Heavy Metals Accumulation and Spatial Distribution in Long Term Wastewater Irrigated Soils.

Risk assessment of domestic wastewater reuse as a water resource for irrigation is an environmental demand. Wastewater could be considered as source of plant nutrients as well as appreciable amounts of trace toxic elements. In this study, spatial distributions and the accumulation among soil depth of lead (Pb), cadmium (Cd), copper (Cu), and nickel (Ni) as consequence of irrigation with domestic wastewater were studied. For this purpose, Elgabal Elasfar farm represents an old agroecosystem in the semi-arid region. It has been influenced by wastewater utilization since it was established on desert land in 1911. Analysis of collected water samples revealed that concentrations of the studied heavy metals are less than the permissible level for irrigation. However the long term irrigation reduced the soil reaction and increased the organic matter content. Remote sensing and geographic information system have been used to manipulate and analyze the agroecosystem data through image classification and spatial variability analysis. Results revealed that 37.5% of the area has more than 100-Pb mg kg-1 and 63.9% of the area has total amount of Cu ranged from 50.0 to 120.5 mg kg-1. Total Cd represents critical level content in the surface layer and ranged from 0.8 to 3.0 mg kg-1. On the other hand Ni content was within the permissible level in most of the study area. Moreover, Pb, Cu, and Ni have high affinity to retain in surface soil layer whereas Cd results showed homogeneous distribution within soil depth. Soil pollution in the investigated area was a result of mismanagement in the aged agriculture practices which used to apply sludge without treatment. In general, the impact of time scale effect on accumulation and spatial distribution of heavy metals indicated the urgent need for remediation and rational management.

 Key words: Heavy metals, Wastewater, Spatial variability, Soil pollution, Elgabal Elasfar