Quantifying temporal and geographic variation in sunscreen and mineralogic titanium-containing nanoparticles in three recreational rivers

Detection of metal nanoparticles (NPs) in the environment is an analytical challenge of interest due to increasing use of nanomaterials in consumer and industrial products. Detecting NPs associated with human activities is affected by both the magnitude and variation in background concentrations of natural NPs. In this work, we investigated the potential release of titanium dioxide (TiO₂) NPs from sunscreen in three recreational rivers, with a time-intensive sampling regime on one river, in order to determine the range and variability of natural, background titania (Ti). Conventional ICP analysis for total metal concentrations, single particle ICP-MS for NP concentrations, and electron microscopy aided in assessing mineralogical morphology and composition. Oxybenzone, a widely-used organic sunscreen, was measured and used as a surrogate for the intensity of recreational activity in the water. Statistically significant increases in Ti concentrations were observed in Clear Creek, CO during one recreation period, but the significance of other instances of recreation-associated Ti increases was unclear, in part due to storm impacts on the natural suspended sediment load of the stream. A comparison of three recreational rivers showed increases in both Ti mass concentrations and NP sizes occur during recreation in both Clear Creek, CO and the Salt River, AZ, but no detectable changes in the Truckee River, NV. However, size distributions were variable in background samples, which make the significance of differences observed during recreation unclear. These results underline that the release of engineered nanoparticles to a natural system cannot be detected without a well-defined background, including measures of its variability during the study period.