Evaluation of water quality via determination of trihalomethanes in water distribution networks of Riyadh city in Kingdom of Saudi Arabia

Trihalomethane (THM) has been classified as a possible human carcinogen by the International Agency for Research on Cancer (IARC). This study was conducted to assess the levels of trihalomethane compounds in the Riyadh city water network in two different years (2015 and 2020) and to understand the factors affecting the formation of these compounds. The study covered 15 sites in Riyadh city, distributed between the north, south, east, west and middle of the city. The results of the study indicated that the sum of the ratio of the concentration of each THM compound to its respective World Health Organization (WHO) guideline value not exceed 1 (unitless) where the combined toxicity of all total trihalomethanes THMs concentrations did not exceed the permissible limits, with values ranging between 0.01 and 0.24 at all study sites. These results were less than the recommended guide values form WHO for individual maximum concentrations of each of the THM in water for human consumption. In addition, the average concentration of free chlorine was 0.04 ppm and 0.09 ppm in 2015 and 2020, respectively, indicating that the disinfection had evolved in 2020 compared to 2015. Moreover, there was a weak relationship between trihalomethane formation and the presence of bromide ions and organic carbon, whereas a positive correlation was observed with free chlorine and pH values. The concentration of heavy metals such as cadmium, arsenic, cobalt and chromium were analysed by inductively coupled plasma-optical emission spectroscopy. The results indicated that all heavy metals were within the standard limits: the maximum values were 123 ppb in 2015 and 70.39 ppb in 2020, with lead having the lowest concentration and aluminium having the highest concentration in both years. Furthermore, the ion chromatography results of the cations (sodium Na +/-, potassium K +/-, calcium Ca2+and magnesium Mg2+) and the anions (fluoride F-, bromide Br-, chloride Cl-, sulphate SO4 2- and nitrate NO3-) indicated that the water in this study was suitable for human consumption based on the WHO regulation and the Ministry of Environment, Water and Agriculture (MEWA) standards for drinking water.