Cycling and ecosystem impact of metals in contaminated calcareous dredged sediment-derived soils (Flanders, Belgium)

Environmental availability and oral bioaccessibility of Cd and Pb in anthroposols from dredged river sediments

Dredging and disposal of sediments onto land sites is a common practice in urban and industrial areas that can present environmental and health risks when the sediments contain metallic elements. The aim of this study was to characterise and study the environmental and toxicological availability of Cd and Pb in anthroposols from dredged river sediments. To do this, 67 surface samples spread over 12 sediment disposal sites in northern France were studied. The results showed substantial heterogeneity for this matrix in terms of physicochemical parameters and contamination degree; however, ascending hierarchical clustering made it possible to classify the samples into eight groups. For each group, the mobile fraction of Cd and Pb was studied using single EDTA extraction, solid-phase distribution was analysed with sequential extractions and toxicological availability was assessed with the oral bioaccessibility test. The results showed that (i) Cd had a higher environmental and toxicological availability than Pb; (ii) this availability depends on the physicochemical characteristics of the matrix; and (iii) it is necessary to take into account the environmental and toxicological availability of contaminants when requalifying these sites in order to propose appropriate management measures. In the first years after sediment disposal, it would appear that the environmental and toxicological availability of Cd and Pb increased (from 52.5 to 71.8% and from 28.9 to 48.9%, respectively, by using EDTA and from 50.2 to 68.5% for Cd with the bioaccessibility test). Further studies would therefore be required to confirm this trend and understand the mechanisms involved.

Comparative analysis of element concentrations and translocation in three wetland congener plants: Typha domingensis, Typha latifolia and Typha angustifolia

This study analyzed the concentrations and distributions of Al, As, Cd, Cr, Cu, Hg, Mn, Ni, Pb and Zn in three different cattail species growing spontaneously in a natural wetland subject to municipal wastewater and metal contamination. The cattail species included Typha domingensis, T. latifolia and T. angustifolia. Results showed that all Typha species have similar element concentrations in roots, rhizomes and leaves, and similar element mobility from sediments to roots and from roots to leaves. This study corroborated three patterns of Typha species growing in metal contaminated environments: high tolerance to toxic conditions, bulk element concentrations in roots, and restricted element translocation from roots to leaves. This study showed that three different Typha species respond similarly to metal inputs under the same polluting field conditions. Given their similar metal content and similar biomass size, our results suggest that T. domingensis, T. latifolia and T. angustifolia may have comparable capacity of phytoremediation. High element uptake and large biomass make Typha species some of the best species for phytoremediation of metal contaminated environments.

Organ- and species-specific accumulation of metals in two land snail species (Gastropoda, Pulmonata)

In order to evaluate the usefulness of terrestrial gastropods as bioindicators there is a need for studies that simultaneously compare (1) concentrations of metals in reference and polluted plots, (2) species within the same polluted habitat, (3) metal accumulation patterns in different organs and (4) metal accumulation patterns in relation to soil physicochemical properties. This study aims to assess metal accumulation patterns in two land snail species. Instead of analyzing an organism as a whole, investigating the partitioning of metals in different organs can provide information on the actual toxicological relevant fractions. Therefore, concentrations of Ag, Cd, Cr, Cu, Ni and Zn were examined in five different organs of Cepaea nemoralis, as well as in the foot and the body of Succinea putris. Snails were sampled at four polluted dredged sediment disposal localities and three adjacent less polluted reference plots situated along waterways in Flanders, Belgium. Due to the small size and problematic dissection of S. putris only the concentrations in the foot of both species could be compared. For this reason only, C. nemoralis can be described as a better bioindicator species that allows a far more detailed analysis of organ metal accumulation. This study showed that organs other than the digestive gland may be involved in the immobilization and detoxification of metals. Furthermore, pH, soil fractionation (clay %, silt %, sand %) and organic matter, correlate with metal accumulation in organs. However, most often the soil metal concentrations did not correlate with the concentrations found in snail organs. Metal concentrations in organs of both species (1) differed among polluted plots but rarely between polluted and reference plots within a locality, (2) were organ-specific (digestive gland>foot>albumen gland=spermoviduct=ovotestis), (3) were species-specific and (4) depended on the metal type (high Cd and Cu concentrations were observed in the digestive gland and foot respectively). Our study emphasizes that background metal levels should be taken into account when using invertebrates as bioindicators of metal contamination and that bioindicators may show substantial differences in accumulation patterns even if they have a highly comparable ecology.

Role of plants in metal cycling in a tidal wetland: implications for phytoremidiation

Accumulation of 8 metals and the semimetal As in 29 plant species was quantified in a restored tidal wetland on a contaminated site. Transfer coefficients between sediment and aboveground plant tissues were lower than in many other systems; from 0.013 (Pb) to 0.189 (Mn). A minor fraction of the sediment metal pool cycled through the aboveground vegetation (≤0.02%). However, during the four years of this study, species composition changed, and plant biomass as well as the metal pool in the vegetation increased (≤0.12%). Succession to either a willow dominated brushwood or a monospecific reed stand can further enlarge this pool (2.5%). Since the amount of trace metals in the wetland soil or in suspended solids deposited during tidal flooding is some orders of magnitude larger than the vegetation pool, phytoextraction is not applicable. The growth of plant species with low accumulation in aboveground tissues, e.g. Scirpus maritimus or Typha latifolia, may be preferred since this might result in lower toxic metal distribution to the wider environment.

Influence of flooding and metal immobilising soil amendments on availability of metals for willows and earthworms in calcareous dredged sediment-derived soils

Soil amendments previously shown to be effective in reducing metal bioavailability and/or mobility in calcareous metal-polluted soils were tested on a calcareous dredged sediment-derived soil with 26 mg Cd/kg dry soil, 2200 mg Cr/kg dry soil, 220 mg Pb/kg dry soil, and 3000 mg Zn/kg dry soil. The amendments were 5% modified aluminosilicate (AS), 10% w/w lignin, 1% w/w diammonium phosphate (DAP, (NH4)2HPO4), 1% w/w MnO, and 5% w/w CaSO4. In an additional treatment, the contaminated soil was submerged. Endpoints were metal uptake in Salix cinerea and Lumbricus terrestris, and effect on oxidation-reduction potential (ORP) in submerged soils. Results illustrated that the selected soil amendments were not effective in reducing ecological risk to vegetation or soil inhabiting invertebrates, as metal uptake in willows and earthworms did not significantly decrease following their application. Flooding the polluted soil resulted in metal uptake in S. cinerea comparable with concentrations for an uncontaminated soil.

Sensitivity to zinc of Mediterranean woody species important for restoration

Heavy metals have increased in natural woodlands and shrublands over the last several decades as a consequence of anthropogenic activities. However, our knowledge of the effects of these elements on woody species is scarce. In this study, we examined the responses of six Mediterranean woody species to increasing levels of zinc in hydroponic culture and discussed the possible implications for the restoration of contaminated sites. The species used, Pinus pinea L., Pinus pinaster Ait., Pinus halepensis Mill., Tetraclinis articulata (Vahl) Mast., Rhamnus alaternus L. and Quercus suber L. represent a climatic gradient from dry sub-humid to semi-arid conditions. Zinc concentrations in shoots ranged from 53 microg g(-1) in Q. suber to 382 microg g(-1) in T. articulata and were well below the levels found in roots. Zinc inhibited root elongation and root biomass and changed the root length distribution per diameter class, but the magnitude of the effects was species-specific. Only P. halepensis and Q. suber showed toxicity symptoms in aboveground parts. Species more characteristic from xeric environments (T. articulata, R. alaternus and P. halepensis) were more sensitive to zinc than species from mesic environments (Q. suber, P. pinaster and P. pinea). According to the Zn responses and bioaccumulation, Q. suber P. pinea and P. halepensis are the best candidates for field trials to test the value of woody species to restore contaminated sites. None of the species tested seemed suitable for phytoremediation.

Phytoremediation as a management option for contaminated sediments in tidal marshes, flood control areas and dredged sediment landfill sites

BACKGROUND, AIM AND SCOPE

Polluted sediments in rivers may be transported by the river to the sea, spread over river banks and tidal marshes or managed, i.e. actively dredged and disposed of on land. Once sedimented on tidal marshes, alluvial areas or control flood areas, the polluted sediments enter semi-terrestrial ecosystems or agro-ecosystems and may pose a risk. Disposal of polluted dredged sediments on land may also lead to certain risks. Up to a few years ago, contaminated dredged sediments were placed in confined disposal facilities. The European policy encourages sediment valorisation and this will be a technological challenge for the near future. Currently, contaminated dredged sediments are often not valorisable due to their high content of contaminants and their consequent hazardous properties. In addition, it is generally admitted that treatment and re-use of heavily contaminated dredged sediments is not a cost-effective alternative to confined disposal. For contaminated sediments and associated disposal facilities used in the past, a realistic, low cost, safe, ecologically sound and sustainable management option is required. In this context, phytoremediation is proposed in the literature as a management option. The aim of this paper is to review the current knowledge on management, (phyto)remediation and associated risks in the particular case of sediments contaminated with organic and inorganic pollutants.

MAIN FEATURES

This paper deals with the following features: (1) management and remediation of contaminated sediments and associated risk assessment; (2) management options for ecosystems on polluted sediments, based on phytoremediation of contaminated sediments with focus on phytoextraction, phytostabilisation and phytoremediation of organic pollutants and (3) microbial and mycorrhizal processes occurring in contaminated sediments during phytoremediation.

RESULTS

In this review, an overview is given of phytoremediation as a management option for semi-terrestrial and terrestrial ecosystems affected by polluted sediments, and the processes affecting pollutant bioavailability in the sediments. Studies that combine contaminated sediment and phytoremediation are relatively recent and are increasing in number since few years. Several papers suggest including phytoremediation in a management scheme for contaminated dredged sediments and state that phytoremediation can contribute to the revaluation of land-disposed contaminated sediments. The status of sediments, i.e. reduced or oxidised, highly influences contaminant mobility, its (eco)toxicity and the success of phytoremediation. Studies are performed either on near-fresh sediment or on sediment-derived soil. Field studies show temporal negative effects on plant growth due to oxidation and subsequent ageing of contaminated sediments disposed on land. The review shows that a large variety of plants and trees are able to colonise or develop on contaminated dredged sediment in particular conditions or events (e.g. high level of organic matter, clay and moisture content, flooding, seasonal hydrological variations). Depending on the studies, trees, high-biomass crop species and graminaceous species could be used to degrade organic pollutants, to extract or to stabilise inorganic pollutants. Water content of sediment is a limiting factor for mycorrhizal development. In sediment, specific bacteria may enhance the mobilisation of inorganic contaminants whereas others may participate in their immobilisation. Bacteria are also able to degrade organic pollutants. Their actions may be increased in the presence of plants.

DISCUSSION

Choice of plants is particularly crucial for phytoremediation success on contaminated sediments. Extremely few studies are long-term field-based studies. Short-term effects and resilience of ecosystems is observed in long-term studies, i.e. due to degradation and stabilisation of pollutants. Terrestrial ecosystems affected by polluted sediments range from riverine tidal marshes with several interacting processes and vegetation development mainly determined by hydrology, over alluvial soils affected by overbank sedimentation (including flood control areas), to dredged sediment disposal facilities where hydrology and vegetation might be affected or managed by human intervention. This gradient is also a gradient of systems with highly variable soil and hydrological conditions in a temporal scale (tidal marshes) versus systems with a distinct soil development over time (dredged sediment landfill sites).

CONCLUSIONS

In some circumstances (e.g. to avoid flooding or to ensure navigation) dredging operations are necessary. Management and remediation of contaminated sediments are necessary to reduce the ecological risks and risks associated with food chain contamination and leaching. Besides disposal, classical remediation technologies for contaminated sediment also extract or destroy contaminants. These techniques imply the sediment structure deterioration and prohibitive costs. On the contrary, phytoremediation could be a low-cost option, particularly suited to in situ remediation of large sites and environmentally friendly. However, phytoremediation is rarely included in the management scheme of contaminated sediment and accepted as a viable option.

PERSPECTIVES

Phytoremediation is still an emerging technology that has to prove its sustainability at field scale. Research needs to focus on optimisations to enhance applicability and to address the economic feasibility of phytoremediation.

Effects of willow stands on heavy metal concentrations and top soil properties of infrastructure spoil landfills and dredged sediment-derived sites

The effects of willow stand development on top soil properties of uncontaminated infrastructure spoil landfills (ISL) and contaminated dredged sediment landfills (DSL) were assessed. For the ISL, significant increases in Cd, Zn and organic C levels in the top soil (0-10 cm) were detected more than 20 years after disposal. The increases in Cd and Zn concentrations in the top soil were attributed to leaf-associated metal transfer and leaf fall: the relatively high Cd and Zn concentrations in willow leaves resulted in top soil enrichment for these elements. Higher absolute amounts of Cd, Zn and Mn were taken up and recycled during leaf fall on DSL than on ISL, but did not result in significant differences between top soil and deeper soil (10-30 cm) for the DSL. Direct comparison of top soil development between both types of sites is not possible due to differences in stand age and time since disposal. The DSL were characterised by a higher short-range variance for the Cd, Cr, Cu, Pb and Zn concentrations in the top soil than the ISL. During the first years of ripening and dewatering, significant sulphate leaching occurred in the top soil of the DSL.