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<br /> mineralization and that 40 i o 50 mg/l of.hydrocarbon have iron present as' mixed oxidation state clay;inine.'ral(glau-
<br /> already been degraded `M' increase in pH was unexpected conte)which may be difficult for microorganisms to use.
<br /> because an.increase in C%,should depress the'pH, not "The The highest benzene, concentrations observed an -
<br /> Y
<br /> increase it. J1 he,obsmed rise in pH is believed to be due to where on the'site"are present in well Al2,approximately 800
<br /> cansum�,tion of when;iron is reduced for use as a ft downgradient from the source welt U10.A•pulse of BTEX
<br /> terminal electron acceptor['equation(3)].This same pattern may have been released into the ground water when the
<br /> continue$ in downgradientwells:where increases in dis- underground storage tanks were/removed and could now be
<br /> solved Fe are accompanied liy an increase in pH.Stoichio- migrating past A 12. This hypothesis is supported by the
<br /> nmetric calculations indicate-that the amount of W con- high chemical oxygen demand(COD)and CO2 concentra-
<br /> 1 J sumed during iron reduction far exceeds that required for tion at this well and anecdotal reports that the contents of
<br /> the observed increase in pH. the tanks(gasoline and water)were drained.back into the
<br /> Thefewas.essentially,no.change in oxygen or nitrate, excavation when the tanks were removed.,Ground-water
<br /> f ! during transport from tlie',{qurce area to 900 ft downgra- in will be continued to determine-if the concentra
<br /> dient since both of tbo .,electron acceptors have already tions decline as the pulse passes A 12.-
<br /> been. depleted:' Over thisiisame travel distance, sulfate At the downgradient'end of the plume(AI8),the aqui-
<br /> decreases from'11 to 0.2 n4/1 as S, total sulfur decrepses fer mineral composition changes and additional biodegra-
<br /> from.l2 to 1.7 nig/l'as S,`aiilddissolved iron increases from `�dation appears to occur.A18 is the first location where shell
<br /> „,. .29 to.6$.mg�1.,The large aline in sulfate-and increase in material is observed in the aquifer.Coincident with appear-
<br /> ' dissolved iron.indicates tlk t.pth sulfate and iron reduction ance of the calcareous shell fragments is an.increase in pH, t
<br /> are occurring. The smallen:decline in total sulfur suggests dissolved calcium,and alkalinity and apparent production
<br /> "that`sulfite is being'reduced-but is not being removed as of dissolved methane. The observed increase in dissolved
<br /> ferrous sulfide precipitates 'Mis pattern of larger reductions sulfate and-decrease in CO2'rnay indicate that well A18 is at
<br /> in sulfate acid smaller redu ons in.total sulfur continues in. the edge of-the hydrocarbon plume:
<br /> the more downgradient wells:These trends could be due to
<br /> formation of colloidal size-ferrous sulfide or production of Variation of BTEX and lndicafor Parameters
<br /> sulfui'corripounds of into!eediate oxidation state(thiosul- PeMendicular to the Direction of Flow '
<br /> fate'(S bri")'or elemental..sulfur(Ss)]. Beller et al. (1992) A seriesof wells was installed in two cross sections
<br /> conducted laboratory studies of toluene biodegradation perpendiculartothe ground-water flow direction to evaluate
<br /> f under mixed iron-sulfate-rl" ucing conditions and hypoth- dilution and biodegradation at the sides of the plume..Fig-
<br /> '' esized that'.the.presenoe.of excess sulfide could result in ure 5 shows BTEX concentrations in cross sections through
<br /> abiotic reduction of iron :hydroxides and production of the fi plume at the sours area(A-Al and 600 ft downgradimt•thiosulfate or elemental sulfur:If such reduction occurred'at (B-B'). The locations of these cross sections:are shown in
<br /> the Rocky Point site,it would explain the smaller decrease in Figure 2.If dilution was the only cause for a concentration
<br /> 'total sulfur.;. reduction,the width of the plume should increase while the
<br /> During:transport fromsource well U10 to well A7(300
<br /> ftdowngradient),toluene and o-xylene decline rapidly fol-
<br /> lowed by til=, -x leve,and benzene.Eth lbenzene does not
<br /> P Y y 212,000
<br /> decline notably with distance.This pattern was unexpected10,000 A A'
<br /> since o-xylene, m-, p-xylene, and ethylbenzene have very
<br /> similar physical and chemical properties. One apparent 50 8,000
<br /> explanation for this pattern is preferential biodegradation of 6,000
<br /> the o-xylene isomer (and-%toluene) by subsurface.micro- .- .4,000
<br /> i organisms. The preferential degradation of o-xylene over 0 2,000
<br /> ethylbenzene and m- -x lene is unusual since revi
<br /> > P Y. p ous 0 .
<br /> laboratory studies in aerobic(Thomas et al.,1990),denitrify- 40 -60 -40 -20 '' 0 20 40.. .60 80
<br /> ing(Hutchins, 1991),and sulfate-reducing(Edwards et al., 12,000
<br /> L t 1992) systems have found"`o-xylene to be less degradable 10,000
<br /> than the other xylene isomers. No documented reports of -B
<br /> C 8,000
<br /> xylene or ethylbenzene degradation under iron-reducing J 6,000 conditions have been found. �°
<br /> Along the center of the plume further downgradient 0 4,000
<br /> Ii I from the source{300 to 800 ft),the rate of biodegradation is 0 2,000
<br /> reduced.The total BTEX:eoncentration is essentially c'on-
<br /> WISM�
<br /> U 0
<br /> stant,.from_A7:to AI2, due to the depletion of readily - -80 -60. -40 -20 0 20 40'. 60 80 4
<br /> Distance from Centerline(ft):
<br /> reducible electron acceptors in the ground water and the low Benzene $m-,p xylene $Es7ylbenzene
<br /> g pH:Downgradient of A7,there is essentially no remaining --$—Toluene — o-xylene
<br /> sulfate,nitrate,or oxygen in the center of the plume.The pH
<br /> Fig. 5. Variation in BTEX constituents perpendicular to.the
<br /> of 6.1 may be low enough to limit the rate of methanogenic direction of ground-water flow at cross sections A-A'and B-B'
<br /> activity.The most prevalent electron acceptor in this area is . gure 2.
<br /> shown In,Figure
<br /> ��� 186 - '
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