Tributary streamflow to the St. Johns River in Duval County is thought to be affected by septic tank leachate from residential areas adjacent to these tributaries. Water managers and the city of Jacksonville have committed to infrastructure improvements as part of a management plan to address the impairment of tributary water quality. In order to provide data to evaluate the effects of future remedial activities in selected tributaries, major ion and nutrient concentrations, fecal coliform concentrations, detection of wastewater compounds, and tracking of bacterial sources were used to document septic tank influences on the water quality of selected tributaries. The tributaries Fishing Creek and South Big Fishweir Creek were selected because they drain subdivisions identified as high priority locations for septic tank phase-out projects: the Pernecia and Murray Hill B subdivisions, respectively. Population, housing (number of residences), and septic tank densities for the Murray Hill B subdivision are greater than those for the Pernecia subdivision. Water-quality samples collected in the study basins indicate influences from ground water and septic tanks. Estimated concentrations of total nitrogen ranged from 0.33 to 2.86 milligrams per liter (mg/L), and ranged from less than laboratory reporting limit (0.02 mg/L) to 0.64 mg/L for total phosphorus. Major ion concentrations met the State of Florida Class III surface-water standards; total nitrogen and total phosphorus concentrations exceeded the U.S. Environmental Protection Agency Ecoregion XII nutrient criteria for rivers and streams 49 and 96 percent of the time, respectively. Organic wastewater compounds detected at study sites were categorized as detergents, antioxidants and flame retardants, manufactured polycarbonate resins, industrial solvents, and mosquito repellent. The most commonly detected compound was para-nonylphenol, a breakdown product of detergent. Results of wastewater sampling give evidence that stream water in the study basins is affected by septic tank effluent. Fecal coliform bacteria concentrations were measured on a monthly basis; of 115 samples, 63 percent exceeded the State of Florida fecal coliform bacteria standard for Class III surface waters of 800 colonies per 100 milliliters of water on any 1 day. Fecal coliform bacteria concentrations ranged from less than 20 colonies per 100 milliliters of sample to greater than or equal to 160,000 colonies per 100 milliliters of sample. Antibiotic resistance patterns of fecal coliform bacteria were used to identify the sources of fecal coliform bacteria. Significant sources of fecal coliform bacteria included wild animals, dogs, and humans. A majority of the fecal coliform bacteria were classified to be from human sources. Because the primary source of fecal coliform bacteria is from human sources, and most likely septic tank effluent, management of human sources may substantially improve microbiological water quality in both the Fishing Creek and South Branch Big Fishweir Creek basins.