**1. Introduction**

Bioremediation is an important type of remediation technology that uses microorganisms (mainly bacteria) to destroy hazardous contaminants or transform them to less harmful forms.

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Microorganisms, through their enzymatic pathways, act as biocatalysts and facilitate the progress of biochemical reactions that degrade the targeted contaminants. Bioremediation has been used in the cleanup of organic (e.g., chlorinated solvents, petroleum hydrocarbons, and pesticides), inorganic (e.g., perchlorate and nitrate), metals, and radionuclides contaminated sites [1-3].

Because of the role microorganisms play in bioremediation, any factors impacting surviv‐ al and growth of these organisms will impact bioremediation. Although microorganisms have been isolated even under extreme conditions, most of them grow optimally over a narrow physical/chemical/biological range. Thus, to effectively and efficiently cleanup a contaminated site through bioremediation, it is important to achieve optimal biogeochemi‐ cal conditions for microbial communities [4]. The conditions to consider include site hydrogeological characteristics, contaminant concentration, pH, redox potential, nutrients, moisture, and temperature [5-7].

In some cases, natural conditions at a contaminated site can provide all essential materials in large enough quantities that bioremediation can occur without human intervention. This process is often referred as intrinsic bioremediation. This is the primary degradation mecha‐ nism behind natural attenuation. However, under the natural conditions of most sites, microorganisms that degrade contaminants may be naturally present in the subsurface, but may not necessarily be there in sufficient quantities required for optimal bioremediation of the site. In these cases, engineered bioremediation (also termed as biostimulation) is needed. Engineered bioremediation relies on accelerating desired biodegradation reactions by encour‐ aging growth of target microorganisms, as well as by optimizing the environment in which the organisms must carry out the detoxification reactions [8]. Engineered bioremediation involves the addition of substrates (electron donors), nutrients, and/or other materials (e.g., pH buffers) into the subsurface to stimulate microbial growth and activity or establish supportive geochemical conditions.

This chapter focuses on the use of additives to stimulate bioremediation of organic contami‐ nants (e.g., chlorinated solvents, petroleum hydrocarbons, and pesticides). Following the introduction, the second section of the chapter discusses major reaction pathways in biorem‐ diation. The third section focuses on the major types of additives and the fourth section discusses in detail how the additives are implemented. The fifth section deals with potential issues associated with additives use in bioremediation. The final section summarizes the chapter and identifies future research needs.
