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TECHNICAL REPORTS

Surface Water Quality

Trihalomethane Formation Potential of Filter Isolates of Electrolyte-Extractable Soil Organic Carbon

^a Hydrology Program, Dep. of Land, Air and Water Resources, Univ. of California, Davis, CA 95616
^b Office of Water Quality, Div. of Environ. Services, California Dep. of Water Resources, P.O. Box 942836, Sacramento, CA 94236-0001
^c Water Management Res. Lab., USDA-ARS, Parlier, CA 93648

^* Corresponding author (fguo{at}water.ca.gov)

Received for publication November 10, 2004. Certain organic C moieties of soil origin in drinking source waters of Sacramento–San Joaquin Delta (Delta) can react with chlorine to form trihalomethanes (THMs) during the disinfection process. Isolation and characterization of them and quantitation of their THM formation potential (THMFP) is necessary for developing effective strategies to reduce their influxes in Delta waters and for removing them during drinking water treatment. In this study, organic C from two Delta soils was extracted using deionized H₂O and four Na- or Ca-based electrolytes of varying electrical conductivity values. Extracts were filtered into particulate, colloidal, fine colloidal, and soluble organic C for quantitation and THMFP determination. Results suggested that <1.5% of soil organic C was electrolyte-extractable. The soluble organic C fraction from both soils dominated in quantity and THMFP. Electrolyte effects were cation dependent. Sodium-based electrolytes at either conductivity level did not significantly decrease extractable organic C (EOC) or THMFP compared with deionized H₂O. In contrast, Ca-based electrolytes reduced EOC and THMFP by >50% even at 1 dS m^–1. Further increase in Ca concentration did not significantly decrease EOC or THMFP. Most reduction in EOC and THMFP by Ca-based electrolytes occurred with the fractions other than the soluble organic C. Results suggested that under natural soil leaching and runoff conditions, the majority of THMFP is associated with organic C of <0.025 µm in diameter. Further molecular characterization of the fractions with high THMFP may help understand the nature of chlorine-reactive organic C from Delta soils.

Abbreviations: COC, colloidal organic carbon • DBPs, disinfection by-products • DOC, dissolved organic carbon • EOC, extractable organic carbon • FCOC, fine colloidal organic carbon • POC, particulate organic carbon • SOC, soluble organic carbon • THM, trihalomethane • THMFP, trihalomethane formational potential • TOC, total organic carbon

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