[Using the Matter-Element Extension Model to Assess Heavy Metal Pollution in Topsoil in Parks in the Main District Park of Lanzhou City].

The UNMIX model was used to analyze the source of heavy metals found to be present in the topsoil of parks in the main district of Lanzhou City. The Hakanson toxicity response coefficient was used concurrently to modify the traditional weights in the model, and the matter-element extension model was used to evaluate heavy metal pollution. The results of the evaluation were compared with the comprehensive pollution index (P_N) and potential ecological risk index (RI). The results were as follows. ①The average heavy metal content in the topsoil at each sampling point was higher than that of the background value of soil in Lanzhou, with the proportion of Ni, Cu, and Co being 100% while the proportion of Cr, V, Pb, and As contents were 58.82%, 14.71%, 20.59%, and 2.94%, respectively. ② The results of source analysis showed that there were three major sources of heavy metal pollution in the topsoil of the parks in the study area. Source 1 is construction pollution, which contributes 56% of the Co present. Source 2 is traffic pollution, which contributes 44% and 52% of Cu and Pb, respectively. Source 3 is natural, and contributes 62%, 60%, 56%, and 56% of V, Cr, Ni, and As, respectively. Thus, this research showed that natural sources are predominant. ③ The weight correction effect for each heavy metal was significant; there was an approximately 44% reduction in both Cr and V, while the corrected weights of Ni, Cu, Pb, As, and Co increased in the order Co < Pb < Cu < Ni < As compared with the conventional weights. The most obvious change in weight was that of As, which increased by approximately 188%. ④ The results of the evaluation using the matter-element model showed that the state of 46% of the topsoil in the parks in the study area was grade Ⅴ (severely polluted), while 41% was grade Ⅳ (moderately polluted) and 3% was grade Ⅲ (lightly polluted); Co was the main pollutant. The results of the model evaluation were roughly the same as of from the P_N and RI, indicating that the matter-element extension model can be used to evaluate heavy metal pollution in soil and the evaluation results are accurate and objective.