Background: Although thallium (Tl) is scarce in soils, it is highly toxic and can accumulate in plants. Previously, we have observed an increment of 100% in its content in urban topsoils monitored across Alcalá de Henares (Spain) in a year in 2001. A further monitoring study was carried out in July 2017 to identify potential risks for inhabitants.

Methods: Tl was quantified in 66 urban, 24 industrial and 4 public garden topsoil samples collected in 2017 by ICP-MS. Data was compared with 97 topsoils collected in the same locations in the urban area in 2001.

Results: The presence of Tl, which was detected in all the 2017-topsoil samples (LoD=0.022 mg/kg), showed a 38.3% significant increment in the urban area when compared with the previous sampling (0.166 vs. 0.12 mg/kg), suggesting some accumulation in these topsoils. The increment of Tl detected may be attributed to its inorganic nature and the growth of the city, which may increase its anthropogenic inputs in manufacturing processes and use of cement. A significantly higher contamination in industrial (0.249) and garden (0.246) topsoils was detected versus urban soils (p-value<0.001), which would be logical due to its uses in the manufacture of electronics, detectors, optical lenses, smelting. Thus, the content of Tl was significantly higher in the suburban area that supports more industries when compared with the other subareas (0.214 vs. 0.184, 0.150, 0.127; all in mg/kg). These levels were lower than, or similar to, the background concentration level suggested in soils from south-east Spain (0.2 mg/kg), and much lower than those detected in two mine-affected catchments in the north-west Madrid Region (0.87-2.65 mg/kg), suggesting a minor contamination by Tl in Alcalá’s topsoils.

Conclusions: The levels of Tl recently monitored in Alcalá’s topsoils would not represent a significant risk for the population derived from the ingestion/inhalation of Tl present in the urban soils.