We used an anthropogenic tracer signal to evaluate downstream solute transport and storage in Valley Creek, a 60 km2 watershed stream near Philadelphia, Pennsylvania, that is subject to urbanization and anthropogenically induced unsteady flow. Bromide-bearing groundwater from an abandoned mineral processing plant enters the upstream reach of Valley Creek through a series of well-defined seeps and springs, producing a steady and significant concentration of bromide in Valley Creek. In addition, at the time of the study, a quarry located near the center of the watershed discharged accumulated groundwater seepage to the stream on a cyclical basis. The quarry discharge reduced the bromide concentration in the main stream and produced a periodicity in the in-stream bromide concentration downstream of the quarry. We used these variations in the ambient bromide concentration to assess solute mixing and transport in Valley Creek. We applied the USGS code OTIS to analyze solute advection, dispersion, transient storage, and groundwater inflow over a 7.5 km stream reach. To apply OTIS for unsteady flow conditions, we independently modeled the variation of stream conditions during a flow cycle and then simulated solute transport using temporal and reach average values of the longitudinal dispersion coefficient (D), transient storage exchange rate (α), and transient storage area (As/A). Calibrated values of D ranged from 0.5 m2 s-1 to 1.4 m2 s-1, As/A ranged from 0.03 to 1.3, and α ranged from 8.7 ¿ 10-6 s-1 to 1.0 ¿ 10-2 s-1. Observed temporal variability in the in-stream bromide concentration was simulated well. The observed transient storage parameters reflect differences in the structure of the three test reaches. Solute storage in the middle reach was dominated by in-stream storage produced by relict hydraulic structures, while solute transport in the other two reaches reflected differences in bed sediment characteristics between upstream and downstream reaches. However, overparameterization of the model and high sensitivity to the dispersion coefficient made it difficult to assess the magnitude of transient storage or hyporheic exchange.