WATER RESOURCES RESEARCH, VOL. 18, NO. 1, PAGES 135-145, FEBRUARY 1982 Modeling of Leachate Organic Migration and Attenuation in Groundwaters Below Sanitary Landfills J. F. SYKES 1 Department of Civil Engineering, University of Waterloo, Waterloo, Ontario, Canada S. $OYUPAK Faculty of Engineering, Technical University of Ankara, Ankara, Turkey G. J: FARQUHAR ß Department of Civil Engineering, Universityof Waterloo, Waterloo, Ontario, Canada A model for predictingthe concentrations of leachate organics,measuredas chemical oxygen demand, in groundwaters below sanitary landfillsites is developed. Simultaneous Michaelis-Menten substrate utilization and microbial mass production equations, with convection and dispersion includedfor the former, were usedfor the modeling of biodegradation. For substrate concentrations significantly less thanthe half utilization rate coefiScient andfor microbial populations approaching a steadystatethe Michaelis-Menten equations are reduced to a first-order reactionkinetic model. The nonlinear model equationsare solved using a Galerkin finite element technique, with a Newton Raphson iteration procedure being used to solve the resulting matrix equations. Model sensitivity studies are undertaken to determine the extent to which COD reduction might vary in the field with respect to time and space.The results indicate that substantial removalof leachate organics can be expected within short distances of the landfill evenunderthe conditions of widely varying biological parameters. Model results are compared to the leachateorganicreductionoccurringbelow the Canadian Forces Base Borden sanitary landfill. INTRODUCTION The movementof moisture throughsolidwastes buried in the soil at landfill sites results in the production of leachates. The concentrations of organicmatter in theseleachates vary from one landfill to the next depending in part on the age and the compositionof the waste material. The partial decomposition of waste organicmatter results in volatile acid formation within th e landfills. The leachate producedmay exhibit chemicaloxygendemands (COD's) as high as 40,000 mg/1 in extreme cases [Roversand Farquhar, 1973] in contrast.to COD's for domesticsewage of 250-1000 mg/1[Metcalf and Eddy, 1972]. This organic matter, along with other leached contaminants, migratesfrom the landfill intothe adjacent soil media. As a result thepotential hazard which these leachatespresent to groundwaterquality adja- cent to landfills i• apparent. Despite the strong potential for ground water pollution, comparatively little•research has been done todetermine the fate of leachate organic matter in soil and to model its transport in the•groundwater system. As a resultthe work described in this paperwas undertaken to develop a model for organic matter transport in soils adjacent to sanitary landfills and to compare model predictions with field data collectedfrom groundwater zonesbelow a sanitarylandfill. ATTENUATION MECHANISMS Mechanisms that reduceorganicmatter concentrations in groundwater during movement through soil involve both •Now at INTERA Environmental Consultants,Inc., Houston, Texas 77079.,. Copyright¸ 1982by the American Geophysical Union. Paper number 1W 1386. 0043-1397/82/001W ' 1386501.00 physicochemicaland biochemical processes. The most ac- tive physicochemical processes are likely to be dispersion, diffusion, and adsorption, with filtration operative for partic- ulate matter. The biochemical processes involve microbial degradation, either aerobic or anaerobic, with the latter expected to be most active, since oxygen supply at depth within soil-groundwater systems can be expected to be limited. It has been well established that landfill leachates are anaerobically biodegradable [Foree and Reid, 1973; Boyle and Ham, 1974;Pohland, 1975; Chian and DeWalle, 1977a, b; Soyupak et al., 1978]. In addition, information is also available on the kinetics of anaerobic biodegradation of leachates from continuously stirredtank reactors[Foree and Reid, 1973; Pohland, 1975; Soyupak et al., 19•78] and soil column studies [Soyupak, 1979]. Chian and DeWnile [!977a, b] defined four distinct bio- degradation phases. They involved the following substrates: carbohydratelike substances, volatile fatty acids, intermedi- ates released during volatile fatty acid metabolism, and refractory organics, all of which may exist together in leachate. Of these, methanefermentationfrom volatile fatty acidsis generallyrate-limitingand accounts for the majority of COD reduction [Lawrence and McCarty, 1970]. Since it has been shownthat the major portion of leachateorganics consistsof volatile fatty acids [Chian and DeWalle, 1973; Pohland, 1975; Chian and DeWalle, 1977a, b], it is reason- able that the study of the anaerobic degradation of leachates should focus on utilizationkinetics of volatile fatty acid. Only limited information is availableregarding the adsorp- tion of leachate organic matter onto soil surfaces. That information which does exist indicates that while microor- ganics such as pesticides andPCB's are strongly adSOrbed, 135