Fact Sheet 2001-01 MMR Groundwater Treatment Technologies
We hope you find this fact sheet informative and we encourage your feedback on the cleanup program. Please visit our web site at www.mmr.org or contact Mr. Doug Karson at (508) 968-4678 ext. 2. Words that appear in italics are defined in the glossary at the end of this fact sheet. -Robert M. Gill Technologies Summary Currently there are 12 groundwater treatment systems operating on 8 different groundwater plumes. Summary of MMR Treatment Systems
NOTE: There are 15 groundwater plumes total. However, FS-1, CS-4, and CS-10 are included twice in the Summary of MMR Treatment Systems chart. The FS-1 leading edge treatment system has been operating since April 1999, and an additional FS-1 system currently is being designed. Likewise, the CS-4 treatment system has been operating since November 1993 but will be modified in the future. The CS-10 plume currently has three systems in place but is also under investigation. Groundwater treatment technologies are selected based on previous experience in treating groundwater contamination at MMR, professional judgment, U.S. Environmental Protection Agency (EPA) databases, input from other regulatory agencies/citizen teams and the Environmental Technology Center on the MMR. The groundwater technologies currently in use or that may be used are:
Since the start of groundwater cleanup:
Extraction, Treatment, and Reinjection (ETR) The most common method of treating groundwater at MMR is to extract the water, treat it at the surface, and return the treated water to the aquifer (Figure 1). There are a variety of methods to return treated water to an aquifer: reinjection wells, infiltration galleries, and surface discharge. All of the groundwater treatment systems, with the exception of the Storm Drain-5 (SD-5) South recirculating wells, use some, if not all aspects of an ETR system. Description of the Technology Contaminated groundwater is pumped out of the aquifer through a series of extraction wells. The extracted groundwater is then treated at the surface. At MMR, the treatment systems use granular activated carbon (GAC), a material that has the ability to adsorb, or grab onto, passing organic molecules and hold them on the surface of the carbon granule. Granular activated carbon, an important component of ETR technology, is used widely in the treatment of contaminated groundwater. It is an effective technology suitable for treating a wide range of contaminants over a broad range of concentrations and is also 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 drinking water purification. The activated carbon system is housed in large cylinders (each containing 20,000 lbs. of carbon) inside a treatment plant and is used to treat groundwater pumped from several extraction wells. A greensand filter is used as a pre-treatment to remove iron and manganese that can clog the system. Eventually the carbon is saturated with contaminants and the carbon must be replaced. Used carbon is sent off-Cape to be recycled.
Figure 1. Cross-section of an ETR system, with DID YOU KNOW? The largest number of wells used in an ETR system on MMR is the FS-12 system. It uses a combination of up to 23 extraction wells and 25 reinjection wells with a total pumping rate of over 1 million gallons per day. After being tested to ensure that no contaminants are being reintroduced into the aquifer, the treated water is then pumped back into the aquifer through a series of either reinjection wells, infiltration galleries, or surface discharge. There are also safety precautions, such as leak detection monitoring. Design Considerations Through the use of computer modeling and other tools, groundwater extraction wells are located to most effectively capture plumes of contaminated groundwater. The number, location, and flow rates of extraction wells are dependent on the size of the plume and the location of the contaminant mass. Reinjection wells generally are placed beyond or near the edges of the plume. Groundwater monitoring wells are used to check the effectiveness of the extraction system.
Mobile Granulated Activated Carbon (GAC) Systems Description of the Technology A mobile granulated activated carbon system shares many of the same features as permanently located treatment plants except its key feature is its transportable platform. Mobile systems can be deployed quickly near extraction wells thereby minimizing construction time and extraction pipeline installation. Design Considerations Mobile carbon treatment could be prioritized for in-plume mass reduction efforts. This technology is less appropriate at leading edge (containment) locations because of the longer treatment periods involved. Pumping rates are limited by system size. Mobile systems also are more manpower intensive than permanent facilities and require operator attention on a daily basis for such tasks as generator refueling. They also require more frequent sampling to ensure effective removal of contaminants. Each system needs to be winterized. Use at MMR A mobile GAC system has been used successfully at the Quashnet Bogs to treat the Fuel Spill 1 (FS-1) groundwater plume, while construction on the permanent treatment plant is completed. DID YOU KNOW? The extraction systems set up along the Coonamessett and Quashnet Rivers to remove EDB contamination each treat approximately 1 million gallons of plume water per day. Discharge systems return treated water to the aquifer to maintain the local hydrology. Technologies include reinjection wells and infiltration trenches. In some instances, surface water discharge is used to return treated water to the environment. Reinjection Wells A reinjection well is a vertical well capable of injecting treated water directly into the saturated portion of the aquifer. Reinjection wells also can be used to minimize drawdown in ecologically sensitive areas. Reinjection wells have proven to be an effective, method of discharging treated water at several sites at MMR- Chemical Spill 10 (CS-10), Fuel Spill 12 (FS-12) and Storm Drain 5 (SD-5). Infiltration Trenches Infiltration trenches consist of a network of trenches located several feet below the frost line with perforated pipes to let water infiltrate throughout the subsurface. The primary difference between infiltration trenches and reinjection wells is that infiltration trenches deliver water above the water table and let gravity settle the water into the aquifer; while reinjection wells inject water directly into the aquifer. Infiltration trenches can function over long periods of time with little need for maintenance.
Figure 2: The Fuel Spill 1 treatment plant bubbler Surface Water Discharge Currently, surface water discharge of treated water is used for FS-1 and FS-28 plumes only. This is because groundwater in the area of the cranberry bogs (where the FS-1 and FS-28 plumes discharge) already discharge to the surface water and there isn't adequate unsaturated material above the water table to absorb treated water for the use of reinjection wells or infiltration trenches.
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 3.) In recirculating well systems, 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. The air containing the VOCs is carried up the well to the surface, where it is passed through 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 previously described in the ETR section. 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 sampled before and after carbon treatment to ensure effective removal of VOCs.
Figure 3. Cross-section of a recirculating well 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 and effluent testing are used to check the effectiveness of the cleanup system. Use at MMR RWT was pilot-tested at two locations, including the Chemical Spill 10 (CS-10) and Ashumet Valley plumes. The purpose of the pilot tests was 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. Currently, the Storm Drain 5 South (SD-5) plume is using RWT. Two recirculating wells have been installed in the Briarwood neighborhood of Mashpee to capture the bulk of the SD-5 South plume. Description of the Technology Reactive wall technology is an in-situ (in-place) process that involves placing a vertical barrier or "wall" of iron filings perpendicular to a groundwater plume. The barrier consists of a porous slurry mixture of sand and iron filings in a gelatin-like consistency. The slurry is commonly known as guar. As the contaminated groundwater flows through the wall, chlorinated compounds, such as trichloroethylene (TCE) and tetrachloroethylene (PCE) react chemically with the iron filings to produce chloride and nontoxic hydrocarbons. The chemical process is called reductive dechlorination. Reactive walls typically are used for contamination at shallow depths. AFCEE estimates that compared to traditional pump and treat systems (such as ETR), reactive wall technology may reduce remediation costs by half. Use at MMR AFCEE installed two parallel 50-foot reactive walls in the CS-10 groundwater plume in June 1998. The reactive walls were placed 20 feet apart, perpendicular to the direction of the groundwater flow. The walls were installed 85 feet below ground and extend downward another 40 feet. The bottom of the plume is currently 115 feet below ground, or 10 feet above the bottom of the walls. This is the first test of reactive wall technology on deep groundwater plumes in the United States. The two walls installed in June 1998 were completed using an innovative installation technique called "vertical hydraulic fracturing" or hydrofractionation. This method involves injecting the guar into the subsurface via injection wells. The injection vertically "parts" or "fractures" the sand layers, leaving a "wall" of iron slurry fluid approximately 3-4 inches thick. After a few days, the slurry is broken down by bacteria to harmless starches and sugars and moves away from the site with the groundwater flow, leaving the reactive iron media behind. In addition, AFCEE also is working with the U.S. Geological Survey (USGS) to evaluate the wall integrity by using a borehole radar technique. This technique works by sending a radar signal through existing monitoring wells across the reactive wall test site. Along with other geophysical methods used onsite, this technique will help determine the effectiveness of the vertical hydraulic fracturing effort. Use at Other Locations The reactive wall technology has shown great promise in treating shallow groundwater contamination. The first full-scale demonstration of the technology at a former semiconductor plant in California resulted in near-complete degradation of chlorinated compounds. TCE levels of 30 to 68 parts per billion (ppb) were reduced to fewer than 0.5 ppb, and levels of cis 1,2 dichloroethylene (1,2-DCE) were reduced from between 393 and 1,916 ppb to fewer than 0.5 ppb. Adapted from an article by Elaine L. Appleton in Environmental Science & Technology, vol. 30, no. 12, 1996. Description of the Technology Monitored natural attenuation (MNA) refers to the strategy of allowing natural processes to reduce contaminant concentrations over time. Natural attenuation involves physical, chemical and biological processes which act to reduce the mass, toxicity, and mobility of subsurface contamination. These processes are naturally occurring and in many cases may reduce risk to human health and the environment. Please see the EPA website: www.epa.gov/swerust1/oswermna/index.htm for more information on MNA. There are several different physical, chemical, and biological processes that comprise MNA. These include:
The use of MNA involves collecting additional data, modeling and evaluating contaminant reduction rates to determine whether MNA is a feasible method for plume treatment. The evaluation of whether or not the treatment method is feasible also must consider existing and potential risk to human health and the environment. Furthermore, to use MNA 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 MNA actually is reducing the mass, toxicity, and mobility of the contamination. Use at MMR Portions of two groundwater plumes at MMR are being monitored for natural attenuation: [1] the southern portion of Ashumet Valley and [2] the central/eastern portion of Landfill-1 (LF-1). In December 1997, another decision was agreed to by AFCEE, EPA, and MassDEP for the western portion of the LF-1 groundwater plume (west of Route 28). Natural attenuation was chosen for the western portion of the LF-1 plume because of concerns regarding: [1] population density and impacts of construction and [2] unproductive nature (i.e. relative closeness to the surface of the salt/freshwater interface) of the aquifer west of Route 28. Use at Other Locations MNA has been implemented successfully in various contaminated sites throughout the United States. A recently issued EPA directive says the use of monitored natural attenuation for the remediation of contaminated groundwater is appropriate only when it will result in protection of human health and the environment, as well as achievement of site-specific remediation objectives, within a reasonable time frame. Glossary adsorb: to attach by physical or chemical attraction aerobic: activity occurring only in the presence of oxygen anaerobic: lacking oxygen aquifer: an underground geological formation containing usable amounts of groundwater 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 cis-1,2 dichloroethene (DCE): a chlorinated solvent, used for industrial purposes dispersion: the process of mixing that occurs when fluid flows though a porous medium drawdown: the measured reduction of the water table due to groundwater extraction from the pumping of municipal, irrigation and private water wells extraction well: a well from which water is pumped out in order to treat it granular activated carbon: a highly adsorbent form of carbon used to remove organic molecules from an air or liquid source. [Carbon has the ability to attract organic molecules and hold them in the pores within the carbon granule.] groundwater plume: a body of groundwater containing contaminants that exceed federal and state drinking water levels or other risk-based levels at multiple test well locations. A groundwater plume results when fuels, solvents, or other contaminants are spilled or released on the ground. When these materials filter through the sandy Cape Cod soil, they encounter groundwater, or the water table, where the soil is saturated with water. As the groundwater moves, the contaminants are carried with it, creating a groundwater plume guar: a porous slurry mixture of sand and iron filings hydrofractionation: a process which induces a fracture in the subsurface formation through the application of pore fluid (water) pressure monitored 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 reductive dechlorination: a process that generally occurs in low oxygen environments (anaerobic) where halogens (chlorine, bromine, flourine) are removed from organic compounds and replaced with hydrogen 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 sorption: the ability of some substances to soak up or attract contaminants and hold onto them tetrachloroethene (PCE): also referred to as perchloroethene; a man-made solvent commonly used for metal degreasing and in dry-cleaning clothes trichloroethene (TCE): a cleaning solvent often used in metal degreasing 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 FOR MORE INFORMATION Doug Karson, Community Involvement Specialist Jim Murphy, Community Involvement Coordinator Ellie Grillo, Community Involvement Coordinator |
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