Dissolved Organic Carbon Concentrations and Compositions, and Trihalomethane Formation Potentials in Waters from Agricultural Peat Soils, Sacramento-San Joaquin Delta, California: Implications for Drinking-Water Quality

By Roger Fujii, Anthony J. Ranalli, George R. Aiken, and Brian A. Bergamaschi



Water-Resources Investigations Report 98-4147

Sacramento, California 2001

Prepared in cooperation with the California Department of Water Resources

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Water exported from the Sacramento-San Joaquin River delta (Delta) is an important drinking-water source for more than 20 million people in California. At times, this water contains elevated concentrations of dissolved organic carbon and bromide, and exceeds the U.S. Environmental Protection Agency's maximum contaminant level for trihalomethanes of 0.100 milligrams per liter if chlorinated for drinking water. About 20 to 50 percent of the trihalomethane precursors to Delta waters originates from drainage water from peat soils on Delta islands. This report elucidates some of the factors and processes controlling and affecting the concentration and quality of dissolved organic carbon released from peat soils and relates the propensity of dissolved organic carbon to form trihalomethanes to its chemical composition.

Soil water was sampled from near-surface, oxidized, well-decomposed peat soil (upper soil zone) and deeper, reduced, fibrous peat soil (lower soil zone) from one agricultural field in the west central Delta over 1 year. Concentrations of dissolved organic carbon in the upper soil zone were highly variable, with median concentrations ranging from 46.4 to 83.2 milligrams per liter. Concentrations of dissolved organic carbon in samples from the lower soil zone were much less variable and generally slightly higher than samples from the upper soil zone, with median concentrations ranging from 49.3 to 82.3 milligrams per liter.

The dissolved organic carbon from the lower soil zone had significantly higher aromaticity (as measured by specific ultraviolet absorbance) and contained significantly greater amounts of aromatic humic substances (as measured by XAD resin fractionation and carbon-13 nuclear magnetic resonance analysis of XAD isolates) than the dissolved organic carbon from the upper soil zone. These results support the conclusion that more aromatic forms of dissolved organic carbon are produced under anaerobic conditions compared to aerobic conditions. Dissolved organic carbon concentration, trihalomethane formation potential, and ultraviolet absorbance were all highly correlated, showing that trihalomethane precursors increased with increasing dissolved organic carbon and ultraviolet absorbance for whole water samples. Contrary to the generally accepted conceptual model for trihalomethane formation that assumes that aromatic forms of carbon are primary precursors to trihalomethanes, results from this study indicate that dissolved organic carbon aromaticity appears unrelated to trihalomethane formation on a carbon-normalized basis. Thus, dissolved organic carbon aromaticity alone cannot fully explain or predict trihalomethane precursor content, and further investigation of aromatic and nonaromatic forms of carbon will be needed to better identify trihalomethane precursors.

Table of Contents



Description of Study Site and Study Design

Hydrologic Framework for Study Design

Sampling and Analytical Approach

Dissolved Organic Carbon Concentrations

Upper Soil Zone

Lower Soil Zone

Drainage Ditch

Composition and Reactivity of Dissolved Organic Carbon

Specific Ultraviolet Absorbance

Fractionation of Dissolved Organic Carbon into Hydrophobic and Hydrophilic Acids

Trihalomethane Formation Potential

Upper and Lower Soil Zones

Trihalomethane Formation Potential of Isolated Fractions

Summary and Conclusions


Summary of Supporting Observations

Dissolved Organic Carbon Concentrations

Quality and Composition of Dissolved Organic Carbon

Trihalomethane Formation

Other Significant Observations

Implications of Study Results

References Cited

Appendix A: Field and Sample Collection Methods

Installation of Samplers

Sample Collection and Field Measurements

Appendix B: Dissolved Organic Carbon and Ultraviolet Absorption Measurements

Appendix C: Dissolved Organic Carbon Fractionation and Isolation

Appendix D: U.S. Geological Survey Trihalomethane Methods


The Chemistry of Disinfection By Product Formation

Methods Employed by the U.S. Geological Survey Laboratory for Trihalomethane

Formation Potential Analysis

Appendix E: Carbon-13 Nuclear Magnetic Resonance Analyses



Appendix F: Preliminary Data for Wetland-Habitat Ponds


Water Resources of California

U.S. Department of the Interior, U.S. Geological Survey
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