API PUBL 4654-1997

$24.70

Field Studies of BTEX and MTBE Intrinsic Bioremediation

Published By	Publication Date	Number of Pages
API	1997	237

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Description

In the last several years, there has been considerable interest in confining the biodegradation of soluble gasoline constituents in groundwater. Recent acceptance of risk-based approaches to corrective action has accelerated the need to better understand the role biodegradation can play in limiting the transport of and possible exposure to dissolved hydrocarbons in groundwater. This study was initiated to document the in situ natural biodegradation (commonly referred to as intrinsic bioremediation) of benzene; ethylbenzene; toluene; o-, m-, and p-xylene; and methyl tertbutyl ether. A rural North Carolina underground storage tank release site was selected for study. The site was instrumented with mom than 50 observation wells monitored for several years to allow quantitative characterization of the down gradient mass transport of the dissolved compounds. Companion laboratory and modeling studies were conducted to facilitate interpretation of the field data.

Three dimensional field monitoring of the dissolved gasoline plume showed rapid decay of toluene and ethylbenzene during down gradient transport with slower decay of xylenes, benzene, and MTBE under mixed aerobic-denitrifying conditions. Background dissolved oxygen concentrations range from 7 to 8 mg/L, and nitrate concentrations range from 7 to 17 mg/L as Nitrogen (N) because of extensive fertilization of fields surrounding the spill.

Sampling results indicate that the plume is not growing and has reached a pseudo-steady-state. Field-scale decay rates were determined by estimating the mass flux of contaminants across four plume cross-sections. First-order decay rates for all compounds were highest near the source and lower farther down gradient. Effective first-order decay rates varied from 0 to 0.0010 d^-1for MTBE; 0.0006 to 0.0014 d^-1for benzene; 0.0005 to 0.0063 d^-1for toluene; 0.0008 to 0.0058 d^-1for ethylbenzene; 0.0012 to 0.0035 d^-1for m-, p-xylene; and 0.0007 to 0.0017 d^-lfor o-xylene. In a companion study, laboratory microcosm studies confirmed MTBE biodegradation under aerobic conditions; however, the extent of biodegradation was limited.

BIOPLUME II and a 3-D analytical model were evaluated for their ability to simulate the transport and biodegradation of MTBE and BTEX at the site. Neither model could accurately simulate contaminant concentrations throughout the length of the plume.

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