Contents and leaching of trihalomethane precursors in soils

Aquatic humic substances can react with chlorine during water treatment to form trihalomethanes (THMs). Soils are important contributors to THM precursors in raw waters. An investigation into the trihalomethane formation potentials (THMFPs) of soils in South China's Guangdong Province was conducted. The results show that the bulk THMFPs (b-THMFP) of aqueous soil extracts range from 0.7 to 36.8 μg g^-1 with a median value of 10.6 μg g^-1, and the THMFPs of dissolved organic matter (d-THMFP) in the 0.45 μm filtered aqueous soil extracts range from 0.5 to 21.2 μg g^-1 with a median value of 3.9 μg g^-1 of soil. Approximately 86% of the 34 soil samples had b-THMFP values ≤20 μg g^-1 and 79% had d-THMFP values ≤10 μg g^-1. The finding that majority of d-THMFP/b-THMFP ratios are less than 0.5 indicates that suspended organic matter in aqueous extract has greater contribution to the bulk THMFP. The results of soil column study indicate that THMFPs of effluents from samples 1, 15 and 21 were higher than 100 or 50 μg L^-1 at various leaching acidities. THMFPs concentrations in leachates increase with increases in the amounts of humic substances in soils. THM presursors appear to more easily flow out with soil water flow under lower acidic leaching conditions. In most cases, THMFPs concentrations in leachates showed decline with the increases of cumulative leaching volumes of feed solutions.