THE GEOCHEMISTRY OF NATURAL WATERS

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2. LEARNING OBJECTIVES. Continue learning how to classify and name organic compounds of relevance to aqueous geochemistry (aromatic hydrocarbons, halogenated hydrocarbons, alcohols, ethers, aldehydes, ketones, etc.).Learn about the behavior of organic contaminant plumes (petroleum, halogenated hydr

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THE GEOCHEMISTRY OF NATURAL WATERS

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1. 1 THE GEOCHEMISTRY OF NATURAL WATERS STRUCTURES, PROPERTIES, AND OCCURRENCE OF ORGANIC COMPOUNDS IN NATURAL WATERS - II CHAPTER 6 - Kehew (2001) Aromatic hydrocarbons

2. 2 LEARNING OBJECTIVES Continue learning how to classify and name organic compounds of relevance to aqueous geochemistry (aromatic hydrocarbons, halogenated hydrocarbons, alcohols, ethers, aldehydes, ketones, etc.). Learn about the behavior of organic contaminant plumes (petroleum, halogenated hydrocarbons). Some of the most dangerous contaminants in many natural waters are organic compounds such as chlorinated hydrocarbons, polychlorinated biphenyls (PCB’s), polycyclic aromatic hydrocarbons (PAH’s), dioxins, tert-butyl ether (the gasoline additive), pesticides, etc. To understand how these chemicals behave in the environment and to develop effective remediation strategies, we must understand the chemical and physical properties of organic compounds. Also, in Lecture 10 we saw that organic carbon is the most important electron donor in most natural waters. Finally, natural organic matter (humic acids, fulvic acids, and others) can play an important role in regulating metal mobility and the behavior of contaminant organic compounds. Thus, in this and subsequent lectures, we will learn some aqueous organic chemistry. Some of the most dangerous contaminants in many natural waters are organic compounds such as chlorinated hydrocarbons, polychlorinated biphenyls (PCB’s), polycyclic aromatic hydrocarbons (PAH’s), dioxins, tert-butyl ether (the gasoline additive), pesticides, etc. To understand how these chemicals behave in the environment and to develop effective remediation strategies, we must understand the chemical and physical properties of organic compounds. Also, in Lecture 10 we saw that organic carbon is the most important electron donor in most natural waters. Finally, natural organic matter (humic acids, fulvic acids, and others) can play an important role in regulating metal mobility and the behavior of contaminant organic compounds. Thus, in this and subsequent lectures, we will learn some aqueous organic chemistry.

3. 3 AROMATIC HYDROCARBONS Include the most common contaminants in ground water, especially near petroleum spills. Aromatic compounds are unsaturated. Most contain the benzene ring (C6H6) The electrons forming the double bonds are not fixed to specific carbon atoms. Instead, they are delocalized.

4. 4 BTEX Benzene, toluene, ethylbenzene, xylene. Closely associated aromatic hydrocarbons.

5. 5 POLYCYCLIC AROMATIC HYDROCARBONS (PAH’S) Compounds consisting of linked benzene rings. Occur at coal gasification, wood-preservation and oil refinery sites, as well as from underground tank and pipeline releases. Low solubilities and high Kd values mean they are not very mobile. Exceed regulatory limits at many sites. Also called polynuclear aromatic compounds. Tend to be carcinogenic.

6. 6 EXAMPLES OF PAH’s

7. 7 PETROLEUM HYDROCARBONS IN GROUND WATER Most commonly detected contaminants. Subtitle I of the Resource Conservation and Recovery Acta (RCRA) requires that underground storage tank (UST) owners in US to upgrade, close or replace all tanks. As of March 1996, 300,000 confirmed cases of leakage from UST’s, mostly involving petroleum, had been reported.

8. 8 BEMIDJI, MINNESOTA Site of best documented petroleum-contaminated site; pipeline rupture. Crude oil consisted of 58-61% saturated hydrocarbons, 33-36% aromatics, 4-6% resins (including compounds containing O, N, and S), and 1-2% asphaltenes.

9. 9 CRUDE OIL DISTILLATES

10. 10 CONTAMINANT PLUMES AT PETROLEUM RELEASE SITES - I An LNAPL may accumulate near the top of the capillary fringe. This may depress the water table and migrate down-gradient. Organic compounds dissolve in ground water and form plumes extending farther down-gradient than the LNAPL. Behavior of organic compounds depends on their Kow value, solubility, molecular weight, Henry’s constant and susceptibility to biodegradation.

11. 11 CONTAMINANT PLUMES AT PETROLEUM RELEASE SITES - II Of BTEX compounds, benzene is in relatively higher concentration near the leading edge of plume owing to its higher solubility and lower Kow. Toluene is more susceptible to biodegradation. It may decrease in concentration rapidly in the plume if there are sufficient electron acceptors present.

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