radiocarbon analysis could be used to determine not only the origin of bulk contaminants, but also
whether a specific chemical originated from industrial waste or originated from the natural biological material such as vegetation.
This research approach was expanded by Spiker and Rubin (1975) who combined radiocarbon and stable carbon isotope analysis of DOC of several river waters in the United States, primarily in Washington, DC and Maryland to investigate petrochemical, domestic waste, and natural decaying matter inputs. They suggested that several biological inputs to DOC, including municipal wastes, soil runoff, agricultural and feedlot drainage, and bioproduction within the aquatic system were modern in their carbon signature. They assumed that petrochemical industrial wastes contained no 14C and that the sources of intermediate 14C signature were negligible. These simplifying assumptions allowed a two component model to be used to calculate the percentage of modern DOC, and percentage of fossil DOC in the river water. Based on 14C data, the authors estimated that the freshwater rivers were approximately 0-37% fossil DOC but there were significant changes at the same site at different sampling times. For example, computed percentage of fossil DOC was ~7% in samples collected in April 1972, and ~30% in samples collected in December 1972 from the Potomac River in Washington, DC. The increase in modern carbon DOC was attributed to increased runoff during flood conditions in December that contributed primarily modern DOC to the water. The 813C values of the river water DOC samples ranged from -24.6 to -34.1%o and were more depleted in 13C than those of terrestrial plants that was attributed to the input of more highly fractionated petroleum products. However Spiker and Rubin (1975) recognized that the 813C range for terrestrial and fluvial plants overlapped with that of petroleum products, so that the differences in 813C values were not as distinct as those of the two-component 14C model. In their study there was no correlation between 813C and 14C of the DOC. Regardless, the 14C DOC method was concluded to be a useful environmental indicator in surface waters.
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