Modeling Variably Saturated Water Flow and Multicomponent Reactive Transport in Constructed Wetlands

important factors in controlling subsurface flow and/or contaminant transport, and/or evaluating regional and Constructed wetlands (CWs) are becoming increasingly popular global water cycles (Scanlon et al., 2002). worldwide for removing organic matter (OM), nutrients, trace elements, pathogens, or other pollutants from wastewater and/or runoff Many or most vadose zone models consider the transwater. We present a multicomponent reactive transport model CW2D port of only one solute and assume that the fate and (i.e., Constructed Wetlands 2D), as an extension of the HYDRUS-2D transport of this solute is independent of all other spevariably saturated water flow and solute transport software package. cies that may be present in the soil solution. In reality, CW2D was developed to model the biochemical transformation and the soil solution is always a mixture of many chemical degradation processes in subsurface-flow CWs. Such wetlands involve species and microorganisms that may mutually interact, a complex mixture of water, substrate, plants, litter, and a variety of create complexed species, precipitate or dissolve, affect microorganisms to provide optimal conditions for improving water each others degradation and decay, and/or compete with quality. The water flow regime in subsurface-flow CWs can be highly each other for sorption sites (van Genuchten and Šimůdynamic and requires the use of a transient variably saturated flow nek, 2004). Many important environmental problems model. The biochemical components defined in CW2D include dissolved oxygen (DO), three fractions of OM (readily and slowly biohence require a simultaneous analysis of prevailing flow degradable, and inert), four N compounds (ammonium, nitrite, nitrate, and transport processes with multiple chemical and bioand dinitrogen), inorganic P, and heterotrophic and autotrophic milogical reactions and transformations. Examples involvcroorganisms. Organic N and organic P were modeled as part of the ing mostly geochemical reactions as listed by Šimůnek OM. The biochemical degradation and transformation processes were and Valocchi (2002) include acid mine drainage (Walter based on Monod-type rate expressions. All process rates and diffusion et al., 1994; Lichtner, 1996), radionuclide transport (Viscoefficients were assumed to be temperature dependent. Heterotrowanathan et al., 1998), and reactive permeable barriers phic bacteria were assumed to be responsible for hydrolysis, mineralfor aquifer remediation (Fryar and Schwartz, 1994). Exization of OM (aerobic growth), and denitrification (anoxic growth). amples that additionally involve biochemical processes Autotrophic bacteria were assumed to be responsible for nitrification, include degradation of NTA (Tebes-Stevens et al., 1998), which was modeled as a two-step process. Lysis was considered to be the sum of all decay and sink processes. We demonstrate the fate and transport of metal–organic mixed wastes (Rittperformance of the model for oneand two-stage subsurface vertical mann and VanBriesen, 1996; VanBriesen, 1998), the flow CWs. Model simulations of water flow, tracer transport, and formation of redox zones in organic-contaminated aquiselected biochemical compounds are compared with experimental fers (Abrams et al., 1998; Essaid et al., 1995), C and N observations. Limitations of the model are discussed, and needs for cycles in soils (Parton et al., 1988), bacteria-induced model improvements are summarized. changes in the soil hydraulic properties (Rockhold et al., 2002), and complex biogeochemical processes in CWs (Langergraber, 2001, 2003). O ability to model flow and transport processes Constructed wetlands are engineering structures used in the vadose zone between the soil surface and worldwide to improve water quality (Kadlec et al., 2000; the groundwater table has increased enormously during Langergraber and Haberl, 2001; Haberl et al., 2003). the past several decades (van Genuchten and Šimůnek, They involve a complex mixture of water, substrate, 2004). Vadose zone models are now routinely used for plants, litter, and a variety of microorganisms to produce a large number of applications, both in research of subthe optimal conditions for removing OM, N, P, and surface processes and management of our soil and water for decreasing the concentrations of toxic trace metals, resources. Typical applications are evaluation of the organic chemicals, and pathogens. Constructed wetlands performance and effectiveness of engineered covers to include surface flow and subsurface flow CWs (Kadlec and minimize infiltration into underlying waste, quantifying Knight, 1996). Surface-flow CWs are generally densely groundwater recharge, evaluating the impact of climate vegetated and typically have water depths of 0.4 m. and land-use changes on subsurface flow, identifying In subsurface-flow CWs no free water level is visible. Subsurface vertical flow CWs with intermittent feeding are now used widely due to their efficiency in removing G. Langergraber, Institute of Sanitary Engineering and Water Polluammonia N (e.g., Langergraber and Haberl, 2001). Altion Control, BOKU Univ. of Natural Resources and Applied Life Sciences, Vienna, Muthgasse 18, A-1190 Vienna, Austria; J. Šimůnek, Dep. of Environmental Sciences, Univ. of California, Riverside, CA Abbreviations: ASM, Activated Sludge Model; COD, chemical oxy92521, USA. Received 25 Nov. 2004. *Corresponding author (guenter. gen demand; CW, constructed wetland; CW2D, Constructed Wetlands [email protected]). 2D; DO, dissolved oxygen; IP, inorganic phosphorus; OM, organic matter; OM CI, inert OM; OM CR, readily biodegradable OM; OM CS, Published in Vadose Zone Journal 4:924–938 (2005). Original Research slowly biodegradable OM; PSCW, pilot-scale subsurface vertical flow constructed wetland; SSP, Small-Scale Plot; TOC, total organic carbon; doi:10.2136/vzj2004.0166 © Soil Science Society of America XANb, autotrophic microorganisms, Nitrobacter; XANs, autotrophic microorganisms, Nitrosomonas; XH, heterotrophic microorganisms. 677 S. Segoe Rd., Madison, WI 53711 USA 924 Published online September 13, 2005