Technical NoteDGT use in contaminated site characterization. The importance of heavy metal site specific behaviour
Introduction
Our understanding of geochemical processes to evaluate metal availability is limited by a lack of simple procedures to measure the entity of exchange between solid and solution pools. On the other hand the evaluation of metal labile pool in soil is a fundamental topic in soil pollution level determination, risk estimation and remediation design (Fava and Sani, 2000, Bonomo, 2005).
Established studies on particle–water interaction are based on the identification and quantification of metals in solid phase, such as carbonates, oxides, organic matter and others, which may be extracted in sequence (Ure et al., 1993). There are several sequential extraction procedures (SEP) available in the literature (Sahuquillo et al., 2002), so it may be difficult to gather comparative information. Furthermore, they may not reflect the real processes in settled soils (Honeyman et al., 1988, McKinley and Jenne, 1991, Benoit and Rozan, 1999). SEP execution is not only difficult but also labour-intensive. In this context, the need for new techniques to study soil metal bindings and their mobility in interstitial pore water has arisen.
The technique of diffusive gradient in thin film (DGT) has been developed (Davison and Zhang, 1994) to measure labile metal species in natural water. Later, this technique was used to measure fluxes of trace metal in sediment (Zhang et al., 1995) and soil (Zhang et al., 1998, Sani et al., 2003). DGT deployment in soil may potentially provide information that can be interpreted directly as pore water concentration. In this way it is possible to quantify the mobility of the heavy metal cations with a parameter called “effective concentration” (CE, μg cm−3).
In the present study the labile pool of several metals (Cu, Fe, Mn, Ni) in a polluted site was evaluated both with SEP and DGT techniques. Our aim was to provide contribution to a close examination of the DGT technique as a test procedure in the frame of soil risk contamination (Zhang et al., 1995, Degryse et al., 2003, Dočekal et al., 2003, Ruello et al., 2004).
Section snippets
Site contamination analysis
An industrial contaminated site (industrial sewage sludge treatment plant) located in the middle of Italy (near the city of Ancona) was thoroughly investigated for suspected metal pollution. The usual area characterization was performed through a geotechnical and geochemical analysis. Site investigations consisted of:
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covering the area of interest (Fig. 1) by a geometrical grid, after which six wells (named SA, SC, SD, SE, SF, SG) were constructed for both groundwater and soil sampling. As shown
Results and discussion
The investigated metals were nickel, manganese, iron and copper. A general overview of the chemical results (Table 2), when compared to the Italian legal limit in soil and water (Ministerial Decree, 1999b), shows higher concentration of Fe, Mn and Ni in groundwater (Cgw), despite the soil concentration (Cs) was found at level similar to the background values.
Both SEP and DGT technique (Table 2) seem to outline the presence of metal speciation in some complex form.
In particular, sample SF, that
Conclusions
The present work deals with the DGT technique as a means to follow the fate of metals in contaminated soil in the frame of environmental pollution assessment. The technique was applied to soil samples from a site committed to industrial sludge treatment. The site has a very high concentration of heavy metals in the interstitial groundwater even if the soil metal concentration is low and not much higher than background values. High water concentration of Fe, Mn and Ni, found using both
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