A decidedly non-virtual reality is setting in among scientists, engineers, and regulators confronted with issues of hazardous waste site remediation. Put simply, we cannot do what we long have hoped and been expected to do--i.e., return the earth to a pre-hazardous-substance state. Moreover, even if those technologies were capable of accomplishing complete cleanup of every existing site, we could not afford the costs of applying them to that end.
If confident responses to concerns of impacts are not to be found in our current technologies and fixed resources, then they must be evolved from fundamental science. Stated another way, we must through basic research come to a better understanding of:
In this GLMAC project, researchers are developing molecular-level knowledge regarding the interactions of organic contaminants with soils and sediments; more specifically, the thermodynamics and rate processes associated with a phenomenon referred to as sorption/desorption hysteresis.
Hysteresis is the failure of a particular effect to retrace itself when the forces acting to cause it are reversed; in other words a condition for which the effect is not reversible. Some degree of hysteresis, or irreversibility, has been observed to occur in the sorption and desorption of organic contaminants by many soils and sediments. This phenomenon is reflected in the distribution of a contaminant between the aqueous and solid phases after its sorption from solution to the solid being different than after its subsequent release from that solid back into solution; i.e.., its desorption. Such differences are also often observed to be sorption-history dependent--i.e., to increase as a function of the period of time that a contaminant remains sorbed to the soil or sediment, an effect commonly referred to as aging.
GLMAC investigators working on sorption/desorption hysteresis have concluded that there is often a certain fraction of any HOC sorbed by a particular geosorbent bound so strongly by the organic matrices of that adsorbent that, even though it may be "leached" by stringent extractions used for analytical assay, will either not desorb under ambient conditions or will desorb at such low rates as to constitute no environmental threat.
No sorption theory or model to date has been found generally applicable for forecasting the occurrence or extent of sorption-desorption hysteresis. In fact, there has not been so much as a generally agreed upon "rule of thumb" for making such determinations and predictions. The molecular-level research at GLMAC may be about to change that circumstance.
The project investigators have developed a hysteresis index (HI) that allows quantification of the incomplete desorption behavior of a particular contaminant with respect to a particular soil or sediment. They have then used elemental molecular analysis (carbon-hydrogen-nitrogen-oxygen) and solid-state carbon-13 nuclear magnetic resonance (C13-NMR) measurements to characterize the soil/sediment organic matter composition and functionality. Finally, they have related values of HI obtained from sorption/desorption experiments with different types of soils and sediments to the respective oxygen-to-carbon (O/C) atomic ratios of these geosorbents.
The hysteresis index is thought by the GLMAC investigators to be a potential method for assessment and quantification of the environmental mobility and bioavailability of organic contaminants in subsurface systems, and thus of the human and ecological risks they may comprise. As such, the HI may serve as a component of a scientifically based approach to establishing alternative remediation standards, one that includes a quantitative assessment of the degree to which sequestering of contaminants by geosorbents alters their environmental mobility and bioavailability. This could significantly impact the need for setting "completely clean" endpoints for soil and sediment remediation in appropriate circumstances, potentially yielding huge savings in resources and time focused on unnecessarily stringent remediation efforts. Basic scientific research could in this example, as in many others, bring us one step closer to being able to cope adequately with our national contaminated site problems.