Groundwater Treatment Technologies

September 1997 Groundwater Treatment Technologies A fact sheet providing information about the groundwater treatment technologies under consideration at the MMR. The purpose of this fact sheet is to describe the technologies that may be used to clean up the groundwater contamination associated with the Massachusetts Military Reservation (MMR). A combination of technologies may be used to clean up the identified groundwater plumes at the MMR. The various technologies or combinations of technologies under consideration for a given plume are called plume response alternatives. Project managers from the Air Force Center for Environmental Excellence (AFCEE), the U.S. Environmental Protection Agency (EPA), and the Massachusetts Department of Environmental Protection (DEP) will be evaluating these alternatives to recommend the most appropriate cleanup solutions for each plume. The alternatives currently under consideration for the Storm Drain 5 (SD-5) South plume are described in the accompanying fact sheet, SD-5 South Plume Response Alternatives. Public involvement during the decision-making period is critical to selecting an alternative that not only cleans up the plume and meets regulatory requirements, but also is most acceptable to the affected communities. This fact sheet (along with an accompanying fact sheet describing the SD-5 plume in general) will provide background information in an effort to help people gather information and develop informed opinions. Words that appear in italics are defined in the glossary at the end of this fact sheet. Extraction, Treatment, and Reinjection Description of the Technology The most common method of treating groundwater is to extract the water, treat it at the surface, and return the treated water to the aquifer. This process is also known as "pump and treat" technology. There are a variety of methods to return treated water to an aquifer, one of which is reinjection wells. (See Figure 1.) Systems using reinjection wells are known as extraction, treatment, and reinjection (ETR) systems. A typical ETR system has three basic components:
Figure 1: Sample ETR Diagram	Contaminated groundwater is pumped out of the aquifer through a series of extraction wells. This requires extraction wells in different locations and at different depths in order to capture the plume. The extracted water is then treated at the surface. At MMR, the treatment system that would be used is activated carbon. Carbon has the ability to adsorb, or grab onto, passing organic molecules and hold them on the surface of the carbon granule. When contaminated groundwater is pumped through a filter of carbon granules, most of the organic contaminants become trapped on the surface of the carbon.
Eventually the carbon is saturated with contaminants and the carbon must be replaced. Used carbon is sent off-Cape to be recycled. The activated carbon system is housed in cylinders inside a treatment plant and is used to treat groundwater pumped from several extraction wells. Safety precautions, such as leak detection monitoring systems, ensure that any leaks are detected immediately. Activated carbon is one way to treat the groundwater after it has been extracted. Other treatment technologies may be considered. After being tested, the treated water is pumped back into the aquifer through a series of reinjection wells. Reinjection wells are used to reduce the impact of the extraction wells on the groundwater hydrology, to help control plume migration, and to return treated water to the aquifer. Design Considerations ETR groundwater collection systems usually consist of a line or arc of wells, called a "fence" placed around the contaminated area or in the path of the groundwater flow. Reinjection wells are generally placed beyond or near the edges of the plume. The specific arrangement of extraction and reinjection wells is determined using groundwater flow information and tools like computer modeling. Modeling and other tools enable engineers to design an effective system, including the number of wells, depths, spacing, and pumping or reinjection rates. A treatment plant housing the granular activated carbon can be located away from the extraction and reinjection wells and can handle water from many wells at once. Monitoring wells are used to check the effectiveness of the treatment system. Use at Other Locations ETR is the most commonly used treatment technology for contaminated aquifers. It has been proven effective over a wide range of site conditions and contaminants for hydraulic control and cleanup of groundwater plumes. Activated carbon, an important component of ETR technology, is used widely in the treatment of organic groundwater contamination. It is an effective technology suitable for treating a wide range of organic contaminants over a broad range of concentrations. It also is used widely in industry for control of air pollution and odors. It has a long history of successful use as a treatment for municipal, industrial, and hazardous wastes, and is commonly used in home water purification. Recirculating Well Technology Description of the Technology Recirculating well technology (RWT) is a recently- developed method of treating volatile organic compounds (VOCs) in groundwater. The primary difference between RWT and ETR technology is that recirculating wells return treated water to the same well, thus minimizing impacts on the water table. (See Figure 2.) In the recirculating well systems being pilot-tested at the MMR, groundwater is drawn into the well near the bottom or from a selected zone in the groundwater. It comes into contact with air which is injected into the well, causing the VOCs to transfer from the water to the air. The cleaned water then is pumped back out of the well into another zone of groundwater at a different depth but at the same location. The combination of contaminated groundwater entering the well from one zone of the aquifer and clean groundwater leaving the well in another zone or depth creates a zone of recirculation in the groundwater near the well. Contaminated water moving through the zone of recirculation is captured and treated within the well.
Figure 2: Sample Recirculating Well Diagram
The air containing the VOCs is carried up the well to the surface, where it may be piped to an activated carbon treatment system. The activated carbon treatment system may be housed above-ground in a small building or in a below-ground vault. Activated carbon treatment removes contaminants from air just as it does from water, as described in the ETR section above. Depending on the design of the system, the treated air is either released to the surrounding environment or directed back into the well for additional removal of contaminants. The air stream is monitored before and after carbon treatment to ensure effective removal of VOCs. Design Considerations The design of a recirculating well system is determined by the volume of groundwater that must be treated, the concentration of contaminants, the thickness of the plume, the pumping rate, the average groundwater flow velocity, and the types of soil in the aquifer. The dimensions of the zone of recirculation determine the number of recirculating wells that would be needed to clean up the plume. A carbon treatment system for air vapors can be housed in a small building designed to look like a garden shed and can handle vapors from several RWT systems at once. Monitoring wells are used to check the effectiveness of the cleanup system. Use at Other Locations RWT is a recently developed groundwater treatment technology. It has been used at over 200 sites in Europe and over 80 sites in the U.S. At the MMR, RWT is being pilot-tested at three locations, including the Chemical Spill 10 (CS-10) and Ashumet Valley plumes. The purpose of the pilot tests at the MMR is to determine the effectiveness of the technology at capturing and removing VOCs in Cape Cod’s geologic environment, and to compare the effectiveness of RWT with the more conventional ETR. Preliminary results of the recirculating well pilot tests indicate that measured cleaning efficiency is consistent with the pilot test design and mass removal of contaminants is being achieved. The pilot test systems currently are being evaluated for consideration as a component of full-scale cleanup alternatives. Natural Attenuation Description of the Technology Natural attenuation refers to the strategy of allowing natural processes to reduce contaminant concentrations to acceptable levels. Natural attenuation involves physical, chemical and biological processes which act to reduce the mass, toxicity, and mobility of subsurface contamination. These processes are always occurring and in many cases may reduce risk to human health and the environment to acceptable levels. There are several different physical, chemical, and biological processes that comprise natural attenuation. These include: biodegradation—breakdown of contaminants by microorganisms in the environment, often forming non-harmful byproducts like carbon dioxide and water chemical stabilization—reduction in contaminant mobility caused by chemical processes dispersion—the process of mixing that occurs when fluid flows through a porous medium sorption—attachment of compounds to geologic materials by physical or chemical attraction volatilization—transfer of a chemical from liquid to vapor; evaporation Natural attenuation, by definition, occurs naturally. However, use of natural attenuation as a specific treatment method is not a "do nothing" approach. It involves modeling and evaluating contaminant reduction rates to determine whether it is a feasible method for plume treatment. The evaluation of whether or not the treatment method is feasible must also consider existing and potential risk to human health and the environment. Furthermore, to use natural attenuation as a cleanup strategy, sampling must be conducted throughout the process to confirm that degradation is proceeding at expected rates. Sampling and analysis will determine whether natural attenuation actually is reducing the mass, toxicity, and mobility of the contamination. Design Considerations Many factors affect whether or not natural attenuation is a feasible treatment alternative. These include: Data must be collected to make modeling as accurate as possible. Intermediate breakdown compounds may be more mobile and more toxic than the original contaminant. Natural attenuation should be used only in low risk situations. Contaminants may migrate before they are degraded. Restrictions on use of groundwater downgradient may have to be enforced until contaminant levels are reduced. Use at Other Locations Natural attenuation has been used at hazardous wastes sites across the country. Although it is a component of remediation activities at many sites, only in the last few years has it been applied specifically as a treatment strategy to be considered along with other engineered solutions. For More Information For more information, please contact the Community Involvement Office for the Installation Restoration Program by calling (508) 968-4678. Glossary adsorb: to attach by physical or chemical attraction aquifer: an underground geological formation containing usable amounts of groundwater that can supply wells and springs. biodegradation: the natural process of contaminant breakdown by microorganisms in the environment, often forming non or less harmful byproducts chemical stabilization: reduction in contaminant mobility caused by chemical processes cleanup: actions for removing contaminants from the environment dispersion: the process of mixing that occurs when fluid flows though a porous medium extraction well: a well where water is pumped out in order to treat the water and to redirect groundwater movement. groundwater plume: a body of groundwater containing contaminants exceeding maximum contamination levels (MCLs) as defined by multiple samples from multiple wells. In the absence of MCLs, a risk-based level will be established. natural attenuation: the process by which a compound is reduced in concentration over time by natural processes. recirculating well: a process for capturing, treating, and releasing groundwater within the same well. The process is not expected to adversely affect the water table. reinjection well: a well where clean water is pumped into the aquifer to replace contaminated water that has been removed. pilot test: demonstrations of technologies/systems to evaluate performance under field conditions. The results are used to develop plume response alternatives and design full-scale treatment systems. plume response alternatives: a specific configuration of treatment system(s) to be compared and evaluated sorption: the ability of some substances to soak up or attract contaminants and hold onto them volatilization: transfer of a chemical from liquid to vapor; evaporation zone of recirculation: a circular zone of groundwater movement created by flow into and out of a recirculating well