Vertical variability of cadmium after one sewage irrigation in long-term sewage-irrigated areas

There is limited development of agricultural economy in the north area of China that has been irrigated with sewage water for a long time, because of the uneven distribution of water resources and different levels of soil pollution. The main contaminants of sewage irrigation are heavy metals, which contamination area has been expanding over the years. A number of large-scale or multi-scale studies have been conducted on heavy metal pollution in surface soils in irrigated areas. However, studies have less focused on the vertical distribution of heavy metal, but mainly concentrated on the characteristics of migration and leaching of heavy metals in the soil. It is therefore necessary to focus on the vertical cross-sectional view of the spatial variability of soil heavy metals in sewage irrigated areas on a small scale. In this study, we investigated the spatial variability and distribution of available cadmium in the soil after once sewage irrigation. Using classical statistics and geo-statistical techniques, we measured the spatial variability and distribution in a 328 m² field in the Niangniangmiao sewage irrigation area in Xinxiang, Henan Province. The classical statistical analysis suggested that the mean available cadmium in soil sections A, B, C and D (the south-north sections with 10.5 m away from each other and across the field from north to south) decreased, respectively, by 0.06 mgkg^-1, 0.11 mgkg^-1, 0.14 mgkg^-1 and 0.23 mgkg^-1 after sewage irrigation; suggesting a moderate degree of dispersion. The dispersion in soil section B increased by 4.32%, while it decreased in other sections by 9.39% (A), 6.12% (C) and 10.98% (D), all of which were within the 28.31%45.16% range after sewage irrigation. Geo-statistical analysis showed that the overall variation in available cadmium in the soil sections was random, C₀/(C₀+C) < 25%. Soil available cadmium had a strong spatial correlation within the 0.3931.308 m range. The optimal empirical semi-variogarm models for simulating soil cadmium content for soil sections A, B, C and D were respectively spherical, spherical, Gaussian and exponential modles. Soil profile B had the highest (1.997) fractal dimension of available cadmium. The fractal dimension decreased after sewage irrigation suggesting a weakening spatial variability of soil available cadmium. Structural factors played a decisive role in the vertical spatial variability and distribution of available cadmium in the soil sections. Kriging interpolation suggested a banded distribution pattern. There was significant available cadmium accumulation in the 015 cm soil layer. The distribution contour of available cadmium in the soil sections varied from dense to sparse along irrigation direction. Correlation and variance analysis showed that available cadmium content was positively correlated with organic matter content, moderately correlated with soil moisture, strongly negatively correlated with pH. The effect of irrigation on each factor was significantly, moreover, irrigation could dampen the impact of the factors on available cadmium. The results on distribution and variation in soil available cadmium showed leaching and migration of soil available cadmium after sewage irrigation. This was more visible for the 015 cm top soil than the 3040 cm sub-soil. The spatial variability of soil available cadmium was mainly influenced by soil structural factors. The study provided the primary investigation for phytoremediation of heavy metal in polluted soils.