TM 5-814-7
(3) Basic equipment for subsurface injection of
(1) Most importantly, injection wells are
wastes consists of a truck or tractor with two or more
considered by EPA policy to be a 'qast resort" means of
chisels attached. Adjustable sweeps are mounted near
disposal. It must be demonstrated that the injected fluids
the bottom of the chisels to open a wide but shallow
will not contaminate ground-water or damage the
underground cavity. Waste is injected into the cavity
environment, and injection is used after all other means
through a tube attached to the back of the chisel. For
of disposal are found unsatisfactory.
repeated application of wastes over long time periods,
(2) In addition, types of wastes to be disposed
underground supply pipes may be installed, with flexible
of may limit disposal options: only liquid wastes may be
tubing used to connect the supply pipe to truck or tractor-
disposed of in injection wells. Injected wastes are strictly
mounted injectors.
covered in UIC; justification for injection must be
presented and pretreatment of waste streams may be
required prior to injection.
5-5. Deep Well Injection
(3) To ensure their separation from drinking
a. Suitable Wastes. Injection wells are used to
water aquifers, injection wells are limited to sites that are
dispose of large quantities of liquid hazardous wastes
in geologically isolated environments.
Extensive
into the subsurface. Injection well disposal is regulated
geologic research and field work must be done to site
by the EPA Underground Injection Control Program
wells and to determine injection zone isolation. Injection
(UICX40 CFR 146) and authorized by subpart C of the
horizons must be tested for waste compatibility to ensure
Safe Drinking Water Act. Currently injection wells may
that the wastes do not contain materials that are
accept large quantities of chemical, waste-water brines
chemically reactive with site soils or rock. Waste
or mining wastes in deep, isolated porous geological
constituents that could pose problems include corrosive
formations. Large volumes of waste, on the order of
mineral salts, acids (capable of dissolving carbonate
hundreds of thousands or millions of gallons, may be
rock), and precipitated salts. In addition, the proposed
disposed by injection. Approximately 160 injection wells
injection area should be tested for overall permeability to
are now operating, with most used by the chemical and
define the injection zone. Typical siting investigations
petrochemical industry.
and well developments and construction information is
(1) A wide variety of wastes can be disposed by
found in comprehensive technical documents (EPA
injection. These wastes include, but are not limited to:
600/2-77-240).
Dilute or concentrated acid or alkaline
(4) Another disposal limitation is the existence
solutions
of unexpected subsurface problems such as pressure
Solutions containing metals
around the formation, induced earthquake activity and
Inorganic solutions
dissolution of injection zone host rock. The precautions
Hydrocarbons and chlorinated hydrocarbons
concerning location of landfills in karst terrain or seismic
Solvents
zones 3 and 4 also pertain to injection wells (see para 5-
Organic solutions with a high biochemical
2b(2)). Pressure mound formation may result in a
oxygen demand
"mound" of injected fluid that forms near the injection
(2) The UIC criteria and standards cover
well hose and interferes with rates of fluid injection and
construction, operating, plugging and closure of deep
ground-water flow. Low magnitude earthquake swarms
wells, and monitoring and reporting requirements. The
may be caused by injecting fluids into deep fault zones;
UIC classification of injection wells is as follows:
such a case was documented at Rocky Mountain
Class I
- Injects hazardous wastes as defined
Arsenal in the 1960's. Finally, host rock may dissolve if it
in 40 CFR 146, subpart A
is incompatible with the injected waste, thereby creating
Class II
- Injects
petroleum
fluids
or
voids at depth and possible subsidence effects.
byproducts
(5) Worst of the subsurface problems is aquifer
Class III
- Injects fluid for mineral extraction
contamination as a result of injection. Contamination
Class IV
- Injects fluids into or above an
could occur as a result of incompletely plugged
underground drinking water source
abandoned injection wells, displacement of saline water
Class V
- Injects fluids not covered in
into potable water, or well bore failure.
Classes I-IV
(6) Finally,
the
substantial
costs
of
b. Disposal constraints. The injection well disposal
implementing injection well disposal systems are a
option is limited by
significant limiting factor; these systems require much
regulations and policy
professional expertise in site evaluation, testing,
waste types
construction and waste stream analysis. Furthermore,
selective geological environment
the system requires stringent monitoring and
construction and operation expense
maintenance to ensure good operation. Costs for typical
Class I-EI type
5-12
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