Effect of irrigation water system's distribution on rice cadmium accumulation in large mild cadmium contaminated paddy field areas of Southwest China

Abstract There are large areas of moderately Cd-contaminated rice paddies (Cd content was less than risk intervention value) in southwest China, under natural conditions, the effect of irrigation water system's distribution on the Cd contamination in soil and rice is still less accurate. In this study, a survey of paired soil-rice (n = 1520) samples was conducted in a large paddy of about 7000 ha in southwest China that originated from the same parent material and grew with the same rice varieties. Specially, three representative characteristic regions (area A, B, C) were selected from north to south to thoroughly investigate the reasons for pollution characteristics. Background soil, irrigation water and sediment, atmospheric deposition, fertilizer were sampled to study the causes of pollution. Results showed the biological accumulation factor (BAF = C (rice Cd)/C (soil Cd)) of area C in the south reached 1.34, which was about 8 times higher than that of area A in the north. The uneven distribution of irrigation water due to geographical reasons was the most important factor leading to this pollution characteristics. The Cd content in soil of north was much higher than that in the south due to the history of Cd-contaminated irrigation and background content of Cd. During farmland formation, river impinges resulted in a gradual decrease in both Cd content and pH in the background soil from north to south. Both of historical polluters and major irrigation systems were distributed in area A. However, when sewage irrigation stopped about 30 years ago, long-term weakly alkaline irrigation increased the pH of the soil from acidic to neutral in area A. Meanwhile, flooding irrigation in area A reduced the absorption of Cd by roots compared area C where was rarely flooded during the rice planting process due to the lack of irrigation water.