## 1 Introduction  
The intensive use of sunscreen in aquatic tourism has increased annually around the world, particularly in coastal areas where tourists apply sunscreen to mitigate the harmful effects sun exposure. Although protective against the risks of skin damage, the widespread application of sunscreen poses a risk for aquatic ecosystems; when people partake in aquatic activities, a percentage of sunscreen applied to their skin is discharged into the water. Aquatic tourism, including tourism at water parks, public beaches, natural and artificial water systems, and hydrological reserves, has increased the presence of chemical contaminants in the surface and groundwater systems. Chemical compounds deriving from sunscreen have been identified in the coastal zone mainly where coral reefs are located, in bodies of water with recreational activities, and water treatment plants. The substances contained in sunscreen are highly dangerous if they continue to reach aquatic systems because they have endocrine and carcinogenic effects, and even with high doses of exposure, their lethality has been demonstrated in indicator organisms. Therefore,sunscreen pollution in ecosystems is directly related to anthropogenic activities, especially in coastal tourist destinations. In this sense, the discharge of sunscreen in aquatic and marine ecosystems is a serious environmental problem for the state of Quintana Roo and the Caribbean coast of the Yucatan peninsula of Mexico. Every year, more than 15 million tourists visit the Riviera Maya, which is the 120 km tourist corridor located between the cities of Cancun and Tulum on the Caribbean coast of the state of Quintana Roo. Another 1.5 million residents live in the state of Quintana Roo [1]. Sunscreen is used intensively by tourists and residents, yet legislation of these products is scarce and ambiguous in the state of Quintana Roo [2]. [PERSON] et al. [3] found that more than 313 tons of sunscreen were released in the region over the last 12 years, and [PERSON] et al. [4] confirmed the presence of organic contamination in the groundwater flows of Quintana Roo, which indicates the transport of pollutants from inland groundwater to the Caribbean coast. According to [PERSON] et al. [5], 10% of the world's population lives in coastal areas, which are used for anthropogenic, industrial, and commercial activities. In addition, tourist activities in coastal areas are considered important for the country because they considerably economically benefit society. Therefore, and according to the estimates of [PERSON] et al. [1], the pollution due to sunscreen use, directly and indirectly, is immense; its discharge is related to the increase in tourism. Notably, 89% of tourists engage in aquatic activities, which increases the potential for discharging chemicals into the water in coastal areas. Sunscreen is a mixture of organic and inorganic substances, e.g., glycerin, vitamins, silicone, benzotriazole derivatives, and nylon-derived compounds, which were devised to protect the skin from solar radiation. This mixture depends on the formulation and regulations of the final composition of each ingredient, that is, each ingredient does not exceed more than 40% of the total composition, and some are even less than 10% of the total mixture [6]. Most of the estimation of sunscreen discharge has been performed in the coastal areas of the world, and mainly on coral reefs. It is estimated that more than 10,000 tons per year are discharged on the reefs. The component most likely to be present in the water, because it is fundamental for mixtures of sunscreens, is oxybenzene; however, nanoparticles such as TiO\({}_{2}\) and ZnO are equally highly dangerous [5]. Contamination by sunscreen components is difficult to quantify and estimate in water, but concentrations of even 1 mg/L were already reported in the ocean [7]. The Yucatan Peninsula, in the state of Quintana Roo, is underlain by karst geology.