United States Environmental Protection Agency Solid Waste and Emergency Response (OS-110W) EPA/542/N-93/006 June 1993 Ground Water Currents Developments in innovative ground water treatment. Funnel and Gate System Directs Plumes to In Situ Treatment By Robert C.Starr and John C.Cherry, Waterloo Centre for Groundwater Research I he Waterloo Centre for Groundwater Research has de- veloped Funnel-and-Gate sys- tems that isolate contaminant plumes in ground water and funnel the plumes through in situ bioreactors. The Funnel- and-Gate consists of low hy- draulic conductivity cutoff walls with gaps that contain in situ reactors (such as reactive porous media), which remove contaminants by abiotic or bi- ological processes. The cutoff walls (the funnel) modify flow patterns so that ground water flows primarily through high conductivity gaps (the gates). Ground water plumes are thus directed through the in situ reactors in the gates where physical, chemical or biologi- cal processes remove contami- nants from ground water. Remediated ground water exits the downgradient side of the reactor. Funnel-and-Gate sys- tems can be installed at the front of plumes, to prevent further plume growth, or im- mediately downgradient of contaminant source zones to prevent contaminants from de- veloping into plumes. This approach is largely pas- sive in that after installation, in situ reactors are intended to function with little or no maintenance for long periods. This contrasts with the energy and maintenance-intensive character of pump-and-treat systems. Additionally, the Funnel-and-Gate system can overcome limitations of the pump-and-treat method, which is usually not effective for restoring aquifers, particu- larly if lighter than water nonaqueous-phase liquids (LNAPLs) and dense non- aqueous-phase liquids (DNAPL s) are present. Funnel-and-Gate systems can be built in several config- urations. They include straight walls with one or more gates, V-shaped funnels with the gate at the point and wide U-shaped funnels with one or more gates along the bottom of the U. The sides of the U extend upstream and can partially surround a contami- nant source. A contaminant source zone can be complete- ly surrounded by cutoff walls, except for a gate in the wall on the downstream side. With this configuration, the cutoff wall on the upstream side deflects clean ground wa- ter around the contaminant source. Any water that infil- trates into the enclosure or flows through the cutoff walls then flows through the gate and out of the enclosure. This configuration minimizes the amount of water that flows through the contami- nant source zone and hence the amount of contaminated ground water that must be treated. This configuration also maximizes the residence time of ground water in the gate, which leads to a more complete treatment. A variety of plume configu- rations and contaminants can be treated. An arrangement of multiple gates in parallel can be used to intercept an excep- tionally wide plume. Or, a complex plume containing a number of different contami- nants can be treated by pass- ing through a series of gates aligned in sequence, each containing a different reactive porous medium. For example, a plume at an electroplating facility that contains both chlorinated hydrocarbons and metals can be treated using one reactor to degrade the organics and a second to pre- cipitate the metals. Multiple parallel treatment gates and gates in series can also be combined. Funnel-and-Gate systems can be constructed through the entire thickness of an aquifer if ground water plumes extend from top to bottom of the aquifer as mightbe the case for DNAPL contamination. If a ground water plume occu- pies only the uppermost por- tion of an aquifer (e.g., if the contaminant source is a LNAPL or a volatile liquid in the va- dose zone), then an installa- tion that extends only through the upper portion of the aqui- fer will be sufficient. The Waterloo modelling analysis is intended to ad- vance the general understand- ing of funnel-and-gate systems. It provides insight into factors that influence plume contain- ment using these systems and the residence time of contam- inants in the gate. Therefore, the system can be located so that all of the water that flows through a contaminant source zone subsequently flows (SEE FUNNEL AND GATE. PAGE 4) This Month in Currents This month s Currents features news and events from our friends North of the Border. Funnel and Gate System Chromium Remediation Waterloo Barrier DNAPL Site Evaluation ------- POINTS OF INTEREST Innovative Remediation of Chromium By Robert W. Puls, Robert S. Kerr Environmental Research Laboratory Imesearchers at EPA's Rob- ert S. Kerr Environmental Re- search Laboratory (RSKERL). in cooperation with research- ers at the University of Okla- homa, are pursuing several different innovative technolo- gies for the remediation of chromium-contaminated ground water and soil. Reme- diation techniques being eval- uated at both the laboratory and field scale include the fol- lowing: (1) in situ application of a geochemical barrier to chromium transport in ground water using elemental iron mixed with site aquifer mate- rials; (2) in situ immobilization of chromium in ground water through stimulation of indige- nous microbial populations to promote biotic reduction of chromate [Cr(VI)]; (3) mobili- zation of Cr(VI) from highly contaminated source area soils where a solubility-controlling equilibrium limits the effec- tiveness of chromate removal via traditional pump-and-treat technology: (4) use of above- ground poly electrolyte ultra- filtration technology to remove Cr(VI) from contaminated ground water pumped to the surface; and (5) assessment of the natural attenuation capac- ity of the system for chromium immobilization through adsorp- tion and reduction processes. The field site used for pilot field-scale studies and the source of soils and aquifer sediments for laboratory-based work is located at the U.S. Coast Guard (USCG) Support Center near Elizabeth City, North Carolina. The USCG is also cooperating in the research. The first two technologies are in situ immobilization techniques which take advan- tage of the fact that chromium exists in either the +3 (re- duced) or +6 (oxidized) oxida- tion states in natural systems. In the oxidized form, as chro- mate or bichromate, chromi- um is toxic and mobile, whereas in the reduced form as chromium+3, it is non- toxic (actually a micronutri- ent) and immobile. The latter species is extremely insoluble and adsorbs to mineral surfac- es almost irreversibly. In most subsurface environments, the trivalent reduced species forms an insoluble mixed chromi- um-iron hydroxide solid phase. The incorporation of elemen- tal iron into aquifer sediment mixtures creates highly reduc- ing conditions which reduce the chromium to the +3 state and result in the formation of the insoluble hydroxide. Labo- ratory batch and column stud- ies using site materials have demonstrated this to be a very efficient and promising tech- nology. Microbial stimulation in laboratory batch and col- umn studies have likewise demonmated reduction of chromium to the insoluble non-toxic formusing benzoate as an electron donor/carbon source. The benzoate micro- cosms have effectively re- duced Cr(VI) levels from 6 parts per million (ppm) to less than detection (0.1 ppm). Column experiments have likewise shown similar results. The third technology has also been laboratory-based to this point. Soil washing stud- ies using various anionic sur- factants have shown promise in desorbing and dissolving sediment-bound and precipi- tated forms of chromate in source area soils. It is expected that soil washing of source area soils will enhance the ef- ficiency of pump-and-treat of chromium-contaminated ground waters in the source area, which otherwise may be in equilibrium with these chromate-laden solid phases at aqueous concentrations ex- ceeding ground water cleanup standards. The fourth technology was recently successfully field- tested. A polyelectrolyte ul- trafiltration (PEUF) pilot plant was constructed for the selective removal of chromate (toxic hexavalent chromium) from ground water at the USCG site. The system was installed during March, 1993. Ground water from three wells having the highest chromium concentrations (3-5 mg/L) was pumped directly and continu- ously through the PEUF sys- tem. PEUF effluent chromate concentrations, monitored on- site using ion chromatogra phy, were less than 0.07 mg/L. The PEUF system produces significantly less waste mass for ultimate disposal compared to other conventional treat- ment systems; and, processed ground water may be reinject- ed into the aquifer. The final approach involves an Investigation of the precipitation-dissolution, ad- sorption-desorption and oxi- dation-reduction processes which govern chromium transport and transformation in subsurface systems. In a sense this research comprises the baseline data against which the other treatments are compared and evaluated. In many natural systems, these chemical interfacial processes can naturally attenuate inputs of hexavalent chromium to the subsurface. In particular, reduction of the toxic hexava- lent form to the reduced tri- valent form often occurs to significant extent due to the presence of organic material and iron-bearing minerals in soil and aquifer sediments. Future research will hope- fully include full-scale field evaluation of approaches 1-3. Plans are underway to scale up the above-ground PEUF sys- tem and use it to reduce chro- mium concentrations in the most contaminated portions of the aquifer at the USCG site For more information, call Bob Puls at RSKERL at 405-436-8543. of HP* Woocrte fecewvr Ground Water Currents ------- Waterloo Barrier-Containment Wall for In Situ Treatment By John Cherry and Robert C. Starr, Waterloo Centre for Groundwater Research new type of containment wall composed of scalable steel sheet piling has been de- veloped at the University of Waterloo's Institute for Groundwater Research. The Waterloo Barrier serves the same general functions as oth- er types of containment wails. However, it has a number of unique advantages over con- ventional sheet piling for con- taining polluted ground water. The materials and construc- tion techniques make the Wa- terloo Barrier less prone to leaking than other types of containment walls, thus pro- viding a greater degree of con- fidence in its performance. The joints of conventional sheet piling are designed for mechanical strength but not watertightness. Leakage of wa- ter through the unsealed joints is acceptable for most civil engineering applications, but generally not for environ- mental applications. With the Waterloo- Barrier, the inter- locking joints between indi- vidual sheet piles incorporate a cavity that is filled with sealant after driving to pre- vent leakage through the joints. The scalable cavities can be formed in two ways. An internal cavity can be formed as the sheet pile itself is manufactured. Or, an exter- nal cavity can be produced adjacent to each joint by at- taching a steel 'L' section to conventional sheet piling. At sites where a very high degree of watertightness is desired, the Waterloo Barrier can be constructed with both an in- ternal and external cavity at each joint. Cavities at the joints provide access for inspection after the sheets have been driven to confirm that the sheet piles were not damaged during construction. With the Waterloo Barrier, excavation of subsurface ma- terials is not required, thus there is less damage to the site and disruption of normal site activities. Also, since workers are not exposed to contami- nated soil, health and safely precautions can be reduced. Disposal of large volumes of contaminated soil is avoided. Installation is relatively clean and rapid; and, comers and ir- regular wall geometries can be easily constructed. Topogra- phy and depth to water table have little effect on installa- non techniques; and, the Bar- rier can even be installed through surface water bodies. The Waterloo Barrier offers a number of design options not available with other types of containment walls. Various options, such as single or double scalable joints, and single or double walls, can be combined on a single project where parts of the wall have one design and other parts have another design. The Barrier can be used for containment purposes only or used in combination with var- ious in situ remediation tech- niques, such as Funnel-and- Gate systems. The volume of sealant required is relatively small, so it is feasible to use special sealants that are par- ticularly resistant to chemical degradation, but are too ex- pensive to use in large quanti- ties. The integrity of the barrier can be confirmed by inspection during construc- tion. Potential leak paths (SEE WATERLOO BARRIER, PAGE 4) NEW FOR THE BOOKSHELF DNAPL Site Evaluation The EPA's Robert S. Kerr En- vironmental Research Labo- ratory has published a manual that is designed to guide in- vestigators involved in the planning and implementation of characterization studies at sites suspected of having sub- surface contamination by dense nonaqueous-phase liq- uids (DNAPLs). DNAPLs, especially chlorinated sol- vents, are among the most prevalent subsurface contami- nants identified in ground water supplies and at waste disposal sites. There are sev- eral site characterization is- sues specific to DNAPL sites including (a) the risk of in- ducing DNAPL migration by drilling, pumping or other field activities; (b) the use of special sampling and mea- surement methods to assess DNAPL presence and migra- tion potential; and (c) devel- opment of a cost-effective characterization strategy that accounts for DNAPL chemical transport processes, the risk of inducing DNAPL movement during field work and the data required to select and imple- ment a realistic remedy. This manual provides informanon to address these issues and de- scribes and evaluates activities chat can be used to determine the presence, fate and trans- port of subsurface DNAPL contamination. The manual discusses the scope of the DNAPL problem, the proper- ties of DNAPLs and subsurface media affecting DNAPL trans- port and fate, objectives and strategies for DNAPL site characterization, invasive and non-invasive methods of site characterization and laborato- ry methods for characterizing fluid and media properties. The manual concludes with several case histories illustrat- ing problems specific to DNAPL sites and priority re- search needs for improving DNAPL site characterization. The manual entitled DNAPL Site Evaluation (Order No. PB93-150217) will be available only from: National Technical Information Ser- vice, 5285 Port Royal Road, Springfield, VA 22 16 1 (tele- phone: 703-487-4650); the cost, subject to change, is $44.50. A free project summa- ry (Document No. EPA/600/ SR-93/002) can be ordered from EPA's Center for Envi- ronmental Research Informa- non, Cincinnati, OH 45268 (telephone: 513-569-7502). Ground Water Currents ------- Funnel and Gate (from page 1) through the gate. The resi- dence time that is critical to the selection and design of re- action media for the gates can be better calculated; and, the least amount of cutoff wall and number of gates can be chosen in order to minimize cost. The modelling analysis does not depend on the type of in situ reactor. Also, the type of cutoff wall used is not critical as long as the gate area does not become plugged with low hydraulic conductivity materi- al during wall construction. The Waterloo Barrier, a seal- able joint sheet piling, devel- oped at the Waterloo Centre for Groundwater Research, is particularly well suited to Funnel-and-Gate construction because it can be easily con- nected to screens that house in situ reactors; and, the area around the cutoff wall does not become plugged with low conductivity material. (The Waterloo Barrier is discussed in more detail in this issue of Ground Water Currents, p. 2.) In situ treatment reactors and Funnel-and-Gate systems are concepts developed very recently; and, research on in situ reaction media is in its in- fancy. Rapid advances on in situ reactors are expected in the next few years. See the previous issue of Ground Wa- ter Currents, Document No. EPA/542/N-93/003, pp. 1-2, for a discussion of one such development—the perme- able reaction wall. Examples of other in situ reactor re- search in progress include: a biotic treatment medium in a removable basket or cassette; an oxygenating medium in a gate for treating hydrocar- bons; and processes for the treatment of nitrate, phos- phate and chromium. For more information on the Funnel-and-Gate concept and modelling analyses and the reactor research, contact Dr. John Cherry at 5 19-888- 45 16 or Dr. Robert C. Starr at 519-M-1211, ext. 6750, at the Waterloo Centre for Groundwater Research. Waterloo Barrier (from page 3) through the Barrier are limit- ed to the sealed joints; so, the joints are the focus of quality control procedures. Rigorous, post-construction hydraulic testing is possible with double- walled configurations or small enclosures. The use of a re- movable sealant, such as a bentonitic grout, allows the sheet piles to be removed from the ground and used elsewhere once a site has been success- fully remediated. The ability to remove the sheet piles makes it easy to: (1) isolate portions of a site for pilot scale tests; (2) progressively remediate a site in sections; or (3) temporarily install the pil- ing for construction purposes. The design specifications of each Waterloo Barrier con- tainment system must be cus- tomized to meet the site requirements. The design is dependent on: surficial geolo- gy; nature and depth of con- tamination and plume morphology; and flow rate. The Waterloo Barrier offers considerable versatility. It can be installed to completely en- close a site to prevent off-site migration of contaminants un- til a remedial plan can be im- plemented, or, to isolate a site while remedial actions are in progress. In some situations an open ended Barrier can be ef- fectively used in conjunction with extraction wells to pro- vide hydraulic containment. The Barrier can be used to di- rect or funnel a contaminant plume into a subsurface treat- ment gate. At new industrial sites the Waterloo Barrier can be installed to enclose the site as a preventive or security measure to control chemical releases that could occur in the future. Enclosures around new landfills can be coupled with caps or infiltration sys- tems to manage the race of waste degradation and leach- ate production. For more information, call John Cherry (519-888-4516) or Robert C. Starr (519- 885-1211, ext. 6750) at the Waterloo Centre for Ground- water Research. To order additional copies of Ground Water Currents, or to be included on the permanent mailing list send a fax request to the National Center for Environmental Publications and Information (NCEPI) at 513-891-6685, or send a mail request to NCEPI, 11029 Kenwood Road. Building 5, Cincinnati, OH 45242. Please refer to the document number on the cover of the issue if available. Ground Water Currents welcomes readers' comments and contributions. Address correspondence to: Managing Editor, Ground Water Currents (OS-110W), U.S. Environmental Protection Agency, 401 M Street S.W., Washington. DC 20460. United States Environmental Protection Agency National Center for Environmental Publications and Information Cincinnati, OH 45242 Official Business Penalty for Private Use $300 EPA/542/N-93/006 ------- |