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Manuscript (Go to Abstract)
Technology Sequencing to Reduce Groundwater Remediation Costs
Introduction At many locations, soil and groundwater have been contaminated by spills and leaks of fuels, solvents, cleaning fluids, process chemicals, etc. Unfortunately metals, chlorinated solvents, gasoline components, fertilizers, pesticides, etc. are frequently detected in soil and groundwater at concentrations which create a threat to human health and/or the environment. It is necessary to define the nature and extent of the contaminated soil and groundwater and to design and implement remedial response action to reduce the threat to acceptable levels. There are available remediation technologies that can address contamination in a timely, cost-effective manner. Figure 1 shows some typical sources of contamination and the general sequence of response action.
Figure 1
Assessment
The first general step in the process of addressing site contamination is a detailed site assessment to determine:
Each site is unique, and it is critical to complete an objective and thorough site assessment so as to establish the basis for timely, cost-effective remedial action. Site assessments tend to be iterative in that the information generated in the initial phases will help focus the subsequent assessment phases. Site assessment planning needs to focus on defining the chemical, physical, and "social" issues while protecting the receptors. Some specific receptor protection action may be required during the assessment if impacted receptors are discovered. Figure 2 emphasizes 8 private drinking water wells impacted by gasoline. Figure 3 shows a typical POET system. Once the receptors are protected, then the project can focus on completing the assessment and on developing and implementing a timely, cost-effective remedial response plan. The assessment activities can be compatible with the overall remedial plan as in the case of a complex site shown in Figure 4; the assessment groundwater monitoring wells were designed to function as dual-phase extraction wells.
Figure 2
Figure 3 Point of Entry Treatment (POET) Systems
Figure 4 Large Scale, Complex Superfund Site
Analytical/QAQC/Data Management
The site assessment, the remedial investigation, the remedial design, the remedial action, and the site closure are driven by sampling and analysis of the affected media (typically: air, soil, sediments, and water). From the initial "discovery" to the final site "closure" analytical results control the entire process. Table 1-1 shows the interaction of the project phases with the sampling, analytical, and QAQC issues.
Sampling, sample handling, analytical QAQC, and data management can be expensive and time-consuming. In some cases, there is a tendency to limit the media tested, the number of samples, the analytical slate, the detection limits, and the QAQC detail in order to save time and cost; it is critical to make objective decisions regarding the analytical issues and to think ahead to the overall project.
Each step of the overall project depends on the data generated during the previous steps; errors, lack of data, and incorrect assumptions (based on limited data) can have a negative impact on the overall project.
An effective progress measurement and control system includes:
a. Flexible analytical and QAQC b. Cover all media and matrices c. Data use drives process
It is critical to understand analytical variability and detection limit issues in order to make cost-effective sequencing decisions; Figures 5 and 6 illustrate the significance of these issues. Figure 7 emphasizes the importance of a close working relationship between the laboratory and the project.
Figure 5
Figure 6 Concentrations Reported as BDL
Figure 7 Program-Controlled Analytical Capability
Anticipated Method Changes from USEPA
Remediation Technologies
There are numerous proven technologies available to remediate contaminated soil and groundwater. These technologies cover source control, free-phase remediation, dissolved-phase remediation, and natural attenuation (passive remediation). The typical remediation phases are:
The goal is to get to monitored natural attenuation in a timely, cost-effective manner.
The various remediation technologies typically have an optimum efficiency and cost range depending on site-specific conditions as illustrated in Figure 8. Table 1 shows some common technologies used to remediate soils and groundwater contaminated by volatile organic chemicals (VOCs). For example, in-situ thermal desorption, combined with soil vapor extraction and thermal oxidation, can be cost-effective for high concentrations (200-300 ppm) of volatile chlorinated organics in the vadose zone; when concentrations reach 20 to 30 ppm, conversion to in-situ bioremediation would be cost effective. Table 2 adds typical costs and duration to the technology sequence. The key is to use the site progress data to make sequencing decisions based on optimum efficiency.
Table 1 Remediation Technology Sequencing
Table 2 Remediation Technology Sequencing
Figure 8
Remediation Principles
The basic remediation principles are:
Most established technologies are easy to apply, and advance planning will accommodate sequencing. For example, the key to effective source control is to address the source quickly. Figures 9, 10, 11, and 12 show typical remediation systems. Tables 3 and 4 list the issues associated with phytoremediation and natural attenuation.
Figure 9 Air Sparging/SVE
Figure 10 Dual Phase Extraction
Figure 11
Figure 12 Mobile Well Service Unit
Case Study #1
Organic Chemical Disposal Site
Background:
Dumped aromatic organics, chlorinated organics, alcohols, and ethers in shallow pits for disposal. The first water-bearing zone at 18' to 28' below ground surface was affected; six separate plumes in 2 to 2-1/2 square mile area; plumes are migrating at about 10 to 20 feet/year. There were several pockets of significant free-phase on the site.
Assessment:
Sixty-five monitoring wells and 70 soil borings defined the nature and extent of the affected soil and groundwater and identified the chemicals of concern as benzene, 1,1-DCE, naphthalene, and TBA. Figure 13 shows the remediation areas.
Figure 13 Case Study #1
Remedial Action Plan:
Excavate and chemically-oxidize 700 yd3 of soil containing free-phase. Excavate, biodegrade, and dispose of 600 yd3 of soil containing free-phase. In-situ thermal desorption, soil vapor extraction, and thermal oxidation to remediate the elevated concentrations of the COCs in the vadose zone. Convert to aerobic in-situ bioremediation with pump and treat when VOC concentrations are < 20 ppm. Sample and analyze the target wells monthly. Convert to monitored natural attenuation when the VOC concentrations are less than 500 ppb. Figure 14 shows the thermal desorption/in-situ bio/soil vapor extraction area. Figure 15 shows effluent natural bioattenuation area. Figure 16 is a typical remediation progress curve.
Figure 14 Case Study #1 - Standardized Equipment
Figure 15 Case Study #1 - Effluent Bio-attenuation
Figure 16 Case Study #1
Case study #2
Marine Terminal Expansion Site (Houston, Texas)
Background:
Spilled chlorinated cleaning solvent contaminated the first shallow water-bearing zone at 10' to 20' bgs; the plume is about 40' long x 80' long near the property boundary; the plume is migrating at about 10' per year. The source and the free-phase have been removed; the shallow (to about 5' bgs) TCE has evaporated; the only issue is the plume in the first water-bearing zone.
The property is prime commercial real estate. The existence of the TCE plume has prevented the sale and economic development of the property.
Assessment:
Installed seven "perimeter" monitoring wells to evaluate the overall site and to target specific areas of concern. Installed six monitoring wells in the single area of concern. Also confirmed that there was no immediate risk to the public health and to the environment. The only issue was the shallow TCE plume. Completed eight, continuously sampled, soil borings in the area of concern; the soil analytical data confirmed that the shallow soils were no longer a source of TCE to the shallow groundwater. Figure 17 shows the site layout.
Figure 17
Response Action Plan:
Pump and treat the affected groundwater from three strategically located wells to reverse the groundwater flow gradient; pump approximately 2.5 gpm; treat with double-pass carbon absorption. Add nutrients (di-ammonium phosphate, potassium nitrate, and potassium sulfate) to the treated water and inject at the edge of the plume at five locations; use a portable unit and "operate" the system eight hours, twice per week. Sample and analyze the pumping wells and the carbon effluent weekly to measure progress. Figure 18 shows the progress data for TCE. Figure 19 shows the mobile remediation unit.
Figure 18
Figure 19 Mobile Well Service Unit
Case Study #3
Gas Station, Mini Mart (Omaha, NB)
Background:
Leaking UST's; two tanks may have leaked for 10 years or more. Soil and groundwater impacted to about 30' deep; shallow potable wells about 600' down gradient have been impacted; point-of-entry treatment (POET) systems have been installed at 28 residences. The chemicals of concern are benzene, MTBE, and TBA. The LUSTs have been removed along with about 300 yd3 of contaminated soils.
The property is prime commercial real estate when combined with adjacent parcels of land; the property is the cornerstone of a planned development; the affected groundwater has impacted the overall project financing.
Assessment:
Completed two phases of soil borings and monitoring wells to define the nature and extent of the affected soils and groundwater. Identified free-phase in the vadose zone soils and in the groundwater; the dissolved plume is about 100' wide by about 700' long in the first water-bearing zone at 20' bgs to 30' bgs. The concentrations of TBA and MTBE are decreasing on the leading edge of the plume. The permeability and transmissivity across the affected zone is relatively uniform. The analytical data confirmed that the source control has been effective. Figure 20 is a schematic of the plume and the response plan.
Figure 20
Response Action Plan:
Soil vapor extraction, thermal oxidation, and pump and treat in the high concentration zones of the dissolved plume and of the vadose zone. Use dual-phase (water and vapor) extraction wells to expand the vadose zone down to the aquitards at about 30' bgs.
Extract approximately 200 scfm of soil vapor from six extraction wells and treat in portable thermal oxidizer. Pump about 4.5 gpm of water and treat in an activated sludge aerobic bioplant; add oxygen and nutrients to the treated water and inject into nine injection wells; start the in-situ bioremediation when the BTEX, TBA, and MTBE concentrations are all below 10 ppm. Convert to monitored natural attenuation when the benzene is less than 120 ppb, the MTBE less than 150 ppb, and the TBA is less than 200 ppb. Figure 21 shows a schematic of the aerobic in-situ bio system. Figure 22 shows the chemical concentrations at two progress monitoring wells.
Figure 21
Figure 22
Case Study #4
Fixed base operation (NW Montana airport)
Background:
Private plane service and maintenance operation adjacent to public commercial airport; fixed base operation has been active for 70 years. Spilled/leaked fuel and maintenance solvents over the years have contaminated the local soil and groundwater to about 60' bgs; the regional groundwater at about 400' bgs has not been contaminated by the site; the groundwater plume could impact private potable wells in the future. The chemicals of concern are benzene, MTBE, 1,1-DCA, and vinyl chloride. Source control has been effective; localized elevated chlorinated levels in soils continue to be a source to the groundwater.
The property has high value as part of the overall commercial development of the airport area. The original owner lacked resources to complete the remediation and manage the liabilities long-term; the contamination restricted the ability to finance the property purchase and the property development.
Assessment:
Soil borings and monitoring wells defined the nature and extent of the soil and groundwater contamination. Some small isolated pockets of free-phase chlorinateds exist in the vadose zone soils; there does not appear to be any free-phase on the groundwater. The dissolved plume is about 90' wide by 400' long and is migrating toward the northeast at about 10' per year; only the first water-bearing zone at 40' bgs to 60' bgs is impacted.
Response Action Plan:
Ozone/peroxide oxidation of localized free-phase; pump and treat and in-situ anaerobic bio to reduce the 1,1-DCA levels to less than 5 ppm; convert to aerobic in-situ bio to reduce all the target chemicals to less than 100 ppb each; convert to monitored natural attenuation.
Case Study #5
Tank Farm, Former oil production area (Houston, Texas)
Background:
Oil and gas production area and fuel and chemical storage tank farm since the 1950's; still some ongoing operations; most of facilities and tanks are inactive. Upscale residential construction has occurred over the last several years up to the perimeter of the site. The property is a prime residential development site. The current local residents are serviced by a municipal utility district, thus the local residents are not directly impacted by the site. Scrap equipment, surface staining, salt water impacts, large excavation pits, etc. create a negative appearance. A residential developer has acquired the site. Figure 23 shows the site and surrounding area in 1995. Figure 23 1995 Aerial Photograph
Assessment:
A phase-1 and a phase-2 environmental site assessment defined the site issues. A detailed remedial investigation generated the data necessary to develop the remedial response plan; several areas with surface (to a depth of 4') organic contamination were delineated; salt water has impacted shallow soils and vegetation in several areas; evidence of surface organic spills and leaks in the process equipment and tank farm area. No significant groundwater contamination. No contamination in the excavation pit sediments. Figure 24 shows the assessment details.
Figure 24 Assessment Details
Response Action Plan:
Conclusion
There are many physical, chemical, and biological processes available to remediate a site. A sequence of technologies/processes is usually cost-effective:
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