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The Role of Natural Attenuation

An On-Line Version of a Column First Published in the:
National Environmental Journal July/Aug. 1995 Vol. 5 No. 4 Pages 23-24

By: David B. Vance

In a time of reappraisal for the allocation of financial resources to environmental action, a question of ever increasing importance is the consequence of no-action concerning the release of petroleum or chlorinated hydrocarbons into groundwater. An important portion of that answer comes from the application of site specific health based risk assessments. However, in instances where human consumption or exposure is not an issue, no-action may be a reasonable alternative, even at elevated dissolved contaminant concentrations. The issue then becomes the determination of the consequence of no-action under conditions where the sole process for remediation is natural attenuation.

The physical, chemical, geological and biological processes that take place in a contaminated aquifer are complex. In most instances a "native" aquifer is in a long standing state of chemical equilibrium between the groundwater and the geologic matrix through which it flows. The release of anthropogenic hydrocarbons into an aquifer upsets that equilibrium. The dissolved concentration of the contaminant as it migrates through the aquifer is controlled by adsorption, dispersion, volatilization and degradation. Adsorption effects the overall residence time of the release and dispersion effects the downgradient shape and dissolved concentration in the plume. Only volatilization and degradation contribute to the removal of contaminant from the aquifer and at low concentrations degradation is the dominant mechanism for attenuation.

The mechanisms for attenuation through degradation can be broadly divided into two categories, biological and abiotic chemical action. This discussion is predicated on relatively "normal" groundwater conditions under which biological action proceeds at a rate orders of magnitude greater than abiotic processes. Extremes of pH, redox conditions, ionic strength or temperature may make an exception to that generalization. Transformation can be chemically complex, dependent upon the environmental conditions described above, and affected by aquifer heterogeneity’s (Granular)(Fractures) . An analytical method of assessment for natural attenuation that negates the requirement for detailed knowledge of those issues is outlined at the end of this column.

The factor controlling the rate of aerobic degradation is the availability of oxygen and the rate at which it can be introduced into the groundwater (through the groundwater table interface) or the rate at which oxygen rich groundwater can pass through the zones of adsorbed contamination. As a "rule of thumb" each pound of hydrocarbon requires 3 pounds of oxygen for complete degradation. Typical in-situ aerobic decay rates for groundwater are in the range of 35 micrograms per liter per day (equivalent to about half an ounce per day per cubic yard of aquifer matrix).

Natural attenuation occurs in both the source zone and in the dissolved phase plume. In the source zone oxygen will be rapidly consumed and portions of the aquifer will then host anaerobic degradation. Anaerobic degradation is limited by the availability of appropriate anaerobic electron acceptors such as nitrate, sulfate or iron. When their availability is limited, degradation will stop after the production of aliphatic and aromatic organic acids, similarly at low levels of Dissolved Oxygen (DO) aerobic degradation may also stop with the production of organic acids.

Optimum aerobic biodegradation occurs with the dissolved oxygen above 2 mg/L. Below that the aerobic degradation rate of aromatic hydrocarbons will decrease dramatically. Conversely, under complete anaerobic conditions nitrate reducing bacteria can effectively degrade hydrocarbons. However, at DO concentrations as low as 0.1 to 0.4 mg/L anaerobic degradation rates will be reduced to just a few percent of optimum.

Because of all the mechanisms described above hydrocarbon plumes tend to achieve a stable shape and size even when there is a continuous source of free phase hydrocarbon release. Steady state is achieved when the area of the plume edge is great enough to provide for a natural degradation rate equivalent to the rate of hydrocarbon infiltration. The interior of the dissolved plume does not have enough DO to support optimum rates of aerobic degradation, but too much DO to allow for optimum anaerobic degradation. However, once the source of hydrocarbon has been removed a dissolved plume will narrow and dissipate from the edges inward, due to the availability of DO from groundwater along those edges.

The selection of a no-action natural attenuation option should be based on an appropriate analysis of data gathered during the assessment of the site. First order decay rates are appropriate for the evaluation of degradation kinetics at low concentrations, less than 1 ppm (an appropriate level to assume at the periphery of a plume). Given first order decay rates, the analysis has a focus that is two-fold, the effect of attenuation over time and the effect over distance.

Attenuation over time is measured at the edges of a plume using concentration measurements gathered repeatedly from specific monitor wells. The minimum recommended time is one year, with quarterly sampling from the selected monitor wells. The data for each well is then semi-log plotted as log concentration against time. The slope of the line is the first order decay constant in per cent per day.

Attenuation with distance more accurately incorporates the effects of aquifer heterogeneity. Data for this analysis is obtained from a minimum of three monitor wells, preferably along the long axis of the plume. This data is semilog plotted as log concentration versus distance. The slope of this line is equal to the decay constant divided by the groundwater velocity.

With this data decisions can be made based on the site specific contaminant dynamics under no-action natural attenuation. This, in conjunction with a health based risk assessment, can allow for sound decision making by the business and regulatory community.

In summary, the adoption of a no-action alternative is most applicable to the dissolved phase plume only. Except for volumetrically small releases, it will still be necessary to remove or remediate the source zone of an impacted aquifer. After which natural attenuation may be a reasonable approach to the residual dissolved phases. Also implicit in this approach, is that no-action does not preclude the performance of requisite assessment activity. Nonetheless, after proper source abatement, assessment, and analysis, the reliance on natural attenuation mechanisms for the final stages of clean-up is a cost effective and if properly managed, environmentally sound resolution to aerially extensive dissolved phase hydrocarbon contamination.

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