北京市耕作土壤重金属统计分布复杂性特征初步研究

为了深入探讨重金属空间分布，用频率分布、双峰拟合和分离的方法研究了北京市耕作土壤重金属Cu、As、Cd、Pb的统计分布特征。结果表明，4种重金属都呈明显的双峰分布，可以用双高斯密度函数很好地拟合经对数变化的实际测量数据，证明北京市耕作土壤中重金属Cu、As、Cd、Pb都包括2个相对独立的对数正态分布总体。根据拟合频率密度函数求得的重金属Cu、As、Cd、Pb的2个子总体的几何均值分别为：(23.35±6.39) mg/kg和(95.84±38.73) mg/kg、(9.35±2.20) mg/kg和(23.67±6.16) mg/kg、(0.1208±0.0334) mg/kg和(0.3548±0.2344) mg/kg、(20.07±6.07) mg/kg和(82.55±39.33) mg/kg，都表现为明显的低值组和高值组。从统计学的角度，可以解释重金属含量呈双峰分布的原因，但从其空间分布上来看，双峰的成因较为复杂。

A preliminary Study on the Complexity Feature of Heavy Metals’ Statistical Distribution in Beijing Cultivated Soil

In order to thoroughly investigate the spatial distribution of heavy metals, the methods of frequency distribution, bimodal fitting and separation were used to study the characteristics of statistical distribution of heavy metals in Beijing cultivated soil. It was demonstrated that, Cu, As, Cd and Pb were bimodal distribution. Double Gaussion Function could be used to fit the empirical distribution well after log-transformation, suggesting the presence of two independent log-normally distributed populations for Cu, As, Cd, and Pb. The geopmetric means of the two populations were (23.35±6.39) mg/kg and (95.84±38.73) mg/kg for Cu, (9.35±2.20) mg/kg and (23.67±6.16) mg/kg for As, (0.1208±0.0334) mg/kg and (0.3548±0.2344) mg/kg for Cd, (20.07±6.07) mg/kg and (82.55±39.33) mg/kg for Pb, respectively. The four heavy metals all could be divided into low value group and high value group. Heavy metals showed obvious bimodal distribution, the reason could be explained from a statistical point and may be complicated based on the spatial distribution of the two groups.