Lanthanum biomonitoring using tree bark: urban vs rural patterns in Leicestershire, the UK

The presence and distribution of lanthanum (La) in the topsoil and wild edible mushrooms in the English city of Leicester could pose some risks to the population. A biomonitoring study was carried out to determine the air quality for La across the region of Leicestershire. Bark samples (2–6 mm thick) were collected from 55 trees in the city and 41 surrounding rural areas. La was measured using ICP-MS in appropriately mineralised samples [LoD = 0.00068 ng/g dry weight (dw)]. Higher levels were found in the bark collected from the trees growing in Leicester (median and ranges, in ng/g dw)—9.679 (2.128-150.769) vs. 8.344 (1.815-59.801)—a distribution consistent with the trend found in the topsoils and which could be attributed to its technological use and traffic volumes. The La content varied between the bark samples collected in the three cardinal sub-areas into which Leicestershire was divided (medians, ng/g dw): 29.772 (NW) > 7.855 (NE) > 2.027 (SE). However, this pattern was different to the distribution of La in the topsoil samples monitored across Leicestershire, which showed statistical significance (p-value = 0.03997, medians, mg/kg): 25.874 (SE) > 24.290 (NE) > 19.470 (SW) > 13.401 (NW). In general, our results showed lower airborne La contamination in Leicestershire, which is supported by the lower levels of La in the bark from the monitored trees compared to those reported in the literature for trees growing in uncontaminated areas, and low enrichment factors (0.383 and 0.393) in the urban and rural areas, which were calculated using the averages in the Earth’s continental crust and the scandium content. Moreover, the enrichment calculated in the topsoils (1.118 and 0.909) also suggests little anthropic influence on the presence/distribution of this metal in the English region studied. Leicestershire's atmosphere represents a minimal risk to the population in terms of La.