Quantifying remediation of chlorinated volatile compounds by sulfidated nano zerovalent iron treatment using numerical modeling and CSIA.

Sulfidated nanoscale zerovalent iron (S-nZVI) has demonstrated promising reactivity and longevity for remediating chlorinated volatile compounds (cVOC) contaminants in laboratory tests. However, its effectiveness in field applications remains inadequately evaluated. This study provides the first quantitative evaluation of the long-term effectiveness of carboxymethyl cellulose-stabilized S-nZVI (CMC-S-nZVI) at a cVOC-contaminated field site. A reactive transport model-based numerical approach delineates the change in cVOC concentrations and carbon isotope values (i.e., δ13C from compound-specific stable isotope analysis (CSIA)) caused by dissolution of dense non-aqueous phase liquid, sorption, and pathway-specific degradation and production, respectively. This delineation reveals quantitative insights into remediation effectiveness typically difficult to obtain, including extent of degradation, contributions of different degradation pathways, and degradation rate coefficients. Significantly, even a year after CMC-S-nZVI application, degradation remains an important process effectively removing various cVOC contaminants (i.e., chlorinated ethenes, 1,2-dichloroethanes, and chlorinated methanes) at an extent varying from 5 %-62 %. Although the impacts of CMC-S-nZVI abundance on degradation vary for different cVOC and for different sampling locations at the site, for the primary site contaminants of tetrachloroethene and trichloroethene, their predominance of dichloroelimination pathway (≥ 88 %), high degradation rate coefficient (0.4-1.7 d-1), and occurrence at locations with relatively high CMC-S-nZVI abundance strongly indicate the effectiveness of abiotic remediation. These quantitative assessments support that CMC-S-nZVI supports sustainable ZVI-based remediation. Further, the novel numerical approach presented in this study provides a powerful tool for quantitative cVOC remediation assessments at complex field sites where multiple processes co-occur to control both concentration and CSIA data.

Compound-specific isotope analysis (CSIA) evaluation of degradation of chlorinated benzenes (CBs) and benzene in a contaminated aquifer.

Compound-specific isotope analysis (CSIA) has become a valuable tool in understanding the fate of organic contaminants at field sites. However, its application to chlorinated benzenes (CBs), a group of toxic and persistent groundwater contaminants, has received less attention. This study employed CSIA to investigate the occurrence of natural degradation of various CBs and benzene in a contaminated aquifer. Despite the complexity of the study area (e.g., installation of a sheet pile barrier and the presence of a complex set of contaminants), the substantial enrichments in δ13C values (i.e., >2‰) for all CBs and benzene across the sampling wells indicate in situ degradation of these compounds. In particular, the 13C enrichments for 1,2,4-trichlorobenzene (1,2,4-TCB) and 1,2-dichlorobenzene (1,2-DCB) display good correlations with decreasing groundwater concentrations, consistent with the effects of in situ biodegradation. Using the Rayleigh model, the extent of degradation (EoD) is estimated to be 47-99% for 1,2-DCB, and 21-73% for 1,2,4-TCB. The enrichments observed for the other CBs (1,4-DCB and chlorobenzene (MCB)) and benzene at the site are also suggestive of in situ biodegradation. Due to simultaneous degradation and production of 1,4-DCB (a major 1,2,4-TCB degradation product), MCB (from DCB degradation), and benzene (from MCB degradation), the estimation of EoD for these intermediate compounds is more complex but a modelling simulation supports in situ biodegradation of these daughter products. In particular, the fact that the δ13C values of MCB and benzene (i.e., daughter products of 1,2,4-TCB) are more enriched than the original δ13C value of their parent 1,2,4-TCB provides definitive evidence for the occurrence of in situ biodegradation of the MCB and benzene.

Anthropogenic lead pervasive in Canadian Arctic seawater.

Anthropogenic Pb is widespread in the environment including remote places. However, its presence in Canadian Arctic seawater is thought to be negligible based on low dissolved Pb (dPb) concentrations and proxy data. Here, we measured dPb isotopes in Arctic seawater with very low dPb concentrations (average ∼5 pmol ⋅ kg-1) and show that anthropogenic Pb is pervasive and often dominant in the western Arctic Ocean. Pb isotopes further reveal that historic aerosol Pb from Europe and Russia (Eurasia) deposited to the Arctic during the 20th century, and subsequently remobilized, is a significant source of dPb, particularly in water layers with relatively higher dPb concentrations (up to 16 pmol ⋅ kg-1). The 20th century Eurasian Pb is present predominantly in the upper 1,000 m near the shelf but is also detected in older deep water (2,000 to 2,500 m). These findings highlight the importance of the remobilization of anthropogenic Pb associated with previously deposited aerosols, especially those that were emitted during the peak of Pb emissions in the 20th century. This remobilization might be further enhanced because of accelerated melting of permafrost and ice along with increased coastal erosion in the Arctic. Additionally, the detection of 20th century Eurasian Pb in deep water helps constrain ventilation ages. Overall, this study shows that Pb isotopes in Arctic seawater are useful as a gauge of changing particulate and contaminant sources, such as those resulting from increased remobilization (e.g., coastal erosion) and potentially also those associated with increased human activities (e.g., mining and shipping).