We estimated the levels and distributions of Cr, Ni, and Cu in the soils of Barcelona Province (NE Spain). This province covers 7731 km2. The geology in this area is varied: granitoids and Paleozoic shales are predominant in the SE-E and Mesozoic and Tertiary limestones, dolomites, marlstones, gypsum, and sandstones in the remainder of the province. In all, 316 topsoil samples (0-20 cm) were collected at 5 km intervals on a regular grid. General soil properties were analysed by standard methods and Aqua regia digests of Cr, Ni, and Cu were determined by Inductively Coupled Spectroscopy.
Exploratory Data Analysis (EDA) was used to detect unusual data behaviour and obtain a robust summary of the dataset. We compared the behaviour of Cr, Ni, and Cu in different data sub-sets (by districts, texture, pH, organic carbon, CaCO3). Some results are: Cr mg kg-1 min 2.5, max 250, med 25.8, Ni mg kg-1 min 2.3 max 186 med 19.4, Cu mg kg-1 min 2.5, max 1153, med 16.7. The maximum contents of Ni, Cr, and Cu versus pH are reached at pH 5-5.5. The highest concentrations of Cu and Cr are associated in the Barcelona area and Ni in Bages district.
This is the first work that combines EDA techniques and pedogeochemical mapping of Cr, Ni, and Cu in the topsoils of Barcelona Province (Catalonia, Spain). The combination of EDA and pedogeochemical mapping is a useful tool for estimating the local background of potentially toxic elements (PTE), which is essential for environmental regulation.
Keywords: Soils; Chromium; Nickel; Copper; Barcelona Province; Pedogeochemical maps; Exploratory data analysis』
2. Material and methods
2.1. Site description
2.2. Soil sampling
2.3. Analytical methods
2.4. Statistical analyses
3. Results and discussion
3.1. Soil properties and single element statistical analysis
3.1.1. Exploratory data analysis
3.1.2. Non-parametric inferential analysis
3.1.3. Correlation measurements
3.1.4. Calculated medians of Cr, Ni, and Cu in the topsoils of Barcelona Province, as compared with published data
3.2. Distribution maps
3.2.1. Maps of soil properties
3.2.2. Maps of total Cr, Ni, and Cu