Historical records of heavy metal accumulation in sediments and the relationship with agricultural intensification in the Yangtze–Huaihe region, China

The extensive use of fertilizers on farmlands during the past several decades in China has led to a rapid deterioration of environmental water quality in recent years. An agricultural watershed with high-yield grain production and characterized by multipond systems in the Yangtze–Huaihe region of China was selected to establish the historic records of heavy metal pollution by 137Cs-dated sediment cores. The experimental results indicated that the contents of most of the heavy metals investigated, such as Cd, Cr, Cu, Ni, Pb, and Zn, continuously increased in the multipond sediments throughout the past three decades. An inflection point appeared in the 1980s, prior to which all heavy metal contents showed little or no increase with time. Thereafter, the heavy metal contents increased dramatically due to the extensive application of phosphate fertilizers. The mean Cd content in 1980 was 0.13 μg/g which increased rapidly to 0.33 μg/g in 2004, but prior to 1980, the level was only 0.08 μg/g. Similar trends were also found for other heavy metals. The enrichment factor (EF) values indicated that Cd was not only the most anthropogenic metal in the multipond sediments but also aggravated anthropogenic impacts on the watershed environment during the past two decades. The high proportion of chemical reactive forms of Cd (10%–30%) implied a moderately high ecological risk. Our results clearly reveal that the extensive use of fertilizers has resulted in significant heavy metal pollution in this watershed, which threatens the water quality of the watershed and downstream water bodies.