Environmental Health - Toxic Substances
Methodology for Estimating Times of Remediation Associated with Monitored Natural Attenuation
By Francis H. Chapelle, Mark A. Widdowson, J. Steven Brauner, Eduardo Mendez III, and Clifton C. Casey
U.S. Geological Survey Water Resources Investigations Report 03-4057
Natural attenuation processes combine to disperse, immobilize, and biologically transform anthropogenic contaminants, such as petroleum hydrocarbons and chlorinated ethenes, in ground-water systems. The time required for these processes to lower contaminant concentrations to levels protective of human health and the environment, however, varies widely between different hydrologic systems, different chemical contaminants, and varying amounts of contaminants. This report outlines a method for estimating timeframes required for natural attenuation processes, such as dispersion, sorption, and biodegradation, to lower contaminant concentrations and mass to predetermined regulatory goals in groundwater systems.
The time-of-remediation (TOR) problem described in this report is formulated as three interactive components: (1) estimating the length of a contaminant plume once it has achieved a steady-state configuration from a source area of constant contaminant concentration, (2) estimating the time required for a plume to shrink to a smaller, regulatoryacceptable configuration when source-area contaminant concentrations are lowered by engineered methods, and (3) estimating the time needed for nonaqueous phase liquid (NAPL) contaminants to dissolve, disperse, and biodegrade below predetermined levels in contaminant source areas. This conceptualization was used to develop Natural Attenuation Software (NAS), an interactive computer program that estimates times of remediation associated with petroleum hydrocarbon and chlorinated ethenecontaminated aquifers. NAS was designed as a screening tool and requires the input of detailed site information about hydrogeology, redox conditions, and the distribution of contaminants. Because NAS is based on numerous simplifications of hydrologic, microbial, and geochemical processes, the program may introduce unacceptable errors for highly heterogeneous hydrologic systems. In such cases, application of the TOR framework outlined in this report may require more detailed, site-specific digital modeling. The NAS software may be downloaded from the Web site http://www.nas.cee.vt.edu/index.php.
Application of NAS illustrates several general characteristics shared by all TOR problems. First, the distance of stabilization of a contaminant plume is strongly dependent on the natural attenuation capacity of particular ground-water systems. The time that it takes a plume to reach a steady-state configuration, however, is independent of natural attenuation capacity. Rather, the time of stabilization is most strongly affected by the sorptive capacity of the aquifer, which is dependent on the organic matter content of the aquifer sediments, as well as the sorptive properties of individual contaminants. As a general rule, a high sorptive capacity retards a plume's growth or shrinkage, and increases the time of stabilization. Finally, the time of NAPL dissolution depends largely on NAPL mass, composition, geometry, and hydrologic factors, such as ground-water flow rates.
An example TOR analysis for petroleum hydrocarbon NAPL was performed for the Laurel Bay site in South Carolina. About 500 to 1,000 pounds of gasoline leaked into the aquifer at this site in 1991, and the NAS simulations suggested that TOR would be on the order of 10 years for soluble and poorly sorbed compounds, such as benzene and methyl tertiary-butyl ether (MTBE). Conversely, TOR would be on the order of 40 years for less soluble, more strongly sorbed compounds, such as toluene, ethylbenzene, and xylenes (TEX). These TOR estimates are roughly consistent with contaminant concentrations observed over 10 years of monitoring at this site where benzene and MTBE concentrations were observed to decrease rapidly and are approaching regulatory maximum concentration limits, whereas toluene, ethylbenzene, and xylene concentrations decreased at a slower rate and have remained relatively high.
An example TOR analysis for chlorinated ethene NAPL was performed at the Kings Bay, Ga., site. NAPL removal action by in situ oxidation was performed here, and the NAS simulations indicated that TOR was highly dependent upon location within the plume (upgradient areas remediate faster than downgradient areas), and the organic carbon content (sorptive capacity) of the aquifer. In general, the NASestimated decreases in chlorinated ethene concentrations at different locations within the Kings Bay plume are roughly consistent with observed decreases over 3 years of monitoring. This comparison, however, also shows that observed patterns of contaminant concentration changes are much more complex than indicated by NAS. This, in turn, illustrates the general principle that hydrologic complexities of ground-water systems are not fully accounted for in simulation tools like NAS, and that TOR estimates made with such tools are inherently uncertain. Thus, although TOR estimates can be useful for evaluating different remediation strategies and goals for particular sites, these estimates should always be verified with site monitoring.
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