Natural attenuation of organic contaminants in groundwater can give rise to a series of complex biogeochemical reactions that release secondary contaminants to groundwater. In a crude oil contaminated aquifer, biodegradation of petroleum hydrocarbons is coupled with the reduction of ferric iron (Fe(III)) hydroxides in aquifer sediments. As a result, naturally occurring arsenic (As) adsorbed to Fe(III) hydroxides in the aquifer sediment is mobilized from sediment into groundwater. However, Fe(III) in sediment of other zones of the aquifer has the capacity to attenuate dissolved As via resorption. In order to better evaluate how long-term biodegradation coupled with Fe-reduction and As mobilization can redistribute As mass in contaminated aquifer, we quantified mass partitioning of Fe and As in the aquifer based on field observation data. Results show that Fe and As are spatially correlated in both groundwater and aquifer sediments. Mass partitioning calculations demonstrate that 99.9% of Fe and 99.5% of As are associated with aquifer sediment. The sediments act as both sources and sinks for As, depending on the redox conditions in the aquifer. Calculations reveal that at least 78% of the original As in sediment near the oil has been mobilized into groundwater over the 35-year lifespan of the plume. However, the calculations also show that only a small percentage of As (∼0.5%) remains in groundwater, due to resorption onto sediment. At the leading edge of the plume, where groundwater is suboxic, sediments sequester Fe and As, causing As to accumulate to concentrations 5.6 times greater than background concentrations. Current As sinks can serve as future sources of As as the plume evolves over time. The mass balance approach used in this study can be applied to As cycling in other aquifers where groundwater As results from biodegradation of an organic carbon point source coupled with Fe reduction.

Uptake of selenium (Se) by plants largely depend on the availability of Se in soil. Soils and plants were sampled four times within 8 weeks of plant growth in pot experiments using four plant species. Sequential extraction and diffusive gradients in thin-films (DGT) method were employed to measure Se concentrations in potted soils in selenite- or selenate-amended soils. Results showed that DGT-measured Se concentrations (CDGT−Se) were generally several folds higher for selenate than selenite amended soils, which were obviously affected by the plant species and the duration of their growth. For example, the folds in soil planted with mustard were 1.49–3.47 and those in soils planted with purple cabbage and broccoli, which grew for 3 and 4 weeks after sowing, were 1.06–2.14 and only 0.15–0.62 after 6 weeks of growth. The selenate-amended soil planted with wheat showed an extremely high CDGT−Se compared with selenite-amended soil, except the last harvest. Furthermore, minimal changes in CDGT−Se and soluble Se(IV) were found in selenite-amended soils during plant growth, whereas significant changes were observed in selenate-amended soils (p < 0.05). Additionally, Se distribution in various fractions of soil remarkably changed; the soils planted with purple cabbage and broccoli showed the most obvious change followed by wheat and mustard. Soluble Se(VI) and exchangeable Se(VI) were likely the major sources of CDGT−Se in selenate-amended soils, and soluble Se(IV) was the possible source of CDGT−Se in selenite-amended soils. In selenate-amended soils, soluble Se(VI) and exchangeable Se(VI) were significantly correlated with Se concentrations in purple cabbage, broccoli, and mustard; in wheat, Se concentration was significantly correlated only with soluble Se(VI) but not with exchangeable Se. CDGT−Se eventually became positively correlated with Se concentrations accumulated by different plants, indicating that DGT is a feasible method in predicting plant uptake of selenate but not of selenite.

Synthetic glucocorticoids (GCs) such as dexamethasone (DEX) and prednisolone (PDS) have been used since the 1940s to cure inflammatory and auto-immune disorders. Their use has been linked to a host of deleterious effects in aquatic ecosystems such as osteoporosis in vertebrates, developmental impairments in molluscs and reduced fecundity and growth in cladocerans. Apart from these handful of studies, the effects of GCs on aquatic biota are largely unknown. The present study is a first of its kind aiming to assess the multi-generational exposure effects of DEX and PDS on the life history parameters of Ceriodaphnia dubia (C. dubia). Multigenerational studies have proved to be an advantage in assessing the cumulative damage caused by aquatic toxicants at the population level of the exposed organisms over a period of successive generations using multiple biological endpoints. Test results demonstrated that C. dubia exhibited varied sensitivities towards both the studied chemicals however were more sensitive to DEX with 48-h EC50 (95% confidence interval) of 0.75 mg/L (CI: 0.59–0.92) in comparison to PDS [19 mg/L (CI: 15–23)]. EC10 values for F0 in a multigenerational chronic bioassays were 48 μg/L (CI: 37.4–61) for DEX and 460 μg/L (CI: 341–606) for PDS and in F3 were 2.2 μg/L (CI: 1.6–3.1) for DEX and 31 μg/L (CI: 19.4–46) for PDS. There was a positive trend of increased toxicity followed by reduced life history traits such as fecundity, brood size and time to first brood and intrinsic rate of population increase and body growth (length and area) of C. dubia in the case of both studied chemicals. The results from the current work highlighted the importance of multigenerational studies in identifying the evolutionary responses of stressed non-target aquatic organisms, and data obtained can be further used in developing water quality guidelines.

The recent discovery of pyrethroid-resistant Hyalella azteca populations in California, USA suggests there has been significant exposure of aquatic organisms to these terrestrially-applied insecticides. Since resistant organisms are able to survive in relatively contaminated habitats they may experience greater pyrethroid bioaccumulation, subsequently increasing the risk of those compounds transferring to predators. These issues were evaluated in the current study following toxicity tests in water with permethrin which showed the 96-h LC50 of resistant H. azteca (1670 ng L⁻¹) was 53 times higher than that of non-resistant H. azteca (31.2 ng L⁻¹). Bioaccumulation was compared between resistant and non-resistant H. azteca by exposing both populations to permethrin in water and then measuring the tissue concentrations attained. Our results indicate that resistant and non-resistant H. azteca have similar potential to bioaccumulate pyrethroids at the same exposure concentration. However, significantly greater bioaccumulation occurs in resistant H. azteca at exposure concentrations non-resistant organisms cannot survive. To assess the risk of pyrethroid trophic transfer, permethrin-dosed resistant H. azteca were fed to fathead minnows (Pimephales promelas) for four days, after which bioaccumulation of permethrin and its biotransformation products in fish tissues were measured. There were detectable concentrations of permethrin in fish tissues after they consumed dosed resistant H. azteca. These results show that bioaccumulation potential is greater in organisms with pyrethroid resistance and this increases the risk of trophic transfer when consumed by a predator. The implications of this study extend to individual fitness, populations and food webs.

Silver nanoparticles (AgNPs) inevitably discharge into aquatic environments due to their abundant use in antibacterial products. It was reported that in laboratory conditions, AgNPs display dose-dependent toxicity to aquatic organisms, such as bacteria, algae, macrophytes, snails and fishes. However, AgNPs could behave differently in natural complex environments. In the present study, a series of microcosms were established to investigate the distribution and toxicity of AgNPs at approximately 500 μg L−1 in aquatic systems. As a comparison, the distribution and toxicity of the same concentration of AgNO3 were also determined. The results showed that the surface layer of sediment was the main sink of Ag element for both AgNPs and AgNO3. Both aquatic plant (Hydrilla verticillata) and animals (Gambusia affinis and Radix spp) significantly accumulated Ag. With short-term treatment, phytoplankton biomass was affected by AgNO3 but not by AgNPs. Chlorophyll content of H. verticillata increased with both AgNPs and AgNO3 short-term exposure. However, the biomass of phytoplankton, aquatic plant and animals was not significantly different between control and samples treated with AgNPs or AgNO3 for 90 d. The communities, diversity and richness of microbes were not significantly affected by AgNPs and AgNO3; in contrast, the nitrification rate and its related microbe (Nitrospira) abundance significantly decreased. AgNPs and AgNO3 may affect the nitrogen cycle and affect the environment and, since they might be also transferred to food web, they represent a risk for health.

Reservoirs typically have elevated fish mercury (Hg) levels compared to natural lakes and rivers. A unique feature of reservoirs is water-level management which can result in sediment exposure to the air. The objective of this study is to identify how reservoir water-level fluctuations impact Hg cycling, particularly the formation of the more toxic and bioaccumulative methylmercury (MeHg). Total-Hg (THg), MeHg, stable isotope methylation rates and several ancillary parameters were measured in reservoir sediments (including some in porewater and overlying water) that are seasonally and permanently inundated. The results showed that sediment and porewater MeHg concentrations were over 3-times higher in areas experiencing water-level fluctuations compared to permanently inundated sediments. Analysis of the data suggest that the enhanced breakdown of organic matter in sediments experiencing water-level fluctuations has a two-fold effect on stimulating Hg methylation: 1) it increases the partitioning of inorganic Hg from the solid phase into the porewater phase (lower log Kd values) where it is more bioavailable for methylation; and 2) it increases dissolved organic carbon (DOC) in the porewater which can stimulate the microbial community that can methylate Hg. Sulfate concentrations and cycling were enhanced in the seasonally inundated sediments and may have also contributed to increased MeHg production. Overall, our results suggest that reservoir management actions can have an impact on the sediment-porewater characteristics that affect MeHg production. Such findings are also relevant to natural water systems that experience wetting and drying cycles, such as floodplains and ombrotrophic wetlands.

Correct characterization of metal speciation and reactivity is a prerequisite for the risk assessment and remedial activity management of contaminated soil. To better understand the intrinsic reactivity of Pb and Zn, nine heavily and poorly contaminated soils were investigated using the combined approaches of chemical extractions, multi-element stable isotopic dilution (ID) method, and multi-surface modelling. The ID results show that 0.1–38% of total Pb and 3–45% of total Zn in the studied soils are isotopically exchangeable after a 3-day equilibration. The intercomparison between experimental and modelling results evidences that single extraction with 0.43 M HNO3 solubilizes part of non-isotopically exchangeable fraction of Pb and Zn in the studied soils, and cannot be used as a surrogate for ID to assess labile Pb and Zn pools in soil. Both selective sequential extraction (SSE) and modelling reveal that Mn oxides are the predominant sorption surface for Pb in the studied soils; while Zn is predicted to be mainly associated with soil organic matter in the soil with low pH and Fe/Mn oxides in the soils with high pH. Multi-surface modelling can provide a reasonable prediction of Pb and Zn adsorption onto different soil constituents for the most of the studied soils. The modelling could be a promising tool to decipher the underlying mechanism that controls metal reactivity in soil, but the submodel for Mn oxides should be incorporated and the model parameters, especially for the 2-pK diffuse layer model for Mn oxides, should be updated in the further studies.

To examine the impacts of urbanization and industrialization on the coastal environment, and assess the effectiveness of control measures on the contamination by chlorinated paraffins (CPs) in East Asia, surface and core sediments were sampled from the urbanized coastal zones in China and Japan (i.e., Pearl River Delta (PRD), Hong Kong waters and Tokyo Bay) and analyzed for short-chain (SCCPs) and medium-chain CPs (MCCPs). Much higher concentrations of CPs were found in the industrialized PRD than in adjacent Hong Kong waters. Significant correlation between CP concentration and population density in the coastal district of Hong Kong was observed (r² = 0.72 for SCCPs and 0.55 for MCCPs, p < 0.05), highlighting the effect of urbanization. By contrast, a relatively lower pollution level of CPs was detected in Tokyo Bay. More long-chain groups within SCCPs in the PRD than in Hong Kong waters and Tokyo Bay implied the effect of industrialization. Comparison of temporal trends between Hong Kong outer harbor with Tokyo Bay shows the striking difference in historical deposition of CPs under different regulatory situations in China and Japan. For the first time, the declining CP concentrations in Tokyo Bay, Japan, attest to the effectiveness of emissions controls.

In this research, adsorption of Cu(II) and Cd(II) by biochars was investigated. To enhance the adsorption of these two metal ions, a simple modification of biochars by phosphoric acid (H(3)PO(4)) was carried out. The surface area was larger and the contents of oxygen-containing functional groups of modified biochars were more than pristine biochars. In comparison with pristine biochar, modified biochars sorbed Cu(II) and Cd(II) much more strongly. Surface area had significant effects on the sorption of Cu(II) and Cd(II) by modified biochars. X-ray photoelectron spectroscopy analyses indicated that the quantities of carboxyl (-COOH) and hydroxyl (-OH) functional groups of modified biochars were larger than those of pristine biochar at the same pyrolysis temperature. Compared with that of pristine biochars, the strong ability of -COOH and -OH of modified biochars to form complexes with Cu(II)/Cd(II) ions resulted in higher adsorption of these two metal ions. The phosphorus-containing groups of modified biochars, such as P=O and P=OOH from the result of Fourier transform infrared spectroscopy, interacted and also formed complexes with metal ions, possibly resulting in the enhanced adsorption of Cu(II) and Cd(II). Thus, sorption of metal ions by modified biochars was controlled by the mechanism of surface complexation between oxygen containing functional groups and metals. In general, the H(3)PO(4) modification was an effective method that prepared biochars with a high affinity for the sorption of heavy metals.

The application of zinc (Zn) isotopes in bivalve tissues to identify zinc sources in estuaries was critically assessed. We determined the zinc isotope composition of mollusks (Crassostrea brasiliana and Perna perna) and suspended particulate matter (SPM) in a tropical estuary (Sepetiba Bay, Brazil) historically impacted by metallurgical activities. The zinc isotope systematics of the SPM was in line with mixing of zinc derived from fluvial material and from metallurgical activities. In contrast, source mixing alone cannot account for the isotope ratios observed in the bivalves, which are significantly lighter in the contaminated metallurgical zone (δ66ZnJMC = +0.49 ± 0.06‰, 2σ, n = 3) compared to sampling locations outside (δ66ZnJMC = +0.83 ± 0.10‰, 2σ, n = 22). This observation suggests that additional factors such as speciation, bioavailability and bioaccumulation pathways (via solution or particulate matter) influence the zinc isotope composition of bivalves.