Effluent from on-site sewage treatment and disposal systems (OSTDS) is generally known to impact groundwaters and surface waters with nitrogen (N) and phosphorus (P) and other contaminants. Little research has quantified this problem along the Indian River Lagoon (IRL), especially in Indian River County (IRC) where there are 26,660 active systems. This study assessed the effects of OSTDS on contamination of surface and groundwaters along three urbanized canals and the St. Sebastian River in IRC, all of which flow into the Central IRL. Multiple lines of evidence were used to define the source of the nutrient loadings including the novel approach of using the artificial sweetener, sucralose, as an indicator of human sewage impact. Results indicate that areas with high densities of OSTDS are contributing N to surface waters and elevating N:P ratios through submarine groundwater discharge and promoting eutrophication in the Central IRL. / Includes bibliography. / Thesis (M.S.)--Florida Atlantic University, 2014. / FAU Electronic Theses and Dissertations Collection