## 5 Conclusions  
Total As determination in the 30 soil samples deriving from the 5 probes of the S53 sampling point and its surroundings showed that As concentration was lower than the As amount detected by \"Sviluppo Italia\". In general, As accumulates in the deepest depth levels. In 16 of the 30 soil samples As concentration exceeds 20 ppm limit, established by D.M. 471/99 for green areas for public use and private and residential use, and in no case did As concentration exceed the limit of 50 ppm, specified by the same decree for commercial and industrial areas. The As sequential extraction protocol showed that the most abundant As amount was contained in the final residue of the sequential extraction procedure. As extracted in the fifth step and As in the final residue is the so-called occluded As that is embedded in rock formations that do not release the metalloid, including some very insoluble sulphides such as arsenopyrite. The graphs showed in this work summarize the different scenarios proposed by us for an evaluation at different levels of approximation of environmental risk related to the presence of As, if the total concentration is not taken into account. This is because the environmental impact of pollution by metals and metalloids in soils depends not only on their total concentration but also on their mobility and availability. The first scenario is derived from considering for risk evaluation only the As extracted by acetic acid in the leaching test. This test allows us to evaluate the amount of extractable As from the leaching effect of rain water. As also described above, the amount of As extracted by acetic acid was very limited, an order of magnitude lower than As extracted by ammonium acetate in the first step of sequential extractions and by ammonium oxalate in the speciation analysis. It was on average 0.16%, both of total As obtained in a unique digestion and total As extracted by the sequential extraction protocol. A more cautious approach may lead to consideration for risk evaluation the amount of As obtained in the first two steps of sequential extractions. This amounts to an average of 9.8% of total As determined in a single mineralization and to 10.5% of As extracted in the sequential extraction procedures. The second proposed scenario also takes into account the As extracted by ammonium oxalate, used for the analysis of chemical speciation and which consists in the percentage of As extracted in mild conditions that do not change the percentages of As(III), As(V) and organic species possibly present. As extracted by 0.2 M ammonium oxalate is on average 4% of total As extracted in the sequential extractions and  
Figure 6: Comparison between the average percentage of As extracted for the leaching test, by ammonium oxalate, and in the first two steps and the first four steps of sequential extraction procedures for soil samples, in all the sampled levels of depth. 3.9% of total As determined in a single digestion. These values are still below the sum of the percentages of As extracted in the first two steps of sequential extractions. Another scenario is derived from considering the As extracted in the first four steps of sequential extractions. We should consider that the third and fourth steps, although carried out using \"strong\" extractants (0.1 M HCl and 0.5 M NaOH) hard to find in natural conditions, mobilize the arsenates bound to the surface of minerals rich in Fe and, in general, to clay and, if present, As bound to organic matrices, which in the more superficial layers of soil can be a substantial fraction. Our results showed that only for total extracted As values in the first four steps of the sequential extraction are the averages found in the upper layer higher than the averages measured in all the other levels of depth, and we attributed this effect to a greater amount of organic matter in the upper layer, a matrix certainly capable of absorbing some of the As species extracted by NaOH in the fourth step of the sequential extraction.