Terminal Electron Acceptor Mass Balance: Light Nonaqueous Phase Liquids and Natural Attenuation

Light nonaqueous phase liquids (LNAPLs) in subsurface systems may contain a relatively large amount of biodegradable organic material. During the biochemical oxidation of the organic compounds in the LNAPL, electrons are transferred to terminal electron acceptors (TEA) [i.e., O2, NO3?, Mn(IV), Fe(III), SO4?2, CO2] via coupled redox reactions. A mass balance between the TEA required for mineralization of benzene, toluene, ethyl benzene, and xylene (BTEX) compounds contained in the subsurface (ground water, soil, LNAPL) and the total TEA available from the ground water and aquifer sediments is proposed and evaluated. The total TEA available is predominantly attributed to the solid phase material; the aqueous phase TEA constitutes a minor amount; and the TEA required for BTEX mineralization is predominantly from the LNAPL. Consequently, a TEA deficit exists in the LNAPL source area. Under these conditions, it may be invalid to assume an infinite supply of TEA and sustained bioattenuation rates. LNAPL removal is one remedial option to reduce the TEA deficit in the source area.