MAIN AREAS
OF EXPERTISE
SPECIALIZED
SERVICES:
| REMEDIAL
TECHNIQUES USED
Sections:
1. DETECTION OF DEFINITION OF THE PROBLEM In most modern installations, vapour or liquid detectors are now installed which allow for early warning of a spill or leak to the station or bulk plant manager. In abandoned, old sites and those sites undergoing repair or replacement of tanks, the first signs of contamination occur as contaminated soil is revealed around the storage tank itself, the feed lines and/or the dispensing units. The initial response is often to excavate as much of the contamination as possible around the tank. Generally, the physical appearance of the soil in this area, especially if this is the source of the leak, is often the worst. Instead of simply trying to dig out all the contaminated soil, it would be better to simply remove soil in the immediate vicinity of the storage tanks especially if new tanks are to be installed and to halt excavation at that point until an investigation of the extent of the contamination can be undertaken. An investigation may take the form of construction of vertical boreholes converted to monitoring wells, collection of soil samples during borehole construction, and later, water sampling from the monitoring wells. The investigation may also obtain data on the history of the site in order to better determine the potential source and extent of the hydrocarbon plume. An alternative to borehole construction, if it is feasible from an access and a site destruction viewpoint, is to undertake test pitting across the site. Soil, gas or vapour surveys are often used where lighter fuels, such as gasoline, have been lost. These surveys may be conducted prior to deeper drilling in order to maximize the use of borehole and monitoring well construction. Vapour gas surveys are often useful where soil removal is considered and a quick assessment needs to be undertaken. Care should be taken in interpreting results of soil gas surveys due to the fact that gas migrates extensively under sealed surfaces such as asphalt. Soil gas vapour presence is complicated where more than one spill has occurred over a period of time, especially if there has been a small amount of surface spillage and an underground line leak. One may find it difficult to use the data to determine where the extent of the plume is located. Borehole construction is relatively standard depending on the soils which are penetrated. These are generally constructed using solid or hollow stem augers to a particular depth, normally several metres into the water table, with soil samples retrieved on an average of 1.5 m intervals. Following the construction of a borehole, a piezometer consisting of slotted PVC screen and PVC solid casing is normally installed. The screen would be wrapped in geosock and the annulus between the wall of the borehole and the screen backfilled with clean silica sand. A bentonite seal is usually placed to prevent any vertical movement of hydrocarbon contaminated water. Soil samples taken during borehole construction are stored in clean mason jars or clean jars, clearly labelled and with a tight seal. Soil samples are taken to a laboratory where the hydrocarbons are analyzed using a gas chromatogram technique. Water samples are normally taken some hours after the completion of the monitoring well and after thorough cleansing or flushing of the well. Flushing is necessary in order to remove any contaminated water which may have entered the well during the construction and not be representative of the zone which is intersected by the screen. It has been the practice to flush the well three times its total volume of water, although some authors claim that four to five times are necessary to thoroughly cleanse a well. As a rule of thumb, one might consider cleansing four to five times where low levels of contamination are present and flushing three times the volume of the well where higher levels of contamination are known to be present. In this case, the volume of the well also includes the sand pack placed during the well construction. Water samples are stored in clean amber or brown colored bottles, clearly labelled and kept cool during transportation to a laboratory. An investigation to determine the extent of a contaminant plume is normally completed through obtaining water elevation data from the various monitoring wells constructed, levelling the site and tying in the boreholes. The resulting data analysis normally allows the definition of a plume for contaminated groundwater, and the extent of free chemical product and residual contamination present in soil. The confidence level for plume definition in all cases must be
balanced against the extent of the monies expended during the investigation.
The most economic approach appears to be a phased investigation
where an initial four to six boreholes are constructed to obtain
a general overview of the levels of contamination and direction
of flow. Following this, a further set of boreholes can be constructed
to more closely define the plume and to determine the levels of
contamination along the plume flow path. The construction of the
boreholes used for the investigation should be such that these can
later be used for monitoring of whatever remediation system is chosen.
As such, they should be constructed to minimize interference with
activities at the site and yet be protected against day to day potential
damage. A typical site assessment will provide data regarding the presence
or absence of free chemical product. If present, the thickness and
distribution of the product is vital in remediation design. The
presence of a dissolved contaminant plume and its vertical and horizontal
extent, also needs to be defined. Other factors necessary are a
knowledge of: the geology and hydrogeology of the site; the presence
or absence of aquifers; aquifer permeability; the presence or absence
of confining or restrictive layers such as silts and clays; the
possible presence of avenues of escape such as sewer lines, buried
pipe lines; the layout and activities on the site; some history
of the site and the present structures on the site. This data can
be used in the preliminary screening of potential remediation technologies.
Refinement of the choice of the technology to treat the site would then depend, to some degree; on the economics of implementation, the applicability of the particular remediation technology to the site, together with the cleanup limits expected and interference with the activities at the site. Often, the best method of cleanup is a combination of available technologies. Future implications also need to be considered. As cleanup proceeds, it is expected that any free product will be collected and therefore the free chemical product plume would decrease both vertically and horizontally. Also, the dissolved contaminant plume would be captured, contained, steadily cleaned and would therefore decrease in size. The activities at the site may also change throughout the remediation period even include transfer of ownership. Spilt contaminants may contain additives that affect its distribution in the environment. For instance, some components of gasoline such as methyl-tertiary-butyl-ether (MTBE), which are added during refining, are not normally identified during analysis. This ether is more soluble that the BTEX hydrocarbons and therefore may arrive at a monitoring well far in advance of any BTEX components. The presence of MTBE may also induce greater dissolution of the petroleum product. The horizontal and vertical composition of the contaminant plume with regards to the presence of different chemical species impacts on the type of remediation chosen. MTBE and fuel oil, for instance, are relatively difficult to remove by air stripping due to the high solubility of the MTBE and the low volatility of the fuel oil. Gasoline/petroleum, however, is relatively easy to air strip. This kind of information will direct, initially, the choice of potential remediation technologies. Remediation of the site needs to be periodically reviewed and may
necessitate changes to ongoing remediation to maximize efficiencies
of cleanup. For example, once the free product plume has been reduced
to a degree where other treatment methods may be applicable, then
the skimmer pump (for instance) may be removed from the site with
the skim of product remaining entering a Granulated Activated Charcoal
(GAC) unit. In other instances, it may be economically and physically
viable to use surfactants after the more mobile product has been
collected to maximize product collection. It is possible that it
may become necessary to redesign or reconstruct the remedial wells
installed to treat dissolved product. The plume may originally have
been present over the entire vertical depth of the aquifer but,
as the lower sections are cleaned so it becomes be more efficient
to reduce the intake screen only to capture contaminated water from
the upper zones of the aquifer. This would lead to savings in the
cost of treatment and reduce the operation and maintenance necessary
at the site. A number of cleanup technologies are commonly used for remediation.
New technologies are also being created and old ones may be modified.
Many of these may not be applicable to the site geology, site size
or economically viable when considering treatment options readily
available. The more commonly used technologies for remediation are
presented. Technologies that are commonly used for in situ treatment of hydrocarbons
are: high vacuum and low vacuum extraction, oxidation and reduction
techniques; in situ thermal; volatilization, leaching or soil washing,
isolation and containment, bioremediation and passive remediation. Volatile compounds are removed from subsurface soils by drawing air through the soil. This can be undertaken in two ways. First, simply creating a vacuum through the use of a surface pump or combining this by also forcing air into the ground some distance away, thus allowing feed air to access the vacuum system. Air can also be allowed entry into the ground through vents or drilled holes. The volatilization system is effective in soils above the water table for the volatile components of spilt chemicals or petroleum hydrocarbons. Exhaust from the soil has usually been discharged directly to the
air thus making the cost of this process, which is confined to a
few vertical or horizontal wells and a vacuum pump, relatively low.
Changes in the way regulatory agencies view this process may, however,
add to the requirement for a carbon absorption unit to treat exhaust
vapours. Even with this, the process remains relatively cost effective. This process requires the application of water and surfactants to in situ soils containing hydrocarbons capable of being leached. The resulting water-surfactant-mixture must then collected downstream of the injection point. Injection or application of the washing solution can be placed into or above the water table. Injection and collection wells are constructed using standard procedures. Difficulties may arise in soils which possess rapid vertical and horizontal changes in permeability which may serve to channel the soil wash solution thus reducing the effectiveness. It is apparent therefore that soil washing is most applicable in granular soils were sufficient geologic knowledge is available to properly inject the solution to achieve a high level of success. The solution pumped from the remedial/correction wells will contain the target chemicals, surfactants and water. Treatment of this effluent will require GAC or biodegradation. The process, if applicable, can be relatively cost effective when
measured against the increased percentage of product collected in
the remedial wells and the length of time that pump and treat would
be required if the soil was not washed. 4(a)(iii) ISOLATION AND CONTAINMENT In some cases, isolation and containment of hydrocarbon contaminated soil may be considered a viable option. The approach would involve the construction of vertical barriers around the contaminated area with the installation completed by an impermeable cap. Barriers may be composed of a synthetic membrane such as high density polyethylene (HDPE), slurry walls composed of a cement, or bentonite mix. The same materials could be used as the final capping agent together with asphalt, soil/bentonite mixtures and clay. In situations where chemicals may be present beneath a building inside the footings or foundations, isolation and containment of the chemicals in this area may be feasible, providing that vapour does not cause problems to the basement itself. This becomes economically attractive where the only other option is removal of a building, excavation or long term in situ treatment. Areas where chemicals are present in low permeability soils, may lend themselves more readily to isolation and containment. The immobilization of the contaminant in this way does not destroy or remove it, therefore whatever liability exists at the beginning of remediation will likely remain for many years after remediation has been completed. Containment techniques may be more efficiently used in conjunction with other remediation techniques such as pump and treat where the vertical barrier would be used to deflect or contain a plume of hydrocarbons with the pump and treat removing the hydrocarbons from the ground. Where contaminants are spilt into soils far above the water table, then capping together with an appropriate vertical barrier may prevent any movement of water into the area thus immobilizing the target chemical from the area. The success of application of isolation and containment depends largely on the choice of barrier material and the method and care with which the barriers are emplaced. Where groundwater containment is required, in situ placement of vertical walls or barriers may require different materials and techniques. Slurry or cut-off walls may be used or grout may be placed in the form of a curtain to cut off groundwater flow. In some cases, sheet piling may be the most appropriate method of forming a cut-off wall. Methods of isolation and containment tend to be more expensive
than other remediation methods and leave some doubt as to their
integrity due to the difficulties of placing these barriers and
achieving a degree of confidence in their continuity and integrity. In situ biodegradation utilizes the natural ability of microorganisms to metabolize organic matter. In the case of hydrocarbons, the petroleum organic forms a specialized substrate for the bacteria, to which they are not normally adapted or able to metabolize. The ability of microorganisms to alter their genetic structure to produce the necessary enzymes to effect degradation lies at the basis of this process. In areas where biodegradable organic chemicals have been spilt over a period of years, the indigenous microflora may have adapted to metabolizing them as a food source. The presence of toxic materials such as heavy metals (e.g., lead) however, may have prevented or restrained adaptation by inducing mortality in the bacterial population. Intermediate degradation products may be produced by the in situ bacteria which, for instance, metabolize only a small range of hydrocarbons. These by-products may exhibit toxic effects on other bacteria which are capable of degrading a different suite of organic products or byproducts resulting in the retardation or destruction of this second group. Adapted bacteria may not be adequately distributed throughout the contaminant plume due to past distribution and availability of the organic contaminant, or previously adapted bacteria may have reverted to a non-adapted status. The above points emphasize the requirement to undertake bioassay and bench test work on samples from potential bioremediation sites. The discussion also points out some reasons why bioremediation may fail to deliver the expected results. For instance, the approach to bioremediation of in situ hydrocarbons utilizes the addition of oxygen and nutrients to the aquifer system to encourage rapid growth of the indigenous biomass. The amplified growth will also provide impetus for adaptation of the non-adapted bacteria. Adaptation may take several months to several years to accomplish. These are factors of concern that can be eliminated by culturing the indigenous bacteria in an ex situ environment. Proven adapted hydrocarbon degrading bacteria are withdrawn from a polluted environment and grown en masse using a fermenter - bioreactor. This fermenter may be available on site and be part of the pump-treat recharge system, or can be located some distance from the site where the fermented bacterial concentrate is delivered to the site. The use of a bioreactor covers several of the concerns raised earlier. Application of oxygen and nutrients is as critical to the process as the presence of adapted bacteria. Bacteria require both oxygen and nitrogen to convert the hydrocarbons to proteins, carbon dioxide, water, energy and minerals. Difficulties often arise in ensuring that both of these necessary parameters are delivered to the in situ biodegradation sites and are available to the microorganisms. Permeability changes in subsurface soils are the main cause of poor distribution of oxygen and nutrients. Other factors also affect bioremediation, such as pH and salinity. The application of oxygen and nutrients is accomplished through
injection ports such as galleries and wells to the aquifer. Where
necessary, and if no bioassay work is available, a bioreactor culture
may also be injected to ensure adapted bacteria are present and
properly distributed throughout the aquifer. Oxygen may be added
as dissolved air, through in situ air sparging, surface aeration
(air stripping), or in the form of hydrogen peroxide or as free
oxygen from solid peroxide chemicals.. The nutrient-oxygen mixture
migrates through the aquifer to a collection well(s) where it is
captured for treatment or recirculation to the surface where additional
nutrients are added and the water is re-oxygenated. It is also at
this stage that a surface bioreactor can be employed and/or culture
added. Recirculation of water is continued to ensure contact of
the solution with all hydrocarbon contaminated soils requiring remediation. In some situations where the environmental risk from spillage of chemicals is low, for instance, in an area where no underground services exist, buildings and structures are located at some distance from the spill, and where low permeability soils are present, passive remediation may be considered as an appropriate approach. In these situations, the spillage and movement of the product or dissolved product would simply be monitored over a period of time. If migration of the product and/or dissolved plume is slow, then in situ degradation processes may be sufficient to essentially contain and treat the hydrocarbons. If migration of the plume in low permeability soils conforms to a timeframe which prevents the dissolved contaminant or free product leaving the passive treatment zone, then such processes such as fixation, dilution, biodegradation, chemical alteration, absorption, etc. may come into play and retard the migration of the plume sufficiently to affect acceptable containment. In high permeability soils where dissolved contamination is present
at low levels, dilution, biodegradation, absorption, etc. may be
highly effective within a reasonable distance from the source of
the chemical and be accepted as a treatment approach. Several technologies exist to treat soil external to the site.
These include: incineration, solidification and stabilization, soil
washing and bioremediation. Incineration of soils has been a fairly common method of remediation although with the advent of climate changes and the Kyoto Protocol, incineration is finding less favour as a remedial treatment for contaminated soil due to the high volume of carbon dioxide discharged. The most common type of incinerator is a Low Temperature Thermal Desorber. This incinerator roasts contaminated soil using a propane gas heat source with the volatilized product being passed through an afterburner. Other forms of incineration techniques employ fluidized bed technology, where the soil is pulverized to a consistency capable of being floated on a bed of air as it is passed through a burner system. Rotary kilns are also used to physically bum the soil in air. Thermal oxidation is also used where the volatized product is oxidized over a catalyst to carbon dioxide and water. The low temperature thermal stripping system would normally comprise of a hopper to receive the feed soil, the thermal processor contains a screw or bed to move soil through the heater. Attached to this is a heating system providing temperatures in the range of 300°C. Volatilized hydrocarbons from the soil are directed to a combustion air blower and afterburner. A rotary kiln operates at much higher temperatures and receives soil feed into the kiln itself. Combusted air is continually fed into the same chamber where the soil is burnt. The resulting ash is passed to a hopper system with the off gas going to a scrubber. An afterburner is used to ensure complete combustion of gases. Few restrictions exist to the use of incineration with regards
to the soils. Debris and large rocks, etc. would, however, have
to be removed from the soil feed stream and treated by some other
means such as bioremediation. 4(b)(ii) SOLIDIFICATION AND STABILIZATION While not widely used, these processes offer an acceptable alternative
to more commonly used methods. Solidification entails the addition
of liquids and/or solids to contaminated materials (e.g. sludges).
Stabilization addresses the addition of 'binding agents' to contaminated
soil to immobilize the contaminants. Soils thus treated can be landfilled
or isolated. The intention of the above processes are to limit solubility,
increase handling ability and decrease reactive surface area of
the contaminant medium. Cement, lime and fly ash based processes
can be used to solidify heavy oily wastes such as sludges. Excavated soil can be treated by washing with a solvent/water or surfactant/water mixture to remove hydrocarbons. The process is similar to that undertaken in situ with the main difference being that excellent control is exercised over the process. Also, the ability to use mixtures of chemicals not suitable for in situ use due to environmental effects is available. This method is highly effective but the costs are high due to the degree of soil handling, chemical costs and the precautions needed. Excavated soil can be placed in a container or on a perforated place or sieve. Solvents or surfactants are added through spraying or flooding the container. Mixing of the soil can also be undertaken to increase efficiency of chemical contact and hence extraction. The soil is then drained or filtered and the extraction agent and hydrocarbons removed. The liquid effluent goes to a clarifier for treatment and finally
to a GAC polisher. 4(b)(iv) LAND TREATMENT - BIOREMEDIATION In the past, landfarming using bioremediation has been commonly practiced by refineries to destroy oily sludges and other by-products. More recently, the construction of closed, monitored piles have been used to effect destruction or organic chemicals in contaminated soil. The latter construction method is preferred over landfarming or venting as emissions of benzene and other undesirable volatile chemicals to the atmosphere are substantially reduced. Landfarming using tillage, turning and air venting are relatively passive methods of remediation. Nutrients may also be added to the soils during these processes to facilitate breakdown. More control of volatile emissions and of the biodegradation process is practised under covered soil pile structures. Nutrients are added to the soils either as the piles are constructed or later through surface ports or vents. Oxygen is normally drawn through aeration tubing laid in the pile or applied under pressure so that oxygen diffuses outward through the soil. The biopile is normally covered until remediation is completed. Where necessary, as in in situ bioremediation, the addition of bioculture may be necessary to provide more timely completion of biodegradation or to ensure the presence of adapted microorganisms if bioassay work has not been undertaken. Successful biodegradation of gasoline and fuel oil contaminated
soils with concentrations up to 26,000 mg/Kg has been accomplished
in a number of months. The rate of biodegradation is greatly affected
by temperature, the presence of moisture and other factors such
as pH, salinity and the presence of toxic materials. Distribution
of nutrients and oxygen in soil piles is also a limiting factor.
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ARC Geobac group inc.
380 Smythe Street
Fredericton, NB
Canada E3B 3E4
Tel: (506) 451-1991
Fax: (506) 457-2100
