Using multiple tracers (F−, B, δ11B, and optical brighteners) to distinguish between municipal drinking water and wastewater inputs to urban streams

Highlights: • Tracers were used to assess drinking water and wastewater inputs to streams. • Municipal waters have distinct tracer (F⁻, B, δ¹¹B, and optical brightener) values. • Municipal water tracer levels were low in rural streams, but high in urban streams. • Drinking water and wastewater respectively contribute up to 54% and 16% of flow. -- Abstract: Releases of municipal waters, including drinking water and wastewater, can considerably alter urban stream chemistry. However, the relative contributions of drinking water versus wastewater to streams have not been quantified previously and are therefore the focus of this study. We sampled streams along a land use gradient that included watersheds with impervious surface areas (ISA) ranging from 1.6 to 62.6%. Samples were analyzed for F⁻, total B, δ¹¹B, and optical brighteners to determine municipal water inputs to streams. We observed low F⁻ (75 ± 20 μg/L), B (29 ± 6 μg/L), and optical brightener (3.66 ± 0.76 RFU) levels in rural streams, but their concentrations increased with urbanization (up to 475 μg/L, 227 μg/L, and 22.09 RFU, respectively). The δ¹¹B values for drinking waters (16.52 ± 0.39‰) and wastewaters (untreated = 6.06 ± 0.88‰ and treated = 6.46 ± 0.93‰) were distinct, but there was poor correlation between δ¹¹B and ISA for the streams (R² = 1 × 10⁻⁵; p = 0.99), likely due to variable lithology in the study area. We used inverse and three-component mixing models to quantify municipal water inputs to the streams. In densely urbanized watersheds, drinking water and wastewater can respectively contribute up to 54% and 16% of the total streamflow. In addition to our spatial sampling, we collected weekly samples at a suburban stream to test the effects of discharge and seasonality on municipal water tracer behavior. We found that tracer levels did not change significantly (p ≥ 0.28) with discharge or season, suggesting that municipal water inputs are fairly constant. Understanding the relative proportions of differing municipal water types to streams is crucial in guiding infrastructure improvements to conserve drinking water and reduce harmful wastewater releases. The unique chemical signatures of municipal waters aid in the widespread applicability of our multi-tracer method for identifying water sourcing to urban streams.