**Acknowledgements**

these metals is also impacted by the prevailing redox conditions at a site. A decrease in pH could also inhibit growth of bacteria communities such as *Dehalococcoides*, thereby stopping

Changes in redox conditions can also enhance solubilization of metals and promote the formation of the following undesirable products (e.g., hydrogen sulfide and methane gases): **1.** If nitrate is used, byproducts including nitrite, nitric oxide, nitrous oxide, and nitrogen gas could be generated. The predominant byproduct depends on the enzymes possessed

**2.** Iron(II) is more soluble than iron(III), so iron reduction could lead to exceedance of iron

**3.** Sulfate is reduced to sulfide under anaerobic conditions. If there are not enough dissolved metals to precipitate metal sulfides, free sulfide and hydrogen sulfide will be generated. Sulfide is toxic to microbial communities and could inhibit degradation of contaminants.

**1.** Bioremediation is widely used in groundwater and soil remediation of organic contami‐ nants such as chlorinated solvents and petroleum hydrocarbons. Additives play an essential role in stimulating microorganism growth and in accelerating contaminant

**2.** Common additives used in engineered bioremediation include organic carbon, oxygen, nutrients, and pH modifiers. Organic carbon substrate is the most important and widely used additives. A wide-range of materials can be used as carbon substrates. These substrates can be generally categorized into soluble, slow-release, and solid substrate

**3.** The use of additives in engineered bioremediation systems depends on the physical properties of additives, site characteristics, treatment goals, and other factors. Additives are usually added to the subsurface environment through direct injection, recirculation,

**4.** Despite the success of bioremediation technology applications in groundwater remedia‐ tion, a number of issues have been identified with the use of additives in bioremediation. These include biofouling, stalling, system bypassing or short circuiting, reduction in hydraulic conductivity, contaminant plume displacement and dilution, and pH and secondary water quality issues. Taking these issues into consideration during remediation design and properly addressing these issues during implementation is essential for

**5.** To enhance the efficiency and promote the application of bioremediation technology in contaminant remediation at contaminated sites, a number of research needs related to

degradation in engineered bioremediation systems.

efficient and cost-effective site cleanup.

the bioremediation process.

160 Advances in Bioremediation of Wastewater and Polluted Soil

by the microbes present.

water quality criteria.

**6. Conclusions**

subgroups.

and trenching.

The authors are indebted to Drs. Ralph Ludwig, Todd Luxton at USEPA, and an anonymous reviewer whose comments helped to improve the manuscript. This review does not reflect EPA's policy.
