Origin of trihalomethane (THM) precursors in Kurose River, one of the sources of potable water in Hiroshima Prefecture, were investigated. THM formation potential (THMFP) at different stations of the river and the contribution from point and nonpoint sources within the river basin were determined. Contributions from both wet and dry precipitations were also evaluated. UV 260, DOC and THMFP were relatively high in water samples collected from stations where inflows of industrial effluents, domestic wastewater and agricultural drains were in the vicinity. Low values were noted at stations near wooded areas. However significant increase in UV 260, DOC and THMFP were observed in the presence of stormwater runoffs which is attributed to water soluble organic substances extracted from forest litter and top soil. The results indicate that the THM precursors in Kurose River could be attributed mainly to industrial effluents, domestic wastewater and agricultural drains. In case of rain, stormwater runoffs contributed significant amounts of THM precursors to the river.

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