By Roger Fujii, Anthony J. Ranalli, George R. Aiken, and Brian A. Bergamaschi
U.S. GEOLOGICAL SURVEY
Water-Resources Investigations Report 98-4147
Sacramento, California 2001
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.
Abstract
Introduction
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
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
Background
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
Introduction
Results
Appendix F: Preliminary Data for Wetland-Habitat Ponds
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