Occurrences and changes of disinfection by-products in small water supply systems

The small water supply systems (WSSs) often report high concentrations of disinfection by-products (DBPs) in drinking water. In this study, occurrences of trihalomethanes (THMs) and haloacetic acids (HAAs) in Newfoundland and Labrador (NL), Canada, were investigated from 441 WSSs for a period of 18 years (1999–2016). The WSSs were divided into groundwater (GWP) and surface water (SWP) systems, which were further classified into eight sub-groups (P1–P8) based on the population served (≤ 100; 101–250; 251–500; 501–1000; 1001–3000; 3001–5000; 5001–10,000; and 10,000+, respectively). The DBPs with probable and possible carcinogenic forms were estimated. Overall, 31.1% of WSSs were GWP, in which averages of THMs and HAAs were 32.2 and 27.7 μg/L, respectively, while the SWP had averages of THMs and HAAs of 97.6 and 129.2 μg/L, respectively. The very small WSSs (P1–P3) of GWP had averages of THMs and HAAs in the ranges of 29.1–43.5 and 15.8–64.3 μg/L, respectively. The P1–P3 of SWP had averages of THMs and HAAs in the ranges of 92.6–112.8 and 108.0–154.0 μg/L, respectively, which often exceeded the Canadian guideline limits. If the samples represented the populations homogenously, the total populations exposed to THMs or HAA₅ above the guideline values would be in the range of 132.08–181.38 in thousands (30.3–41.6% of total populations). The probable and possible carcinogenic forms of THMs in GWP and SWP were in the ranges of 4.8–48.8 and 4.4–7.0% of THMs, respectively. In HAAs, carcinogenic forms were in the ranges of 82.6–98.4 and 97.6–98.7%, respectively. The findings indicated that the SWP might need further attention to better protect human health.