Petroleum oils and lubricants (POLs) containing contaminants such
as benzene, toluene and xylene and solvent nonaqueous phase liquids (NAPLs)
containing contaminants such as trichloroethylene (TCE) and perchloroethylene
(PCE) are the most common problem at DoD, DOE, and EPA contamination sites.
The Department of Defense alone is responsible for cleaning up contamination
at over 10,000 active sites at more than 700 military installations (Tennesen,
1993). Up through 1996 DoD had spent approximately $15 Billion in its environmental
restoration program (Budget of the US, 1997). Contaminants in soil and
ground water limit reclamation at 75% of the sites. The remaining
25% of the contaminated sites were reclaimable through surface activities.
NAPL source areas provide a long-term source of contamination impacting
aquifers and surface waters. "Pump-and-Treat" is currently the technology
of choice at 90% of sites. However, extraction of contaminants by "pump-and-treat"
systems has been shown to be ineffective. Extraction of contamination from
the subsurface is limited by:
To understand the significance of an immiscible phase, consider a situation
where PCE was disposed with other organic liquids and the mole fraction
of PCE in the NAPL is 0.1. Also assume: the mixed NAPL is trapped uniformly
in the soil pores at a saturation of 5% of the porosity of a homogeneous
formation; PCE concentration in the aqueous phase as predicted by Raoult's
law and the mole fraction remains constant throughout the dissolution process.
Under this hypothetical situation the maximum concentration of PCE in water
is 16 mg/l where the solubility of PCE is 160 mg/l. PCE has a density of
1.6 g/cc. Under this hypothetical situation approximately 5,000 pore volumes
would be needed to dissolve the PCE. For the most part, this scenario represents
optimum conditions for PCE dissolution, however, PCE removal would be somewhat
accelerated if the mole fraction of PCE increased during the dissolution
process (the NAPL was composed predominantly of compounds having higher
water solubilities than PCE). Conversely, if PCE is more soluble
than most other NAPL constituents, the mole fraction of PCE would decrease
over time resulting in lower aqueous concentrations and slower removal.
1.1.2 Aquifer heterogeneity
Soils and aquifers are not homogeneous and the contamination is rarely (if ever) uniformly distributed. Before a contaminant can be dissolved in a pump-and-treat system, a flow line must pass by the contaminant. Therefore, extraction efficiency is rarely (if ever) 100%. If one estimates the extraction efficiency to be 5% and there was no transformation of the contaminant, the extracted water from a pump-and-treat system described in 1.1.1 would have a PCE concentration of 800 mg/l, and it would require 100,000 pore volumes to achieve remedial objectives.
1.1.3 Sorption to aquifer solids
Sorption to aquifer solids acts to retard the movement of the contaminants.
For most solvents and POL contaminants of concern, this is usually a small
factor. For PAHs and PCBs, however, the retardation can be significant,
due to their high partition coefficient, increasing the time or pore volumes
required before remedial objectives are achieved.
As illustrated above when a NAPL is present, the time required to
achieve remedial objectives using pump-and-treat or natural attenuation
is long. Either of these alternatives leads to monitoring costs that do
not have a foreseeable end. Intera (1998) reported that the typical operational
cost for a pump-and-treat system is $400 to $500 thousand per year. Many
are suggesting that achieving remedial objectives is technically impracticable
at NAPL sites. The fiscal commitments being made to pump-and-treat
systems often will have little impact on environmental cleanup and will
be limited in their effectiveness to the prevention of the spread of contaminants
to previously uncontaminated areas. The objective of this report is to
document research findings that suggest remedial objectives are technically
achievable in a reasonable time frame. The program demonstrated in the
field the effectiveness of emerging technologies using side by side comparisons
in controlled experiments. The performance of each approach was compared
to "pump-and-treat" technology to normalize the variability at the site.
The first site selected for evaluation was Operable Unit 1, Hill Air Force
Base, Utah, USA. This site is highly contaminated with a light nonaqueous
phase liquid (LNAPL) containing a large variety of contaminants of environmental
concern. Nine different technologies were evaluated and compared to pump-and-treat
technology. Table 1.2.1 outlines these technologies
Table 1.2.1 Summary of Technologies Investigated
| Technology | Remedial fluid | Advantages | Design issues |
| Pump-and-treat | Water | Accepted technology | Very slow process requiring continuous monitoring and operation and maintenance cost, need to establish institutional controls suitable for decades or centuries |
| Cosolvent solubilization | Low molecular wt alcohols | Fast, easy to manage and operate, can achieve remedial objectives in reasonable time frame using food grade biodegradable materials | Potential unstable flow conditions, potential density override of remedial fluid, waste handling |
| Cosolvent mobilization | High molecular wt alcohols | Fast, easy to manage and operate, can achieve remedial objectives in a reasonable time frame | Potential loss of hydraulic control at DNAPL sites, unstable flow conditions, density override potential, waste handling |
| Surfactant solubilization | Hydrophillic surfactant | Faster than pump-and-treat, products used as surfactants can be food grade additives. | Waste handling, potential creation of liquid crystals at DNAPL sites |
| Surfactant mobilization | Hydrophobic surfactants | Fast, can achieve remedial objectives in a reasonable time frame | Potential for the formation of macro molecules or liquid crystals, potential loss of hydraulic control at DNAPL sites, potential separation of remedial components, waste handling |
| Surfactant micro-emulsion | Surfactant and alcohol cosolvent | Fast, can achieve remedial objectives in a reasonable time frame | Waste handling |
| Macro molecule | Cyclodextrin | Reasonably fast, remedial fluid biodegradable | Remedial fluid biodegradable some may be lost during reprocessing, waste handling, preferentially removes smaller compounds |
| Steam | Steam | Fast for volatile substances, relatively low cost remedial fluid | May move contaminants to new locations without removal, preferentially removes more volatile components |
| Sparging/Venting | Air | Low cost, stimulates oxidation of bio-degradable contaminants, no water brought to surface | Slow for low volatility compounds, difficult to get good distribution of air |
| In-well aeration | Air | No water brought to surface, stimulates oxidation | Oxidation primary source for removal, slow for low solubility compounds |