Groundwater Contamination by Organic Chemicals

The contamination of groundwater by organic chemicals is a major concern. Many organic substances decay rapidly or are immobilized in the soil, so the number of compounds that are sufficiently persistent and mobile to travel to the water table and to contaminate poundwater there is relatively small.

The compounds that are most often detected in groundwater-based U.S. community public water supplies, including those near hazardous waste sites, are summarized in Table 14-1. Municipal landfills as well as industrial waste

Organic Compounds Commonly Found in U.S. Groundwater-Based

Community Water Supplies

and Their Properties

Chemical

Density (g/mL)

Water Solubility (g/L)

Present at 25-50% of sites:

Chloroform (trichloromethane)

1.48

8.2

Bromodichloromethane

1.98

4.4

Dibromochloromethane

2.45

2.7

Bromoform (tribromomethane)

2.89

3.0

Present at a smaller fraction of sites:

Trichloroethene

1.46

1.1

Tetrachloroethene (perchloroethene)

1.62

0.15

1,1,1 'Trichloroethane

1.34

1.5

1,2-Dichloroethenes

1.26, 1.28

3.5, 6.3

1,1 -Dichloroethane

1.18

5.5

Carbon tetrachloride

1.46

0.76

Dichloroiodomethane

1.58

Xylenes

0.86-0.88

0.18 (o)

1,2-Dichloropropane

1.16

2.8

Benzene

0.88

1.8

Toluene

0.87

0.54

Also commonly present at wells close to

hazardous waste sites:

Methylene chloride

1.33

20

Ethylbenzene

0.87

0.15

Acetone

0.79

sol

1,1 -Dichloroethene

1.22

2.3

1,2-Dichloroethane

1.24

8.5

Vinyl chloride (chloroethene)

gas

8.8

Methyl ethyl ketone

0.80

268

Chlorobenzene

1.11

0,47

1,1,2 -Trichloroethane

1.44

4.5

Chloroethane

0.90

5.7

Fluorotrichloromethane

gas

1.1

1,1,2,2-Tetrachloroethane

1.60

2.7

Methyl isobutyl ketone

0.80

19

Source; Based on U.S. EPA surveys of about 2% of U.S. water supplies.

Source; Based on U.S. EPA surveys of about 2% of U.S. water supplies.

disposal sites are often the source of the contaminants. Liquid that contains dissolved matter that drains from a terrestrial source, such as a landfill, is called a leachate. In rural areas, the contamination of shallow aquifers by organic pesticides, such as atrazine (Chapter 10) leached from the surface, has become a concern. The insecticide dieldrin (Chapter 10), which has been banned since 1992, is the pesticide found most often to exceed human-health guideline levels in U.S. groundwater. Ironically, shallow groundwater aquifers used to supply drinking water are often more polluted by pesticides at greater than acceptable levels than are those in agricultural areas in the United States.

The typical organic contaminants in most major groundwater supplies are:

• Chlorinated solvents, especially trichloroethene (TCE, "trie," also called trichloroethykne), C2.HCI3, and perchlorocthene (PCE, "perc," also called perchbroethylene or tetrachbroethene), C2C14. These molecules contain a C=C bond, with three or four of the four hydrogen atoms of ethene (ethylene) replaced by chlorine:

cr xi ci xi

TCE PCE

By a large margin, chlorinated solvents are the most prevalent organic pollutants in groundwater.

• Hydrocarbons from the BTX component of gasoline and other petroleum products: benzene, C6H6, and its methylated derivatives toluene, C6H5(CH3), and the three isomers of xylene, CgH^CHj^. (See Chapter 7 for structures.)

• MTBE (methyl tertiary-butyl ether) from gasoline (see Chapter 8)

The chemicals in the groups mentioned above occur commonly in groundwater at sites where manufacturing and/or waste disposal occurred, especially from 1940 to 1980. In that period, little attention was paid to the ultimate fate and residence following the in-ground injection of these chemicals. The sources of these organic substances also include leaking chemical waste dumps, leaking underground gasoline storage tanks, leaking municipal landfills, and accidental spills of chemicals on land.

Trichloroethene is an industrial solvent, used to dissolve grease on metal, as is perchloroethene. The U.S. MCL for TCE in drinking water is 5 ppb, and the same limit is now used in Canada as well. A 2006 report by the U.S. National Academy of Sciences concluded that TCE is a possible cause of kidney cancer, can impair neurological function, and can cause reproductive and developmental damage. A link between TCE exposure and an abnormally low sperm count in males has been established. The International Agency for Research on Cancer has classified TCE as "probably carcinogenic to humans."

PCE is used not only in metal degreasing but also finds wide application as the solvent in dry-cleaning operations, so it is released from a large number of small sources. A group of women in Cape Cod, Massachusetts, who were inadvertently exposed over several decades to high levels of PCE in their drinking water were found to have small to moderate increases in their risk of contracting breast cancer.

Gasoline enters the soil via surface spills, leakage from underground storage tanks, and pipeline ruptures. Before 1980, underground gasoline storage tanks were made from steel; almost half of them were sufficiently corroded to leak by the time they were 15 years old. Once they descend to groundwater, the water-soluble components of the gasoline are preferentially leached into the water and can migrate rapidly in the dissolved state. The BTX component, which is the most soluble of the hydrocarbons, often occurs at concentrations of 1-50 ppb in groundwater. However, the alkylated benzenes are rapidly degraded by aerobic bacteria and consequently are not long-lasting.

The MTBE component of gasoline (Chapter 7) is more water-soluble than the hydrocarbons, but unlike them, it is not readily biodegraded. It is not highly toxic. The main problem is the odor and taste that it gives to water; as little as 15 ppb in water can be tasted or smelled. MTBE contamination of well water, albeit at low levels, has become of concern in the United States since it has occurred at about a quarter of a million sites.

The Ultimate Sink for Organic Contaminants in Groundwater

The subsequent behavior of the organic compounds that do migrate to the water table depends significantly upon their density relative to that of 1.0 g/mL of water. Liquids that are less dense ("lighter") than water and have low solubility in it form a mass that floats on the top of the water table. All hydrocarbons having a small or medium molecular mass belong to this group, including the BTX fraction of gasolines and other petroleum products (see Table 14-1). In contrast, polychlorinated solvents are more dense ("heavier") than water and insoluble in it, so they tend to sink deeply into aquifers; important examples are methylene chloride, chloroform, carbon tetrachloride, 1,1,1 'trichloroethane, TCE, and PCE (see Table 14-1). Nonchlorinated but insoluble high-molecular-weight organic materials, such as creosote and coal tar, also belong to the heavier-than-water group. These substances are sometimes referred to as dense nonaqueous-phase liquids, DNAPLs.

Although the oily liquid blobs that these organic compounds form generally are found in an aquifer at a position either directly below their original point of entry into the soil or close to it, the implication that they are horizontally immobile is misleading. Very slowly—in a process that often takes decades or centuries to complete—these low-solubility compounds gradually dissolve in the water that passes over the blob and so provide a continuous supply of contaminants to the groundwater. The complete removal of such deposits usually is not feasible since they may exist as several blobs whose exact location is difficult to pinpoint. In addition, disturbance of the deposit during removal or treatment may increase its net exposure to the water phase. Even removing 90% of the substance does not necessarily result in reduction

FIGURE 14-6 The contamination of groundwater by organic chemicals.

of its groundwater concentration. Thus, plumes of polluted water grow, in the direction of the water's flow, and thereby contaminate the bulk of the aquifer (see Figure 14-6). Because of such contamination, many wells used for drinking water have had to be closed.

Decontamination of Groundwater: Physical and Chemical Processes

In the last two decades, considerable energy and money have been spent in the United States on attempts to control aquifer pollution by the oily liquids discussed above. Dense organic leachates, especially PCE and TCE, have contaminated the groundwater that lies below the waste sites associated with the U.S. Superfund remediation initiative (see Chapter 16). Unfortunately, no easy cure to the problem of contamination has been found. Control usually consists of pump-and'treat systems that pump contaminated water from the aquifer, treat it to remove its organic contaminants (using methods of the type described later in this chapter), and return the cleaned water to the aquifer or to some other water body. Alternatively, a fine mist of the contaminated groundwater is sprayed into the air above agricultural land using a long, moveable sprinkler system; the contaminant volatile organic compounds (VOCs) evaporate into the air and the cleansed water is used for irrigation.

Aquifer

Direction of ->

groundwater flow

Ground Water Chemistry

Surface source o • o • o • o • o • o • o •

Aquifer

Direction of ->

groundwater flow

The volume of water that must be pumped and treated in a given aquifer is huge. For organic contaminants with low water solubility, recontamination of water returned to the aquifer by additional dissolution from the blob will occur. Consequently, the treatment systems must operate in perpetuity, and there are already thousands of them spread across the United States.

Both in situ heating, to vaporize the organic liquids so their vapors rise to the soil surface, and the addition of oxidants to convert the substances to products such as carbon dioxide have been tried in some locations. Typically, temperatures close to 100°C are used in heating, though it is not known if this is optimal in most cases. Heating and/or the production of gases or precipitates by oxidation may inadvertently change the geologic and biological conditions in the immediate vicinity of the treatment, with unforeseen effects on the distribution and mobility of the remaining pollutant.

Continue reading here: Decontamination of Groundwater Bioremediation and Natural Attenuation

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    Can metal degreasing compounds contaminate groundwater?
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