Spatial distribution of metals in soils under agriculture in the Jordan Valley

Abstract

The aim of this study was to evaluate levels, sources, and spatial distribution of 6 metals in soils under agriculture in the Jordan valley. A total of 127 surface soil samples (101 irrigated with treated wastewater and 26 with fresh water) were collected at a density of one sample per 2 km2, and concentrations of As, Cd, Cr, Cu, Pb, and Zn were analyzed. In addition, clay content, pH, salinity, sodicity, organic carbon, total phosphorus and total nitrogen were measured for all samples. Principal component analysis and Pearson correlation matrix were used to identify possible metal sources. Average soil metal levels in treated wastewater were higher than in fresh water except for Cd. The mean values of the heavy metals in treated wastewater irrigated soils were 1.07, 2.87, 27.39, 14.46, 43.93, and 97.21 mg kg−1, while average metal contents in fresh water irrigated soils were 0.61, 3.22, 18.68, 8.03, 33.66, and 65.87 mg kg−1. The mean concentrations of all metals were below the maximum permissible concentration according to WHO/FAO threshold values for soils under agriculture. Multivariate and geostatistical analyses suggested that soil metals were mainly derived from agricultural practices (fertilizer, organic manure, and treated wastewater use) and parent materials. Results showed that Zn, and Cu were mainly derived from fertilizer use, As mainly from soil parent materials, and Cd, and Cr, and Pb mainly from treated wastewater and organic fertilizer use. Documenting metal levels in these soils may help in understanding their sources and assisting to take certain undertaking measures and actions to protect soil quality. Caution should be taken into consideration to decrease the excessive use of inorganic fertilizers and manage more efficiently TWW use.