A new bacterial ratio has been developed and proven by multiple case studies to provide information on both potential source and fecal age. This ratio utilizes the conventional total coliform test, comparing the relative concentrations of different colonies that form on a membrane filter fed by m-Endo media. These bacterial colonies can be classified into 3 types; typical (TC), atypical (AC), and background. The ratio obtained by dividing the AC by the TC concentrations (AC/TC) defines shifts in populations between indigenous and introduced bacteria, with the indigenous bacteria providing a baseline against which the concentrations of the introduced, fecally-associated bacteria are evaluated. Different values of the AC/TC ratio can be ascribed different relative pathogen risk levels. When the AC/TC ratio is low (<5), fresh fecal material is being added to surface water and pathogen risk can be expected to be higher. As time passes, the AC/TC ratio increases (>20) and can be related to healthier water quality conditions with respects to pathogens that die-off in the environment. Different types of fecally impacted runoff are reported to have statistically different AC/TC values with human sewage at the lowest end of the spectrum with a value of 1.5 under normal conditions. Applications of the ratio for detecting hot-spots of human sewage in a watershed and as a control standard for watershed quality are presented, along with a modeling approach that can reliably predict human enteric virus in stormwater impacted surface water using the AC/TC ratio. Copyright ASCE 2005.