Remediation Technologies - Environmental Investigation and Remediation - page 330

7.2.5 S UMMARY
The following sorptive media have been tested to evaluate their capacity to remove 1,4-dioxane
from water: SMZs, SMZ with ZVI, Tri-Base Carbon, MPPs, GACs made from agricultural by-
products, organoclays, and palladium-111. Although some of the media tested demonstrated removal
rates of up to 50%, the test conditions required high media:water ratios that would not be feasible in
many i eld applications. Specii c sorbents may be a consideration in situations with low levels of
1,4-dioxane and low groundwater extraction rates.
7.3 NATURAL ATTENUATION
Natural attenuation, or monitored natural attenuation (MNA), has been used to demonstrate passive
remediation at sites contaminated with a variety of organic and inorganic contaminants. MNA is
most commonly applied to sites contaminated with chlorinated ethenes and petroleum compounds,
primarily because these are among the most common contaminants at hazardous waste sites and
there are published protocols for leveraging natural attenuation of these compounds for site reme-
diation. The U.S. Environmental Protection Agency's (USEPA's) policy directive on natural attenu-
ation describes MNA processes as those that “act without human intervention to reduce the mass,
toxicity, volume or concentrations of contaminants in soil or groundwater” (USEPA, 1999). Natural
attenuation of contaminant mass is attributable to a variety of physical, chemical, or biological pro-
cesses that can include biodegradation, dilution, dispersion, advection, sorption, volatilization,
radioactive decay, and chemical and biological stabilization, transformation, and destruction of con-
taminants; destructive processes are preferred over nondestructive processes (USEPA, 1999).
7.3.1 E VIDENCE OF THE E FFECTIVENESS OF N ATURAL A TTENUATION
Demonstrating that MNA will reliably achieve the desired passive remediation typically requires
identifying the active attenuation process and then establishing its effectiveness through several
different lines of evidence. USEPA (1999) has established the following evaluation elements, or
lines of evidence, that should be assessed:
1. Establish historical trends
2. Indirectly verify the attenuation process by using hydrogeologic and chemical data
3. Directly coni rm the attenuation process; for instance, microbial activity can be proven
through microcosm studies
Implementing an MNA solution typically involves demonstrating any or all available lines of
evidence, the availability of MNA protocols for the contaminant of interest, or other factors. Because
MNA has not been commonly used for 1,4-dioxane, implementing MNA for 1,4-dioxane will prob-
ably require a more rigorous weight of evidence to satisfy regulatory authorities. Furthermore, many
of the MNA processes commonly invoked for chlorinated solvent sites may be less effective with
1,4-dioxane because of its chemical characteristics, especially its miscibility, low soil partitioning
coefi cient, and resistance to biodegradation under natural conditions. Demonstrating a natural
attenuation mechanism for 1,4-dioxane is expected to be difi cult.
7.3.2 D IFFUSION INTO I MMOBILE P OROSITY
One potentially applicable attenuation mechanism relates to the hydraulic characteristics of i ne-
grained materials in the subsurface. Finer-grained, less permeable aquifer materials such as clays
and silts in contact with more transmissive, coarser-grained aquifer materials may trap contami-
nants migrating through the aquifer. Fine-grained sediments reduce the effective porosity of the
aquifer to the point that molecules of the contaminant can diffuse from the transmissive zones