Citation:

Zepp, R., S. McWhorter, M. Cyterski, M. Molina, AND B. Acrey. Contaminant photoreactions at Gulf of Mexico beaches. National American Chemical Society National Meeting, San Francisco, CA, August 13 - 17, 2023.

Impact/Purpose:

Increasing human activities result in beach contamination by harmful chemicals, bacteria and viruses.  However, little information is available about the fate and exposure concentrations of these contaminants.  This presentation provides new data indicating that photochemical transformations are the major fate of these contaminants, and that light attenuation and photosensitization are the major competing processes that affect contaminant exposure concentrations.  

Description:

The Gulf of Mexico receives inputs from numerous tributaries that discharge natural organic matter (NOM) and other natural photosensitizers to its coastal beaches. Much of this NOM is colored dissolved organic matter (CDOM) which affects photoreactions of contaminants by light attenuation, a retarding effect, and photosensitization, an accelerating effect.   To obtain data for modeling contaminant photoreaction rates, water samples were collected throughout the summer at three beaches located in Florida, Mississippi, and Texas.  In addition to its important role in attenuating solar radiation, absorption of sunlight by CDOM initiates photoproduction of reactive oxygen species that mediate indirect (sensitized) photoreactions of contaminants in Gulf waters.  Hydrogen peroxide and singlet oxygen are reactive oxygen species that play dominant roles in indirect photoreactions.   For example, singlet oxygen reactions inactivate bacteriophages that are being evaluated as indicators of viral pathogens in recreational waters and hydrogen peroxide influences cyanobacterial blooms that plague the Gulf coast.  Bill Cooper was among the first to demonstrate the widespread occurrence of hydrogen peroxide in natural waters.  UV-visible spectral data, action spectra (wavelength studies), solar spectral irradiance and other data and relationships were used to simulate the photoproduction of reactive oxygen species in the three coastal beaches of the Gulf that were studied.  Results of these modeling studies show that indirect photoreactions of contaminants at the beaches are strongly linked to concentrations of CDOM in the waters. The net effect on depth-integrated rates is surprisingly invariant from one beach area to another.   Modeling results indicate that this lack of variability is largely attributable to compensating effects of retardation caused by UV light screening which offsets acceleration by photosensitization. This interesting effect on transformation rates mediated by CDOM will be discussed during the presentation.

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