Submitted to: Chemosphere
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/22/2005
Publication Date: 8/10/2005
Citation: Chow, A.T., Guo, F., Gao, S., Breuer, R.S. 2005. Size and xad fractionations of trihalomethane precursors from soils. Chemosphere, 62 pp 1636-1646. Interpretive Summary: Soil organic matter is an important source of allochthonous dissolved organic matter inputs to the Sacramento-San Joaquin Delta waterways, which is a drinking water source for 22 million people in California, USA. Knowledge of trihalomethane (THM) formation potential of soil-derived organic carbon is important for developing effective strategies for organic carbon removal in drinking water treatment. In this study, soil organic carbon was extracted with electrolytes (deionized H2O and Na- or Ca-based electrolytes) of electrical conductivity bracketing those found in Delta leaching and runoff conditions. The extracts were physically and chemically separated into different fractions: colloidal organic carbon (0.45-0.l um), fine colloidal organic carbon (0.1-0.025 um), and dissolved organic carbon (DOC) (<0.025 um); hydrophobic acid (HPOC), transphilic acid, and hydrophilic acid. Two representative Delta soils, Rindge Muck (a peat soil) and Scribner Clay Loam (a mineral soil) were examined. Results showed that less than 2% of soil organic carbon was electrolyte-extractable and heterogeneous organic fractions with distinct THM reactivity existed. Regardless of soil and electrolytes, DOC and HPOA fractions were dominant in terms of total concentration and THMFP. The amounts of extractable organic carbon and THMFP were dependent on the cation and to a lesser extent on electrical conductivity of electrolytes. Along with our previous study on temperature and moisture effects on DOC production, we propose a conceptual model to describe the impacts of agricultural practices on DOC production in the Delta. DOC is mainly produced in the surface peat soils during the summer and is immobilized by accumulated salt in the soils. DOC is leached from soils to drainage ditches and finally to the Delta channels during winter salt leaching practices.
Technical Abstract: Two soils (peat and mineral soils) of contrasting texture and organic carbon contents were extracted with deionized H2O and four electrolytes (Na+ and Ca2+). Water extractable organic carbon was fractionated by different pore size filters into colloidal organic carbon (COC), Fine colloidal organic carbon (FCOC), and dissolved organic carbon (DOC) and fractionated by XAD resins into hydrophobic acid (HPOA), transphilic acid (TPHA), and hydrophilic acid (HPIA). Results suggested that reactivity of DOC and COC from the Rindge Muck were the same, but DOC was more reactive than COC from Scribner Clay Loam. HPOA from both soils had the highest reactivity in forming THM compared to other chemical fractions. The effect of electrolytes on the disbrituion of organic carbon fractions is minimal, although the pressence of divalent Ca can significantly reduce the amount of water extractable organic carbon (WEOC) from soils. Results suggested that, under natural leaching and runoff conditions where electrolytes with both monovalent and divalent cations are present, the DOC fraction (0.025 um or smaller) and HPOA fraction are the most reactive THM precursors.