Soil low-density geochemical mapping of technology-critical elements (TCEs) and its environmental implications: The case of lithium in Portugal

Abstract

Increased use of technology-critical elements (TCEs) like lithium (Li), and their socio-environmental impacts, make it an issue of national and global importance. In Portugal, new Li exploration/exploitation projects are a very likely scenario. Thus, it is essential to establish geochemical backgrounds/thresholds for Li in soil, which can have several applications. Here, Li contents were determined and mapped from a previous low-density geochemical survey that covered the entire continental area of Portugal, following UNESCO's IGCP 259 project recommendations. The sampling sites were chosen in undisturbed/uncultivated land to ensure a reliable representation of “natural” soils. A total of 152 samples (0–20 cm; <2 mm) were taken for this study. Soil Li analyses were done by Flame Atomic Absorption Spectrometry (FAAS) after aqua regia (AR) extraction (geoavailable Li), while a subset of 55 samples underwent further digestion with a strong acid mixture to measure total Li (FAAS). This was done to ascertain the relationship between the two Li fractions and its environmental significance. Soil Li spatial distribution was produced with GIS software. Median values of 14 mg/kg for geoavailable Li and 60 mg/kg for total Li were estimated from these datasets. The first value is comparable to the median Li (11 mg/kg) from an AR-extraction for agricultural/grazing soils in Europe (GEMAS project). Based on spatial analysis, Cambisols overlying granitoids in northern/central Portugal contain the highest AR-extractable Li (40 mg/kg). Such areas are recognized for hard-rock Li mineralizations, mainly associated with aplite-pegmatites. Principal Component Analysis identified an important Li-Al relationship, linked to Cambisols and Leptosols overlying granitoids/metamorphic rocks. The geoavailable/total Li ratios revealed that >60 % of the samples have a relatively high proportion (>45 %) of Li that can be mobilized/dispersed in the surface environment. These findings are intended to support the management of potential concerns regarding Li mining in mainland Portugal.