Fate Of Contaminants Research Articles

A Unit-Concentration Method to Quantify Source Contribution.

Solute migration is typically simulated to describe and estimate the fate and transport of contaminants in groundwater. The unit-concentration approach is investigated here as a method to enable solute transport simulations to expand the capabilities of groundwater flow modeling. The unit-concentration method uses a concentration value of one to identify sources of water to be assessed and a concentration of zero for all other water sources. The distribution of concentration thus obtained, unlike particle tracking methods, provides a more intuitive and direct quantification of the contribution of sources reaching various sinks. The unit-concentration approach can be applied readily with existing solute transport software for a range of analyses including source allocation, well capture analysis, and mixing/dilution calculations. This paper presents the theory, method, and example applications of the unit-concentration approach for source quantification.

Implementation of Soil Washing in Remediation of Contaminated Soil

Many human anthropogenic activities, including as deforestation, fossil fuel combustion, industrialisation, and solid waste production, have contaminated and endangered the entire environmental ecosystem in this age of pollution. Recently, heavy metal contamination in soil particles has attracted the attention of governments around the world, as many agricultural farmlands are contaminated with heavy metal pollutants such as copper, nickel, cobalt, iron, and lead, which have negatively impacted crop development. In addition, long-term exposure of the human body to heavy metals will cause severe illnesses, including neurological disorders, cardiovascular diseases, and chronic diseases. These contaminated soils are extremely tough and demanding to remediate. Soil washing is one of the most effective, rapid, and straightforward ways for decontaminating heavy metal-contaminated soil. The primary purpose of soil washing is to clean the sand and gravel fractions while concentrating contaminants in the clay and silt fractions. This will aid in the removal of heavy metal particles from the soil and their transfer to the washing solution. This study will examine the fate and transport of heavy metal contaminants as well as the many forms of soil washing mechanisms. In addition, the obstacles of implementing soil washing as well as its advantages and disadvantages were explored. Further research and possibly new directions, in addition to the possibility of soil washing, would also be discussed.

Potential interaction mechanisms between PAHs and glomalin related-soil protein (GRSP)

Glomalin-related soil protein (GRSP) has gained widespread attention because of its benefits to carbon sequestration, improving soil quality and fixing heavy metals. However, studies on how GRSP affects the environmental fate of organic contaminants are scarce. In this study, different types of GRSPs were isolated from forest soils and characterized to study the binding of GRSPs and PAHs under different environmental conditions. The results indicated that GRSPs contain abundant functional groups (such as –NH, –COOH, and CO) and material composition, like humic acid, proteins, and lipids. For the tested GRSPs, EE-GRSP has lower DOC, SUVA260 and SUVA280 values, as well as higher E2/E3 values, indicating that EE-GRSP has lower hydrophobicity and molecular weight. These properties can lead to strong interactions between GRSP and PAHs, especially with benzopyrene, which has a high Kow and Ksw and a large molecular size, with binding constants ranging from 16,119 to 163,697 L·kg−1. Furthermore, low pH (pH = 3) and temperature (15 °C) could increase GRSP's aggregation, enhance the GRSP binding ability with PAHs, whose binding constants were 11,595 and 5067.3 L·kg−1. Therefore, the binding between GRSP and PAHs may lead to changes in the fate of PAHs in the soil and affect the environmental risk of PAHs. The results presented here will deepen our understanding of the environmental function of GRSPs and provide a theoretical basis to further elucidate the mechanisms of GRSPs and organic pollutants.

Heterogeneous Aggregation of Humic Acids Studied by Small-Angle Neutron and X-ray Scattering.

Aggregation of humic acids (HAs) was studied by small-angle neutron and X-ray scattering techniques. The combination of these techniques enables us to examine the aggregation structures of HA particles. Two HAs with distinctive compositions were examined: a commercial HA (PAHA) and a HA extracted from deep sedimentary groundwater (HHA). While macroscopic coagulation tests showed that these HAs were stable in solutions except for HHA at pH < 6, small-angle neutron scattering (SANS) and small-angle X-ray scattering (SAXS) revealed that they formed aggregates with sizes exceeding the sub-micrometer length scale. The SAXS curves of PAHA remarkably varied with pD = log aD+, where aD+ stands for the activity of deuterium ions, whereas the SANS curves did not. With the help of theoretical fittings, it was revealed that PAHA aggregates consisted of two domains: poorly hydrated cores and well-hydrated proton-rich shells. The cores were (dis)aggregated with pD inside the aggregates of the shells. The SANS and SAXS curves of HHA resembled each other, and their intensities at low q, where q stands for the scattering vector, increased with a decrease of pD, indicating the formation of homogeneous aggregates within the spatial resolutions of SANS and SAXS. This study revealed that distinctive aggregation behaviors exist in humic substances with nm-scale heterogeneous structures like PAHA, which is important for their roles in the fate of contaminants or nutrients in aqueous environments.

Global variability and controls on the accumulation of fallout radionuclides in cryoconite

The accumulation of fallout radionuclides (FRNs) from nuclear weapons testing and nuclear accidents has been evaluated for over half a century in natural environments; however, until recently their distribution and abundance within glaciers have been poorly understood. Following a series of individual studies of FRNs, specifically 137Cs, 241Am and 210Pb, deposited on the surface of glaciers, we now understand that cryoconite, a material commonly found in the supraglacial environment, is a highly efficient accumulator of FRNs, both artificial and natural. However, the variability of FRN activity concentrations in cryoconite across the global cryosphere has never been assessed. This study thus aims to both synthesize current knowledge on FRNs in cryoconite and assess the controls on variability of activity concentrations. We present a global database of new and previously published data based on gamma spectrometry of cryoconite and proglacial sediments, and assess the extent to which a suite of environmental and physical factors can explain spatial variability in FRN activity concentrations in cryoconite. We show that FRNs are not only found in cryoconite on glaciers within close proximity to specific sources of radioactivity, but across the global cryosphere, and at activity concentrations up to three orders of magnitude higher than those found in soils and sediments in the surrounding environment. We also show that the organic content of cryoconite exerts a strong control on accumulation of FRNs, and that activity concentrations in cryoconite are some of the highest ever described in environmental matrices outside of nuclear exclusion zones, occasionally in excess of 10,000 Bq kg−1. These findings highlight a need for significant improvements in the understanding of the fate of legacy contaminants within glaciated catchments. Future interdisciplinary research is required on the mechanisms governing their accumulation, storage, and mobility, and their potential to create time-dependent impacts on downstream water quality and ecosystem sustainability.

Bioretention Cells Provide a 10-Fold Reduction in 6PPD-Quinone Mass Loadings to Receiving Waters: Evidence from a Field Experiment and Modeling.

Road runoff to streams and rivers exposes aquatic organisms to complex mixtures of chemical contaminants. In particular, the tire-derived chemical 6PPD-quinone (N-(1,3-dimethylbutyl)-N'-phenyl-p-phenylenediamine-quinone) is acutely toxic to several species of salmonids, which are critical to fisheries, ecosystems, and Indigenous cultures. We therefore urgently require interventions that can reduce loadings of 6PPD-quinone to salmonid habitats. Herein, we conducted a spike and recovery experiment on a full-scale, mature bioretention cell to assess the efficacy of stormwater green infrastructure technologies in reducing 6PPD-quinone loadings to receiving waters. We then interpreted and extended the results of our experiment using an improved version of the "Bioretention Blues" contaminant transport and fate model. Overall, our results showed that stormwater bioretention systems can effectively mitigate >∼90% of 6PPD-quinone loadings to streams under most "typical" storm conditions (i.e., < 2-year return period). We therefore recommend that stormwater managers and other environmental stewards redirect stormwater away from receiving waters and into engineered green infrastructure systems such as bioretention cells.

Site-specific spatiotemporal occurrence and molecular congener distributions of naphthenic acids in Athabasca oil sands wetlands of Alberta, Canada

The Athabasca oil sands region (AOSR) of Alberta, Canada is notable for its considerable unconventional petroleum extraction projects, where bitumen is extracted from naturally-occurring oil sands ore. The large scale of these heavy crude oil developments raises concerns because of their potential to distribute and/or otherwise influence the occurrence, behaviour, and fate of environmental contaminants. Naphthenic acids (NAs) are one such contaminant class of concern in the AOSR, so studies have examined the occurrence and molecular profiles of NAs in the region. We catalogued the spatiotemporal occurrence and characteristics of NAs in boreal wetlands in the AOSR over a 7-year period, using derivatized liquid chromatography-tandem mass spectrometry (LC-MS/MS). Comparing median concentrations of NAs across these wetlands revealed a pattern of NAs suggesting NAs in surface waters derived from oil sands deposits. Opportunistic wetlands that formed adjacent to reclaimed overburden and other reclamation activities had the highest concentrations of NAs and consistent patterns suggestive of bitumen-derived inputs. However, similar patterns in the occurrence of NAs were also observed in undeveloped natural wetlands located above the known surface-mineable oil sands deposit that underlies the region. Intra-annual sampling results along with inter-annual comparisons across wetlands demonstrated that differences in the spatial and temporal NA concentrations were dependent on local factors, particularly when naturally occurring oil sands ores were observed in the wetland or drainage catchment.

The Role of Sediment Records in Environmental Forensic Studies: Two Examples from Italy of Research Approaches Developed to Address Responsibilities and Management Options

The coupling of scientific evidence from sediment cores with historical information represents an effective way to reconstruct and quantify recent anthropogenic impacts in transitional and marine-coastal areas. These are both key points pertaining to studies that aim at establishing the responsibility for envi ronmental pollution. Good practices for the selection of sampling sites and specific survey techniques are fundamental to understand pollution histories and dynamics, together with reliable dating methods and analytical procedures. In addition, a certain degree of flexibility and willingness to explore different research pathways is necessary, particularly when unexpected questions arise from scientific data or from requests posed by authorities in charge of preliminary investigations or court debates. In this paper, two different study cases are reviewed, and the approaches developed to tackle with specific issues are presented. Its main purpose is both to explain study paths undertaken to answer challenging scientific-legal questions and to provide examples for developing countries that present similar risks of uncontrolled industrialization. Results were used in preliminary investigations or court debates for the attribution of responsibility for environmental pollution to past or present industrial managements. In addition, they were fundamental for other studies aiming at implementing models that simulate the fate and distribution of contaminants and human exposure. In the Augusta Harbor, an integrated approach merged archive information, bathymetry, and high-resolution seismic profiles with the results of an independent tracer (hexachlorobenzene, HCB). This approach helped attribute the presence of high surficial Hg concentrations to resuspension and redistribution of deep sediments caused by dredging and maritime traffic and not to active outfalls. In the Venice Lagoon, an extensive literature search supported analytical results for the correct identification of industrial processes responsible for the contamination by polychlorinated dibenzo-p-dioxins and furans (PCDD/Fs) in the canals of the Porto Marghera Industrial Area. In addition, sedimentary profiles of PCDD/Fs in local salt marshes (“barene”) recorded well the events relative to the industrial development and management of the area reported by historical documents, confirming their potential for this kind of investigation.

Spatial and temporal water chemistry variations of hydraulic fracturing flowback and produced waters from the Duvernay Formation

Chemical characterization of hydraulic fracturing (HF) flowback and produced water (FPW) is important to the environmental sustainability of HF operations. In this research, we analyzed and compared HF FPW samples from different well locations as well as FPW samples from a single well at different stages of FPW production in the Duvernay Formation using a suite of analytical techniques and statistical methods. The results provide vital information to assess the reusability of FPW and support the choice of water handling, treatment, or disposal options. Additionally, this work contributes to a more complete understanding of regional FPW chemical composition and has the potential to provide guidance on best practices for future applications of HF fluid additives. While reuse of FPW is desirable, it is not always feasible. In these scenarios, the transportation of FPW offsite for treatment and/or disposal increases the risk of spills to near-surface environments. This work provides data to aid spill clean-up efforts and to determine the environmental fate of FPW contaminants. Overall, the information presented in this work may be used to provide economic benefits to the oil and gas sector by increasing FPW reuse that will directly decrease wastewater disposal costs and reduce environmental impacts of HF operations by decreasing the volume of used surface water and the risk of FPW surface spills.

Diffusive fluxes of persistent organic pollutants between Arctic atmosphere, surface waters and sediments

Arctic communities are disproportionately exposed to pollutants from sources including global atmospheric transport and formerly used defense sites (FUDS). The effects of climate change and increasing development in the Arctic have the potential to exacerbate this problem. Yupik People of Sivuqaq, or St Lawrence Island, Alaska are one such community with documented exposures to pollutants from FUDS, and their traditional lipid-rich foods such as blubber and rendered oils of marine mammals. Troutman Lake, adjacent to the Yupik community of Gambell, Alaska, was used as a disposal site during the decommission of the adjacent FUDS, leading to community concern about exposure to military pollution and intrusion from historic local dump sites. In collaboration with a local community group, this study utilized passive sampling devices deployed in Troutman Lake. Air, water and sediment deployed samplers were analyzed for unsubstituted and alkylated polycyclic aromatic hydrocarbons (PAHs), brominated and organophosphate flame retardants and polychlorinated biphenyls (PCBs). PAH concentrations were low and comparable to other remote/rural locations. PAHs were generally in deposition from the overlying atmosphere into Troutman Lake. Of the flame retardants, brominated diphenyl ether-47 was detected in all surface water samplers while triphenyl phosphate was detected in all environmental compartments. Both were at concentrations equivalent or lower than other remote locations. Of particular interest, we measured higher atmospheric concentrations of tris(2-chloroethyl) phosphate (TCEP) (0.75–2.8 ng/m3) than previously reported in the literature for remote Arctic sites (<0.017–0.56 ng/m3). TCEP was found to be in deposition to Troutman Lake at magnitudes from 290 to 1300 ng/m2/day. No PCBs were detected in this study. Our findings demonstrate the relevance of both modern and legacy chemicals from local and global sources. These results help us to understand the fate of anthropogenic contaminants in dynamic Arctic systems providing valuable data for communities, policy makers and scientists.

Rapid Accumulation of Soil Inorganics on Plastics: Implications for Plastic Degradation and Contaminant Fate

As plastics degrade in the environment, chemical oxidation of the plastic surface enables inorganics to adsorb and form inorganic coatings, likely through a combination of adsorption of minerals and in situ mineral formation. The presence of inorganic coatings on aged plastics has negative implications for plastics fate, hindering our ability to recycle weathered plastics and increasing the potential for plastics to adsorb contaminants. Inorganic coatings formed on terrestrially weathered polyethylene were characterized using synchrotron spectroscopy and microscopy techniques across spatial scales including optical microscopy, nano-X-ray-fluorescence mapping (nano-XRF), nano-X-ray absorption near edge structure (nano-XANES), and high-energy resolution fluorescence detected-XANES (HERFD-XANES). Results indicate a heterogeneous elemental distribution and speciation which includes inorganics common to soil terrestrial environments including iron oxides and oxyhydroxides, aluminosilicates, and carbonates.

Prediction of alkaline earth metal ion adsorption on goethite for various background electrolytes with the CD-MUSIC model

As water scarcity drives the use of more saline water sources, contaminant fate and transport models must capture the impact of high concentrations of alkaline earth metal ions (AEMs) and background electrolytes in these more complex waters. By utilizing macroscopic adsorption data from various electrolyte systems, a Charge Distribution – Multisite Complexation (CD-MUSIC) model, capable of incorporating electrolyte adsorption, was able to accurately simulate the adsorption behavior of alkaline earth metal ions onto goethite. The modeling effort was guided by previous spectroscopic and surface complexation modeling of alkaline earth metal adsorption and built on previous CD-MUSIC modeling that accounted for changes in crystal face contributions to the surface site density as a function of specific surface area. The model was constrained to consider only two dominant surface complex species for each metal ion adsorption reaction. These two species were selected from 44 possible species through objective curve fitting of single-solute macroscopic adsorption data. While most of the alkaline earth metal surface complexes formed outer-sphere complexes at the goethite surface, an inner-sphere species was utilized for Mg2+. With the surface complex species and equilibrium constants obtained from this study, the calibrated model successfully predicted alkaline earth metal ion adsorption over a wide range of solution and surface conditions; the model predictions encompassed a wide range of pH (5–11), solute/solid ratio (1.37 × 10−5– 8.33 × 10−4 mol-solute/g-solid), ionic strengths (0.01 M – 0.7 M), and background electrolytes (Na+, Cs+, Rb+, Cl−, and NO3−) using the same crystal face contribution methodology for site density, capacitance values, and surface acidity constants adopted for proton and cadmium adsorption in previous work (Han and Katz, 2019). Model simulations for a range of background water chemistries demonstrated the potential for Mg2+ to reduce Cd2+ adsorption to goethite in model seawater and oil- and gas-produced waters.

Modeling the Effect of Hyporheic Flow on Solute Residence Time Distributions in Surface Water

Understanding the dynamics of hyporheic flow is important for managing water resources, since this interfacial flow exchange affects the fate and transport of contaminants in rivers. This study numerically quantifies the effect of hyporheic exchange on solute residence times in surface water systems by simulating solute transport in unified turbulent open-channel and hyporheic zone systems. Interfacial hyporheic fluxes (qint) increase with increased Reynolds number (Re) that produces an enhanced bottom pressure gradient over the ripple bed. Heavy-tailed breakthrough curves emerge when hyporheic flow is considered in transport simulation. This reveals that hyporheic flow is a dominant driver of non-Fickian transport in surface water as this interfacial flow exchange delays solute transport with slow porewater flows. Furthermore, the increase in Re extends the longitudinal spreading of solute tracers because a higher surface flow velocity intensifies the magnitude of hyporheic flow and associated storage effects. This can be confirmed by the ratio of the maximum residence time to the peak arrival time that increases with the increase in Re, following a power-law relationship with both Re and qint.

The Sorption Behaviors of Barium during Reinjection of Gas Field Produced Water into Sandstone Reservoir: An Experimental Water-Rock Interaction Study

Identifying the fate of contaminants (such as barium) during gas field produced water reinjection could be a feasible method to evaluate the environmental risks of the reinjection project and thus improve its sustainability. To assess the barium sorption behaviors during gas field produced water reinjected into sandstone reservoirs, a series of water–rock interactions experiments were conducted to systematically investigate the effects of brine/rock ratio (5:1~500:1), pH (3~10), temperature (20, 50 and 80 °C), brine salinity (NaCl solution, 0~100 g/L), competitive cations (Sr and Mg, 0.5 g/L), and organic compound (methanol, 0~5 g/L) on the sorption of barium in sandstone. The rock samples were collected from the Triassic formation of the Yanchang Group in the northern Ordos Basin, China. The results indicated that the sorption of barium in sandstone strongly depends on the brine/rock ratio. Under the same brine/rock ratio, the severity of the impact on the barium sorption from high to low was competitive cation, salinity, pH, temperature, and methanol concentration. The sorption process of barium onto the sandstone could be well fitted by a pseudo-second-order kinetics model. The presence of competitive cations would restrain the sorption of barium, while the existence of methanol hardly affects the barium sorption. The chloro-complexation reaction and the reduction of rock surface electrical potential are mainly responsible for the inhibition effects of NaCl salinity on barium sorption, and the corresponding relationship can be characterized by an exponential function. Barium sorption in sandstone decreases with increasing temperature, while it is positively correlated to the initial pH of the solution. The water-rock system is weakly alkaline with a value of 7.7–8.5 when the barium sorption reaches an equilibrium state, regardless of the initial pH of the reactive solution. The results are meaningful in understanding and predicting the fate of barium after the gas field produced water was reinjected into the underground.

Bioengineering remediation of former industrial sites contaminated with chemical mixtures

Former gasworks sites are known to be contaminated with complex chemical mixtures that require remediation before redevelopment. Bioamendments such as biochar and spent mushroom compost (SMC) offer a green and sustainable remediation approach to help tackle this issue. However, the effectiveness of different biochar types and their interactions with the soil microbial community is still not well understood. To address this, a full factorial microcosm experiment was carried out using biochar derived from rice husk (RHB) and wheat straw (WSB) mixed with soil from a former gasworks site at varying concentrations (0%, 2.5%, and 5%), with and without SMC. The experiment aimed to evaluate the fate of contaminants including alkanes, PAHs, and metals, and their effect on the soil microbial community, as well as the implications for remediation endpoints. The results showed that the bioamendments had an average TPH reduction of 92%, with SMC and WSB-SMC having the highest degradation rates at 93%. While the bioamendments did not significantly affect the extent of TPH removal compared to the control, they did improve the degradation of high molecular weight (HMW) PAHs, particularly in RHB-SMC for EC17-20 (60%) and EC21-35 (62%) of total PAH concentration, and in WSB-SMC for HMW bioavailable PAH concentration (89%). The bioamendments also affected the partitioning and distribution of metals after 120 days of treatment, leading to decreased available phase fractions. The treatments increased microbial abundance in the soil, with Gram positives, Gram negatives, and fungi increasing by 4%, 8%, and 38%, respectively, after 120 days, particularly in SMC and mixed treatments. This was mirrored in increased microbial soil respiration. After 120 days, low metal (178 ± 5 mg/kg) and TPH (21 ± 7 mg/kg) bioavailability translated into higher EC50 (10624 ± 710 mg/L), indicating lower toxicity. There was a strong correlation between bioavailability and toxicity of TPH and metals with microbial relative abundance and activity. In summary, while green and sustainable remediation may accelerate the remediation process, monitored natural attenuation may be sufficient for site reclamation. However, this strategy, as demonstrated here, can reduce metal bioavailability, and promote the biodegradation of HMW PAHs.

Total organic carbon concentrations in clastic cave sediments from Butler Cave, Virginia, USA: implications for contaminant fate and transport.

Clastic cave deposits are representative of sediments throughout the karst aquifer and thus are an abundant and accessible resource through which to study the chemistry of karst aquifer. Clastic cave sediments are attributed to depositional facies based on cave location, sorting, and particle size. These facies settings may influence different chemical parameters of the sediments, like concentrations of total organic carbon (TOC). The TOC concentrations in clastic cave sediments have not been well constrained nor has the role of clastic sediments in contaminant fate and transport through karst systems been well described. In this study, particle size, TOC, and total nitrogen were measured in sediments representing different facies in Butler Cave, Virginia, USA. TOC concentrations ranged from 0.08 - 0.87 weight percent and C:N molar ratio ranged from 3 - 15, indicating a possible terrestrial source of organic carbon in these sediments. The diamicton facies was sandier and but had similar TOC concentrations compared to the channel facies. TOC concentrations measured in Butler Cave were within the same range as those observed in above water, eogenetic clastic cave sediments from two caves in Puerto Rico. Estimated retardation factors calculated based on the TOC concentrations in the Butler Cave sediments indicate the range of TOC in this cave could be responsible for 39 - 987% increase in retardation of selected contaminants. This study highlights the importance of measuring the ranges of TOC in clastic cave sediments across different facies and their role in contaminant fate and transport.

Electrochemical reactions driving Mn-enrichment in Fe[sbnd]Mn supergene ores: A mineralogical perspective

Iron and manganese oxides embody a geochemical system of great environmental, biological, and economical relevance. Chemical equilibria and the stability of FeMn phases under surface or near-surface conditions can influence the fate of contaminants in the environment, impact biological metabolic processes, or Fe and Mn phase distribution in weathered ores. In the present work, we focus on the textural, mineralogical, and chemical study of FeMn ores from the weathered zone of a vein-hosted deposit outcropping in the Iberian Pyrite Belt, SW Portugal, by means of micro-Raman spectroscopy coupled with electron microprobe microanalysis. The aims of our investigation are i) identifying the several Fe and Mn phases occurring in differently enriched ores samples, ii) relating their chemical composition with possible mineralization mechanisms, and iii) defining the mineral paragenetic pathway related to the observed textural features. Our approach enabled both the identification of the coexisting Fe and Mn phases, and unravelling paragenetic pathways leading to supergene enrichment mineralizations. Collected evidence demonstrates that changes in Eh/pH can lead to goethite dissolution under reductive conditions, promoting the release of Fe2+ into solution, whose electrochemical interaction with Mn4+ results in the formation of several types of Mn oxides, and secondary goethite. Our data shows a clear relationship between the type of Mn oxide crystallized and the ratio of aqueous Mn3+/Mn4+, alongside other prevalent cations, incorporated into tunnel or interlayer structural sites, which may be also desorbed/solubilized from primary goethite.

Compound-Specific Carbon, Hydrogen, and Nitrogen Isotope Analysis of Nitro- and Amino-Substituted Chlorobenzenes in Complex Aqueous Matrices.

Compound-specific isotope analysis (CSIA) is an established tool to study the fate of legacy groundwater contaminants but is only emerging for nonconventional contaminants, e.g., nitro- and amino-substituted chlorobenzenes that are widely used as industrial feedstock and the target of this work. To date, CSIA of the target compound groups used special combustion interfaces and the potential matrix interferences in environmental samples has not been assessed. We validated CSIA methods for δ13C, δ2H, and δ15N of four analytes from each chemical group and developed a solid-phase extraction (SPE) method to minimize matrix interferences during preconcentration of complex aqueous samples. The SPE recovery was >80% and the method quantification limits of SPE-CSIA for δ13C, δ2H, and δ15N were 0.03-0.57, 1.3-2.7, and 3.4-10.2 μM aqueous-phase concentrations, respectively, using 2 L of spiked MQ water. The SPE-CSIA procedure showed negligible isotope fractionation for δ13C (≤0.5‰), δ15N (≤0.5‰), and δ2H (≤5‰ for nitroaromatics and ≤10‰ for aminoaromatics). In addition, solvent evaporation, water sample storage up to 7 months, and SPE extract storage for 1.5 years did not change analytes' δ13C signatures beyond ±0.5‰. However, to avoid significant δ2H and δ15N fractionation of aminoaromatics, cartridge breakthrough should be avoided and SPE preconcentration must be conducted at pH > pKa + 2. Application of the method at a contaminated site showed excellent precision, at ≤0.3‰ for C and N, and ≤1.5‰ for H. The methods validated here now allow the use of multielement CSIA to track the environmental fate of nitro- and amino-substituted chlorobenzenes in complex aqueous samples.

Long-term monitoring and modeling of PAHs in capped sediments at the Grand Calumet River

The assessment of a cap for remediation of sediments requires long-term monitoring because of the slow migration of contaminants in porous media. In this study, coring and passive sampling tools were used to assess the transport and degradation of polycyclic aromatic hydrocarbons (PAHs) in an amended cap (sand + Organoclay® PM-199) in the Grand Calumet River (Indiana, USA) during four sampling events from 2012 to 2019. Measurements of three PAHs (phenanthrene (Phe), pyrene (Pyr) and benzo[a]pyrene (BaP), representing low, medium, and high molecular weight compounds, respectively) showed a difference of at least two orders of magnitude between bulk concentrations in the native sediments and the remediation cap. Averages of pore water measurements also showed lower levels in the cap respective to the native sediments by a factor of at least 7 for Phe and 3 for Pyr. In addition, between the baseline (BL), which corresponds to observations from 2012 to 2014, and the measurements in 2019, there was a decrease in depth-averaged pore water concentrations of Phe (C2019/CBL=0.20−0.07+0.12 in sediments and 0.27−0.10+0.15 in cap) and Pyr (C2019/CBL=0.47−0.12+0.16 in sediments and 0.71−0.20+0.28 in the cap). In the case of BaP in pore water, no change was observed in native sediments (C2019/CBL=1.0−0.24+0.32) and there was an increase in the cap (C2019/CBL=2.0−0.54+0.72).Inorganic anions and estimates of pore water velocity along with measurements of PAHs were used to model the fate and transport of contaminants. The modeling suggested that degradation of Phe (t1/2=1.12−0.11+0.16 years) and Pyr (t1/2=5.34−1.8+5.3 years) in the cap is faster than migration, thus the cap is expected to be protective of the sediment-water interface indefinitely for these constituents. No degradation was noted in BaP and the contaminant is expected to reach equilibrium in the capping layer over approximately 100 years if there exists sufficient mass of BaP in the sediments and there is no deposition of clean sediment at the surface.

Prediction of Photochemical Properties of Dissolved Organic Matter Using Machine Learning.

Apparent quantum yields (Φ) of photochemically produced reactive intermediates (PPRIs) formed by dissolved organic matter (DOM) are vital to element cycles and contaminant fates in surface water. Simultaneous determination of ΦPPRI values from numerous water samples through existing experimental methods is time consuming and ineffective. Herein, machine learning models were developed with a systematic data set including 1329 data points to predict the values of three ΦPPRIs (Φ3DOM*, Φ1O2, and Φ·OH) based on DOM spectral parameters, experimental conditions, and calculation parameters. The best predictive performances for Φ3DOM*, Φ1O2, and Φ·OH were achieved using the CatBoost model, which outperformed the traditional linear regression models. The significances of the wavelength range and spectral parameters on the three ΦPPRI predictions were revealed, suggesting that DOM with lower molecular weight, lower aromatic content, and a more autochthonous portion possessed higher ΦPPRIs. Chain models were constructed by adding the predicted Φ3DOM* as a new feature into the Φ1O2 and Φ·OH models, which consequently improved the predictive performance of Φ1O2 but worsened the Φ·OH prediction likely due to the complex formation pathways of ·OH. Overall, this study offered robust ΦPPRI prediction across interlaboratory differences and provided new insights into the relationship between PPRIs formation and DOM properties.