Lecture No. 26. Chlorinated Organics in Groundwater

Among the contaminants that find their way into groundwater are chlorinated organics. Some originate in leaking underground storage tanks ("LUST"), while others originate in long-term surface spills, industrial sites, landfills, poor application practices used with pesticides, etc. Some examples:

Solvents and their breakdown products:

Fungicides and pesticides:

Most hydrocarbons (other than asphaltenes) are less dense than water and so tend to float. Most are only sparingly soluble in water, so they form "light non-aqueous phase liquids" (LNAPLs). On the other hand, most of the chlorinated compounds are denser than water and so form "dense non aqueous phase liquids" (DNAPLs) that sink to the bottom of an aquifer. Since most of them are sparingly soluble in water, a common situation is a concentration of a few parts per million dissolved in water and one or more pools of pure DNAPL at the bottom that act as sources and continue to contaminate fresh groundwater moving through the aquifer. Adsorption of NAPL to aquifer sediments is also important.

John Cherry (Waterloo University, Ontario) describes the common DNAPL contaminant plume in groundwater as a core with relatively high, maybe saturated, concentration of contaminant, surrounded by a fringe with only a few ppm concentration. In most cases investigators never find the core because they do not drill enough test wells. The core flows away from the source, which can be one or more DNAPL pools. If only the dissolved contaminant is treated, the source will keep on supplying more contaminant, and depletion of the source zone may take decades or even centuries.

Mechanisms of Biodegradation

Some chlorinated organics degrade aerobically and some anaerobically.

Aerobic Mechanisms

  1. Direct Metabolism: In a few cases the chlorinated compounds are metabolized directly by microorganisms. They oxidize the carbon and excrete the chlorine as inorganic chloride. This is fairly uncommon, because there is little or no energy to be gained by oxidizing a carbon-chlorine bond.
  2. Aerobic Cometabolism: This is a more common mechanism. A microorganism that lives on a nonchlorinated organic produces an enzyme that happens to break down the chlorinated compound. The microbe gains nothing by the dechlorination; the process works because the microbe is inefficient. One of the best examples of this is the decomposition of one- and two-chlorine compounds by methanotrophs, bacteria that metabolize methane. One type of methanotroph produces a soluble version of the enzyme called methane monooxygenase (sMMO). There has to be just enough methane present in the water for the decomposition of the chlorinated compound to proceed at a reasonable rate. If there is not enough methane, the bacteria will not multiply. If there is too much methane the sMMO will tend to react with methane rather than with the target compounds. The methanogens that produce sMMO are nitrogen-fixers, so they are favored by nitrogen-deficient conditions; in the presence of high nitrate or ammonium concentrations the other strains will out-compete them.

Methanotrophs are microaerophilic, i.e., they do best in slightly oxygen-deficient (suboxic) conditions. Under oxygen-saturated conditions other aerobic species dominate.

Other types of bacteria that cometabolize chlorinated organics include toluene oxidizers, butane metabolizers, and ammonia oxidizers (nitrifying bacteria).

Anaerobic Mechanisms

These processes generally treat organic chlorine as an electron acceptor, i.e., as an oxidizing agent.

1. Anaerobic Cometabolism: In process similar to aerobic cometabolism, bacteria living on other electron acceptors (e.g., sulfate) produce soluble enzymes that happen to catalyze reductive dechlorination. Example:

H2 + C2HCl3 ¨ C2H2Cl2 + H+ + Cl–

Sulfate reducers, methanogens, and acetogens are important anaerobic cometabolizers.

2. Dehalorespiration: A diverse group of bacteria, none of which were known before 1993, use chlorinated organics as their principal electron acceptor. Example:

H2 + C2HCl3 ¨ C2H2Cl2 + H+ + Cl–

In some cases no one knows what these bacteria live on if no chlorinated organics are available. Some may be able to live by fermentation.

One species, Dehalococcus ethogens, needs vitamin B-12. Under cobalt-deficient conditions it cannot dechlorinate organics.

These first two types of reductive dechlorination or hydrogenolysis are essentially similar. The main distinction between them is based on whether the bacteria actually derive any benefit — either energy or anabolic carbon — from the dechlorination.

Besides the types of bacteria already mentioned, nitrate reducers and iron (III) reducers sometimes promote reductive dechlorination.

Dissolved hydrogen is important to many of these processes.

3. Dichloroelimination: Some bacteria promote the elimination of two chlorine atoms from one organic molecule, e.g.,

C2H2Cl4 ¨ C2H2Cl2 + Cl2

No one has come up with a definite explanation of why bacteria would produce chlorine. It may be a chemical defense for some species, just as many fungi and actinomycetes produce antibiotics as a chemical defense and the immune systems of mammals produce hypochlorous acid to kill bacteria.

4. Dehydrochlorination: This is an abiotic process, often catalyzed by bases, by which one hydrogen and one chlorine atom are eliminated from an organic molecule. It is not a redox process. An example is the conversion of vinyl chloride to acetylene:

CH2=CHCl ¨ CH¼ CH + H+ + Cl–

Intermediates can be very important, since a breakdown product may be more toxic than the original compound. For example, vinyl chloride is more toxic than trichloroethylene. If VC breakdown is slower than TCE breakdown, there will be a relatively high steady-state concentration of VC, and this may worsen the situation.

Soil Vapor Extraction (SVE) is commonly used to remove LNAPLs. It is of little use in removing DNAPLs, since they are usually under the water. However, if DNAPL's are still moving down through the vadose zone, SVE may be useful.

The Pump and Treat approach is a process whereby water is pumped out of an aquifer, treated by biological or chemical processes, and then discharged either to the aquifer of to surface waters. It is essential that the treated water meet all criteria for all pollutant concentrations, or it cannot legally be discharged.

This means that a company or agency that is interested in removing a pollutant for which it is responsible ends up cleaning up everyone else's mess as well. Suppose an aquifer that is already contaminated with high levels of nitrate from bad farming practices or high sulfate from bad mining practices is subject to chloroform contamination. A company using "pump and treat" to remove the chloroform will also have to clean up the nitrate or sulfate before discharging the water.

"Pump and treat" is an ex situ process and as such is relatively expensive. In situ bioremediation is preferable when possible. Supplying the native microorganisms with the nutrients they need and possibly with the redox conditions they require is generally cheaper, more efficient, and less disruptive to the natural system than ex situ treatment would be. Sometimes all that is needed is aeration; "air sparging" of aquifers is a common way to supply oxygen to aerobic bacteria. Sometimes hydrogen peroxide is used instead as an oxygen source, especially in low-conductivity aquifers. However, this discriminates in favor of peroxide-tolerant bacteria and against some efficient cometabolizers like methanotrophs, which are poisoned by peroxide.

Reactive wells have gotten a good deal of attention in the past 5 years or so as a way to remove certain inorganic contaminants and as a way to establish suitable anoxic conditions for reductive dechlorination. Jim Spain at Batelle Pacific Northwest Laboratories is a leading researcher in this field. The most commonly described reactive well is one in which the usual gravel packing around the screened interval is replaced by iron shot.

 

Reactive well (Vertically compressed diagram)

 

Water is pumped into the well through the screened interval where it contacts the iron and then back out into the aquifer through another screened interval. The iron metal reacts with inorganic oxidizing agents and reduces then to less harmful species. For example, chromate and nitrate are reduced respectively to chromium (III) and to ammonia or dinitrogen:

2 CrO42– + 3 Fe + 8 H2O ¨ 2 Cr3+ + 3 Fe2+ + 16 OH–

NO3– + 4 Fe + 6 H2O ¨ 4 Fe2+ + NH3 + 9 OH–

2 NO3– + 5 Fe + 6 H2O ¨ N2 + 5 Fe2+ + 12 OH–

Alloy steels tend to produce ammonia from nitrate, whereas mild steel or pig iron shot produces N2.

For remediation of chlorinated organics, an important reaction is the reduction of water by iron:

Fe + 2 H2O ¨ Fe2+ + H2 + 2 OH–

As was already pointed out, methanogens and other bacteria that promote reductive dechlorination require hydrogen.

Case Study: Degradation of PCA in a Freshwater Tidal Wetland

(Lorah, Michelle M. and Lisa D. Olsen, 1999, "Degradation of 1,1,2,2-tetrachloroethane in a freshwater tidal wetland: field and laboratory evidence," Environmental Science and Technology 33(2), 227-234.)

A semi-confined aquifer at Aberdeen Proving Ground, Maryland, is contaminated with 1,1,2,2-tetrachloroethane (PCA). PCA was the first chlorinated solvent to be widely used in the years before World War I. It was later replaced by solvents that were considered less toxic, such as PCE and TCE.

The contaminated aquifer is oxic until the water approaches the discharge area in a tidally-influenced freshwater marsh. The water then moves into iron-reducing, anoxic-sulfidic, and finally anoxic-methanic regions. (See overhead.) In the reducing regions, PCA was found to be degraded anaerobically.

Three mechanisms were considered possible:

  1. Hydrogenolysis, which would produce in turn 1,1,2-trichloroethane, 1,2-dichloroethane, chloroethane, and finally ethane.
  2. Dichloroelimination, which would produce cis- and trans-1,2-dichloroethylene from PCA or vinyl chloride from 1,1,2-trichloroethane.
  3. Dehydrochlorination, which would produce 1,1,2-trichloroethylene.

Field and laboratory (microcosm) studies were both performed.

In the field study, 1,1,2-trichloroethane and 1,2-dichloroethane appeared simultaneously with 1,2-dichloroethylene, indicating that both hydrogenolysis and dichloroelimination were at work. Both processes occurred in the iron-reducing region and continued in the more reducing sulfidic and methanic zones.

In the laboratory study, microcosms containing groundwater and wetland sediments under methanogenic conditions were incubated. In this case the predominant products of degradation were 12DCE and VC, indicating that dichloroelimination of PCA and possibly hydrogenolysis of 12DCE were the main pathways. TCE was not detected in the laboratory study, which indicated that dehydrochlorination of PCA was probably not occurring.

To previous lecture: Lecture No 25. Special Topics III

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