Most of earth’s population depends on onsite sewage systems to dispose of their wastewater to the environment.
Samples used to estimate contaminant transport in the environment have typically been collected from nitrogen loads external to the septic tank. Here we characterize the isotopes of boron, nitrate–nitrogen and nitrate–oxygen in partially clarified liquid collected directly from within septic tanks located in Florida. Among other parameters, total Kjeldahl nitrogen (TKN) and total boron were also evaluated. Nitrate–nitrogen isotopes from three samples ranged from +1.25 to +10.00‰, and boron isotopes from five samples ranged from +8.8 to +31.3‰. Nitrate–oxygen isotopes ranged from −3.23 to +19.96‰ and were indicative of an anoxic environment. On average, residential septic tanks were found to contain 96 µg L−1total boron and 63 mg L−1 TKN. Utilizing isotopes of boron, among other parameters, our findings show that the partially clarified liquid in septic tanks contains a distinguishable δ11B signature.