Methodology for Applying Monitored Natural Attenuation to Petroleum
Hydrocarbon-Contaminated Ground-Water Systems with Examples from
South Carolina
by:
Francis H. Chapelle, John F. Robertson, James E. Landmeyer, and
Paul M. Bradley
U.S. Geological Survey Water-Resources Investigations Report 00-4161
Abstract
Natural attenuation processes such as dispersion, advection, and
biodegradation serve to decrease concentrations of dissolved contaminants
as they are transported in all ground-water systems. However, the
efficiency of these natural attenuation processes and the degree
to which they help attain remediation goals, varies considerably
from site to site. This report provides a methodology for quantifying
various natural attenuation mechanisms. This methodology incorporates
information on (1) concentrations of contaminants in space and/or
time; (2) ambient reduction/oxidation (redox) conditions; (3) rates
and directions of ground-water flow; (4) rates of contaminant biodegradation;
and (5) demographic considerations, such as the presence of nearby
receptor exposure points or property boundaries. This document outlines
the hydrologic, geochemical, and biologic data needed to assess
the efficiency of natural attenuation, provides a screening tool
for making preliminary assessments, and provides examples of how
to determine when natural attenuation can be a useful component
of site remediation at leaking underground storage tank sites.
At a site in the Piedmont Physiographic Providence (Laurens, South
Carolina), hydrologic and water-chemistry data indicate that the
natural attenuation capacity for benzene is approximately 5 percent
per foot of flowpath. As a result, benzene concentrations would
decrease from about 28,000 micrograms per liter in ground water
at the source area to less than 5 micrograms per liter 200 feet
downgradient and prior to discharging to a stream. Because of this
rapid attenuation, contaminants do not presently impact the stream
downgradient of the site. In contrast, at a site in the Coastal
Plain Physiographic Province (Charleston, South Carolina), hydrologic
and water-chemistry data indicate that, even though the site has
a substantial natural attenuation capacity, it may not be sufficient
to fully protect a nearby point of ground-water discharge.
These two sites illustrate how the efficiency of natural attenuation
processes acting on petroleum hydrocarbons can be systematically
evaluated using hydrologic, geochemical, and microbiologic methods.
These methods in turn can be used to assess the role that the natural
attenuation of petroleum hydrocarbons can play in achieving overall
site remediation.
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