Chloroform associated health risk assessment using bootstrapping: A case study for limited drinking water samples

Chlorine is the most commonly used disinfectant for water treatment but when in contact with organic precursors it produces toxic compounds known as trihalomethanes (THMs). Chloroform is the most commonly occurring THM and constitutes approximately 90% of the THM concentration. A study was conducted in Newfoundland for estimating the chloroform content in the drinking water supplies in Clarenville and St. John's. The sampling program was conducted in two stages, July and October-November. Higher concentrations of chloroform were observed in Clarenville but in St. John's the concentrations were within Health Canada guidelines. The chloroform is a probable human carcinogen and is categorised as 'B2' by U.S. Environmental Protection Agency (U.S. EPA). The human health risk associated with consumption of drinking water was estimated by performing probabilistic analysis. In the first phase bootstrap simulations (B = 250) were performed to calculate the variability in the data. The normal distribution was found the best-fitted distribution to chloroform concentration data. For each bootstrapped run 1000 Monte Carlo Simulations (MCS) were performed in the second phase. The cancer risk was estimated using dose-response relationship of chloroform. Appropriate probability density functions (PDFs) were defined subjectively for intake rate, body weight, exposure duration, exposure frequency and averaging time. The excess lifetime human cancer risk varied from 0.5 x 10(-4) to 4.0 x 10(-4), and zero to 1.0 x 10(-4) for Clarenville and St. John's, respectively. The average cancer risks in Clarenville and St. John's communities were 1.2 x 10(-4) and 2.5 x 10(-5). The estimated probabilities of exceedences above 100 ppb (mug L-1), were 100 and 2.5% for Clarenville and St. John's, respectively which emphasised the need for water treatment. Granular Activated Carbon (GAC) was proposed as one of the potential treatment technology for risk reduction in St. John's community. The treatment efficiency of a proposed technology was assumed uniformly distributed and revised risks were calculated. The average risk in St. John's can be reduced to 8.0 x 10(-6) after treatment.