Environmental Engineering Reference
In-Depth Information
forces—determined to a large extent by the soil fractions and the pH of the soil-water
system. The various types of soils and their different soil fractions have different sorption
capacities, dependent on the nature and distribution of the HMs and pH of the system.
A four-tiered approach has been developed by the USEPA (2007a) for inorganic contami-
nants as follows:
1. It must be demonstrated that the groundwater plume is not enlarging and that if
immobilization is a dominant attenuation process, then sorption of the contami-
nant onto aquifer solids is occurring.
2. Both the
rate
and
mechanism
of the attenuation process must be determined.
3. The capacity of the aquifer to attenuate the contaminant within the plume and the
irreversibility of the immobilization must be determined.
4. The monitoring program and a contingency plan must be designed based on the
determined mechanisms for the attenuation process and site characteristics.
A second volume in the series looks at speciic MNA information for various non-nuclides
including arsenic, cadmium, chromium, copper, lead, nickel, nitrate, perchlorate, and sele-
nium (USEPA, 2007b). The third volume addresses radionuclides (USEPA, 2010) includ-
ing tritium, radon, strontium, technetium, uranium, iodine, radium, thorium, cesium, and
plutonium-americium. In addition to the previously mentioned mechanisms, radioactive
decay processes are emphasized for these contaminants.
Precipitation of HMs as hydroxides, sulides, and carbonates generally classify as part of
the assimilative mechanism of soils because the precipitates form distinct solid material
species, and are considered as part of the attenuation process. Either as attached to soil
particles or as void pluggers, precipitates of HMs can contribute signiicantly to attenua-
tion of HMs in contaminant plumes. Ion exchange can also play a role. Determination of
the chemistry including pH and
Eh
for assessment of lines of evidence should not neglect
examination of possibilities of precipitation and solubilization of metals as part of the evi-
dence phase. Hydroxide precipitation is favored in alkaline conditions, for example, when
Ca(OH)
2
is in the groundwater in abundance. With available sulfur and in reducing condi-
tions, sulide precipitates can be obtained. Sulide precipitates can also be obtained as a
result of microbial activity—except that this will not be a direct route. Sulfate reduction by
anaerobic bacteria will produce H
2
S and
HCO
3
−
, thus producing the conditions for forma-
tion of metal sulides.
Signiicant knowledge has been gained over the last decade, particularly with regard
to site characterization (USEPA, 2008a,b). For example, at the U.S. Department of Energy
Handford Site, liquid waste with uranium entered an unconined aquifer. It was esti-
mated that natural attenuation could reduce the uranium levels to less than 20 μg/L
in the next 3 to 10 years. Characterization was revised to reduce the uncertainty in
the natural attenuation processes. The newer components include laboratory tests for
sorption-desorption testing measuring mass luxes in the smear zone and evaluating
reversibility of sorption, uranium speciation measurement with X-ray spectroscopy/
diffraction, and electron microscopy with chemical extraction tests, identiication of
clay minerals in the aquifer, and determining uranium distribution according to parti-
cle size. By controlling the lux of uranium into the aquifer and by better characterizing
the mass, and speciation of uranium, it is estimated that MNA can play an important
role in the remediation.
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