A study of biological contaminants in rainwater collected from rooftops in Bryan and College Station Texas

Abstract

This study was conducted to identify the presence of biological contaminants and the factors responsible for these contaminants in rainwater collected from rooftops of buildings in Bryan and College Station, Texas. Two main biological contaminants were identified: total coliform bacteria and turbidity. The study is based on the hypothesis that the presence and level of contamination of rainwater in terms of turbidity and total coliform bacteria collected from rooftops will depend on type of roof, area of the roof, intensity of rainfall and dry spell between each rain. Two different roof types were identified, namely, asphalt shingles and metal. A total of sixty samples was collected with thirty from each roof type. The samples were analyzed for turbidity and presence of total coliform bacteria. An external laboratory, Acquatech Inc., in Bryan, Texas, did the analysis of the rainwater samples. Data from the test results were analyzed for statistical significance by multiple regression techniques. A regression model was developed between turbidity and total coliform numbers as the dependent variables and roof type, roof area, intensity and dry spell as the predictor variables. Analysis of the turbidity model was done by the multiple linear stepwise regression technique. The analysis of the total coliform model was done using stepwise logistic regression technique, as the laboratory results for the total coliform tests were binary. The resulting model indicates that turbidity is directly related to the roof area and the dryness of the roof. The other variables, namely, roof type and rainfall intensity were found to be statistically insignificant by the analysis. Results of the logistic regression for total coliforms indicated that the presence of total coliform bacteria is directly affected only by the intensity of rainfall. The study has shown factors are likely to affect the presence of biological contaminants such as total coliform bacteria and turbidity in roof water run-off. This will help in optimizing the design of water treatment units for rainwater harvesting systems. It has been shown that a dry spell has an effect on turbidity levels indicating that the first flush would be more contaminated than other water flows.