## 4 Discussion
The collected data for the soil samples from the S53 sampling point and its close surroundings suggest that As is present mainly in an occluded form. Figure 4 describes the comparison between the As amount extracted by sequential extractions and the As quantity extracted by ammonium oxalate 0.2 M in the chemical speciation procedure. In general, As extracted with ammonium oxalate was always less than the sum of the As fractions extracted in the first and second step of sequential extractions. Moreover, in the superficial levels As extracted by oxalate was very similar to that extracted in the first step of sequential extractions, whereas when increasing the depth of the soil sample As extracted by oxalate became more abundant. As extraction by ammonium oxalate for the chemical speciation analysis could be considered like a leaching test which allows the release of the highest possible amount of As, without changing his chemical properties. Analyzing As leaching in ammonium oxalate 0.2 M, we noticed that the highest levels of As release was obtained from the deepest soil sample, with a maximum generally between 2 and 3 m of depth (core S53/1, S53/1A e S53/1B). The As amount extracted from soil samples of S53 sampling point and its surroundings was on average 4% of total As. This value was very different from As extracted by ammonium oxalate from other areas located near the site analyzed in this study. For example, in the Sviluppo Industriale e dei Servizi Reali alle Imprese (SISRI) area of Brindisi, As extracted by ammonium oxalate 0.2 M was 62% of total As. Comparing As amounts extracted by acetic acid with As extracted by ammonium oxalate, we observed much higher values of As extracted for the chemical speciation analysis. In fact, As extracted by acetic acid ranges from 6 to 126 ppb, whereas As extracted by ammonium oxalate ranges from 152 to 1962 ppb. In this regard, Figures 5 and 6 show the 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 the most superficial layers on the one hand, and the corresponding average values of all levels of depth on the other hand. For total extracted As values in the first four steps of the sequential extraction only, the averages found in the upper layer were higher than the averages measured in all the other levels of depth. This is probably due to the increased presence of organic matter in the upper layer, the matrix on which some of the As species extracted by NaOH in the fourth step of the sequential extraction are adsorbed. Further studies concerning the determination of As release due to the removal of the organic substance performed in the third and the fourth step of the sequential extraction and due to the possible use of fertilizers, especially phosphates with an anionic component with high exchange capacity for As oxyanions, are needed. Figure 5: 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 the most superficial layer.