Assimilative Capacity Is an Under-Utilized Concept for Regulating Soil and Groundwater Contamination with Important Implications for Site Closure

The beginning of modern in situ subsurface cleanup arguably began in the early 1970s with the application of hydrogen peroxide to shallow aquifers to drive gasoline spill remediation, in what became known as the Raymond method (Committee on In Situ Bioremediation 1993). Since then, this idea has sparked remarkable remediation innovations where processes that occur naturally in the subsurface, including biodegradation, sorption, diffusion, abiotic reactions, and volatilization are enhanced by actions specifically designed to boost their role in lowering contaminant concentrations in groundwater.

By the 1990s, the age of “Natural Attenuation” (NA) was upon us, together with vigorous ethical and technical discussions aimed at understanding and explaining why the technology was not simply a euphemism for “do nothing.” Natural attenuation proponents held that natural processes tend to be protective of human and environmental health, are more cost- and resource effective than active remediation, and less risky by minimizing the aggressive movement of fluids in the subsurface. On the other hand, critics contended that NA amounted to a delay tactic aimed primarily at cost savings, with little or no risk reduction over reasonable time periods. Critically, since the end of the twentieth century, advancements in site characterization and monitoring tools have refined conceptual site models and established multiple lines of evidence that NA is associated with (1) well-constrained plume development and (2) diminished risk over space and time to an acceptable level without active remediation.

As NA gained acceptance, its adoption at sites became primarily dependent on evidence of biologically driven contaminant transformation as the primary attenuation mechanism. In the process, other important and effective processes, including the collective physical, geochemical, and hydrogeologic influences that can govern or complement contaminant mitigation were often overshadowed or completely overlooked. We know that chemical and physical mechanisms can play primary roles in reducing contaminant concentrations. This includes reactions with iron or manganese oxides, hydrolysis, sorption, diffusion, and dispersion, volatilization, and other mechanisms that lower a specified mass of contaminant, or contaminant mixture, in the subsurface. These processes can be influenced by, and feedback to, the biological activity in aquifers; the interconnectedness of biology, chemistry, and flow are better appreciated now than ever in the past. It is therefore intellectually, technologically, and economically wasteful to disregard any part of this compilation of processes when dealing with aquifer restoration. To improve site management and closure efforts, we need process-based conceptual site models that account for all the processes occurring in a groundwater system. Herein, we contend this can be realized by extending the current NA framework and giving proper recognition to a well-known but under-utilized concept: assimilative capacity (AC).

The idea of AC is decades old (e.g., Falk, 1963). Depending on the literature consulted, the usage of the term varies somewhat, but generally alludes to the ability (by any mechanism) of an environmental system to transform or sequester various materials without deleterious effects to the system itself. Most often, AC has been associated with surface water resources (e.g., rivers and wetlands) and atmospheric studies (e.g., greenhouse gases). AC has also been integral in our approach to disposal of wastes (e.g., wastewater treatment effluents, septic systems and land application of biosolids). About 25 years ago the term was discussed in the context of sustainability and the need to avoid exceeding the AC of natural systems. By doing so, our growing and technologically advanced society could coexist with ecosystem services indefinitely (Cairns Jr 1999).

In the specific context of groundwater systems literature, AC has been used since at least the 1970s (e.g., Willis 1976), in arguably vague terms, with little hydrogeological context. Furthermore, following the rationale for NA, these discussions of AC appear to have emphasized intrinsic processes within a flow system rather than specific quantities, as suggested by the term “capacity.” This tendency has blurred the lines between NA and AC. Going forward, it may be useful to make a clear distinction between AC and NA by defining AC as the mass of contaminant(s) that can be transformed or sequestered by a volume of subsurface over a specified period of time, via the chemical and physical mechanisms mentioned previously. This definition frees hydrogeologists and engineers to design aquifer restoration programs and risk reduction projects that take full advantage of all nature's mechanisms for improving groundwater quality. Moreover, this definition applies to all hydrogeologic settings; it is relevant to cases involving small receptors and large plumes in quasi-steady state with no apparent migration. It is relevant to aquifers and aquitards. In the latter case, the concept of AC may be especially relevant because aquitards may be diffusion-controlled environments where time scales are extended and reactivity enhanced. High-resolution aquitard characterization techniques can provide accurate assessment of ‘assimilation’ rates simply through the analysis of concentration profiles (diffusion halos).

As alluded to previously, a major issue facing the application of the AC concept, extending the scope of NA, lies in our ability to characterize the subsurface for its capacity to “assimilate.” In diffusion-controlled settings, concentration profiles may be sufficient to accomplish this task. But, in many cases, even open and dynamic aquifers can be characterized for AC with confidence, using high-definition characterization tools and in situ flux characterization technologies. With these recent developments in tools, and a growing appreciation of conceptual site model development, we should now be able to shed light on the complete nature of the complex, and interwoven processes affecting contaminant transport and fate over space and time. For example, the familiar transect approach for assessing contaminant losses from groundwater within an aquifer volume is easily adapted using the new tools. In principle, measurement of contaminant discharges (or fluxes) at two bounding transects, upgradient and downgradient of the test volume, provide the data needed for the AC of that volume to be estimated in situ.

Another potentially fruitful application of AC is in documenting contaminant behavior at sites that have undergone NA or active remediation and are now exhibiting low but persistent concentrations of contaminants. Back-diffusion from low-permeability strata is thought to be a common source of such long-term dilute plumes. NA (as biodegradation) may be difficult to prove in such settings, since the aquifers may have returned to an oligotrophic state in which ambient microbial activity is no longer sufficient to drive contaminant concentrations to regulatory limits. A resulting lack of understanding and confidence that risk is under control will likely delay site closure, sometimes unnecessarily. On the other hand, other processes included in the AC concept may act to fill the gap, and be protective of aquifers to concentrations below safe thresholds. This could open the future to specific benefits including improved risk assessment, enhanced future land use, superior site-specific remediation standards, refined monitoring strategies, and successful technology adoption.

In summary, AC may be the concept of greatest relevance in thoughtful site closure decision making. AC is central to extending natural attenuation processes beyond those that have come to dominate NA thinking and be more versatile for distinct hydrogeologic and contaminant types. The recent high-definition characterization technologies and increasing emphasis on process-based site conceptual models are giving site management strategies more potency than ever before. The time has come for our body of groundwater research to seek to standardize the quantification of AC through the use of process-based site conceptual models developed using modern characterization and monitoring tools. As such, regulatory frameworks can be shaped by a more informed understanding of hydrogeologic principles to safeguard human health and the environment.