Trying to understand the grounds for the high trihalomethanes formation potential of four surface water bodies employed as source waters

Abstract

Because of their historically high records of disinfection by-products, four water bodies serving as sources for drinking water in four cities of Castilla y León, Spain, were selected for a further study. With moderate to high contents of organic matter, an important part of it corresponded to algal biomass, as confirmed by the enormous values of chlorophyll a (Chl a) – means of 130–140 μg.L−1 in two of them. These two water bodies presented a moderate to low aromatic character of the dissolved organic carbon (DOC), as indicated by their low values of specific ultraviolet absorbance (SUVA means of 1.61 and 1.89 L.mg−1·m), which suggests an important contribution of the algal-derived compounds to this DOC. Upon chlorination, three of the water systems studied presented mean values of trihalomethanes potential formation (THMFP) in the order of 250 μg.L−1, but the specific one (STHMFP) revealed great differences of reactivity between them and, moreover, waters with higher SUVA showed lower STHMFP. This confirms the hypothesis that non-aromatic structures can also be reactive with chlorine. To investigate if the reactivity of the global DOC increases as its algal-derived DOC becomes more abundant, THMFP was tested versus Chl a/DOC, but the regression coefficients did not show any tendency. As compared with other studies, the values for THMFP can be considered of moderate to high (of up to 370 μg.L−1), but those of STHMFP are among the highest ever reported (some of them above 125 μg.mg−1), which means very reactive organic matter and, moreover, in waters with a very low SUVA.