Laboratory studies of steam stripping of LNAPL‐contaminated soils

Bench‐scale laboratory experiments were conducted to evaluate the effectiveness of steam injection for in situ remediation of soils contaminated by light nonaqueous‐phase liquids (LNAPLs). Several parametric studies were performed with various combinations of soils, LNAPLs, and steam injection conditions.

An increase in steam injection pressure produced a significant increase in LNAPL recovery efficiency. An increase in steam injection pressure from 12.4 to 44.8 kPa resulted in increased LNAPL recovery efficiency from 86 to 95% after one pore volume of steam injection. Higher steam injection pressure yielded maximum LNAPL recovery efficiency in significantly less time and required a smaller amount of steam than at low pressure.

An increase in soil grain size or an increase in grain‐size‐distribution slope resulted in increased LNAPL recovery efficiency. The final LNAPL residual saturation was approximately 0.5% for coarse‐grained soils and 1.8% for soils with finer grain sizes. Soils with finer grains required more time for treatment than soils with coarser grains.

Steam injection experiments with No. 2 heating oil and with jet fuel showed no significant variation in steam front propagation, temperature profile, and maximum LNAPL recovery efficiency. The LNAPL residual saturation after steam injection was essentially independent of the starting LNAPL saturation.     

Supercritical Fluid Extraction of a Light PAH Contaminated Sand

This work was aimed at evaluating the feasibility of a remediation treatment performed by means of a supercritical carbon dioxide extraction on a sandy soil recently contaminated by light polycyclic aromatic hydrocarbons.

The soil utilized in this study was artificially contaminated by naphthalene and anthracene. The artificial contamination process was intended to simulate a recent accidental spillage of hydrocarbon fuels.

Several extractions, aimed at singling out the operating parameters (pressure, temperature, supercritical fluid mass flow rate) that are able to obtain the residual required concentration (50 mg/kg dry soil) in the shortest time, were carried out on a on-purpose made system.

The best extraction conditions were 120 bar and 40°C for a naphthalene contaminated soil and 200 bar and 80–100°C for an anthracene contaminated soil.

The results obtained in the experimental tests made it possible to build an analytical model able to correlate, for the given soil, the extraction length to the operating parameters such as supercritical fluid density, temperature and mass flow rate.

In order to evaluate the economic feasibility of the process, a unit treatment cost was evaluated for the case of an extraction carried out in a 10 m³ reactor in the presence of the best extraction conditions that were previously determined. The extraction unit cost was therefore equal to 35 000–65 000 €/t for a soil with a starting contaminant concentration equal to 1000 mg/kg of dry soil.     

Solubilization and Plant Uptake of Zinc and Cadmium from Soils Treated with Elemental Sulfur

In growth chamber experiments we studied the potential use of elemental sulfur (S₈) as an acidifying agent to enhance the uptake of Cd and Zn from three different polluted soils by candidate phytoremediation plants (Brassica juncea, Helianthus annuus, Salix viminalis). Two of the three soils were calcareous, the other slightly acidic. One of the calcareous soils had been contaminated by dust emissions from a nearby brass smelter. The pollution of the other two soils had resulted from sewage sludge applications.

Sulfur was added to soils in quantities of 20 to 400 mmol sulfur kg^-1 soil. Plants were grown under fluorescent light in 1.5 l ($OS 13 cm) pots for 28 d.

Within 700 h soil pH decreased significantly in all soils, depending on S₈ dosage. In the acid soil, pH decreased from pH 6.5 to about 4 at the highest treatment level, while pH in one of the calcareous soils dropped even below pH 4. The effect was smaller in the second calcareous soil.

NaNO₃-extractable Cd and Zn increased up to 26-and 13-fold, respectively, in the acid soil, while in the calcareous soils, maximum increases were 9-and 11-fold, respectively.

Increased NaNO₃-extractable concentrations translated well into shoot concentrations (dry matter) in plants. Shoot Zn concentrations in H. annuus, for example, increased from 930 in the controls to 4300 mg kg^-1 in the highest S₈ treatment. However, effects observed in the plants were generally smaller than in the soils. In addition, in some variants growth was negatively affected, resulting in reduced metal removal from the soils.     

Hexavalent Chromium Reduction in Soils Contaminated with Chromated Copper Arsenate Preservative

The toxicity and mobility of chromium in the environment greatly depends upon its speciation. The reduction of hexavalent chromium to trivalent chromium in a soil environment was examined by spiking three soil types (sandy, clayey, and organic soils) with a common wood preservative solution known as chromated copper arsenate (CCA). Chromium in the CCA preservative solution exists in the hexavalent form. The total and hexavalent chromium concentrations (mg/kg) were measured over a period of 11 months. Leachable chromium concentrations (mg/L) were assessed using the synthetic precipitation leaching procedure (SPLP). The degree and rate of hexavalent chromium reduction were similar for the sand and clayey soil, but much greater for the organic soil. Most of the chromium reduction occurred within the first month of the experiment. At the end of the experiment, approximately 50% of the hexavalent chromium was converted to the trivalent form in the sand and clayey soils. Hexavalent chromium concentrations were below detection in the organic soil at the end of the experiment. Nearly all of the chromium observed in the SPLP leachates was in the form of hexavalent chromium. Chromium leaching was thus greatest in the sand and clay soils where the hexavalent chromium persisted. The results indicate that hexavalent chromium in soils can persist for considerable time periods, in particular in soils with low organic matter content.

     

Prediction of leachate concentrations in petroleum‐contaminated soils

The efficacy of cleanup methods in reducing gasoline contamination at spill sites is typically determined by measuring benzene, toluene, xylene (BTX), and total petroleum hydrocarbon (TPH) concentrations in soil samples. Although these values may provide a direct measurement of soil contamination, they may not be indicative of what is transferred to percolating water. This study addresses this issue by measuring TPH, toluene, m‐ and p‐xylene, and naphthalene levels in gasoline‐contaminated soil columns before and after forced‐air venting and relating these values to the aqueous‐phase concentrations measured when water is percolated through the same columns.

Sandy soils with and without organic matter were packed into glass columns. The soils were brought to residual water and residual gasoline saturations by applying a vacuum to a ceramic pressure plate at the column bottom. Venting was performed by passing clean, moist air through the columns. The columns were subsequently leached under unsaturated conditions.

Soil samples were taken from the bottom of the columns upon completion of the venting or leaching phases of the experiments. Toluene, m‐ and p‐xylene, naphthalene, and TPH values were measured in soil samples extracted with either freon or methanol. Aqueous phase concentrations of these compounds were predicted using measured soil concentrations and either Raoult's law or organic matter‐water and fuel‐water partitioning theory (Boyd and Sun, 1990). The predicted results were compared with measured leachate concentrations from the same columns.

Mole fractions estimated from soil concentrations and TPH values used in Raoult's law gave good predictions of aqueous phase concentrations for compounds that had a high mole fraction in the residual nonaqueous phase liquid (NAPL). For compounds at low concentrations in the residual NAPL, an approach using a distribution coefficient that accounted for both the organic matter and residual NAPL in the soil provided better estimates than those based on Raoult's law.     

Organic emissions from petroleum‐contaminated soil fixed in concrete

This study investigated a solidification treatment process for soils that are contaminated with hydrocarbons at levels of 0.5 and 3.0% by weight of benzene. The contaminated soils were bound in a concrete matrix and the migration of organics from the concrete to air was evaluated. If the hydrocarbon emissions are sufficiently attenuated, the concrete containing such contaminated soil can be used for exterior construction applications.

The experimental specimens consisted of concrete mixtures in which 40% of the sand was replaced with the contaminated soil. The mixtures’ ratio of cement, aggregate, sand, and water is 1:1.5:1.5:0.5 by weight. The study included specimens with and without class C fly ash replacing 10% of the cement. In addition, two unfixed control specimens were prepared for each contamination level. The concrete‐soil mixtures were placed in sealed jars and air was passed through the head space of the jars and then through carbon adsorption tubes for measurement of contaminant flux from the surface of the specimens. Measurements were made during and after concrete curing. The results were fitted to a Fickian diffusion model to estimate effective diffusivity in the concrete‐soil specimens.

The test results showed that the effective diffusivity of the contaminant within the concrete was reduced by three to five orders of magnitude over the molecular diffusivities in unfixed contaminated soil used as control. It was observed that the presence of fly ash in the concrete affects the hydrocarbon release and causes an additional decrease in effective diffusivity of about one order of magnitude. Contaminant emissions during the curing phase were found to exceed rates predicted by the Fickian model. This is apparently due to the water used in the concrete. Total emissions, however, never exceeded values emitted from the unfixed controls. This study indicates that fixation of low hydrocarbon levels within concrete is a technically viable and safe technology for recycling petroleum‐contaminated soil.     

The role of elevation and soil chemistry in the distribution and ion accumulation of floral morphs of Streptanthus polygaloides Gray (Brassicaceae), a Californian nickel hyperaccumulator

Background: The flora of serpentine/ultramafic soils provides an excellent model system for the study of natural selection in plant populations. Streptanthus polygaloides is a nickel hyperaccumulator that is endemic to serpentine soils in the Sierra Nevada of California, and has four floral morphs (yellow, purple, yellow-to-purple and undulate).

Aims: We investigate three hypotheses: (1) the purple morph occurs in colder, wetter climates than the yellow morph; (2) tissue–soil ionic relationships differ among morphs; and (3) morphs occur on soils with differing elemental concentrations.

Methods: We queried herbarium records to investigate patterns of occurrence among the yellow and purple floral morphs, and analysed soil and tissue samples from wild populations of all four morphs.

Results: The purple morph inhabited serpentine outcrops with colder temperatures and greater precipitation levels than the yellow morph. Concentrations of elements in leaf tissue and rhizosphere soil differed little among populations of the morphs, but showed substantial within-site variation.

Conclusions: Our results suggest that a climatic gradient may be responsible for divergence in floral colour among populations of S. polygaloides. Because of the large within-site variation in soil and tissue elemental concentrations, plants appear to have a varied physiological response to edaphic factors, regardless of morph membership.     

Multiple biomarker approach in the fiddler crab to assess anthropogenic pollution in tropical coastal environments

Context: Fiddler crabs are important to the ecology of estuarine systems around the world, however, few studies have incorporated them as bioindicators. Urias estuary represents one of the most urbanized lagoons in the Gulf of California region and received discharges from different sources: shrimp farm, thermoelectric plant, fish processing plants, and untreated domestic and sewage wastes.

Objective: Assess the effects on anthropogenic contamination on female fiddler crabs reproduction, survival and genetic stability.

Methods: Exposition of wild crabs from a less impacted (reference) site to naturally contaminated sediments on under controlled laboratory conditions. Reproductive parameters, levels of DNA damage and mortality rates were measured, together with chemical analyses of sediments.

Results: The most contaminated sediments corresponded to the site where fish processing plants were located and the integrated biomarker response analysis revealed that the most adverse effects were produced by exposure to sediments from this site; these crabs showed higher mortality (67%) and poorer ovarian development than those crabs exposed to sediments from other sites.

Conclusions: Female crabs under pollution stress are able to trade-off reproduction for survival, and surviving animals were able to restore genetic stability possibly by activating DNA repair mechanisms. Multiple biomarker approach discriminates different coastal contamination scenarios.     

Feasibility of Phytoextraction to Clean Up Low-Level Uranium-Contaminated Soil

The potential to phytoextract uranium (U) from a sandy soil contaminated at low levels was tested in the greenhouse. Two soils were tested: a control soil (317 Bq ²³⁸U kg^-1) and the same soil washed with bicarbonate (69 Bq ²³⁸U kg^-1). Ryegrass (Lolium perenne cv. Melvina), Indian mustard (Brassica juncea cv. Vitasso), and Redroot Pigweed (Amarathus retroflexus) were used as test plants.

The annual removal of the soil activity with the biomass was less than 0.1%. The addition of citric acid (25 mmol kg^-1) 1 week before the harvest increased U uptake up to 500-fold. With a ryegrass and mustard yield of 15000 kg ha^-1 and 10000 kg ha^-1, respectively, up to 3.5% and 4.6% of the soil activity could annually be removed with the biomass.

With a desired activity reduction level of 1.5 and 5 for the bicarbonate washed and control soil, respectively, it would take 10 to 50 years to attain the release limit.

A linear relationship between the plant ²³⁸U concentration and the ²³⁸U concentration in the soil solution of the control, bicarbonate-washed, or citric acid-treated soil points to the importance of the soil solution activity concentration in determining U uptake and hence to the importance of solubilising agents to increase plant uptake.

However, citric acid addition resulted in a decreased dry weight production (all plants tested) and crop regrowth (in case of ryegrass).     

Mycorrhizal ecology on serpentine soils

Background: Serpentine ecosystems support different, often unique, plant communities; however, we know little about the soil organisms that associate with these ecosystems. Mycorrhizas, mutualistic symbioses between fungi and roots, are critical to nutrient cycling and energy exchange below ground.

Aims: We address three hypotheses: H1, diversity of mycorrhizal fungi in serpentine soils mirrors above-ground plant diversity; H2, the morphology of mycorrhizas and fungi on serpentine soils differs from that on non-serpentine; and H3, mycorrhizal fungal communities of the same or closely related hosts differ between serpentine and non-serpentine soils.

Methods: This review focuses on whether plant diversity on serpentine soils correlates with the below ground diversity of mycorrhizal fungi.

Results: Studies show that plants and fungi formed abundant ectomycorrhizal and arbuscular mycorrhizal symbioses on and off serpentine soils. No serpentine-endemic fungi were identified. Molecular analyses indicate distinct serpentine isolates for Cenococcum geophilum and for Acaulospora, suggesting adaptation to serpentine soils. While fungal sporocarp assemblages on serpentine sites resembled those off serpentine, fruiting of hypogeous fungi was greatly reduced.

Conclusions: Ectomycorrhizal fungal communities did not differ between soil types; however, arbuscular mycorrhizal communities differed in some cases but not others. The additive response to multiple factors, described as the serpentine syndrome, may explain part of the response by fungi.     

A direct solid‐phase assay specific for heavy‐metal toxicity. II. Assessment of heavy‐metal immobilization in soils and bioavailability to plants

We have used the solid‐phase MetPLA TE, an enzyme assay that is specific for heavy‐metal toxicity, to investigate metal toxicity of soils that have been amended with urban wastewater sludges or contaminated with dry deposition from metal‐plating industries. We have shown that soil toxicity, using MetPLA TE, ranged from 21 to 72.5% inhibition of enzyme activity. Evin soil, which displayed the highest toxicity, also had the highest concentrations of Pb and Zn. Metal uptake studies with ryegrass grown on Evin soil, showed Zn, Cd, and Pb accumulation in the plant that exceeds the standard levels reported for grasses

Solid‐phase MetPLA TE was also used as a tool to study the reduction of heavy‐metal toxicity following soil amendments to immobilize metals in soil and thus reduce their toxicity. It was found that the addition of 1% hydrated manganese oxide significantly reduced dissolved metals in soil, their accumulation by ryegrass, and soil toxicity as shown by MetPLA TE.     

Stabilization and solidification of hydrocarbon‐contaminated soils in concrete

This article describes an experimental program developed to investigate the potential for using hydrocarbon‐contaminated soils as a fine aggregate replacement in concrete. Five different contaminated soil types with a total petroleum hydrocarbon content of less than 1% were investigated. For each soil type, three concrete mixtures were obtained by replacing sand with contaminated soils (10, 20, and 40% replacement ratio). The resulting concrete was tested for setting times, compression strength, flexural strength, durability, and teachability of benzene to water.

The results indicate that the addition of hydrocarbon‐contaminated soil adversely affects the strength of concrete. The strength reduction at each soil replacement level depends on contamination concentration, contaminant type, and soil type. The durability of the tested concrete is comparable to normal concrete. For all five soils at a 40% replacement ratio, the leachability of benzene was nondetectable after 24 h and after 10 d. After testing the leachability of artificially contaminated soils (0.5 and 3% neat benzene contamination) for 24 h, it was found that the leaching of benzene increases with the percentage of contamination. However, the fraction of benzene that leached was about 95% lower than the values for loose soils.     

Estimation of effective cleanup radius for soil‐vapor extraction systems

Soil‐vapor extraction (SVE) is a standard and effective in situ treatment for the removal of volatile contaminants from vadose‐zone soil. The duration of SVE operation required to reach site closure is quite variable, however, ranging up to several years or more. An understanding of the contaminant recovery rate as a function of distance from each vapor‐extraction well allows SVE systems to be designed so that cleanup goals can be achieved within a specified time frame.

A simple one‐dimensional model has been developed that provides a rough estimate of the effective cleanup radius (defined as “the maximum distance from a vapor extraction point through which sufficient air is drawn to remove the required fraction of contamination in the desired time") for SVE systems. Because the model uses analytical rather than numerical methods, it has advantages over more sophisticated, multidimensional models, including simplicity, speed, versatility, and robustness.

The contaminant removal rate at a given distance from the vapor‐extraction point is assumed to be a function of the local rate of soil‐gas flow, the contaminant soil concentration, and the contaminant volatility. Soil‐gas flow rate as a function of distance from the vapor‐extraction point is estimated from pilot test data by assuming that the infiltration of atmospheric air through the soil surface is related to the vacuum in the soil. Although widely applicable, the model should be used with some caution when the vadose zone is highly stratified or when venting contaminated soil greater than 30 ft below grade. Since 1992, Groundwater Technology, Inc. has been using this model routinely as a design tool for SVE systems.     

Spatial variation of soil properties and plant colonisation on cut slopes: a case study in the semi-tropical hilly areas of China

Background: Transport infrastructure has severe impacts on ecosystems and results in large numbers of cut slopes, which are difficult to revegetate. To increase successful revegetation, it is crucial to understand the relationships of soil properties and vegetation during spontaneous vegetation recovery on cut slopes.

Aims: To assess the effects of different slope positions on soil properties and vegetation on a cut slope and to determine the key factor(s) affecting vegetation distribution on a cut slope in a semi-tropical environment.

Methods: Soil samples were collected in three slope positions: upper slope (US), middle slope (MS) and foot slope (FS). Soil pH, moisture and bulk density and concentrations of soil organic carbon (C), total nitrogen (N_T), available nitrogen (N_A), total phosphorus (P_T), available phosphorus (P_A), total potassium (K_T) and available potassium (K_A) were determined. Vegetation composition and cover were recorded along the slope. One-way analysis of variance (ANOVA), indicator species analysis (ISA) and detrended canonical correspondence analysis (DCCA) were applied to analyse differences in soil properties among slope positions and vegetation distributions.

Results: N_T, N_A, P_T, P_A, K_A, C and pH tended to increase from the US to the FS. Two indicator species were abundant in their respective slope positions: Achyranthes bidentata in the FS and Dicranopteris dichotoma in the US. DCCA showed that pH and some soil nutrients (N_A, P_T, P_A and C) influenced the vegetation distribution on cut slope.

Conclusions: Soil pH and some soil nutrients including N_A, P_T, P_A and C had large impacts on vegetation distribution along slope positions in a semi-tropical area of China. We suggest increasing soil pH to provide a better soil environment for plant colonisation in further research concerning the restoration of such cut slopes.     

Riverbank Stabilization of Lead Contaminated Soils Using Native Plant Vegetative Caps

Hamburg is a small borough located in Berks County, Pennsylvania. During the 1940s and 1950s, crushed automobile battery casings, containing high levels of lead, were used as fill in and around Hamburg. Several of the fill areas were along the eastern bank of the Schuylkill River and the Schuylkill River Canal. To reduce exposure to human and ecological receptors, the United States Environmental Protection Agency (U.S. EPA) initiated actions at several of the fill areas. Remediation actions at three of these fill areas, the Berry Property, the Hamburg Playground City Playground, and the Port Clinton Avenue site, utilized native plants, slope stabilization, and soil caps.

The Berry Property consisted of a flat, wooded area adjacent to the river. The Hamburg Playground consisted of a steep wooded slope between the river and the parking lot for the municipal park. The Port Clinton Avenue site consisted of flat and sloped, wooded, and old-field areas between the canal and Port Clinton Avenue. At each of the three sites, some of the contaminated soils were excavated and the remainder was graded and capped. The clean soil cap was then covered with an erosion control mat, seeded with native grasses, and planted with native shrubs. At the Hamburg Playground and Port Clinton Avenue site, the existing trees and much of the existing vegetation were maintained to preserve the slope stability and the natural environment. Great care was taken to ensure community access to the municipal park. Some of the important considerations included retaining the existing trees, dealing with invasive species, maintaining the plants during a drought, and channeling storm-water runoff. The work was coordinated with the Hamburg Borough Council, the Schuylkill River Greenway Association, and the Pennsylvania Department of Environmental Protection (PADEP).

The actions resulted in a stabillized slope with channelized storm water to control erosion and protect the river, a clean soil and plant cover that eliminates exposure to human and animal receptors, and an aesthetically pleasing and usable area that meets the needs of the community and the local conservation/environmental organization.     

Influence of Soil Organic Matter on Copper Extraction from Contaminated Soil

Column experiments of copper extraction from four contaminated soils characterized by a content of Soil Organic Matter (SOM) ranging from 1% to 25% are presented and discussed. The extraction was performed by flushing the soil with an aqueous solution of a sodium salt of ethylene diamminotetraacetic acid (EDTA). Preliminary tests were performed on a soil containing 25% of organic matter, to investigate the influence of pH, concentration and volumes of EDTA on its chelant action and on the dissolution of SOM. Having selected the optimal conditions for the extraction process, a further series of tests was conducted on the four soils to evaluate the influence of organic content on copper extraction yields. EDTA solutions at 0.01 M, 0.05 M, 0.1 and 0.2 M were injected at 0.33 ml/s; copper and organic matter extraction yield were determined. At a pH of 5, 15 pore volume (PV) of a solution containing 0.05M EDTA, extracted about 99% of copper contained by the soil with the higher organic matter content. Under the same conditions, and for soil with > 6% SOM, extraction yields over 80% were achieved, while at lower organic content, copper extraction was dramatically reduced. This was attributed to the formation of highly stable copper-humate complexes and to their increasingly dissolution that occurred in the soils with higher organic matter level.

Experimental tests performed at different contamination levels (1200 mg/kg, 2400 mg/kg) showed that EDTA extraction effectiveness also depended upon initial soil Cu concentration.     

Application of Diffusive Gradient in Thin Film to Estimate Available Copper in Soil Solution

In Chile, there are several agricultural areas with soils containing high levels of copper of both anthropic and geochemical origin. The diffusive gradient in thin film (DGT) technique is a promising tool for the evaluation of the bioavailability of metals in situ in different environmental systems. The objective of this study was the preparation, validation, and application of DGT to soil solutions and soil containing copper over 1000 mg kg^?1.

The results show that a resin gel thickness of 0.3 mm gives the best reproducibility and response in the absorption of copper by DGT. The amount of copper extracted in a period of 4 h by the devices from the soil solution corresponds to 13% of the total metal present in the solution.

The DGT allowed a more representative estimation of the amount of Cu available in the soil, more in agreement with the absence of symptoms of phytotoxicity in cultivated species. This shows that the determination of available Cu by DTPA must be handled cautiously because in soils with high Cu content the amount of metal that can have direct influence on absorption by the plant is overestimated.     

Vapor Pressure Characterization to Predict Contaminant Releases from MGP Site Source Area Soils

Risk-based management of contaminated sites often requires timely screening and identification of source areas that release contaminants to groundwater and other reception of concern. Over-reliance on total concentrations often results in costly site characterization, delays in remediation, and overestimation of the soil volumes targeted for removal or treatment. Advances in diagnostic tools to accelerate source area characterization will improve the reliability of management decisions. The objective of this work was to determine the potential of using vapor pressure measurements as a rapid indicator of the mass release potential of suspected source area soils.

The current work has focused on determining the relationship between partial pressure and mobility for monoaromatic and polycyclic aromatic hydrocarbon (PAH) contaminants in source area soils. This work ascertained that for a wide spectrum of non-ionized, aromatic contaminants in MGP soils, total vapor pressure and benzene partial pressure could be measured with good repeatability. Under controlled conditions, photoionization detector (PID) measurements on 16 soil samples from two MGP sites were found to correlate well to laboratory measurements of equilibrium partitioning and resin-mediated soil contaminant desorption with R² values of over 0.9. Results indicate that vapor pressure correlates well to contaminant mobility in soils.     

Assessment of terrain sensitivity on high plateaux: a novel approach based on vegetation and substrate characteristics in the Scottish Highlands

Background: High plateaux in the Scottish Highlands are vulnerable to disturbance and erosion, but there is a lack of quantitative measurements of terrain sensitivity.

Aims: To apply new quantitative methods to assess the sensitivity of such terrain to physical stress.

Methods: We investigated two components of the mechanical properties of the terrain on 10 plateaux underlain by several different lithologies: the tensile strength of the vegetation mat and underlying root zone, and the shear and compressional strengths of the substrate.

Results: Significant differences in tensile strength occur amongst plant communities, but there is also large within-site and between-site variation for particular communities. A significant component of such variability is attributable to the proportional representation of co-dominant species within communities, and inter-site variability is partly explained by substrate granulometry: particular communities exhibit lower strength characteristics when rooted in sandy substrates derived from coarse-grained lithologies than the same communities on silt-rich soils derived from fine-grained lithologies.

Conclusions: Terrain sensitivity to physical stress is conditioned by the interaction of vegetation and substrate characteristics. Generally, grass-dominated (particularly Nardus-dominated) communities tend to be most robust, and communities dominated by bryophytes and prostrate Calluna vulgaris are typically most sensitive. We identify a continuum of substrate strength: peat is the most sensitive substrate type, followed by other organic-rich soils, sandy matrix-supported substrates and silt-rich matrix-supported substrates. Clast-supported substrates and openwork blockfields are the most robust substrate types. Because the near-surface layers of mineral substrates are weakest, erosion is likely to remove these to expose the underlying robust but sterile clast-supported layers, altering soil status and inhibiting plant recolonisation on eroded substrates.     

Exposure assessment of potentially toxic trace elements via consumption of fruits and vegetables grown under the impact of Alaverdi's mining complex

This study is aimed to investigate the transfer of potentially toxic trace elements from soils to plants grown under the impact of Alaverdi's mining complex and assess the related dietary exposure to local residents. Contamination levels of potentially toxic trace elements (Cu, Ni, Pb, Zn, Hg, As, Cd) in soils and plants were determined and afterwards, transfer factors, estimated daily intakes, target hazard quotients, and hazard indexes were calculated.

Some trace elements (Pb, Zn, Cd) exceeded the maximum allowable levels. EDIs of Cu, Ni, Hg for the majority of studied fruits and vegetables exceeded the health-based guideline values. Meanwhile, in case of combined consumption of the studied food items, the estimated cumulative daily intakes exceeded health-based guideline values not only for the aforementioned trace elements but also for Zn in the following sequence: Zn > Hg > Ni > Cu. HI > 1 values highlighted the potential adverse health effects for local population through more than one trace element.

Detailed investigations need to be done for the overall assessment of health risks, taking into consideration not only adverse health effects posed by more than one toxic trace element but also through other exposure pathways.