Filter pore size selection for characterizing dissolved organic carbon and trihalomethane precursors from soils
Introduction
The Sacramento-San Joaquin Delta is a source of drinking water for 22 million people in California, USA. There is a significant increase in organic carbon when water passes through the Delta. High organic carbon in water is of concern because it is a precursor of carcinogenic disinfection by-products, such as trihalomethane (THM). The increase in organic carbon or THM precursors in Delta waters is partially attributed to agricultural drainage in the Delta, which is estimated to contribute an average of 36 Mg C day−1 into Delta waterways (Amy et al., 1990; Jassby and Cloern, 2000). Organic-rich peat soils in the area have been identified as a major source of organic carbon and THM precursors in the drainage waters (California Department of Water Resources, 1994; Fujii et al., 1998). Thus, understanding the chemical characteristics of organic carbon leaching from Delta soils is essential for water treatment facilities that use Delta waters as a drinking water source.
Conventional filtration commonly uses a pore size filter as a cutoff point for defining dissolved organic carbon (DOC). However, a pore size of is so large on a colloidal scale that heterogeneous organic fractions possibly exist in filtrates (Thurman, 1985). Studies have shown that different size organic matter fractions, such as particulate organic matter, colloidal organic matter, and dissolved organic matter have different chemical properties (Guo et al., 2003). We speculate that these fractions may also have different reactivity in forming THM during chlorination. Importantly, inconsistent results have been observed in THM research when using filtrates (Chow et al., submitted). The heterogeneity in filtrates is probably one of the main reasons for the inconclusive results. While most research focuses on THM precursors in the fraction (Owen et al., 1993), a recent study showed that suspended organic matter from soil extracts likely serves as an important pool of THM precursors (Li et al., 2003). The reactivity from organic matter in THM formation has seldom been studied. Thus, organic matter should be further divided into more homogeneous fractions for analysis. One objective of this study was to evaluate THM reactivity and chemical characteristics of filtrates from different pore-size filters with a special focus on application to studies of organic carbon from Delta soils.
The amounts of water extractable organic carbon (WEOC) and THM precursors in soils are probably proportional to the content of soil organic carbon (SOC). Environmental factors and farming practices also control the partitioning of WEOC and THM precursors with SOC. Current agricultural practices in the Sacramento-San Joaquin Delta include intensive summer irrigation and winter flooding. Soil salinity and sodicity increase in summer due to evapotranspiration of soil water, whereas soil salinity and sodicity decrease in winter due to salt leaching. The change in soil water composition affects soil chemistry, and eventually affects the amount of WEOC and THM precursors leaching from the soils. A controlled laboratory study conducted by Chow et al. (2003) showed that an increase in salinity significantly decreased DOC concentrations in soil water from agricultural peat soils in the Delta. The specific UV absorbance at 254 nm (SUV254), a surrogate for THM precursors, decreased with an increase in salinity and Ca2+ concentrations. This study only examined the dissolved phase of organic soils passing a filter. Other physical fractions of organic soils as well as organic carbon from mineral soils were not studied. In this study, we further examine salt effects on the solubility of different size organic carbon fractions along with the contributions of organic carbon from a mineral soil. Results from the study contribute to a better understanding of the role of agricultural soils in the production of THM precursors and the carbon budget of Delta waterways.
Section snippets
Soil sampling and characterization
Soil samples from Twitchell Island (N38°07′02.3′′, W121°38′20.5′′) and Webb Tract (N38°04′01.5′′, W121°35′41.6′) in the west-central Delta were collected for this study. The soil from Twitchell Island is classified as a Scribner Clay Loam (Cumulic Endoaquoll) and the soil from Webb Tract is classified as a Rindge Muck (Typic Haplosaprist) (USDA, 1977; USDA, 1993). These two soils were selected because they are representative of major mineral and organic soils in the Delta. Soil samples were
General soil properties—Rindge Muck vs. Scribner Clay Loam
Table 1 presents some general physical and chemical properties of the soils used in this study. First, significant differences in SOC and CEC are noted between the two soils. The Clay Loam contained 4% SOC and had a CEC of 37 meq 100 g soil−1. In contrast, the Rindge Muck had much higher SOC and CEC, 23% and 149 meq 100 g soil−1, respectively. Second, the soil salinity of Rindge Muck (2.1 dS m−1) was five times higher than that of the Clay Loam (0.4 dS m−1). Although high soil salinity may reduce the
Conclusion
Water extracts of representative organic and mineral soils from the Sacramento-San Joaquin Delta were filtered through a series of pore size filters (1.2, 0.45, 0.1 and 0.025 μm) and filtrates examined for TOC, UV254, and THMFP. Results showed that the conventionally defined DOC potentially contains heterogeneous organic fractions, such as COC. COC was particularly important in extracts from the Clay Loam and it caused high turbidity that resulted in abnormally high UV254 absorbance.
Acknowledgment
The authors gratefully acknowledge financial support for this project from the California State Water Contractors. Alex Chow was also supported in part by an NIGMS MORE Institutional Research and Academic Career Development Award to UC Davis and San Francisco State University, http://prof.ucdavis.edu, Grant no. K12GM00679.
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