WILLEMSTAD – Heavy rainfall can dramatically increase the amount of human-related chemical pollution reaching Curaçao’s coastal waters, with a new scientific study finding contamination levels more than 3.5 times higher during an exceptionally wet period.
Researchers studying Curaçao’s coastal environment found that rainfall was the single most important factor determining the abundance of land-based xenobiotics — chemical substances associated with human activity, including personal care products, plastic additives, pharmaceuticals and other compounds.
Three sampling campaigns were conducted between 2021 and 2022. During the 2021 wet season, average rainfall during the research period was 64 millimeters per month. During the 2022 dry season it was 37 millimeters, while the wet-season campaign later that year coincided with average rainfall of 144 millimeters per month.
The difference in chemical pollution was striking.
During the very wet 2022 period, overall xenobiotic abundance was approximately 3.5 times higher than during the 2022 dry season and 3.2 times higher than during the wetter period sampled in 2021.
Researchers concluded that seasonal precipitation was the primary driver of chemical contamination in coastal waters.
Rain can wash pollutants accumulated on roads, soil and other surfaces toward the sea. It can also increase underground groundwater discharge and potentially cause wastewater overflows in areas where stormwater and sewage systems are not adequately separated.
The effect can be particularly strong following dry periods, when accumulated pollutants are suddenly washed into the environment during heavy rainfall — a phenomenon known as the “first flush.”
The increase was not identical for every type of chemical.
During the 2022 wet season, different groups of xenobiotics recorded concentrations between 1.4 and 7.1 times higher than during the dry season. Plastic additives, personal care products, substances associated with indoor environments and food-contact chemicals showed some of the largest increases.
Personal care products, biocides and plastic additives increased particularly sharply during wetter conditions, rising by more than fivefold. Other groups, including certain drugs, plant protection products and persistent, mobile and toxic substances, showed considerably less seasonal variation.
Researchers also found higher concentrations of nutrients during the exceptionally wet 2022 campaign. The Willemstad area stood out in several measurements.
During the 2021 wet season, average phosphate and ammonia concentrations near Willemstad were higher than in the other zones studied. During the 2022 dry season, nitrate and nitrite concentrations also peaked in nearshore waters around Willemstad, apart from higher levels recorded on the North Shore in some measurements.
The study warns that the relationship between extreme rainfall and pollution could become increasingly important as climate change affects the frequency and unpredictability of severe weather.
More intense rainfall and flooding could result in stronger pulses of contaminants from land entering Curaçao’s marine environment.
The findings suggest that monitoring Curaçao’s coastal water quality immediately before and after major rainfall events could provide important information about when reefs and other marine ecosystems experience their greatest exposure to chemical pollution.