Upward Flow Constructed Wetland Groundwater Remediation - Promising New Technology Available Commercially

By: Jacob Elder

Fitting into our expanding green and sustainable remediation portfolio, Cox-Colvin recently entered into a Cooperative Research and Development Agreement (CRADA) to assist in the development and commercialization of an upward flow wetland shown to be effective at treating groundwater contaminated with chlorinated VOCs. The US Air Force, in cooperation with researchers at Wright State University, has been developing the technology since 2000. A pilot-scale demonstration wetland, which mimics the upward flow hydrology of natural fen wetland systems, is located at the Wright-Patterson Air Force Base (WPAFB) in Dayton, Ohio [1]. The WPAFB treatment wetland was constructed in a lined excavation (120 ft x 60 ft, & 7 ft deep) filled with a shallow gravel layer at the bottom, and 5 feet of hydric soil. The first third of the hydric soil was amended with wood chips to act as a source of organic matter. Between the gravel and soil, sewage sludge and freshly collected mud from a local anaerobic marsh was used as microbial seed. Contaminated groundwater, with initial perchloroethene (PCE) and trichloroethene (TCE) concentrations of 34 and 0.6 ug/L, respectively, is fed into the gravel manifold at the bottom of the wetland system at a rate of ~10 gpm. By design, the water flows upward through hydric soil and roots of wetland plants, emerging at the wetland surface. Treated water ultimately discharges to the sanitary sewer system.

Results indicate the system is capable of sequential dechlorination of PCE through both reductive and aerobic processes as groundwater moves upward through the system [1]. The organic-matter rich hydric soil promotes strongly reducing conditions that facilitates the anaerobic degradation of PCE into less chlorinated species. PCE degradation products, including TCE, cis-1,2-dichloroethene (cis-DCE) and vinyl chloride (VC), were found within the system during groundwater samples collected from 2003 through 2009 [2]. To promote aerobic processes, the treatment wetland was planted with multiple species of wetland plants appropriate to an up-flow (fen) wetland. Soil cores indicate that some roots rapidly grew to the full depth of the wetland, producing a robust rhizosphere within the system. This plant rhizosphere supports oxidizing microenvironments in an otherwise reducing wetland soil, and facilitates mineralization of TCE, cis-DCE, and VC in the root rich subsurface [3]. Overall, PCE was reduced by > 90%, with both PCE and TCE well below applicable MCLs at the water outflow weir. In addition, TCE, cis-DCE and VC were not detected at the outflow.

Based on these favorable results, Air Force and Wright State University researchers estimate that the WPAFB treatment wetland will save over $500,000 annually on operation and maintenance costs compared to an air stripper used for the same purpose, while enhancing the natural environment at the Air Force base. Cox-Colvin currently is working with its clients to design and implement a full-scale version of this green and sustainable remediation technology.

Articles Referenced:

[1] Amon, J.P., Agrawal, A., Shelley, M.L., Opperman, B.C., Enright, M.P., Clemmer, N.D., Slusser, T., Lach, J., Sobolewski, T., Gruner, W., Entingh, A.C., 2007. Development of a Wetland Constructed for the Treatment of Groundwater Contaminated by Chlorinated Ethenes. Ecological Engineering. 30, 51-66.

[2] C.L. Powell, A. Agrawal, M.N. Goltz, 2011, M.L. Shelley, K.T. Krejny, 2011. Pore-Water Geochemistry of an Upward-Flow Treatment Wetland Showing Natural Attenuation of Chlorinated Ethenes. Biogeochemistry (In Press).

[3] C.L. Powell and A. Agrawal. 2011. Biodegradation of trichloroethene by methane oxidizers naturally associated with wetland plant roots. Wetlands. 31(1), 45-52.

Jacob Elder is a project manager at Cox Colvin & Associates, Inc. He received his BS degree in Environmental Science from Bowling Green State University and his MS in Environmental Science, specializing in soil physical property dynamics, from The Ohio State University. His areas of expertise include soil physical properties analysis and VAP project implementation and funding. Mr. Elder also leads Cox-Colvin & Associates' research efforts in the use of natural systems for water treatment.