## 5 Discussion
### Soil, Vegetables and Risk Analysis  
Arsenic is not an essential nutrient for plants and can reach toxicity levels in cases of high concentration in the soil [62]; the soil pH regulates its availability, speciation and fate in plants [63]. The sub-acidic conditions of the investigated soils, in addition to the relatively high As concentration, affects arsenic mobility, increasing the phytoavailability and uptake by plants. The background concentration of As in soil mainly depends on the  
Figure 8: Distribution of arsenic concentration in groundwater.  
lithology and type of soils, ranging from 5 to 10 mg/kg [64]. Several authors showed that As concentration in soils of polluted areas varied from 126 to 1600 mg/kg [64]. In our study, the soil samples of five horticultural gardens had As contents higher than 100 mg/kg.  
Usually, the As concentration in plants is less than 1 mg/kg (dw) in uncontaminated soils; however, the As amount can increase up to 0.1 wt% in contaminated soils [65]. According to several authors, the threshold of 0.5 mg/kg can be considered as the maximum allowable limit [66]. In this case, the vegetables growing in the Baccatoio area should not be consumed since, for most vegetables, the edible parts exceed this limiting value.  
According to the literature [67], vegetables like radish (turnip, leek, fennel, onion) that grow in contaminated soils accumulate higher amounts of As than leafy vegetables (black cabbage), as here observed. In addition, it can be inferred that plant metabolism plays an important role, and leafy vegetables (such as black cabbage) accumulate more arsenic than tomatoes.  
Soil risk assessment indicates that the major threat for health associated with As is through ingestion. Dermal contact yields potential adverse health effects considering carcinogenic effects only; inhalation pathways do not produce any significant health risk. As regards vegetables, a carcinogenic risk exists for tomato and black cabbage ingestion. For non-carcinogenic effects, the chronic daily intake exceeds the reference dose for tomatoes in the case of children only.  
The calculated SSLs reveal that the average As concentration measured in soil should be considered of potential toxicological concern. It is worth noting that the calculated SSL value is based on a target cancer risk of \(10^{-6}\). Widely divergent cleanup targets, guidelines and standards for arsenic in soils have been established by many regulatory organizations. In addition, many countries use naturally occurring background or baseline soil arsenic levels as their screening guidance. It should also be considered that risk assessments carried out following international guidelines are often based on conservative assumptions and default exposure parameters, which represent a \"reasonable maximum exposure\" condition for long-term/chronic exposure. For example, in the case of the soil ingestion pathway, it is assumed that, for the whole period over which the outdoor exposure is averaged, the human target, child or adult, can accidentally ingest 200 and 100 mg of soil, respectively, every day. The SSL concentration values obtained in this study, intended as cautionary, are lower than the As concentration threshold in soil imposed by the current Italian regulations. This opens a question about the actual validity of regulatory threshold values for arsenic in soil and highlights the need for site-specific health risk assessments and to revise conservative assumptions before planning any remedial action.