Remediation of Contaminated Sites-The Pump and Treat System
By Dr. Yashpal Singh
Ex-situ, contaminated site remediation technologiesinvolve the extraction and subsequent treatment of contaminated ground water. Ex-situ remediation generates two streams one of the contaminated residues which has to be immobilized and disposed safety. The other stream of purified ground water has also to be disposed of either by reinjection into the aquifer, passive infiltration into the aquifer, disposal to land/marine/surface water bodies, evaporation and even disposal to foul sewer systems.Reinjection of remediated ground water and upgrading of a contaminated region effectively combines the technology of hydraulic flushing with the ex-situ treatment. Reinjection, downstream of the contaminated region can serve as a hydraulic barrier to flow from the contaminated region. Whichever disposal option is used, it is important that the environmental impact of that particular disposal is considered.
Pump and Treat remedies often operate for long periods, in some cases decades and can significantly contribute to the energy and environmental foot print of a Pump and Treat remedy. Environmental Impacts may be related to energy use, material handling, wastes generated, impact on ground water (dewatering of wet lands), land use conflicts etc. through the implementation of various processes such as ground water extraction, process equalization, metals removal by chemical reaction and precipitation etc.,air stripping, off gas treatment,granulated active carbon filtration, Effluent tanks, discharges to surface water, building and other long-time operations. It may also lead to net withdrawal of local ground water resources when extracted water is discharged to surface water.Pump and treat is highly effective in remediating highly contaminated waters in relatively short time. Once this high contamination is removed, technologies that are more effective at low contaminant concentrations can be used. Pump and treat can also be used for gradient control to prevent migration of chromium from the site while other clean up technologies are being implemented.
While the concentration of contaminants is initially high in the extraction wells, they decrease significantly with continuous pumping. One of the major problems with the pump and treat systems is that these residual contaminants are much higher than the maximum contaminant levels and would persist for a long time. This tailing is an area of concern.
Early efforts at aquifer remediation relied mostly on the pump and treat systems which involved, extraction of contaminated ground water from the aquifer, treating and either returning it to the aquifer or otherwise disposing elsewhere. The extracted water was replaced by clean water. An assessment done by E.P.A. on 19 sites in 1989 found that many of these systems had achieved mass removal and some degree of plume control as well as a reduction in contaminant concentrations but most of them were not achieving aquifer restoration. The causes of poor performance included sub surface heterogeneity, sorption of chemicals to aquifer solids, the presence of NAPLS, inadequate site characterizations and poor system design. Subsurface heterogeneity generally leads to more accumulation of contaminants in the low permeability zone and it becomes difficult to move these contaminants out of the low permeability zone. Some other limitations in the pump and treat system are the persistence of contamination source zones over a longer period of time, long treatment times, large volumes of ground water to treat and discharge and the high OM costs with loss of land use. Abiotic technologies overcome this problem by the addition of energy to heat the aquifer using steam, hot water, electrical resistance heating or radio frequency heating or electro kinetics’. Permeable reactive barriers are also a cost-effective method.
Where contamination has existed for decades, contaminants reach the low permeable zones from the highly permeable zones through the process of molecular diffusion. During pump and treat remediation, clean water is moved through the more permeable layer at a higher rate them through the low permeable rate. The removal of contaminants from the high permeable areas is therefore more while the removal of contaminants from the lower contaminant lenses is limited by the rate of diffusion into high permeability layers. This diffusive flow of contaminants from the low permeable layers (lenses) maintains the concentration of contaminant in the more permeable lenses at a low level but higher than MCL (Tailing). As the pumping is discontinued the velocity of rates in the more permeable layer decreases and water stays longer in the contaminated zone picking up more contaminants causing the concentration in the ground water in the more permeable zone and hence the extraction water to rise.
Remediation of fractured bed rook is complicated by the difficulty in characterizing the flow system, the potential for small aperture fractures that restrict flow, dead end fractures that become contaminant sinks and diffusion of contaminant in the rock matrix. It is here that the pump and treat systems are more useful as they do not have to rely on a detailed knowledge of contaminant location.
Contaminants can exist within the subsurface in relatively large reserves either as solid-state precipitates or as residual and pooled non aqueous liquids.
Decades of operation may be required for pump and treat remediation to decrease contaminant levels below MCL yet, it is a very important tool.
The potential capacity of the pump and treat system is limited by the nearby topography.If too many buildings or structures are located near the remediation site, abstraction of too much ground water may destabilize the foundations and hence pump and treat may have to be exercised with caution.
The cost of remediation could be made more effective if the decontaminated ground water is used for industrial or irrigation purposes or even for augmenting the drinking water supply.
In order to minimize the environmental impacts, best management practices need to be examined for designing the Pumpand treat. Early planning can include a renewable energy assessment to determine whether solar, wind or other resources could meet all in part of the energy demand of the Pump and treatoperations.
Optimization of extraction rates leads to optimum energy consumption and resource optimization. A reduction in the duration of operations helps in reducing cumulative energy consumption and chemical and material use and waste disposal. The approaches for duration reduction could include collecting information on appropriate use of monitored natural attenuation for the diffuse part of the plume, considering technologies that can operate in conjunction with Pump and treatsystems such as in situ chemical oxidation, thermal remediation or bioremediation in the source area and planning options for implementing a remediation polishing technology at a stage when contaminant concentrations are reduced to a target level.
Injection of chemical agents that can react with contaminants and enhance their rate of removal in pump and treat methods have also been tried. It can shorten the time required for treatment and is advantageous given the high costs. The major areas of concern in this approach of chemical enhancement are
- Delivery of the reactive agent to where it is needed within the aquifer
- The reaction between the reactive agent and the contaminant
- The removal of the contaminant and
- The treatment of the extracted water and disposal of the resultant sludges.
A reactive agent may compete with the contaminant for adsorption sites, complex the contaminant, change its redox state or any combination of these processes. Chemical enhancers may also need to be removed from the ground water. If the effort used in removing the chemical reactants is equal to more than that for removing the contaminant or if leads to shifting the remediation problem from removing chromium to removing the reactant then there is no point in adding chemical enhancers.
Complexing agents are not selective. They may complex a number of ions and hence the extracted water may after treatment produce high number of sludges, the disposal of which could be expensive. The increased load of contaminants due to reduced time may also require bigger facilities for treatment. It may be cheaper to build smaller facilities that may operate over a 10-year period than a larger facility that only operates for one or two years.
Capping involves placing a cover over contaminated areas in order to keep the contaminated areas in place and is used where excavation is expensive or difficult. Caps do not clean up the environment. They just keep it in place. A cap stops rain water from seeping through and prevents leaching, it prevents wind blowing up the hazardous material and it also keeps people and animals from coming into contact with the contaminant. Caps can be single or multi layered, consisting of an upper layer of soil with grasses and plants to prevent erosion, a second layer for collection of any water that seeps in from the 1st layer, a third layer for gas collection and a bottom layer lying directly on the contaminated material. This layer is of day covered by a geo membrane. All these layers prevent the seepage of rain water down ward. Ground water wells are placed around the edges of the cap so that any leakage can be detected. Caps can be effective for a number of years. They are also often used with the pump and treat systems. The pumping treats up polluted ground water while the cap prevents contaminated materials from reaching the ground water.
The type of contaminant influences the capital cost of a Pump and Treat System. In general, capital costs and annual operating costs were lower for sites at which chlorinated solvents were present, alone or with other VOC’s than for sites at which other combination of contaminants such as VOC’s with metals are present.
For sites with more complex contaminants more complex above ground systems are used and the type of treatment influences the cost. The cost of remediation through permeable Reactive Barrier was generally lower than those of the pump and treat systems. Economics of scale play an important role in the capital and operating costs .
Plume containment coupled with ex-situ pump and treat was also examined as an alternative remedial measure. Plume containment can be achieved by the Strategic extraction of ground water to induce draw down and direct flow of the plume towards extraction points and ground water injection where ground water sourced from outside the plume creates a hydraulic barrier that prevents the flow of the plume towards potential sensitive sites. Ex-situ treatment technologies include chemical reduction/precipitation, MBFR(Membrane Bio-film Reactor) and ion exchange. It was estimated that the remediation would take 140 years to achieve the desired remediation objective. Modeling suggested that the pump and treat is expected to be less effective and more difficult at reducing the plume core to the back ground concentration than other technologies such as the in-situ technologies. This does not also promote the agricultural use of ground water but a 100% reinjection of de-contaminated water minimizes ground water draw down.
Influent water characteristics to thePump and treatsystem govern the size and specification of treatment required. Project managers should carefully evaluate nuisance contaminant constituents such as iron and manganese which can easily foul system components or load to more complex treatment systems that may involve additional energy on resources.
Concentration of chemicals of concern in system influent may unexpectedly change over time. Frequent monitoring and use of real time methods for concentration measurements will identify changes quickly and prepare for treatment modification throughout the project life. Continued use of an equipment that has become one sized over time can be a major cause of inefficiency.
Water also plays a crucial role and different disposal methods like into surface waters, reinjection to subsurface and discharge to treatment waters may have an impact on energy and environment considerations. In general resource efficiencies may be gained by using more than one treatment technology for each aspect of the treatment train, planning for eliminations of treatment train components that will become unnecessary as site conditions change and using a form of renewable energy or waste heat, solar thermal panels, combined heat and power or water source heat pumps to provide the desired heat and heat exchanges to utilize heat rather than dispose it along with the effluents.
Chemicals and materialsmay also contribute directly to the Ecological footprint of a Pump and treat system by influence treatment option. Short distance sourcing of materials and chemicals results in lowering of transportation costs. Chemicals and material use options have to be examined to decide the lowest foot print option. The following factors also govern the foot print.
- Collection and disposal of treatment waste-waste recycle can be a good option
- Effluent management and related standards
- Electricity use
- Storm water discharge controls
- Green structures and housing for above ground treatment processes.
- Fuel consumption and alternatives
- Equipment maintenance
- Sampling and analysis of Process water
- Sampling and analysis of Ground water in monitoring wells.
- Routine checks and Balances
- Annual Energy consumption of a common P&T system.
- Energy deployment in Extraction Systems
- Space and power requirements in building.
- Above ground process water treatment.
- Air Stripper off gas emission treatment
- Total Annual Electrical Consumption
Carbon Foot Print equivalency 94 metric tons of C2.
A report on the cleanup operations at the Odessa Chromium Super fund site Texas covers a period from 11/93 to 1/98 on a then ongoing pump and treat system for remediation.
Improper disposal practices have led to a buildup of chromium in ground water which was detailed to be as high as 72 mg/L in 1955. The regulatory regime required that the treated effluent to be reinjected into the aquifer must have a chromium concentration of less than 0.05 mg/L and contain an inward gradient toward the site to contain the plume.
Ground water is extracted using 6wells constructed at the trinity sand aquifer to a depth of 138 feet below ground level at an average pumping rate of 60 gpm. Extracted ground water is treated for Cr removal with chemical treatment (Ferrous ion produced at site), pH adjustment, flocculation, precipitation and multimedia filtrations. Treated water is reinjected through 06 injection wells. Ground water is found 30-45 bgs and 125 million gallons was treated up to January 1998. Start up problems included clogging of well by iron and calcium. This was subsequently taken care of.
Chromium concentrations were removed but not to the desired .05 mg/L. Average Cr+6 reductions of 48% were observed from 1992 to 1997. Treated effluent has met the performance standard of .05 mg/L. Plume contaminant has been achieved since 1995 and 1143 pounds chromium removed.
The actual cost was appx $2742000 ($1954000 in capital and $728000 in O&M) which corresponds to $30 per gallon of ground water extracted and $2,400 per pound of contaminant removed.
Suggested Readings–
- Palmer, C.D. and Wittbrodt, P.R. 1991 Processes affecting the remediation of Chromium contaminated sites, Environment Health Perspectives, Vol. 92. PP. 25-40.
- S. Environmental Protection Agency 2001, A citizens guide to capping, E.P.A. 542 F-01-022, EPA 542-F-01-022.
- S. Environmental Protection Agency, 2012-CLU-IN/Contaminants/Fractured Rock-Fractured Rock overview-Technology Innovation and Field Services Division clu-in.org, July 27, 2012.
- S. Environmental Protection Agency, 2001, Cost Analyses for selected Ground Water Cleanup projects: Pump and Treat Systems and Permeable Reactive Barriers, Solid Waste and Emergency Response (5102G), EPA 542-R-00-013, February 2001, clu-in.org.
- Haley and Aldrich, ING 2010, Feasibility Study, Pacific Gas and Electric Company, Hinkley Compressor station, Hinkley, California.
- Pump and Treat of Contaminated Ground Water at the Odessa Chromium 1 Super fund site, OU2, Odessa, Texas, Federal Remediation Technologies Round Table 2012.
- United State Environment Protection Agency , 2009, Green Remediation Best Management Practices, Pump and Treat Technologies, Office of Solid waste and Emergency response (5102G), EPA 542-F-09-005, Dec 2009.
- International Atomic Energy Agency, IAEA, 1999 Technical Options for the remediation of contaminated ground water, IAEA-TECDOC-1088, ISSN 1011-4289.
- Hazardous and other Wastes (Management and Transboundary Movement) Rules 2016 – An Overview
- Remediation of Contaminated Sites
- Remediation of Contaminated Sites- Permeable Reactive Barriers