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4 12 acceptors carried into this zone by the floU groundwater. a Benzene 10 Toluene Evidence of these processes is seen in the'.q_Ages in cottccn- El One m-,p-Xylene trations of BTEX and indicator pa.rarq betwee� wells c 8 ene e Etho-Xylbenzene inside versus outside the plume. g During transport from U9 and U10, total dissolved r 4 BTEX increases from 0 to 25.6 mg/1,oxyged'and nit>:ate are !! depleted,dissolved iron increases to 29 mg/l(0.52;meq/1), I 2 and the redox potential drops from approx A14* !#•190 to 0 --132 mV. The rapid decline in oxygenpOpitra te and 3 increase in dissolved iron results..from the„ of these B tF 2(0.125 meq per mg) compounds as terminal electron acceptors d�biodegra- 2.5 03(0.357 meq par mg) dation of the hydrocarbons.The decline p;Jlp ,})otential �i e(0.017 meq per mg) 2 04(o.0e33 meq per mg) reflects the change from oxidizing conditig pt gkdient of (0.500 meq per mg) the source area (absence of dissolved ho rbons) to 1.5 La reducing conditions in the source area(presectQe of�issolved 1 hydrocarbons). C 0.5 The relative importance of the differegt#jWftgn accep- tors can be seen by plotting the concentrati *iequiv- alents of electron acceptor per liter.lmeq/4).; e,Vxeq/l of 120 electron acceptor should be able to degrade one peq/1 of C ih BTEX as c electron donor(hydrocarbon). During traqWpyt fzom well 1100 3 CO2 as o U9 to U10,0.26,2.3,and 0.5 meq/l of oxy ni ate,and 80 iron are reduced. These data indicate.tbatt; gi94ometri 0 60 tally,the most important electron hcceptgr;jts. s�egion is nr convert 40 nitrate,followed by iron and oxyge .Factapl'ging milligrams to milliequivalents for each eleMonAweptor are 0 20shown in Figure 4B. 0 At the upgradient edge of the plume,:dissolved CO2 -200 0 200 400 600 800 1,000 increases from 16 to 60 mg/1 as carbon(C)and the.pH rises Distance from Well U10(ft) from -- 4.7 to 5.8. Whole rock analysis of-the aquifer Fig.4.Variation in BTEX components(A),electron acceptors sediment indicates that the carbonate minerals arebeiow the (B),and indicator parameters(C)in a longitudinal profile along detection limit(<0.01%)in both contaminated and uncon- plume center line.Electron acceptors are reported in meq/1 for easy comparison.Conversion factors are shown in legend. taminated areas (Becker, 1992). This india Ates.,that the increase in dissolved CO2 must be due to;organic matter Table 1.Average Concentration of BTEX and Indicator Parameters in a Profile Along the Plume Center!1ne Monitoring well. U11 U9 U10 A7 A19 Al2 A18 Distancefrom source(ftf: -180 --128 0 300 600 800 1,074 Benzene(µgll) BQL* _BQL 1,326 316 628 2,025 615 Toluene(µg/1) BQL BQL 10,436 117 80 566 26 Ethylbenzerne(ILgll) BQL BQL 1,814 1,824 1,925 1,507 42 M-,p-Xylene Gig/l) BQL BQL 8,179 4,501 2.752' 1;995 335 o-Xylene(µg/1) BQL BQL 3,873 111 38 1,293 13 .. Total BTEX(µgel) BQL BQL 25,628 6,869 5,423 7,387 1,031 Methane(Mg/1), BQL BQL 0.1 0..1 0.1 0.1 0.4 COD(mgt 1) 9 14 104 25 21 77 20 Redox potential(mV) 196 190 -132 -181 -118 -161 . -187 Dissolved oxygen(mg/1) 3.1 2.4 0.3 0.3 0.2 0.3 0.7 Nitrate(mg/l as N) 1.4 6.6 0.1 0.1 0.1 0:2 0.1 Dissolved iron(mg/1) 0.2 0.2 29 65 52 .3 2 Sulfate(mg/1 as S) 6.3 9.3 11 0.2 1.3 0.5 4.3 Total sulfur(mgrl as S) 7.0 10 12 1.7 2.3 6.3 6.0 Alkalinity(mg/1 as C8CO3) 6 7 54 97 122 49 148. pH 4.6 4.7 5.8 6.2 6.1 5.7 7.1 Carbon dioxide(mg/l as Q' 16 29 60 66 70 62 25 I Total dissolved solids(mg/1) 62 105 123 165 303 "S 224 Temp(°C) 18.3 19.3 19.9 19.4 17.3 18.9 17.6 , Calcium(Mg/1) 5 14 7 13 7 1>5 75 Simi +Distances are measured in a longitudinal transect along the plume centerline.Negative distances are upgradient of U distances are downgradient. *BQL-Below quantitation limit. .185