Evaluation of Ecological Risk from Heavy Metals in Lake Sediments in Xizang, China

We measured the content of six heavy-metal elements (Co, Cu, Zn, As, Cd, Pb) in 72 lake surface-sediment samples from the Xizang Autonomous Region (referred to as Xizang) of China, and calculated four indices: enrichment factor (EF), geo-accumulation index (I_geo), pollution load index (PLI), and a comprehensive potential ecological risk index (RI) to explore the differences in the accumulation and the major sources of heavy metals, and to reveal the role of human activities in the ecological environment of high-elevation cold regions. Results show that the heavy-metal contents in lake surface sediment samples in eastern Xizang are much higher than those in central and western areas, and are also higher than the background value of surface soils. Their distribution can be related to regional sources of heavy metals: natural parent material (western Xizang), agriculture (eastern Xizang), mining-related industrial sources (central Xizang), and degree of development of road network (all of Xizang). The majority of lakes in Xizang have a relatively low risk of ecological harm, but there are notable regional peculiarities in the element content and evaluation indices and the lakes in eastern Xizang are at higher risk than those in central and western parts. Lakes at higher ecological risk (such as Lake Butuo (Lake 71) and Samu Co (Lake 68)), remind us that the potential influence of heavy-metal enrichment on lake sediments and aquatic plants should not be overlooked.