Integrated study of metal behavior in Mediterranean stream ecosystems: a case-study

Mercury Bioaccumulation in Benthic Invertebrates: From Riverine Sediments to Higher Trophic Levels

Riverine sediments are important sites of mercury methylation and benthic invertebrates may be indicators of Hg exposure to higher organisms. From 2014 to 2018, sediments and invertebrates were collected along a mercury gradient in the Toce River (Northern Italy) and analyzed for THg and MeHg. Concentrations in invertebrates, separated according to taxon and to Functional Feeding Group, ranged from 20 to 253 µg kg⁻¹ dry weight (d.w.) for THg, increasing from grazers (Leuctra, Baetis, Serratella) to predators (Perla). MeHg ranged from 3 to 88 µg kg⁻¹ d.w. in biota, representing 6–53% of THg, while in sediments it was mostly below LOD (0.7 µg kg⁻¹), accounting for ≤3.8% of THg. The Biota-Sediment Accumulation Factor (BSAF, ranging 0.2–4.6) showed an inverse relation to exposure concentrations (THg in sediments, ranging 0.014–0.403 µg kg⁻¹ d.w.) and to organic carbon. THg in invertebrates (up to 73 µg kg⁻¹ wet weight), i.e., at the basal levels of the aquatic trophic chain, exceeded the European Environmental Quality Standard for biota (20 µg kg⁻¹ w.w.), posing potential risks for top predators. Concentrations in adult insects were close to those in aquatic stages, proving active mercury transfer even to terrestrial food chains.

Characterization of river biofilm responses to the exposure with heavy metals using a novel micro fluorometer biosensor

River biofilms are a suitable indicator of toxic stress in aquatic ecosystems commonly exposed to various anthropogenic pollutants from industrial, domestic, and agricultural sources. Among these pollutants, heavy metals are of particular concern as they are known to interfere with various physiological processes of river biofilm, directly or indirectly related to photosynthetic performance. Nevertheless, only limited toxicological data are available on the mechanisms and toxicodynamics of heavy metals in biofilms. Pulse Amplitude Modulated (PAM) fluorometry is a rapid, non-disruptive, well-established technique to monitor toxic responses on photosynthetic performance, fluorescence-kinetics, and changes in yield in other non-photochemical processes. In this study, a new micro-PAM-sensor was tested to assess potential acute and chronic effects of heavy metals in river biofilm. Toxicity values across the three parameters considered in this study (photosynthetic yield YII, non-photochemical quenching NPQ, and basal fluorescence F₀) were comparable, as determined EC50 were within one order of magnitude (EC50 ∼1-10 mg L^-1). However, the stimulation of NPQ was more clearly associated with early acute effects, especially in illuminated samples, while depression of YII and F₀ were more prevalent in chronic tests. These results have implications for the development of functional indicators for the biomonitoring of aquatic health, in particular for the use of river biofilm as a bioindicator of water quality. In conclusion, the approach proposed seems promising to characterize and monitor the exposure and impact of heavy metals on river periphyton communities. Furthermore, this study provides a fast, highly sensitive, inexpensive, and accurate laboratory method to test effects of pollutants on complex periphyton communities that can also give insights regarding the probable toxicological mechanisms of heavy metals on photosynthetic performance in the river biofilm.

Performance of Chlorella Vulgaris Exposed to Heavy Metal Mixtures: Linking Measured Endpoints and Mechanisms

Microalgae growth inhibition assays are candidates for referent ecotoxicology as a fundamental part of the strategy to reduce the use of fish and other animal models in aquatic toxicology. In the present work, the performance of Chlorella vulgaris exposed to heavy metals following standardized growth and photosynthesis inhibition assays was assessed in two different scenarios: (1) dilutions of single heavy metals and (2) an artificial mixture of heavy metals at similar levels as those found in natural rivers. Chemical speciation of heavy metals was estimated with Visual MINTEQ software; free heavy metal ion concentrations were used as input data, together with microalgae growth and photosynthesis inhibition, to compare different effects and explain possible toxicity mechanisms. The final goal was to assess the suitability of the ecotoxicological test based on the growth and photosynthesis inhibition of microalgae cultures, supported by mathematic models for regulatory and decision-making purposes. The C. vulgaris algae growth inhibition test was more sensitive for As, Zn, and Pb exposure whereas the photosynthesis inhibition test was more sensitive for Cu and Ni exposure. The effects on growth and photosynthesis were not related. C. vulgaris evidenced the formation of mucilaginous aggregations at lower copper concentrations. We found that the toxicity of a given heavy metal is not only determined by its chemical speciation; other chemical compounds (as nutrient loads) and biological interactions play an important role in the final toxicity. Predictive mixture effect models tend to overestimate the effects of metal mixtures in C. vulgaris for both growth and photosynthesis inhibition tests. Growth and photosynthesis inhibition tests give complementary information, and both are a fast, cheap, and sensitive alternative to animal testing. More research is needed to solve the challenge of complex pollutant mixtures as they are present in natural environments, where microalgae-based assays can be suitable monitoring tools for pollution management and regulatory purposes.

Gold mining tailing: Environmental availability of metals and human health risk assessment

The gold ore from sulfide minerals is, in general, ore dressed by means of nine stages among which stand out flotation and cyanidation. The residues of these steps, containing potentially toxic elements, such as As, Cd, Cr, Mn, Zn and Pb disposed of tailings dams, which might be a source of environmental contamination if not suitably disposed and/or in cases of accidents and overflows. Sequential extraction schemes (SES) have been used to estimate the potential environmental availability of contaminants from environmental matrices and, lately, from residues. This research evaluates the environmental availability of As, Cr, Cd, Mn, Pb, and Zn, by using two different SES, Tessier and Marin (BCR) in cyanidation residues. The analytes were quantified by inductively coupled plasma with optical emission spectrometry (ICP OES). A human health risk assessment was performed considering a scenario of soil contamination by the tailing after failure dam, based on the potential environment availability of metals, resulted from distinct SES studied. The results showed that Mn and Pb are the most labile, and therefore the most dangerous and bioavailable for the surrounding environment (≥75%). Moreover, the scenario simulated demonstrated the risk for human health mostly due to As, Cd and Zn.

The importance of drying and grinding samples for determining mobile chromium fractions in polluted river sediments

A possible impact of sample preparation on the chemical fractionation results is generally underestimated in studies of forms of occurrence of heavy metals in river sediments. Our analysis of the recently published results of sequential extraction of chromium has revealed the effect of sample grinding on the result of determination of mobile chromium fractions in river sediments. This observation has been experimentally verified along with the analysation of potential effect of river sediment drying conditions on chromium distribution pattern. The studies were carried out on river sediments polluted with tannery effluents (Cr, 29.2-233 mg/kg). The determined content of chromium bound to carbonates in powdered samples was 2 to 7 times higher than those in raw river sediment samples. It was shown that the main reason was the different kinetic characteristics of chromium leaching in these sediments. Using the shrinking core model, it was found that diffusion through the "ash layer" was the rate-controlling step during the extraction of the carbonate fraction of chromium. It has been additionally confirmed that common air drying of sediment samples does not affect the results of chemical fractionation of chromium.The results of our studies are also vital for the assessment of environmental risk posed by river sediments polluted with heavy metals. In the case of sediment samples used in this study, powdering changed the risk category (RAC) from low risk to high risk. Hence, in order to achieve a realistic assessment of chromium mobility and environmental risk, it is advisable to use raw samples, despite their poorer homogeneity, and thus, lower precision of chemical fractionation results.

Predicting mercury bioavailability in soil for earthworm Eisenia fetida using the diffusive gradients in thin films technique

In general, the diffusive gradients in thin films (DGT) technique is an effective tool for evaluating metal bioavailability; however, its applicability is subject to the type of metal and organism involved. In this study, the accumulated masses of Hg in DGT probes and in the earthworm species Eisenia fetida were monitored for 10 days, to test if the DGT technique can be used as a predicting method for the bioavailability of soil Hg to earthworms. In the Hg exposure tests using soils prepared with different peat moss concentrations of 5, 10, 15, and 20% and varying pH values of 4.6, 5.6, and 6.2, the experimentally determined DGT-soil accumulation factor (DSAF) and biota-soil accumulation factor (BSAF) both increased as the peat moss content decreased and the pH increased. According to a one compartment model, this was a result of the increased Hg uptake rate constant (k₁) and the relatively stable Hg elimination constant (k₂) under lower peat moss and higher pH conditions. It is interesting to note that the Hg uptake rates by DGT and earthworms were considerably higher for fresh soils than for aged soils, while porewater (and acid-extractable) Hg concentrations were rather similar between the two types of soils. Across diverse soil properties, steady-state Hg in earthworm tissue showed a strong positive correlation with DGT-measured Hg flux ([earthworm Hg] = 354(DGT-Hg flux)-34, r² = 0.88), while meager correlations were found between Hg concentration in earthworms and that in porewater (and acid-extractable). The overall results indicate that DGT-measured Hg flux is a better tool than conventional methods for predicting Hg bioavailability for earthworms inhabiting diverse types of soil.

Priority Pollutants in Water and Sediments of a River for Control Basing on Benthic Macroinvertebrate Community Structure

Understanding the drivers of macroinvertebrate community structure is fundamental for adequately controlling pollutants and managing ecosystems under global change. In this study, the abundance and diversity of benthic macroinvertebrates, as well as their chemical parameters, were investigated quarterly from August 2014 to April 2015 in four reaches of the Huai River basin (HRB). The self-organizing map (SOM) algorithm and canonical correspondence analysis (CCA) were simultaneously applied to identify the main factors structuring the benthic community. The results showed that the benthic community structure was always dominated by gastropoda and insecta over seasons and presented obvious spatial and temporal heterogeneity along different pollution levels. The insects were always the top contributors to number density of the benthic community, except for the summer, and the biomass was mainly characterized by mollusca in all seasons. Statistical analysis indicated that TN and NH3-N in water, as well as Hg, As, Cd, and Zn in sediments, were the dominant factors structuring the community, which determined the importance of sediment heavy metal concentrations in explaining the benthic community composition in comparison with other factors. These major factors should be given priority in the process of river pollutant control, which might be rated as a promising way to scientifically improve river health management and ecological restoration.

Relation between different metal pollution criteria in sediments and its contribution on assessing toxicity

Several tools have been developed and applied to evaluate the metal pollution status of sediments and predict their potential ecological risk assessment. To date, a comprehensive relationship between the information given by these sediment tools for predicting metal bioavailability and the effective toxicity observed is lacking. In this work, the possible inter-correlations between the data outcoming from using several qualitative evaluation tools of the sediment contamination (contamination factor, CF, the enrichment factor, EF, or the geoaccumulation index, Igeo), metal speciation on sediments (evaluated by the modified BCR sequential extraction procedure) and free metal concentrations in pore waters were studied. It was also our aim to evaluate if these assessment tools could be used for predicting the pore waters toxicity data as toxicity proxy. Principal component analysis and cluster analysis revealed that two quality indices used (CF and EF) were highly correlatable with the more labile fractions from BCR sediment speciation. However, neither of these parameters did correlate with the toxicity of pore waters measured by the chronic toxicity (72 h) in Pseudokirchneriella subcapitata. In contrast, the toxic effects of the given total metal load in sediments were better evaluated by using an additive metal approach using pore water free metal concentrations.

Use of Chemical Indicators and Bioassays in Bottom Sediment Ecological Risk Assessment

This study is was designed to assess the ecological risk associated with chemical pollution caused by heavy metals and PAHs on the basis of their ecotoxicological properties in sediments collected from the Rzeszów dam reservoir (Poland). The sediment samples were collected from three sampling stations: S1-inlet, backwater station, S2-middle of reservoir, S3 outlet, near the dam. The sediments' toxicity was evaluated using a battery of bioassays (Phytotoxkit, Phytotestkit, Ostracodtoxkit F, and Microtox). The highest content of metals (120.5 mg Zn; 22.65 mg Pb; 8.20 mg Cd ∙ kg-1 dw) and all PAHs (∑9361 μg ∙ kg-1 dw) in sediments was found at station S1. The lowest content of metals (86.72 mg Zn; 18.07 mg Cu; 17.20 mg Pb; 3.62 mg Cu; 28.78 mg Ni; 30.52 mg Cr ∙ kg-1 dw) and PAHs (∑4390 μg ∙ kg-1 dw) was found in the sediment from station S2. The ecological risk assessment of the six metals and eight PAHs revealed a high potential toxicity in sediments from stations S1 (PECq = 0.69) and S3 (PECq = 0.56) and a low potential toxicity in sediments from station S2 (PECq = 0.38). The studies also showed the actual toxicity of sediments for the test organisms. The sediment pore water was least toxic compared to the whole sediment: solid phases > whole sediment > pore water. The most sensitive organism for metals and PAHs in bottom sediments was Lepidium sativum, and in pore water-Sorghum saccharatum. The concentration of metals and PAHs in bottom sediments generally did not affect the toxicity for other organisms. Clay content and organic C content are likely to be important factors, which control heavy metal and PAH concentrations in the sediments. Data analysis by PCA found the same origin of metals as well as PAHs-mainly anthropogenic sources. The obtained information demonstrated the need to integrate ecotoxicological and chemical methods for an appropriate ecological risk assessment.

Prediction of the bioavailability of potentially toxic elements in freshwaters. Comparison between speciation models and passive samplers

The aim of this work is to predict the bioavailability of the Potentially Toxic Elements (PTEs) Cd, Pb, Hg, Ni, Cu, Zn, As, Cr and Se in 6 sites within the Ebro River basin. In situ Diffusive gradient in thin-films (DGTs) and classical sampling have been used and compared. The potentially bioavailable fractions of each PTE was estimated by modelling their chemical speciation using three programs (WHAM 7.0, Visual MINTEQ 3.1 and Bio-met), following the suggestions published in recent European regulations. Results of the equilibrium-based models WHAM 7.0 and Visual MINTEQ 3.1 indicate that As, Cd, Ni, Se and Zn, predominate as free metals ions or forming inorganic soluble complexes. Copper, Pb and Hg bioavailability is conditioned by their affinity to dissolved humic substances. According to Visual MINTEQ 3.1, Cr is subjected to redox reactions, being Cr (VI) present (at low concentrations) in the studied rivers. According to Bio-met model, the bioavailability of Cu and Zn is highly influenced by soluble organic matter and water hardness, respectively. For most PTEs, the bioavailability estimated by deploying DGTs in river waters tends to be slightly lower than the estimation obtained with speciation models, since in real conditions more environmental factors take place comparing to the finite number of parameters considered in models.

Performance of Raphidocelis subcapitata exposed to heavy metal mixtures

Microalgae growth inhibition assays are candidates for referent ecotoxicological assays, and are a fundamental part in the strategy to reduce the use of fish and other animal models in aquatic toxicology. In the present work, the performance of Raphidocelis subcapitata exposed to heavy metals following standardized growth inhibition assays has been assessed in three different scenarios: 1) dilutions of single heavy metals, 2) artificial mixture of heavy metals at similar levels than those found in natural rivers and, 3) natural samples containing known mixtures of contaminants (heavy metals). Chemical speciation of heavy metals has been estimated with Eh-pH diagram and Visual MINTEQ software; heavy metal and free heavy metal ion concentrations were used as input data, together with microalgae growth inhibition, for Dr. Fit software. The final goal was to assess the suitability of the ecotoxicological test based on the growth inhibition of microalgae cultures, and the mathematic models based on these results, for regulatory and decision-making purposes. The toxicity of a given heavy metal is not only determined by its chemical speciation; other chemical and biological interaction play an important role in the final toxicity. Raphidocelis subcapitata 48h-h-EC50 for tested heavy metals (especially Cu and Zn) were in agreement with previous studies, when ion metal bioavailability was assumed to be 100%. Nevertheless, the calculated growth inhibition was not in agreement with the obtained inhibition when exposed to the artificial mixture of heavy metals or the natural sample. Interactions between heavy metal ions and the compounds of the culture media and/or the natural sample determine heavy metal bioavailability, and eventually their toxicity. More research is needed for facing the challenge posed by pollutant mixtures as they are present in natural environments, and make microalgae-based assays suitable for pollution management and regulatory purposes.

Application of the Multimedia Urban Model to estimate the emissions and environmental fate of PAHs in Tarragona County, Catalonia, Spain

The Multimedia Urban Model (MUM-Fate) was used to estimate the emissions, fate and transport of polycyclic aromatic hydrocarbons (PAHs) in Tarragona County, Catalonia, Spain, where the largest chemical/petrochemical industrial complex of Southern Europe is located. MUM-Fate is a Level III steady-state fugacity model consisting of seven bulk media compartments (lower and upper air, surface water, sediment, soil, vegetation, and an organic film that coats impervious surfaces). The model was parameterized according to environmental conditions in Tarragona County, and used to back-calculate emissions from measured air concentrations of naphthalene, anthracene, phenanthrene, fluoranthene, pyrene, and benzo[a]pyrene. Modelled results in soils were within the range reported for measured concentrations. Estimated emissions of ∑₆PAH were 42ty^-1, with phenanthrene having the greatest value (16ty^-1). The fate and transport of ∑₆PAH were subsequently estimated by running the model an illustrative emission rate of 1molh^-1. Organic film on impervious surfaces was the compartment that achieved the highest concentrations of PAHs, being up to 2·10⁸ngm^-3. However, as the film conveys chemicals to surface waters, the persistence in this compartment was <1day. Soils and sediments were the greatest sinks for PAHs, with a persistence of 100-1000days. The greatest loss of PAHs was due to advection from air, followed by photodegradation from air. These results provide a first approximation of the current emissions and fate of PAHs in Tarragona County.

In situ bioavailability of DDT and Hg in sediments of the Toce River (Lake Maggiore basin, Northern Italy): accumulation in benthic invertebrates and passive samplers

DDT and mercury (Hg) contamination in the Toce River (Northern Italy) was caused by a factory producing technical DDT and using a mercury-cell chlor-alkali plant. In this study, DDT and Hg contamination and bioavailability were assessed by using different approaches: (1) direct evaluation of sediment contamination, (2) assessment of bioaccumulation in native benthic invertebrates belonging to different taxonomic/functional groups, and (3) evaluation of the in situ bioavailability of DDT and Hg using passive samplers. Sampling sites were selected upstream and downstream the industrial plant along the river axis. Benthic invertebrates (Gammaridae, Heptageniidae, and Diptera) and sediments were collected in three seasons and analyzed for DDT and Hg content and the results were used to calculate the biota sediment accumulation factor (BSAF). Polyethylene passive samplers (PEs) for DDT and diffusive gradients in thin films (DGTs) for Hg were deployed in sediments to estimate the concentration of the toxicants in pore water. Analysis for (DDx) were performed using GC-MS. Accuracy was within ±30 % of the certified values and precision was >20 % relative standard deviation (RSD). Total mercury concentrations were determined using an automated Hg mercury analyzer. Precision was >5 % and accuracy was within ±10 % of certified values. The results of all the approaches (analysis of sediment, biota, and passive samplers) showed an increasing contamination from upstream to downstream sites. BSAF values revealed the bioavailability of both contaminants in the study sites, with values up to 49 for DDx and up to 3.1 for Hg. No correlation was found between values in sediments and the organisms. Concentrations calculated using passive samplers were correlated with values in benthic invertebrates, while no correlation was found with concentrations in sediments. Thus, direct analysis of toxicant in sediments does not provide a measurement of bioavailability. On the contrary, analysis of bioaccumulation in benthic organisms provides the most realistic picture of the site-specific bioavailability of DDx and Hg, but this approach is time-consuming and not always feasible. On the other hand, the in situ deployment of passive samplers proved to be a powerful tool, providing a good surrogate measure of bioaccumulation.

Metal bioavailability in freshwater sediment samples and their influence on ecological status of river basins

The general aim of this work has been to check the ecological impact of metals on the Ebro river basin. In order to evaluate this, metal behavior considering water, sediment as well as metal bioaccumulation in fish has been studied. Total concentrations of metals, as well as the potentially bioavailable fraction of metals in sediment has also been analyzed by the application of the sequential extraction method (BCR method). In order to evaluate the influence of metal pollution on the river ecological status, according to the Water Framework Directive (WFD), diverse biological indices such as macroinvertebrates (IBMWP), diatoms (IPS) and macrophytes (IVAM), have been considered from an integrated point of view. Considering both water and sediment, metals which contributed in higher extend to the reduction of biological quality have been demonstrated to be Pb and Zn, as they presented a negative influence on macroinvertebrates, diatoms and macrophytes communities. As and Cr that seemed to have a significant influence on macroinvertebrates and diatoms too, while Ni negatively influenced only diatom communities. This study also demonstrated that monitoring programs only based on total metal determination in water are inefficient, as metals present even at undetectable concentrations in water are strongly accumulated in fish. Moreover, the high concentrations of Hg found in sediments indicated that this river basin may present pollution problems regarded to this metal, as demonstrated by the high Hg levels found in fish.

Assessment of sediment ecotoxicological status as a complementary tool for the evaluation of surface water quality: the Ebro river basin case study

According to the European Water Framework Directive (WFD), assessment of surface water status is based on ecological and chemical status that is not always in coherence. In these situations, ecotoxicity tests could help to obtain a better characterization of the ecosystems. The general aim of this work is to design a methodology to study the ecotoxicological status of freshwater systems. This could be useful and complementary to ecological status, for a better ecological characterization of freshwater systems. For this purpose, sediments from thirteen sampling sites within the Ebro river watershed (NE Spain) were collected for ecotoxicity characterization. The ecotoxicity of pore water has been evaluated employing the test organisms Vibrio fischeri, Pseudokirschneriella subcapitata and Daphnia magna, while whole sediment ecotoxicity was evaluated using Vibrio fischeri, Daphnia magna, Nitzschia palea and Chironomus riparius. An analysis of acid-volatile sulfide (AVS) and simultaneously extracted metals (SEM) was performed to evaluate the sediment toxicity associated to bioavailable metals. Moreover, data about priority pollutants defined by the WFD in water, sediment and fish as well as data of surface water status of each sampling point were provided by the Monitoring and Control Program of the Ebro Water bodies. In general terms, whole sediment bioassays have shown more toxicity than pore water tests. Among the different organisms used, P. subcapitata and C. riparius were the most sensitive in pore water and whole sediment, respectively. Our evaluation of the ecotoxicological status showed high coincidences with the ecological status, established according to the WFD, especially when ecosystem disruption due to numerous stressors (presence of metals and organic pollution) was observed. These results allow us to confirm that, when chemical stressors affect the ecosystem functioning negatively, an ecotoxicological approach, provided by suitable bioassays in pore water and whole sediment, could detect these changes with accurate sensitivity.

Assessment of river water quality using an integrated physicochemical, biological and ecotoxicological approach

In order to maintain and improve the water quality in European rivers, the Water Framework Directive (WFD) requires an integrated approach for assessing water quality in a river basin. Although the WFD aims at a holistic understanding of ecosystem functioning, it does not explicitly establish cause-effect relationships between stressors and changes in aquatic communities. To overcome this limitation, the present study combines the typical WFD physicochemical and biological approaches with an ecotoxicological approach. The main goal was to assess river water quality through an integrated manner, while identifying potential risk situations for aquatic communities in the Cértima river basin (Portugal). To achieve this goal, surface water samples and macroinvertebrate specimens were collected under contrasting hydrological conditions (autumn and spring seasons) at three river sites exposed to distinct pollution levels defined according to the WFD (low, moderate and highly polluted). Physicochemical water quality status was defined according to the Portuguese classification for multipurpose surface waters, whereas biological water quality was assessed in accordance with the South Invertebrate Portuguese Index. Ecotoxicological assays included four standard species, a bacterial species (Vibrio fischeri), a unicellular algae (Pseudokirchneriella subcapitata), a macrophyte (Lemna minor) and a crustacean (Daphnia magna), which were exposed to different river water concentrations. The study sites represented a clear and pronounced gradient of pollution, from the unpolluted reference site to the sites under moderate to high anthropogenic pressure. In the latter sites, clear signs of organic pollution were found, such as low dissolved oxygen concentrations, high nutrient loads and prevalence of highly tolerant macroinvertebrate species. Despite the evident signs of pollution, no clear evidence of toxicity was observed in test species, suggesting that ecotoxicological assays using standard laboratory species and methodologies might not be suitable for assessing the effects of organic pollution. Nevertheless, the integrated methodology presented in this study provided important additional information on the Cértima's water quality status. Its wider use could contribute to a more comprehensive assessment of the effects of anthropogenic pollution on the status and functioning of aquatic ecosystems under the WFD and, thereby, improve the scientific foundations for the sustainable future management of surface water resources.