Temporal and spatial occurrence of oxybenzone and octocrylene in the coastal waters of Oʻahu

Abstract

Organic ultraviolet filters (UVFs) are gaining increasing attention as contaminants of emerging concern in aquatic and marine ecosystems, entering the environment via plastics and personal care products, such as sunscreens. At heavily used beaches, an important source of UVFs are sunscreens; therefore, the number of people in the water may be an important driver of spatial and temporal variation in UVF concentrations ([UVF]), along with the environmental characteristics of the beach itself, such as the retention time of nearshore waters or flushing due to tide. Presented here are two time series monitoring the temporal and spatial occurrence of the UVFs oxybenzone (BP3) and octocrylene (OC) at popular beaches on Oʻahu in Hawaiʻi. The first time series reports the monthly, diurnal occurrence of BP3 and OC at four beaches on the South Shore of Oʻahu. Samples were collected one Saturday at month in the morning – prior to the arrival of most beachgoers, midday – when beachgoers were most abundant, and sunset – after the accumulated exposure to the days' beachgoers. The second time series reports the bimonthly, diurnal occurrence of BP3 and OC at four sampling locations in the Hanauma Bay Marine Life Conservation District. Hanauma Bay is a protected Bay and popular tourist destination that is open to the public Wednesday through Sunday. Samples were collected on Wednesdays (the first day open each week) and Sunday (the last day open each week) at the time of beach opening, noon, and beach closing every month for seven months. Analytes were extracted via reversed-phase solid phase extraction, extracts were analyzed via LCMS-QQQ in multiple reaction monitoring, and concentrations were calculated using an 11-point standard curve. Using a linear mixed-effects model, I quantified the effects of collection time, sampling location, people, and tide height on [BP3] and [OC]. Both time series occurred during an unprecedented time on Oʻahu, when travel restrictions were being lifted and beaches and parks were being reopened after pandemic closures, which included changes in management practices at Hanauma Bay, as well as when the ban on BP3 went into effect on January 1, 2021. Therefore, additional analyses evaluated whether 1) there are differences in the occurrence of BP3 and OC at South Shore sites before and after the ban on BP3 went into effect; and 2) there are differences in the occurrence of BP3 and OC at Hanauma Bay on the Wednesday morning, on the day the Bay opens, and Sunday morning, on the day the Bay closes. [BP3] and [OC] in surface water from the South Shore sites ranged between 1.00 ng/L to 1,006.99 ng/L and 1.12 ng/L to 7,159.12 ng/L (n = 396). [BP3] and [OC] in surface water at Hanauma Bay ranged from 1.73 ng/L to 1,941.91 ng/L and 2.16 ng/L to 29,986.79 ng/L (n =504). The linear mixed effect model showed that at both the South Shore beaches and Hanauma Bay, interactions between sampling location, collection time, and tide height have the greatest significant effect of [BP3] and [OC]. People density has a main effect on the occurrence of BP3 and OC in waters from the South Shore. Total visitors does not have a main effect on the occurrence of BP3 and OC in waters from Hanauma Bay. The South Shore time series t test results revealed that significant difference in [BP3] and [OC], before and after both the ban on BP3 and the reopening of beaches after pandemic closures, were site dependent. The Hanauma Bay time series t test results showed that there was no significant difference in [BP3] on Wednesday and Sunday mornings; significant difference in [OC] on Wednesday and Sunday mornings varied by month with no distinct seasonal trends. Overall, the occurrence of BP3 and OC in surface waters from the South Shore and Hanauma have high diurnal variation due to anthropogenic activity and grab sampling techniques are limited by sampling frequency. Predicted [BP3] and [OC] generated by the linear mixed effect models can be used for guidance in possible UVF monitoring programs, especially best times to sample given the high diurnal variation.