RESUMO
Um dos problemas comumente encontrado na análise estatística de dados provenientes do monitoramento da qualidade da água envolve os chamados dados censurados. Este estudo mostra como os valores calculados das concentração médias de amostras que contenham dados censurados são influenciadas pelo tamanho da amostra, percentual de dados censurados e método de cálculo. As concentrações médias foram calculadas com os métodos de substituição e maximum likelihood estimator (MLE) para amostras de demanda química de oxigênio e fosfato. O intervalo de confiança da média foi utilizado como métrica de comparação. Os resultados revelam que as concentrações médias calculadas com o método de substituição simples se posiciona fora do intervalo de confiança. Já o método MLE calcula valores aceitáveis, porém, a eficácia do método depende de um limite de elasticidade do nível de censura e do tamanho da amostra.
One of the most common problems in statistical analysis of data from water quality monitoring programs involves the so-called censored data. This study highlights how the sample mean concentration of chemical oxygen demand and phosphate variables are influenced by the estimation method, sample size and percentage of censored data. The confidence interval was used as a metric for comparison of calculated sample mean concentrations on both simple substitution and maximum likelihood estimator (MLE) methods. The results show that sample mean concentrations calculated by simple substitution method are positioned outside the confidence interval. Otherwise, the MLE method produces acceptable values for sample mean concentration, although the magnitude is constrained by sample size and percentage of censored data. The effectiveness of the method depends on an elastic limit of censoring level and sample size.
RESUMO
This study evaluates the spatial patterns of land occupation and their relationship to water quality in the Cuiabá River watershed, one of the main affluents of the Pantanal floodplain. The impact of farming and other land occupation forms were studied using a three year time series. Monitoring included 15 parameters at 21 stations with a total of 1266 different samples. Ten stations along the Cuiabá River were ordinated by Principal Component Analysis (PCA). For an exploratory analysis in the spatial domain, sub-basins of the Cuiabá watershed were classified according to mean concentrations of selected water quality parameters. Supervised classification of digital Landsat ETM imagery and standard GIS techniques were applied to parameterize land use and occupation according to a watershed scale. Redundancy Analysis (RDA) was then used to evaluate impacts of environmental and socio-economic factors on water quality. A Cuiabá headwater station only shows slightly elevated total coliform counts and concentrations of nutrients in the river after it passes regions of extensive cattle farming. After the confluence with the Manso River, nutrient and COD concentrations increase significantly, receiving loads from sub-basins under intensive agricultural land use, with mean annual concentrations up to 1.74 mg/L of total nitrogen (Kjedahl). Sub-watersheds with intensive fishing culture activities were shown to have significant impact on nitrogen concentrations, reaching mean concentrations of 2.66 mg/L of total nitrogen in the affluents. Most serious biological and chemical water pollution can be observed at stream outlets in the urban agglomeration of Cuiabá/Várzea Grande. Affluent pollution is reflected in the water quality of the Cuiabá River: subsequent monitoring stations in the urban area are ordinated on a gradient of increasing degradation of chemical and biological water quality. The auto-depuration capacity of the Cuiabá River is intact, but elevated concentrations of Phosphorous and Chemical Oxygen Demand can be observed as far away as the Pantanal floodplain, about 120 km downstream from the urban agglomeration.
