Geochemical Background in Heavy Metals on Basaltic Soils from the Lisbon Volcanic Complex

Anthropic activities have increasingly influenced soil quality due to growing urban development. This study aims to establish a methodology for determining reference levels for heavy metal concentrations in soils by using X-ray fluorescence, an analysis technique that relies on simple and relatively fast sample preparation. To test the proposed methodology, a study was carried out on soils and outcrops originating from the basalts of the Lisbon Volcanic Complex in Odivelas. To verify the concentrations of heavy metals obtained via X-ray fluorescence, soil samples were also tested by atomic emission spectrometry. The results compare favourably between the pairs of V, Cr, and Pb measurements, showing a concordance between the methods. The obtained concentrations and their cumulative frequency curves were used to determine the upper limit for the background concentration in V, Cr, Ba, Ni, Zn, Cu, and Pb. The 95th percentile was chosen as the upper limit of background for the studied soils. These soils show a large variability in their composition that prevents a clear definition of the boundary between which of them correspond to natural concentrations, and which are most likely caused by anthropogenic influences. Some of the values obtained are higher than those found in guidelines for use at contaminated sites and deserve further investigation to confirm contamination or not.