This book deals with a quantitative analysis of environmental issues related to the water quality of natural hydrosystems. More specifically the questions of risk and reliability in water quality are analysed, from the engineering point of view and a methodology is developed to evaluate environmental impacts on rivers, groundwater and coastal areas from wastewater disposal. In a biological environment with many risks and unexpected events, an attempt to apply a rigorous analysis to uncertain and complex environmental issues may appear ambitious, if not utopic. In fact, environmental problems related to water resources are complicated, unstable and difficult to understand. What would be, for example, the effect on algal blooms and eutrophication in a coastal area if the pollutant loads from a river doubled? Even more complicated is the quantitative prediction of coastal water quality from a possible climate change (e.g. doubling of the atmospheric C02).