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.