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pling the oxidation of toluene to the reduction of Fe(III)in the fate of dissolved hydrocarbons in the subsurface.While
<br /> ferric hydroxide[Fe(OH)3)can be represented-as follows: there are well-documented reports of toluene biodegrada-
<br /> tion via sulfate reduction(Beller et al., 1992)and methanb-
<br /> genesis(Grbi6-Galib and Vogel,1987),the extent and signili-
<br /> 7 CO2+36 Fe+2+94 H2O (3) canoe of hydrocarbon biodegradation in shallow aquifers by
<br /> these processes is poorly quantified.This void may be par-
<br /> The reduction of Fe(II1)results in elevated concentrations tially due to the characteristics.of these microorganisms.
<br /> of dissolved Fe(II)in contaminated aquifers. Lovley et al. Sulfate-reducing and methanogenic consortia are known to
<br /> (1989)found that in an aquifer contaminated by a crude oil be very sensitive to a variety of environmental conditions
<br /> ' spill,the selective removal of benzene,toluene,and xylenes including temperature,inorganic nutrients(nitrogen,phos-
<br /> from the plume was accompanied by an accumulation of phorus, trace metals), toxicants, and pH (Zehnder, 1978).
<br /> dissolved Fe(II)and depletion of Fe(III)oxides in the con- An imbalance in any of these factors could reduce the rate
<br /> I tan-dr ated sediments. Although the exact mechanism(s)of and extent of anaerobic biodegradation.-
<br /> 1 microbial ferric iron reduction is poorly quantified, the
<br /> available evidence suggests that iron reduction is an impor-
<br /> tant mechanism in the subsurface biodegradation of dis- Site History, Regional and Local Hydrogeology
<br /> solved hydrocarbons. The research site examined in this study is located near
<br /> the town of Rocky Point, North Carolina. Ground-water
<br /> } Other Anaerobic Biodegradation Mechanisms contamination was identified at the site in 1987 as a result of
<br /> Past research has shown that a wide variety of problem leakage from two USTs. North Carolina State University
<br /> organics may be biodegraded by sulfate-reducing and/or began field investigations at the site in the Spring of 1991
<br /> methanogenic(methane-generating)microorganisms.These with funding provided by the American Petroleum Institute.
<br /> compounds include creosol isomers(Smolenski and Suflita,
<br /> -- 1987),homocyclic and heterocyclic aromatics(Berry et al., Geology
<br /> 1987), alkylbenzenes (Grbi&Galit and Vogel, 1987), and The research site is located in the outcrop area of the
<br /> unsaturated hydrocarbons(Schink, 1985).Sulfate reducers peedce Formation which has been interpreted by Swift and
<br /> could potentially biodegrade toluene using sulfate in the Heron(1969)as having been deposited in a shallow,marine,
<br /> following reaction(Beller et al., 1992): open-shelf environment during the late Cretaceous.
<br /> r C6H5-CH3+4.5 SO4-2+3 H2O- Stephenson(1923)describes this unit as consisting of"dark-
<br /> - -'
<br /> +i green or gray,finely micaceous,more or less glauconitic and
<br /> 2.25 H2 S +2.25 HS +7 HCO3- +0.25 H (4) argillaceous sands,layers of which are calcareous and some
<br /> Methanogenic consortia (groups of microogranisms that of which are sufficiently calcareous to form an impure
<br /> generate methane) could potentially biodegrade toluene limestone."
<br /> using water as an electron acceptor in the following reaction: The geology of the Rocky Point site consists mainly of
<br /> dark gray and green micaceous fine sand,overlain by 5 to 15 .
<br /> C6115-CH3+5 H2O—4.5 CH4+2.5 CO2 (5) ft of silts,clays,and clayey sands(Figure 1).These sands and
<br /> -� At this time,few field studies have examined the effect clays are typical in the Peedee Formation.The lower per-
<br /> i • of sulfate reduction and methanogenic biodegradation on meability clays and clayey sands form a surface-confining
<br /> i layer over the sand aquifer in theareaof the plume. In the
<br /> contaminant source area north of Arvida Road,there are 12
<br /> L_
<br /> to LS ft of interbedded clays and silts overlying the sand
<br /> Norm South aquifer.South of Arvida Road,the ground elevation drops
<br /> ff about 10 ft and the clays and silty clays are generally absent.
<br /> L..J .r s°"° A21 Overlying the fine sand in this area are 3 to 5 ft of medium to
<br /> U11 \uta Aa
<br /> g U11 40 _ A2 Road' coarse,often gravelly,clayey sand.
<br /> Ati
<br /> m ' W =_' _� A9 Ai The sand that comprises the aquifer at the Arvida site is
<br /> �__ -.-- A — All Ali
<br /> 0 -
<br /> 30 mostly fine-grained,dark gray or greenish gray,micaceous,.
<br /> cc
<br /> _ -- glauconitic, slightly silty, and compact quartz sand. The
<br /> 20 sand is very homogeneous throughout the site with only a
<br /> c
<br /> few
<br /> exceptions.ns.In the southern m portion of the site s
<br /> outh of
<br /> well Al$),a 1-to 24n.layer of shell material and carbonate-
<br /> m _ cemented sand was encountered at a depth of about 15 ft.
<br /> w ° -z°o o zoo 400 Boo Soo 1000 Below this layer,rare shell material was scattered through-
<br /> Distance from well U10 (feet) out the sand.A deep exploratory boring completed adjoin-
<br /> ing well U2, roughly 50 ft to the west of the source area,
<br /> 0 FlA.B�IM 0 u.yrs.ne a,Y �'�Law* 0 s""h showed 15 ft of gray clay overlying about 40 ft of fine,gray
<br /> �....�
<br /> sand interbedded with local,thin,black-clay lenses.The clay
<br /> Fig. 1. Geologic cross section along a longitudinal transect
<br /> through the contaminant plume.Water-table elevations are for lenses cannot be traced between borings indicating they are
<br /> of limited lateral extent.
<br /> October 15,1991.
<br /> LJ
<br /> 182
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