Effects of Treated Wastewater Irrigation on Heavy Metals Concentration, Distribution and Contamination of Soil

Over the past decades, due to climate change and water scarcity, the recovery and use of urban wastewater, especially in arid and semi-arid climates, has increased. But since wastewater is considered as an unconventional source of water, its use in agriculture requires special management which, while benefiting from it, does not have environmental and health hazards in soil, plant and surface water and underground water resources. On the other hand, sewage systems often have significant amounts of heavy and toxic metals, the type and amount of which varies from place to place, and even in the specific location, over time. The soil also has a limited capacity to absorb and maintain these elements, and if their concentration exceeds the permitted range, they can cause pollution of the water, soil, plant and human cycle. Therefore, the present study was conducted to investigate the effect of irrigation with treated wastewater in Urmia city on concentrations, distribution and contamination of Zn, Cu, Cd, Pb and Ni elements.

In field work, 6 soil profiles (5 profiles from the wastewater-irrigated soils and a profile from the well-irrigated soil as control soil) were dug, described, and sampled. At around each profile, composite soil samples were also obtained in the root depth of the area (Ap horizon, the depth of 30 cm). Soil samples were first air-dried and passed through a 2-mm sieve and then analyzed for the determination of heavy metals. The available and total fraction of zinc (Zn), copper (Cu), cadmium (Cd), leads (Pb), and nickel (Ni) were extracted by DTPA method and concentrated acid (HNO3) procedure, respectively. The content of Zn, Cu, Cd, Pb and Ni were determined by an atomic absorption spectrophotometer (Shimadzu AA-6300). Descriptive statistics were conducted using SPSS 16 for Windows. In order to study the effect of irrigation with treated wastewater on the extent of contamination of heavy metals, the AP (availability percentage), PI (Single-factor pollution index), NPI (Nemerows pollution index), and PLI (Pollution load index) in the affected soils with this wastewater was calculated. Also, all soil and water experiments were performed in 3 replicates and then, using the excel data software category, tables and charts were plotted.

The soils were alkaline and calcareous as characterized by high pH, ranging from 7.6 to 8, and calcium carbonate equivalent, ranging from 30 to 42%. On average, the value of the available fraction of the examined metals in the wastewater-irrigated soils ranged from 1.9 to 3.5 mg kg-1for Zn, 2.5- to 3.5 mg kg-1for Cu, 0.4 to 0.62 mg kg-1for Cd, 2 to 2.9 mg kg-1for Pb, and 1.34 to 1.75 mg kg-1for Ni. Comparing to the control, irrigation with wastewater resulted in a considerable build-up in the available fraction of the metals in the rank of Ni (79-142%)> Cd (54-125%)> Zn (35-73%)> Cu (13-87%)>Pb (6-32%). These patterns can be due to the quality and quantity of the used wastewater and impact of the used wastewater with its receiving soils. Similar to the available fraction, there was an increasing trend in the total fraction of metals in the order of Cd> Zn>Pb> Ni> Cu following wastewater irrigation. In this context, the mean content of total Zn, Cu, Cd, Pb, and Ni in wastewater-irrigated soils was as 51-157%, 10-32%, 243-310, 11-203%, and 13-126% higher than those of control soil, respectively. In spite of such enrichment, only the Cd values exceeded the maximum acceptable limits. The AP index is an appropriate index to compare the mobility potential and the toxicity of heavy metals in soil. In this study, the highest rate of this index among the heavy metals was related to Cd and its lowest level was related to Pb, which showed more toxicity and more mobility of Cd compared with other elements. The average of single-factor pollution index of five elements was observed in sequence Cd> Zn> Ni>Pb> Cu that the element of Cd had the highest class of PI (class 4). The highest and lowest of NPI values of five elements were observed in profiles 4 and 2, respectively. Also, the greatest effect of the five elements of this study is on the elements of Cd and Zn in the generation of this level of contamination. The pollution index of the five studied elements in irrigated soils with treated wastewater was similar to the NPI, its maximum was observed in profile 4 and Cd showed the highest effect on increasing the value of this index.

The results of this study showed that irrigation with sewage significantly increased the available fraction of the metals in the order of Ni (78.9-141.8%)> Cd (54.4-125%)> Zn (35.7-73.3%>Cu (13-87%)>Pb (6-32.3%) compared to the control. However, with the exception of cadmium, the available fraction of other elements was within the permissible limit. Compared to the control, in the majority of studied soils, the total fraction of the metals (with the exception of copper) was significantly increased and the lowest and highest increase associated with Cu (10-32%) and Cd (2 - 3 times). Also, the results of pollutant indices showed that the majority of the studied soils were in the low to high contamination and Cd was known as the major metal affecting the indices yield.