Abstract: Zhaxigang wetland is a typical plateau meadow swamp and an important ecological function reserve in Lhasa. Wetlands in Tibet are extremely fragile due to climate and topography, which should be given full consideration. To accurately assess the pollution status and sources of heavy metals in the soil of Zhaxigang wetland, we analyzed the contents of seven heavy metal elements (Cr, Ni, Zn, Pb, Cd, As and Hg) in 18 surface soil samples of Zhaxigang wetland. The pollution risks of heavy metals were evaluated by using the method of geo-accumulation index (I_geo) and pollution loading index (PLI). Based on PCA, the APCS-MLR model was used to analyze the pollution sources of heavy metals. The results showed that: (1) The contents of Cr and Ni in the surface soil of Zhaxigang wetland were lower than the background value of Tibetan soil. The contents of the other five elements were accumulated in different degrees. Except for Cd and As, the measured values of the other five elements were lower than the risk screening value in “Soil Environmental Quality Risk Control Standard for Soil Contamination of Agricultural Land of  (GB 15618-2018)”. (2) Except for Cd and Hg, the I_geo values of other elements were less than 0, indicating a clean level. The overall PLI in the study area was 1.44, indicating light ecological hazard. Both evaluations showed that Cd and Hg were the main pollution elements in this area. More attention should be paid on those two elements, and routine monitoring should be strengthened. (3) The sources of heavy metals were mainly mixed industrial-transportation sources, natural sources, and mixed agricultural-coal combustion sources. Ni, Zn, Pb, Cd, and As were mainly from mixed industrial-transportation sources, Cr was mainly from natural sources, and the sources of Hg were mainly affected by human activities such as agriculture and coal combustion.

Key words: Zhaxigang wetland, sediment heavy metal, pollution assessment, APCS-MLR model