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J1 1 1 ' CLEARWATER GROUP Natural Attenuation Processes and Recommended Monitoring Guidelines 1 The following document details the processes Involved to the natural attenuation of petroleum hydrocarbons in soil ' and groundwater and presents recommendations for monitoring and confirming these processes By confirming natural attenuation,a conceptual basis is provided for regulatory site closure Natural Attenuation Processes ' The predominant attenuation process is intrinsic biodegradation (aerobic and anaerobic) mediated by hydrocarbon degrading bacteria Other factors in natural attenuation Include physical and chemical processes such as volatilization, dispersion, sorption and hydrolysis Unless otherwise referenced, the following information was ' derived from McAllister and Chiang(1994) Aerobic de radatton In aerobic respiration, microbes utilize dissolved oxygen (DO) as an electron acceptor during hydrocarbon oxidation ' (degradation), producing carbon dioxide, water, and microbial biomass The electron acceptor is a substance that facilitates the reaction by taking up the electrons released by oxidation, the electron acceptor then becomes reduced during the process of biodegradation ' The aerobic process is the most Important form of biodegradation wherever DO concentrations exceed 1 to 2 mg/L Under hypoxic conditions (0 1 to 2 mg/L, DO), aerobic degradation may occur along the edges of the plume while anaerobic degradation predominates in the center of the plume ' Anaerobic degradation Microbes may also degrade hydrocarbons via anaerobic processes by utilizing alternate biochemical pathways when DO concentrations are Insufficient for aerobic degradation Anaerobic degradation is much slower than the aerobic process and not all BTEX compounds (benzene, toluene, ethylbenzene, and xylenes) are consistently degraded Some studies Indicate benzene is recalcitrant to anaerobic degradation while others have demonstrated limited degradation (Rlfai et al, 1995) Anaerobic degradation generally occurs in the center of the plume where DO has ' been depleted by aerobic degradation Research Into the efficacy of anaerobic processes is ongoing Anaerobic electron acceptors Include [in order of sequential use and decreasing redox potential (Eli)] ' nitrate(NO3), • oxides of ferric iron(Fe 3,), • sulfate(SO4 2+), ' water The associated biochemical processes are denitrification (or nitrate reduction),iron reduction, sulfate reduction, and methanogenesis Manganese (Mn 4+) may also function as an electron acceptor Nitrate and sulfate reduction do not ' degrade alkanes such as methane,propane, and butane Volatilization ' Dissolved plume mass can be reduced by volatilization of contaminants to the vapor phase in the unsaturated zone Normally volatilization is a negligible component of natural attenuation, however, it may contribute 5% or more of total mass loss to shallow (<15 feet), warm and/or fluctuating water table conditions in permeable soils (Rifat et al, 1995) ' Dispersion Mechanical/molecular mixing reduces dissolved concentrations substantially by lateral spread No dissolved contaminant mass is removed from the system by this process Dispersion (D) is generally modeled based on the I length of the plume (x) Conservative practice calls for dispersion In the downgradient direction (longitudinal dispersivity, Dx) to be modeled at 0 1 times the plume length Dispersion in the transverse direction (transverse ' CLEARWATER GROUP(NATURAL ATTENUATION) 1 revised October 3,2002