Aquifer Heterogeneity at the Norman, Oklahoma Landfill and its Effect on Observations of Biodegradation Processes

By Martha A. Scholl, Isabelle M. Cozzarelli, Scott C. Christenson, George N. Breit, and Jamie L. Schlottmann

ABSTRACT

Biodegradation processes in the leachate plume at Norman landfill probably vary among the distinct hydrologic and chemical environments at the site. These environments include zones of mixing with surface water, rainfall recharge or background ground water, and areas of varying permeability in the part of the aquifer occupied by the contaminant plume. The alluvial aquifer at the Norman landfill site consists of sediments ranging from mud to pebbly sand, which, except for the mud layers, have a range of measured hydraulic conductivity from 2.4 x 10^-7 to 2.8 x 10^-4 m/s. Core descriptions from the site indicate that average aquifer composition is 75% sand (fine, medium and coarse), 10% silt and clay in discrete layers, 10% medium-grained sand matrix with pebbles or mud clasts, and 5% coarse sand with pebbles. A numerical model (BIOMOC) was used to explore how permeability might affect observed biodegradation processes and rates. Results of the simulation suggest that low-permeability areas in the aquifer may be relatively uncontaminated, and act as reservoirs of electron acceptors to the surrounding leachate-contaminated areas. A moderate permeability area in the model had relatively low contaminant concentrations and biodegradation appeared to be most efficient in that area. In the highest-permeability areas, the simulation results suggested that preferential electron acceptors are depleted very quickly, leading to higher contaminant concentrations due to less efficient degradation processes and high rates of transport.