Water quality index modified by using multivariate analysis: a case study of Pelotas Stream, RS, Brazil

ABSTRACT This research aimed at developing a modified water quality index (WQIMod) by using multivariate analysis, from a comparison with the WQI used by the Environmental Company of São Paulo (Companhia Ambiental do Estado de São Paulo-Cetesb) (IQACETESB), to be applied in the Pelotas Stream, RS, Brazil. Secondary data from Pelotas Stream surface water quality were used, collected in the sampling site GER 48, located in the municipality of Pelotas, where they were monitored and provided by the State Foundation of Environmental Protection Henrique Luís Roessler (Fundação Estadual de Proteção Ambiental Henrique Luís Roessler-FEPAM) between 1997 and 2013. Among the multivariate analyses, the factor analysis/principal component analysis (PCA) was used. The results indicated that the PCA extracted two principal components, which explained 100% of the total data variance in the correlation matrix, while the factorial scores for WQIMod calculation were extracted from the first factor, which explained 70.6% of the total data variance. We concluded that the WQIMod exhibits biochemical oxygen demand (BOD5), turbidity (TH) and total solids (TS) as the most important variables for the index construction, differing from the WQICETESB, which shows the variables dissolved oxygen (DO), thermotolerant coliforms (TC) and hydrogenionic potential (pH) as the most relevant ones, indicating the need of WQICETESB calibration for applying in this specific ecosystem.摘要：本研究旨在通过多元分析构建改良水质指数（modified water quality index, WQIMod），并与巴西圣保罗州环境公司（Companhia Ambiental do Estado de São Paulo-Cetesb）所采用的IQACETESB水质指数进行对比，以期将该改良指数应用于巴西南里奥格兰德州佩洛塔斯溪。本研究使用了佩洛塔斯溪地表水水质的二手监测数据，采样点位于佩洛塔斯市GER 48区域，数据由亨里克·路易斯·罗斯勒州环境保护基金会（Fundação Estadual de Proteção Ambiental Henrique Luís Roessler-FEPAM）于1997年至2013年间采集并提供。本研究选用的多元分析方法为因子分析/主成分分析（factor analysis/principal component analysis, PCA）。结果显示，主成分分析提取出两个主成分，二者在相关矩阵中可解释全部数据总方差；而用于计算WQIMod的因子得分则提取自第一个主成分，该成分可解释70.6%的总数据方差。研究结论表明，WQIMod将五日生化需氧量（biochemical oxygen demand, BOD5）、浊度（turbidity, TH）及总固体（total solids, TS）作为构建该指数的核心变量，这与IQACETESB的核心变量选择存在显著差异：IQACETESB以溶解氧（dissolved oxygen, DO）、耐热大肠菌群（thermotolerant coliforms, TC）及氢离子活度（hydrogenionic potential, pH）作为关键指标，这说明在该特定生态系统中应用IQACETESB时，需对其进行校准优化。