Ecological risk assessment for metal(oid)s in microplastics and sediments at different depths from cenotes (sinkholes) of Yucatan, Mexico

Karst aquifers, such as cenotes, are important sources of drinking water and economic income, as well as representing natural and cultural wealth. In this study, metal(oid)s were determined in microplastics and sediments from eight cenotes (sinkholes) in the Yucatan Peninsula, Mexico. Sediment samples were collected from eight cenotes by diving at different depths ranging from 2 to 32 m. The mean (±SD) of microplastic particles (MPs) in the sediments was 43.10 ± 50.58 MP/kg. The most abundant polymers were polyethylene (PE), polyethylene terephthalate (PET), polyamides (PA) and polyester (PES). A multiple correlation (r: 0.37, p = <0.001) was found between the density of MPs and depth. Al, Cr, Sn, and Cu were the metals that showed the highest concentrations in both MPs and sediments, unlike the others metals. Hg (r = 0.58; p < 0.05, multiple correlation) showed a significant correlation between the concentrations determined in sediments and microplastics. A multiple correlation analysis was also found between the amount of organic matter and the concentrations of lead (r = 0.57; p = 0.001) and tin (r = 0.63; p = 0.0003). The potential ecological risk index (PERI) for karst aquifers such as cenotes ranged from 121.44 to 323.52 in MPs and from 95.65 to 288.29 in sediments, showing a considerable ecological risk of metal(oid) contamination in microplastics and sediments in most cenotes. This is the first study of microplastics in sediments at different depths in karst sinkholes. Moreover, it also provides data on the contamination and ecological risk of metal(oid)s related to microplastics and sediments, which could be linked to the use of cenotes as garbage dumps in earlier times.