| Investigation of a 45-m thick, 177-hectare, urban solid-waste landfill indicates that microbes mineralize (C{dollar}sb{lcub}rm min{rcub}{dollar}) about 10-25% of the dry-waste mass. Flux of 36,000 mt yr{dollar}sp{lcub}-1{rcub}{dollar} of mineralized carbon is reported. CH{dollar}sb4{dollar} and C){dollar}sb2{dollar} in landfill gas occur in a ratio of 3:2 and account for more than 97% of carbon flux. Dissolved carbon accounts for the remainder. Steady gas composition is explained by a methanogenic microbial consortium in which non-methanogens produce H{dollar}sb2{dollar} and acetate (6:5) and methanogens consume these products by combined pathways of acetate-dissimilation (75% of CH{dollar}sb4{dollar} production) and CO{dollar}sb2{dollar}-reduction (25%). Gas flux accounts for more than 99% of the mass discharge of identified hydrocarbon compounds (such as toluene and xylenes) and is controlled by affinity for the gas phase and abundance in waste. Leachate is composed primarily of mineral salts (Na, Cl, K) and byproducts of C{dollar}sb{lcub}rm min{rcub}{dollar} (DIC, ammonia) and is deficient in oxidized solutes.; Layered landfill structure results in increased age and substrate depletion with depth, and superposition of microbial populations at different stages of evolution. Interlayer transport of leachate, gas and heat is responsible for substantial deviation from classic landfill models. Leachate flow is an important mechanism for transport of microbial substrates and produces a geochemically-active interface between pre-methanogenic waste (above) and methanogenic waste characterized by non-conservative behavior of numerous solutes (dissolved carbon, calcium, magnesium, sulfate, trace metals) and precipitation of carbonate and sulfide minerals. Landfill thickness and waste residence time are important controls on landfill temperature and leachate strength. Leachate strength increases with depth of flow, although solute uptake decreases with depth in older wastes. Thermal modeling of landfill heat production and heat transport is a promising method for study of C{dollar}sb{lcub}rm min{rcub}{dollar} rates. Methane production of 56-121 l kg{dollar}sp{lcub}-1{rcub}{dollar} dry-waste is reported and is substantially complete within 5-7 years. A large proportion of organic matter is recalcitrant ({dollar}approx{dollar}50%) and enables long-term, low-level methane production. Dominance of thermophilic microbial populations is reported. Cycling of hydrocarbons occurs by volatilization in heated landfill interior, and condensation and accumulation in cool surface soils. Landfill methanogenesis makes {dollar}deltasp{lcub}13{rcub}{dollar}C{dollar}sb{lcub}rm DIC{rcub}{dollar} and total dissolved carbon promising leachate tracers. |
