Establishing local background values for evaluating potentially toxic element risk in semi-arid Moroccan agricultural soils

Abstract

Accurate assessment of potentially toxic element (PTE) risk in agricultural soils requires distinguishing natural geochemical variability from anthropogenic contamination through locally derived background values (SGB). This study established SGB for eight PTEs (As, Ba, Cd, Cu, Mn, Pb, Sr, Zn) across three semi-arid Moroccan regions using five statistical methods, evaluated pollution risk using contamination factor (CF) and geo-accumulation index (Igeo), and examined soil quality’s role in contamination assessment. Sixty composite soil samples from Beni Amir (limestone), El Jadida (ultrabasic), and Kenitra (sandstone) were analyzed by Inductively Coupled Plasma Optical Emission Spectrometry ICP-OES following HF + aqua regia digestion. SGB was determined using Mean + σ, Mean + 2σ, Q3 + 1.5×IQR, Median + 2×MAD, and cumulative frequency analysis. Modified soil quality index (mod. SQI) quantified soil fertility-contamination relationships. The cumulative frequency method yielded the lowest SGB values across all regions. Regional variations reflected lithological controls: El Jadida showed elevated As(17.32 mg kg− 1), Sr (497.09 mg kg− 1), and Ba (109.08 mg kg− 1) from ultrabasic materials; Beni Amir exhibited higher Cd (1.88 mg kg− 1) from agricultural inputs, while Kenitra displayed minimal values (As: 2.55, Sr: 4.86, Ba: 52.74, Cd: 0.60 mg kg− 1). SGB-based indices (CF: 1.048–2.527, Igeo: − 0.517 to 0.752) were lower than literature values (CF: 4.08–5.2, Igeo: 3.079), demonstrating systematic overestimation by non-localized approaches. Soil quality inversely correlated with contamination indices (r = 0.998, p < 0.05): superior quality soils (El Jadida, mod. SQI = 1.12) exhibited enhanced metal retention yielding conservative risk assessments despite elevated concentrations, while degraded soils (Beni Amir, mod. SQI = 1.29) showed increased metal mobility despite moderate concentrations. SGB coupled with soil quality assessment provides scientifically defensible contamination risk evaluation by accounting for lithological variability and metal retention capacity, preventing systematic misclassification and enabling targeted soil management in semi-arid agricultural systems.