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s }``� i to, other solid iron compounds by slight amounts of dts- --8-Do (ppmr t, solved oxygen Thus geochemical data are useful indica- + M tions of such phenomena as consumption of electron e r t donors and acceptors and production of CO, which in E 3 �� r t°� a turn indirectly indicate microbial degradation of con- oi �1 �� tamtnants } Laboratory Microcosm Studies I �� Laboratory microcosm studies can be used to deter- --&—BTEX (ppb) ti, mine if the contaminated aquifer has the capability of E degrading(biologically or chemically)the contaminants 9 ' ,a� of interest Although microcosm studies have been con- W7 W2 ru W19 WM Pzi w, wi+ nA ducted for BTEX at many sites the, results indicate that Monuonue well BTEX degrading microbes are nearly ubiquitous and in general it is not necessary to conduct microcosm stud- Figure 12 Comparison of dissolved oxygen and BTEX concen- ies to determine if the BTEX coin ound% can be tratrons in ground water at a Rhode island service station P degraded at a site Microcosm studies should hL con- ducted for compounds whole biological or chemical Additional Indicators de,radabihty has not bLLn estabh%ha,d or for some. other Other indicators and methods for demonstrating site-specific reason A typical microcosm consists of a natural attenuation include geochcmical characteristics soil and ground water slurr} placed in a scaled vial or laboratory microcosm studies and detailed modeling flask with the D O pH temperature and other relevant studies variables either measured or controlled AflLr scaling the microcosm contaminant concentrations are moni- Geochemical Characterist cs tored over umi_ to detcrminL if thL contaminant is The aeochemieal characteristics of an aquifer in the degrading Microcosm swdies of tics type conducted arca of a petroleum hydrocarbon spill can he used as with Samples from the Michigan gas plant and manv secondary indicators of natural attenuation As other site hawill, demonstrated that naturally (iccurr:nZ described by Siegel ct al (1990 1992) microbial oxid,t- microhe,s capable of �ierohiC hiodc.,_r.id,ition of 131 EX tion of hvdrocarbons utiliving D O as a terminal elec- are prc%Lnt in soils and ground water The. microcosm Iron acceptor will produce dissolved carbon dioxide and results obtained from thr_ Michigan gas plana studies organic acids A region of reduced pH should develop (Chiang et al I989) demonstrated that at least I to 2 which corresponds roughly to the soluble contaminant mg/L D O were required for rapid a,robrc hiodegrada- plume due to the acidification of ground water Associ- lion of hen7enc ated with the region of reduced pH will be plumes of Detailed Ground Water Modeling increased carbonate hardness and alkalinity The subse- If sufficient site characterisation information is avail- quent hydrolysis of carbon dioxide to carbonic acid acidifies ground water within the soluble contaminant able detailed numerical modeling of ground water flow plume resulting in dissolution of aquifer minerals such and contaminant transport can be conducted to help as CaCO3 In situations where electron acceptors other demonstrate natural attenuation Numerical modeling than oxygen are being utili-rcd, different behavior such can illustrate the behavior of a plume undergoing as increases in pH and associated chemical equilibrium natural attenuation howc%cr the limitat,ons of such shifts may be observed Thus careful evaluation of geo- models regarding data requirements data uncertainties chemical data may provide insights into attenuation and predictive capabilities should be understood and mechanisms considered when deciding to conduct extensive model- In addition to the above geochemical traits the dis- ing and when drawing conclusions from modeling solved iron concentrations can indicate bioattenuation results The time required to generate and calibrate a as well As reported by Lovley et al (1989), studies of site-specific ground water model and the uncertainty an oil spill site in Bemidji Minnesota, have shown that associated with many of the input parameters make it the microbial oxidation of aromatic hydrocarbons under more appropriate to rely directly on actual site data and anaerobic conditions is coupled to the reduction of the indicators previously described The preferred Fe(IIi) to Fe(iI) (ferric to ferrous) As D O levels are approach for most sites would be to utilize an easily applied analytical or`mean-behavior"numerical model depleted from aerobic biodegradation, conditions to help interpret actual data but not as a substitute far within the contaminant plume become anaerobic result- 'rig in the use of Fe(111) as an electron receptor Thus data Ee(III)is reduced and transferred from aquifer minerals to soluble Fe(II) in ground water Siegel et al (1992) Conclusions and Recommendations have observed that soluble iron concentrations tend to As discussed here and in the literature,natural atten- decrease shortly downgradient of the hydrocarbon cation of contaminants in ground water occurs as a source as ferrous iron is oxidized to ferric hydroxide or result of biological degradation processes,physical phe- SPRING 1994 GWMR r 171