Simulation of biodegradable organic contaminants in groundwater: 2. Plume behavior in uniform and random flow fields

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The behavior of dissolved organic contaminants in groundwater is often difficult to interpret in field settings because of the complex interaction of physical, chemical, and biological processes. The organic solute transport model presented by MacQuarrie et al. (this issue) is used to examine plume migration in a shallow aquifer where dissolved oxygen is the sole electron acceptor. In uniform groundwater flow a plume originating from a high‐concentration source will experience more spreading and slower normalized mass loss than a plume from a lower initial concentration source because dissolved oxygen is more quickly depleted. Low background dissolved oxygen concentrations also cause organic mass loss to decrease, while the initial size of the microbial population has little effect. Large groundwater velocities produce increases in the rate of organic solute mass loss because of increased mechanical mixing of the organic plume with oxygenated groundwater. Because sorbed organic mass is unavailable for biodegradation, increasing the retardation factor of an organic solute causes slower mass loss. For easily biodegraded organics the mass loss is only weakly dependent on the kinetic biodegradation parameters because the amount of mixing of the organic and dissolved oxygen controls the biodegradation process. This implies that for these types of compounds the kinetic parameters do not need to be known with precision. For all simulations the rate of mass loss for the organic plumes decreased with time because the concentrations of organic and dissolved oxygen decrease with time. In complex‐structured flow fields caused by heterogeneity, the bulk velocity, position, and rate of spreading of an organic plume is strongly dependent on local‐scale transport parameters such as the groundwater velocity in individual beds. Because an organic plume emanating from a small to moderate sized source does not evolve in a manner similar to a conservative plume in heterogeneous materials due to a decrease in the size of the plume, it is suggested that average transport parameters such as macrodispersivity may not be applicable to such organic plume transport. If this is the case, then reliable predictions concerning the ultimate fate of organics in groundwater exemplifies the need for obtaining knowledge about the bedding features and degree of heterogeneity of the host geologic materials.