The Environment research group centers its work in the use of isotopic tools to identify the contamination sources and to study the evolution of contaminants in the environment. Current research lines are focused on agricultural (nitrate, pesticides) and industrial (DNAPL's, PAHs, BTEX, fuel oxygenates) pollution sources, with a special aim in the isotopic characterization of natural and induced attenuation processes.
Nitrate and volatile organic compounds are among the most common contaminants found in groundwater. Nitrate pollution is mainly linked to the intensive use of synthetic and organic fertilizers as well as to wastewater discharges. Chlorinated volatile organic compounds, including carbon tetrachloride (CT), chloroform (CF), tetrachloroethylene (PCE), trichloroethylene (TCE) and cis-1,2-dichloroethene (c-DCE), have been widely utilized for many years as solvents, cleaners, dry-cleaning and degreasing agents. Environmental pollution caused by these compounds usually results from their industrial manufacturing and use, commonly by their inappropriate handling and storage. Contamination of groundwater by petroleum hydrocarbons, such as BTEX (benzene, toluene, ethylbenzene and xylenes) and methyl tert-butyl ether (MTBE), usually is the result of leakages from underground storage tanks or accidental spills. Due to the increased use of pesticides, contamination of soil and groundwater by pesticides has become one of the major threats to the subsurface environment. For improving water management policies and remediation actions, it is vital to assess the source and fate of these pollutants in the environment.
Natural attenuation of these compounds in groundwater can take place by the action of indigenous microorganisms able to degrade the contaminants under natural conditions. Attenuation processes can be induced by the provision of suitable electron donors/acceptors and/or energy sources to stimulate these indigenous microorganisms. Where the appropriate degrading bacteria are not present, bioaugmentation can accelerate the removal of the contaminants by introducing specialized microorganisms. Contaminant attenuation can also be induced by means of abiotic remediation strategies, such as permeable reactive barriers (PRBs) filled with specific reactive media or the injection of a chemical oxidant during in situ chemical oxidation (ISCO) to degrade the target contaminants.
Multi-isotopic methods are useful tools to identify the sources of these contaminants, to assess and quantify their natural attenuation processes and to monitor the success of remediation strategies at contaminated sites. Non-destructive abiotic natural processes such as dispersion, sorption or volatilization generally do not cause significant isotope fractionation. In contrast, biotransformations and chemical reactions involve the formation or cleavage of chemical bonds, leading to enrichment of heavy isotopes in the remaining substrate because light isotopomeres (e.g., 12C, 1H, 35Cl, 14N, 16O) typically react faster than heavy isotopomeres (e.g., 13C, 2H, 37Cl, 15N, 18O). Significant changes in isotope ratios over time and space can be used to evaluate and quantify natural and induced attenuation processes. To these ends, the interpretation of concentration data alone is often insufficient, and isotopic data are required.
Isotopic tools can be used for source identification of pollutants. The isotopic composition of dissolved nitrate for example can be used to identify the origin of the contaminant in diffused nitrate pollution areas because the isotopic signatures are generally different among various nitrate sources such as fertilizers, animal manure, sewage, mineralization of soil nitrogen, atmospheric deposition, etc.
On the other hand, organic compounds have a characteristic “isotopic fingerprint”, defined by the isotopic composition of their precursor materials and by the formation processes. Although the concentration of a compound in the environment can change owing to dilution or phase transfer processes, its isotopic composition will remain largely unaffected and can thus be used to elucidate its sources.
Assessment and characterization of natural attenuation
Isotopic tools can also be used as an indicator of the geochemical processes that control the fate and attenuation of the contaminants in groundwater. Regarding nitrate, the most significant natural attenuation process is denitrification, i.e. the reduction of nitrate to dinitrogen gas by anaerobic facultative bacteria (and a few archaea) that utilize nitrate as the electron acceptor. During denitrification, as nitrate decreases, residual nitrate becomes enriched in the heavy isotopes 15N and 18O. Isotopic characterization of dissolved nitrate in groundwater can thus be used to identify natural attenuation processes. A further step in the investigation of denitrification processes is to determine the processes governing the reaction. A coupled approach of chemical data with the δ15N and/or δ18O of dissolved nitrate and the isotopic composition of the ions involved in denitrification reactions, as, for example, the δ34S and δ18O of dissolved sulfate, and/or the δ13C of dissolved inorganic carbon, can be used to determine the relative role of heterotrophic and autotrophic processes in natural denitrification.
Measuring stable C, Cl or H isotope fractionation of individual organic contaminants employing compound specific isotope analysis (CSIA) is a useful approach to assess biodegradation in contaminated aquifers. CSIA may provide direct evidence of the degradation of the target contaminant if an isotope shift occurs along a groundwater flow line or at least between the source area of the contamination and downstream wells. In addition, monitoring changes of isotope signatures of two (or more) elements is recommended to obtain a more reliable assessment of degradation and to derive the extent and relative contribution of different reaction mechanisms.
Quantification of contaminant degradation in both natural and enhanced attenuation
Shifts in stable isotope ratios of pollutants can be used to calculate the extent of natural or induced degradation. Contaminant transformations usually describe Rayleigh distillation processes. Quantification of the extent of contaminant transformation based on stable isotopes requires experimental determination of the isotope fractionation (ε) associated with the reactions under consideration. Istotope fractionation values can be determined in groundwater field studies or in laboratory degradation experiments inoculated with a strain capable of degrading the target compounds or using microorganisms enriched from contaminated sites. These studies are extremely useful because they provide a basis for the interpretation of field data selecting appropriate and representative isotope fractionation values for the active biodegrading microbial communities within a specific aquifer.
Isotopic data coupled to chemical data are very useful to evaluate the efficiency and performance of enhanced attenuation approaches. For example, the presence of contaminant downgradient of a PRB may result from incomplete degradation within the barrier or from bypassing under or around the barrier. The isotopic tools are useful to distinguish both processes. In the case of an ISCO approach for example, isotopes can serve to distinguish areas where the oxidant injection has been effective and areas where the oxidant is not adequately reaching the target treatment area. This better understanding can lead to better project management decisions, such as the design of more effective injection plans.