Effect of seawater intrusion on human health risk and toxicity of disinfection by-products

Around the world, coastal groundwater is increasingly subject to seawater intrusion (SWI). The quality and characteristics of such waters differ from those of surface and groundwater. In the current study, trihalomethane (THM) formation under varying levels of SWI, its human health risks, and toxicity were evaluated. Various levels of SWI were simulated by mixing deionized water synthetic seawater (SSW) by varying seawater volumes from 0% to 3%. Chlorination of these samples was carried out as per uniform formation condition (UFC). Chlorine demand increased with increasing SWI.  THM concentration increased from 12.64 μg/L to 105.34 μg/L after 24 h and to 115.8 μg/L after 48 h for an increase from 0% to 3% volume of seawater. Human health risks due to THMs were determined using probabilistic risk assessment models considering ingestion, dermal contact, and inhalation as three exposure routes. Risk assessment was carried out using 1,00,000 iterations of Monte Carlo simulations. Total cancer risk increased 4 times for an increase of SWI from 0% to 0.25%. Further, the toxicity of THMs to mammalian cells due to increasing degrees of SWI was calculated. For this, the lethal concentration that reduced the Chinese hamster ovary (CHO) cell density by 50% (LC₅₀) by all four THMs reported in the literature was considered. The highest total toxicity value of 1.07 × 10^-04 was observed at SWI = 1% by volume. In general, an increase in SWI of up to 1% resulted in maximum health risk and toxicity. The results of the current study are useful for coastal water utilities and treatment plants to reduce human health risks due to disinfection by-products.