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At some sites Monitored Natural Attenuation (MNA) can manage metals and metalloid contaminants (“metals”) if the contaminants can be safely held in place on aquifer materials by sorption and/or precipitation processes. The degree and permanence of precipitation and adsorption can be evaluated using the concept of geochemical gradients, where key aquifer properties such as pH, redox potential, and ionic strength are different within the plume and can change over time as the plume moves through the subsurface. The U.S. Environmental Protection Agency (USEPA) developed an extensive three-volume guidance document and a Directive that can be used to determine if MNA can be applied at a site with metal contaminants in groundwater. Using technical aspects of this guidance document as a foundation, we also overview a “Scenarios Approach”, developed by the U.S. Department of Energy for evaluating MNA for metals in groundwater that shows the mobility chart for a number of metals for six different geochemical scenarios.

Related Article(s):

CONTRIBUTOR(S): Dr. Miles Denham and Dr. Charles Newell, P.E.

Key Resource(s):

• A Decision Framework for Applying Monitored Natural Attenuation Processes to Metals and Radionuclides
• Use of Monitored Natural Attenuation At Superfund, RCRA Corrective Action, And Underground Storage Tank Sites

Introduction

Monitored natural attenuation (MNA) of metal and metalloid contaminants in groundwater is a remediation strategy that relies on natural processes occurring in an aquifer that minimize risk to human health and the environment by attenuating contaminant migration from their source to a compliance point such as a drinking water well or property boundary. With the exception of short-lived radionuclides, metals and metalloids are not destroyed by natural attenuation processes and will remain sequestered in the aquifer when sufficiently attenuated. Therefore, gaining acceptance of MNA as a remedy for metal and metalloid contamination requires demonstrating, to an acceptable degree of uncertainty, that the contaminants will remain in place and pose a minimal risk for decades to centuries, depending on the contaminant.

Attenuation of Metals and Metalloids

Natural attenuation processes of metals and metalloids that occur in aquifers are adsorption, precipitation, radioactive decay, and dispersion. Adsorption and precipitation limit the mobility of metals and metalloids by causing them to partition from groundwater to solid phases. Dispersion dilutes the concentration of contaminants, rather than limiting their mobility. Radioactive decay applies to only those radionuclides that have short enough half-lives to prevent their migration to compliance points. “Short enough” varies with the contamination scenario. Radioactive decay may be sufficient to achieve successful MNA if the groundwater travel time to the compliance point is much longer than the half-life of the radionuclide.

Geochemical Gradients

Geochemical gradients are the spatial variations in geochemical conditions created by waste disposal or other phenomena. Geochemical gradients can evolve over time as geochemical conditions change; for instance, as neutral pH water displaces low pH water. When present, geochemical gradients can strongly affect contaminant mobility and thus identification and characterization of geochemical of gradients allows the contaminant attenuation-affecting conditions of a site to be projected into the future.

Demonstrating that adsorption and/or precipitation are sufficiently limiting the mobility of a contaminant metal or metalloid is the strongest evidence that MNA is an appropriate remedy. Adsorption and precipitation may occur when a metal or metalloid enters groundwater because the contaminant, and often the composition of fluids carrying the contaminant, cause perturbations of the near steady-state condition of the groundwater system. This promotes reactions that tend to return the groundwater system toward its original state and these often result in contaminant adsorption, precipitation, or both.

Consider the evolution of a contamination plume in an aquifer (Fig. 1). As contamination enters the aquifer, a geochemical gradient forms at the leading edge of the plume. This leading gradient may simply be a concentration gradient of the contaminant, where the concentration is higher on the side of the plume opposite of the direction of flow. In many cases, the leading gradient also includes gradients in concentrations of other constituents associated with the contaminant source. For example, contamination plumes often have different pH, oxidation-reduction potential, or ionic strength than the native groundwater. In any event, reactions tend to occur that counter the geochemical gradient and can cause adsorption of the contaminant to aquifer mineral surfaces or precipitation of the contaminant.