Long-overlooked coumarins in swimming pool water and their contributions to the formation of disinfection byproducts

Abstract

Non-targeted analysis was used to characterize the dissolved organic matter in water from two swimming pools in Nanjing and Changzhou, eastern China. A total of 112 coexisting compounds (level 2b) were detected in both pools using the MassBank database. Source tracking identified 20 compounds from tap water, 19 from sweat, 1 from urine, and 2 from mosquito repellents, including amino acids, coumarins, alcohols, N-heterocycles, alkaloids, carboxylic acids, terpenoids, ketones, and esters. The number of coumarins accounted for 10% of the common compounds in tap water, 21% in sweat, 100% in urine, and 50% in repellents, confirming tap water, biological excretions, and cosmetics as the coumarin sources in pool water. Concentrations ranged from ng L⁻¹ (pool and tap water) to μg L⁻¹ (sweat and urine) and mg L⁻¹ (repellents). Breakthrough tests revealed that sand filtration, China's primary treatment, removed <5.8% of the coumarins due to their hydrophilicity (log P < 5). In simulated chlorination of tap water spiked with 100 ng L⁻¹ of each coumarin, only 0.26 ng L⁻¹ of 3-Cl-7-OH-4-CH₃-coumarin was detected, while the concentrations of trichloroacetic acid, iodoacetic acid, bromodichloromethane, dibromochloromethane, and trichloroacetaldehyde increased by 0.38–3.3 μg L⁻¹ (4.1–36%), 0.16–2.9 μg L⁻¹ (1.7–31%), 0.70–3.9 μg L⁻¹ (3.8–21%), 2.2–8.0 μg L⁻¹ (4.5–12%), and 0.9–2.4 μg L⁻¹ (4.8–13%), respectively, due to coumarin transformation under high chlorine doses and long reaction times (12 h). This study demonstrates that coumarins act as disinfection byproduct precursors in swimming pool water, and their accumulation may increase DBP levels, posing health risks to swimmers, thereby filling a knowledge gap in pool water DOM research.