Sustainable Water Management - Sustainable Practices in Geoenvironmental Engineering - page 90

larger framework known as the National Hydrography Dataset (NHD) that allows sharing
across programs.
3.4.1 Techniques for Soil and Groundwater Treatment
Contamination of groundwater occurs when chemicals are introduced on or into the
ground through such actions as application of pest control aids, direct and indirect dis-
charge of liquid wastes, transport of leachates containing contaminants escaping from
landills, etc. Decontamination of the groundwater requires reduction (elimination?) of
the toxicity of the contaminants by (a) sequestration, or by (b) chemically and biologically
mediated transformation of the contaminants, and/or (c) removal of the noxious contami-
nants from the groundwater. In most instances, the groundwater of concern is porewater
(water in the void spaces of compact soil). Several techniques are available to manage and
control contamination of groundwater by contaminant plumes, to minimize adverse envi-
ronmental and health impacts. These include (a) construction of impermeable barriers and
liner systems for containment facilities, (b) remediation techniques designed to remove
or reduce (attenuate) the contaminants in the ground such as soil lushing, and (c) passive
procedures relying on the properties of the ground to reduce contaminant concentrations
in leachate streams and contamination plumes.
3.4.1.1 Isolation and Containment
Contaminants can be isolated and contained to (a) prevent further movement, (b) reduce the
permeability to less than 1 × 10 −7 m/s, and/or (c) increase the strength (USEPA, 1994). Physical
barriers made of steel, concrete, bentonite, and grout walls can be used for capping, verti-
cal, and horizontal containment. Liners and membranes are mainly used for protection of
groundwater systems, particularly from landill leachates. A variety of materials are used
including polyethylene, polyvinyl chlorides, asphalt materials, and soil-bentonite or cement
mixtures. Monitoring is a key requirement to ensure that the contaminants are not mobilized.
Most in situ remediation techniques are potentially less expensive and disruptive than
ex situ ones, particularly for large contaminated areas. Natural or synthetic additives can
be utilized to enhance precipitation, ion exchange, sorption, and redox reactions (Mench
et al., 2000). The sustainability of reducing and maintaining reduced solubility conditions
is key to the long-term success of the treatment. Ex situ techniques are expensive and can
disrupt the ecosystem and the landscape. For shallow contamination, remediation costs,
worker exposure, and environmental disruption can be reduced using in situ remediation
techniques.
Sequestration of contaminants can be obtained using encapsulation techniques. The com-
mon ones are known as solidiication/stabilization (S/S) techniques. These are designed to
detoxify and incorporate the contaminants in a solid matrix. Some metals such as arsenic,
mercury, and chromium(VI) are less suitable for these techniques. Monitoring is required
to ensure that the process is stable, and the contaminants are not mobilized. For inorganic
contaminants, the two groups of solidiication/stabilizing agents used are (1) cement, ly
ash, kiln dust, clays, zeolites, and pozzolonic materials and (2) bitumen products, epoxy,
urea-formaldehyde, polyethylene, and resins. Strict requirements for weathering (leach-
ability, etc.) and durability of solidiied and stabilized products have been speciied by
many regulatory agencies. Performance assessment of such products, as with most treat-
ment procedures, is a standard requirement. X-ray diffraction and scanning electron
microscopy may also be beneicial for examining crystalline structures.