Several processes can impact the mobility and/or the transformation of contaminants in plumes, and these processes may provide adequate attenuation and protection after source control is implemented. However, a greater understanding of the spatial extent and rate of these natural attenuation processes is needed, both for currently understood processes and particularly for processes that previously have not been studied in detail. Of particular interest is the kinetics associated with contaminant back diffusion. Further, although some of these processes may have been previously identified, the extent to which they contribute to contaminant attenuation is still not known.
It would be particularly helpful to have a better understanding of the fluxes from less mobile regions and the degradation near and within those regions. These parameters can have significant impacts on the costs and risks associated with the long term management of contaminated sites following partial restoration. Unfortunately, the uncertainty associated with current measures or estimates of these key variables is large, limiting the confidence in predictions of future risks and restoration timeframes.
An important need is to develop and validate field-scale methods for estimating these parameters. These tests may include (but are not limited to) stressing the subsurface systems via activities such as push-pull tests or shut-down tests, if active remediation systems are currently in place. The stress imposed on the subsurface system by the field test must be large enough to allow for unambiguous interpretation of the results. At the same time, the new field tests must be practical enough to allow for widespread adoption at active sites. The field-scale methods developed need to take into account the likely spatial and temporal variability in the parameters.
It may be important at sites relying on natural attenuation to ensure that biological activity or other degradative processes persist over time. Cost-effective techniques are needed to provide ancillary diagnostic evidence during the long natural attenuation phase, when contaminant 3 concentration monitoring may occur infrequently. For instance, field-scale diagnostic evaluations, such as push-pull tests or tracer tests, could be performed at intervals to determine whether the natural attenuation processes are still occurring and to estimate the associated attenuation rate.
In particular, cost-effective methods are needed to verify that specific degradation processes are occurring, and to estimate the in situ degradation rates over time. Different tools will likely be needed for anaerobic or aerobic treatment systems, for evaluating biotic or abiotic processes and for reducing or oxidizing conditions. These diagnostic tools may be useful to assess initial rates, to determine whether natural attenuation mechanisms are still operating at original or predicted rates, and to determine whether degradation rates are equal to or greater than back diffusion rates so that the plume is contained. Similarly, diagnostic tests and data analysis methods are needed to determine whether degradation rates in a downgradient plume are in line with mass flux rates from a source area following source area remediation. Such data could be used to verify or modify the rates used in the original attenuation modeling or remedy decision basis.