Norman Landfill

The Norman Landfill Research Site is a closed municipal solid waste landfill, formerly operated by the City of Norman, Oklahoma. The landfill now is a research site operated by the U.S. Geological Survey (USGS) through its Toxic Substances Hydrology Program. Scientists from the USGS, the University of Oklahoma, the U.S. Environmental Protection Agency, and numerous other institutions are investigating the chemical, biological, and hydrologic processes in ground water and surface water affected by landfill leachate. Norman Landfill provides an opportunity to investigate how landfill leachate affects the environment. Our project has been investigating the biogeochemical processes associated with the natural attenuation of organic contaminants in the leachate plume. Natural attenuation refers to processes that naturally transform contaminants to less harmful forms or immobilize contaminants so that they are less of a threat to the environment; see “Natural Attenuation for Groundwater Remediation,” by the National Research Council (National Research Council, 2000). Many of the processes we study are biologically mediated and can aid in understanding the fate of contaminants in other subsurface environments.

Intrinsic bioremediation is a type of natural attenuation that uses the innate capabilities of the in-situ microorganisms to degrade the contaminants of concern. This aspect of natural attenuation has received much attention because contaminants are chemically transformed to less harmful products. Previous studies have shown that microbial processes change over time in contaminant plumes, resulting in different rates of degradation. The availability of electron acceptors, such as iron oxides or dissolved sulfate, is an important factor for evaluating the efficacy and sustainability of intrinsic bioremediation in contaminated aquifers. We have used a combined geochemical and microbiological approach to identify the important biogeochemical processes occurring in the aquifer contaminated by leachate from the Norman Landfill (Cozzarelli et al., 2000a, Harris et al., 1999). Water samples have been collected from multilevel wells (MLS) installed in the plume and by extracting water from cores (Fig. 1).

In the core of the plume (Fig. 2) the non-volatile (NV) DOC concentrations show little change with distance, indicating that NVDOC is not efficiently degraded in this zone. Most of the degradation occurs at the boundaries of the plume where electron acceptors are available (Fig. 3). Direct rate measurements made in the laboratory combined with field observations suggest that sulfate reduction and methanogenesis are the most important microbial reactions affecting aquifer geochemistry downgradient from the Norman Landfill. Although the core of the plume is strictly anaerobic and supports both sulfate reduction and methanogenesis, the edges of the plume appear to support iron reduction and, to a greater extent, sulfate reduction, due to the increased availability of readily reactive electron acceptors at these boundaries. The non-uniform availability of electron acceptors and the mixing of the contaminant plume with oxygenated water at the plume boundaries have a significant effect on biogeochemical processes. High rates of sulfate reduction near the water table appear to be controlled by the rate of sulfate supply from sulfide oxidation and/or dissolution of sulfate during recharge events. Sulfate reduction within the core of the plume is limited by the availability of sulfate, which is supplied by the slow process of barite dissolution. The type of approach used in this study for assessing the active microbial processes can be used at other sites contaminated with leachate.

We have been investigating volatile organic compounds (VOCs) in the leachate plume at the Norman Landfill and the microbial processes that may lead to their degradation (Eganhouse et al., 2001). VOCs are organic compounds that tend to vaporize at room temperature and pressure. Examples of VOCs include some of the compounds in gasoline, lubricants, and solvents. Some VOCs are highly toxic or carcinogenic. VOCs can be put in landfills in many ways, including the disposal of ordinary household items such as cleaners or marking pens. Although VOCs make up less than 0.1 percent of the mass of organic carbon in the leachate plume, they are useful indicators to show that biodegradation is occurring in the leachate plume. In Eganhouse et al. (2001) we compare concentrations of two different alkylbenzene isomers, isopropylbenzene and n-propylbenzene, in landfill leachate. Isomers are molecules that have the same number and types of atoms but slightly different structures. Isomers have similar physical properties, so they should be affected by volatilization, dilution, and sorption in a similar manner. The concentration of n-propylbenzene decreases much faster as leachate flows away from the landfill than do the concentrations of isopropylbenzene. The different behavior of the propylbenzene isomers indicates that biodegradation is the process primarily responsible for attenuation of these compounds in the aquifer.

In-situ field experiments of microbial processes in zones with different chemical and physical properties are being conducted at Norman Landfill using push-pull test technology and small-scale tracer tests in collaboration with USGS hydrologists Martha Scholl and Scott Christenson (Scholl et al., 2001) and Jack Istok or Oregon State University (Istok et al., 1997). Push-pull tests are single-well injection-withdrawal tests. During the injection phase of the test, a solution consisting of ground water amended with tracers, electron donors, or electron acceptors is injected or "pushed" into the aquifer. During the extraction phase, the test solution is pumped ("pulled") from the same location and concentrations of tracer, reactants, and possible reaction products are measured as a function of time in order to construct breakthrough curves and to compute mass balances for each solute. Reaction rate coefficients are computed from the mass of reactant consumed and/or product formed. These tests can be conducted anywhere in the aquifer, making it possible to investigate processes and rates in different geologic textures and geochemical environments.

At Norman Landfill we are using these field injection techniques to investigate how biodegradation rates may vary with aquifer permeability (Scholl et al., 2001). Push-pull tracer tests were conducted to measure in-situ biodegradation rates of simple organic acids in the leachate plume. Replicate wells were placed in 3 layers: medium sand, silt/clay lenses in sand, and poorly sorted gravel. In-situ biodegradation rates of two simple organic acids, formate and lactate, were compared in three different permeability zones within the anoxic leachate plume at the site. These organic acids were used as microbial process probes since they degrade at different rates depending on the dominant microbial processes. The results show that there are differences in biodegradation in areas of different permeability that may be related to microbial community structure, sediment chemistry, and water flow regime.