Long-Term Biogeochemical Evolution of a Leachate Plume from a Closed Municipal Landfill, Norman, Oklahoma

Leachate from the decomposition of landfill wastes can emanate from closed municipal landfills, generating ground-water contaminant plumes that may last for centuries. Understanding the evolution of redox zones in a leachate-impacted aquifer has important implications for the natural attenuation of contaminants, including the transport and fate of metals. An interdisciplinary team of USGS and university researchers has been studying the temporal evolution of redox zones downdgradient from the Norman Landfill, Norman, Oklahoma, at different temporal and spatial scales, for more than a decade. The leachate plume contains elevated concentrations of non-volatile dissolved organic carbon (NVDOC, ≤ 300mg/L), methane (≤ 16 mg/L), ammonium (≤ 650 mg/L), ferrous iron (≤ 23 mg/L), chloride (≤ 1,032 mg/L), and bicarbonate (≤ 4269 mg/L). Investigations of the distribution of plume constituents in two dimensions reveal NVDOC and methane biodegradation consume electron acceptors in the aquifer, depleting the natural attenuation capacity over time. The core of the plume is depleted in sulfate and has expanded in size by a factor of two over ten years. Shorter time-scale (monthly) variability in chemical constituents is related to seasonal hydrologic changes, and shows that the boundaries of the plume and mixing interfaces between ground-water and surface water are zones of increased reactivity. The conceptual model of biogeochemical processes developed for the Norman Landfill study provides a framework for understanding the transport of organic contaminants, and provides insight into the natural attenuation of leachate compounds in the aquifer. In the context of this biogeochemical framework investigators are conducting detailed experiments on the rates of processes and transport and fate of compounds of concern.