517 13 Organic Chemicals Organic compounds are major constituents of biochemical oxygen demand (BOD) or chemical oxygen demand (COD) in municipal wastewater, as discussed in Chapter 8. However, there is an increasing number of treatment wetlands that tar- get specic individual or groups of organic chemicals. These chemicals pose a new and somewhat more difcult set of problems because of their possible toxicity to plants and the limitations of aerobic and anaerobic degradation. The major routes for removal of hydrocarbons from wetland waters include: volatilization, photochemical oxidation, sedimenta- tion, sorption, biological (microbial) degradation, and plant uptake. Three types of microbial processes can contribute: fermentation, aerobic respiration, and anaerobic respiration. The general principles and chemistry of processes affecting carbon compounds are discussed in Chapter 8; see Equations 8.10 through 8.18. Wetlands manufacture and contain a wide spectrum of organic compounds. These compounds range from small molecules such as methane to humic acids of very high molecular weight. Many wetland soils are organic in nature and possess an afnity for introduced organics, via sorption and other binding mechanisms. Aliphatic hydrocarbons, both straight-chain and branched, are present as natural waxes. As a result, trace (background) amounts of some hydrocarbons are present in all wetlands, whether constructed or natural. 13.1 PETROLEUM HYDROCARBONS There is considerable information on the use of treatment wetlands in the petroleum industry. Some of the general prin- ciples and available data have been summarized in a 1998 industry report (Knight et al., 1997; Knight et al., 1999). However, that compilation did not focus on specic hydro- carbon classes, such as BTEX and its constituent components (benzene, toluene, ethyl benzene, and xylenes). Two other hydrocarbon designations of interest include Gasoline Range Organics (GRO) and Diesel Range Organics (DRO). There is some overlap and ambiguity in these designations. Typically, GRO consists of hydrocarbons with 6–9 carbon atoms, while DRO contains 10–40 carbon atoms (Chapple et al., 2002). Total Petroleum Hydrocarbons (TPH) is a measure of the sum of parafnic and aromatic constituents. BTEX Biodegradation Biodegradation of BTEX in a wetland environment is compli- cated by the existence of biolms on submerged plant parts, plant litter, and gravel. Although small in terms of mass per unit volume, these biolms are very active in biodegrada- tion, and consequently serve as important sinks for organics. In this aspect, treatment wetlands resemble conventional attached-growth treatment processes. Chang et al. (2001) established that all BTEX constituents degrade rapidly (half-lives of one to two days) when inocu- lated with a microbial consortium conditioned with toluene. Their study showed that benzene, toluene, and ethylbenzene were directly consumed as carbon sources, while xylene was removed by co-metabolism. Moore et al. (2002) measured both aerobic and anaerobic biodegradation of BTEX in peat. Aerobic degradation was tracked via oxygen consumption and carbon dioxide produc- tion, and averaged 56 mg/kg·d. Anaerobic degradation was inferred from the consumption of other electron acceptors, including nitrate, sulfate, iron, manganese, and methane. About one third of the BTEX loss could have been attributed to anaerobic degradation. Volatilization BTEX constituents are volatile and may be easily lost from water, especially shallow water bodies, such as FWS wet- lands. Lee et al. (2004a) have reported that rst-order loss rate constants for BTEX constituents exhibit a xed ratio to the loss rate constant for benzene, independent of the water- mixing regime (Table 13.1). Further, the presence of more than one BTEX compound, or of surfactants, had only minor effects on this ratio. Of direct interest are the estimates, based on wetland data, of Keefe et al. (2004a) for an FWS wetland in Arizona. Values were determined of K w 0.015 m/hr (130 m/yr) for toluene. This is a relatively high rate, compared to other pollutants treated in FWS wetlands. Moore et al. (2002) evaluated volatilization losses from eld vapor collection in a peatland contaminated with BTEX. Losses averaged 2,500 mg/m 2 ·d over all seasons. This high rate may have been in part due to the existence of a Light Non-Aqueous Phase Layer (LNAPL) in the peat. Plant Uptake Willows (Salix babylonica) were shown to materially contrib- ute to the removal of benzene from water under hydroponic conditions (Corseuil and Moreno, 2001). Volatilization was effectively suppressed in the experiments, as demonstrated by control mesocosms. About 80% reduction in benzene was found with an HRT of one day. Corseuil and Moreno sug- gested that benzene was initially sorbed onto root biomass, followed by plant uptake and biological degradation. © 2009 by Taylor & Francis Group, LLC 518 Treatment Wetlands Sorption BTEX components are expected to partition strongly to organic wetland substrates, although a denitive wetland study is lacking. Wetland sediments typically have high organic content, and therefore sorption may be an important rst step in overall removal. Benzene sorption onto peat was found to be strong, with a linear sorption coefcient of K OC 5.6 L/kg (Moore et al., 2002), whereas binding to clay was much weaker, with K OC 0.12 L/kg. Wetland System Studies In all existing petrochemical plant applications, wetlands have been accompanied by pretreatment. However, other applications, such as those involving remediation, do not involve pretreatment. Hydrocarbon contamination of ground- water includes closed and operating sanitary landlls, army ammunition plants, and former oil renery sites. These facil- ities were often the recipient of solvents and other organics, which in the course of time have contaminated nearby waters. Concentrations of petrochemicals in landll leachate are typically lower than those associated with reneries and terminals. Gulf Strachan, Rocky Mountain House, Alberta HSSF wetlands were tested for the ability to reduce hydro- carbons, notably BTEX and TPH (Moore et al., 2000a). The wetlands were planted with Phragmites australis and Typha latifolia. Reductions of 40–60% were achieved within 14 days of detention time with no aeration, while aeration produced 100% removal in the same detention time. Inuent BTEX concentrations ranged from 4.5–12.1 mg/L, and inu- ent ows ranged from 7–33 L/min. At the same site, Moore et al. (2002) reported on the natural attenuation of BTEX in a natural peatland, which received both aqueous and nonaqueous phase hydrocarbons for more than 15 years. No BTEX was detected leaving the peatland. Companion studies elucidated some of the candi- date removal mechanisms, including sorption, aerobic deg- radation, volatilization, and anaerobic degradation. Aerobic degradation, which was stimulated by air injection, was the dominant removal mechanism. Former Oil Refinery—United Kingdom HSSF wetlands planted with Phragmites were tested for the ability to reduce hydrocarbons, notably DRO (Chapple et al., 2002). Reductions of 40–64% were achieved in less than one day’s detention time. Gravel-based beds performed better (k 270 m/yr) than soil-based beds (k 137 m/yr) for DRO removal. Mobil Oil AG Terminal at Bremen, Germany The information here is taken from Altman et al. (1989), which summarizes three years of research results at Bremen. Highly contaminated wastewater (COD up to 14,000 mg/L) was brought through an API separator, a parallel plate sepa- rator, and a percolating reactor to a two-train pilot wetland system patterned after the concepts of Seidel (1966; 1973) at the Max Planck Institute. Each train had ve subsurface ow wetlands in series, each being 2.5 m wide, 4.0 m long, and 0.8 m deep. Beds 1 and 2 were vertical ow, with passive aeration to the underdrains. Beds 3, 4, and 5 were horizontal ow. Each bed was lled with a stratied media, from the bot- tom: 5 cm of 8/16 mm (pea) gravel, 20 cm of 36/72 mm stone, 25 cm of 8/16 mm g ravel , 10 cm of shar p sand, a nd toppe d wit h 2 cm of organic soil. Water is intermittently dosed to the sur- face, where it spreads and inltrates, ultimately reaching the (10 cm) under-drains in the second layer. The passive air is admitted to the rst layer via perforated plastic pipe. Details of this type of system are given in Vymazal et al. (1998). The plants that proved best for Bremen were cattails (Typha angustifolia) and bulrushes (Scirpus lacustris). Hydrocarbon removal performance of this system was primarily measured as total hydrocarbon, but BTEX and its constituents were also measured less frequently. The inuent BTEX in August was 3.8 mg/L (1.5 mg/L benzene), and the outow contained no detectable BTEX, with all constituents being below 0.01 mg/L. The ow of 5 m 3 /d corresponded to about 5 cm/d hydraulic loading, or about ve days’ detention. Although these results are encouraging, they do not answer the question of how much of the ve-bed system was needed to achieve a given reduction. However, inferences about BTEX removal may be made from the removal of total hydrocarbon (TH). Data were taken at the outlet from each bed of the trains, and therefore a removal model may be calibrated. A rst-order areal TH model yields k 66 m/yr, with no evidence of temperature dependence. This is consistent with the benzene removal rate coefcient, which cannot be less than 90 m/yr for the data in the preceding paragraph. Williams Pipeline, Watertown, South Dakota An aerated HSSF wetland was operated to reduce BTEX from petroleum contact waters. Complete removal of TABLE 13.1 Volatility of BTEX and Chlorinated Benzenes Compared to Benzene Compound Volatilization Ratio Benzene 1.000 Toluene 0.966 Ethylbenzene 0.922 m-Xylene 0.930 1,3 Dichlorobenzene 0.882 1,3,5 Trichlorobenzene 0.569 1,2,3,4-Tetrachlorobenzene 0.530 Pentachlorobenzene 0.493 Note: The volatilization ratio is the ratio of rst-order loss rate coefcients. Source: Data from Lee et al. (2004a) Water Research 38(2): 365–374. © 2009 by Taylor & Francis Group, LLC Organic Chemicals 519 hydrocarbons was reported (Wallace, 2002), despite high inuent concentrations (CBOD 5 16,000 mg/L, NH 4 -N 200 mg/L, and BTEX 1 mg/L), but this facility operated at an extremely low hydraulic loading (1 mm/d), and conse- quently had no water discharge. ESSO, Chilliwack, British Columbia A cardlock facility produced stormwaters contaminated by diesel fuel. FWS wetlands were found successfully reduc- ing water-phase diesel range organics to permit levels (Nix, 1995). Removal rate constants were about 10 m/yr. Marathon-Pitchfork, Wyoming Produced Water Project The Colorado School of Mines tested a combined pilot sys- tem for hydrocarbon removal (Caswell et al., 1992). Overland ow gravel beds (shallow) were loaded at 20–80 cm/d, and reduced benzene from 15 to 2 µg/L. Further removal in SSF wetlands reduced benzene to below detection. Isanti-Chisago, Minnesota Leachate Treatment System The Isanti-Chisago Sanitary Landll, an unlined municipal solid waste facility located near Cambridge, Minnesota, was closed in 1992 (Loer et al., 1999; Kadlec, 2003c). Leach- ing of soluble wastes had contaminated the surcial and increasingly deeper aquifers with toxic organic compounds and heavy metals. Extraction wells permit pumping of leach- ate to the top surface of the landll mound, about eight meters above the surrounding landscape. Volatile Organic Compounds (VOCs), including BTEX, are largely removed by cascading the water down the side of the landll into a sedimentation basin which serves to settle and store iron precipitates. The next component of the treatment train is a 0.6-ha FWS wetland. During ve seasons of treatment, oper- ating results indicated that the system efciency ranges from 85–100% for VOCs and 98% for iron. Only low levels of BTEX enter the system: (benzene 3.7 µg/L; toluene 0.6 µg/L; ethylbenzene 1.1 µg/L, xylene 1.8 µg/L). The cascade and settling basin do all the BTEX removal, although the FWS wetland does polish out any remaining traces of BTEX. Benzene, toluene, ethyl benzene, and xylenes are all individually below detection (0.1 µg/L) in the system outow. Saginaw Township, Michigan, Leachate Treatment System Saginaw Charter Township’s Center Road Landll was closed in the early 1980s (Kadlec, 2003c). Finger drains were installed in the seepage zones, which connect to a collection pipe. Leachate is then pumped to an aerator, which provides some ammonia and BTEX stripping. The water is then held in a sedimentation basin, to promote removal of solids. Water is discharged periodically to one of two intermittent sand/gravel lters, which provide ltration and nitrication. Underdrains then convey the water to two parallel free water surface wet- lands. BTEX and its constituents were monitored for ve years after startup. A mean total BTEX of 39 µg/L was removed, with only one detection in weekly samples (5.9 µg/L). Phoenix, Arizona Wastewater Polishing A demonstration wetland project was studied for VOC removal (Keefe et al., 2004a). The wetland was a 1.4-ha free water surface system of depth about 60 cm. The deten- tion time was 3.9 days. An 80% reduction of toluene from inlet to outlet was measured. Keefe et al. (2004a) attrib- uted the reduction to volatilization, but concluded that theo- retical predictions were only good for order-of-magnitude estimation. Alcoa, Tennessee Groundwater Remediation A pilot wetland project was initiated and operated for a period of over one year, in a moderate north temperate con- tinental climate. DRO and GRO were among the targeted substances (Gessner et al., 2005). GRO entered at monthly average values of 0.04–0.37 mg/L, and never exceeded the detection limit of 0.01 mg/L at the outlet of the FWS wet- land. DRO entered at monthly average values of 0.29–1.08 mg/L, and exited at 0.11–0.44 mg/L. GRO removal rate constants could not have been less than 100 m/yr. In con- trast, the DRO removal rate constants were 12 m/yr annual average. The rst wetlands at the site were SSF wetlands, which were abandoned in favor of FWS because of continual operational difculties. Casper, Wyoming, Groundwater Remediation A pilot scale subsurface vertical ow wetland system was con- structed at the former British Petroleum Renery in Casper, Wyoming, in order to determine benzene, toluene, ethylben- zene, and xylene (BTEX) degradation rates in a cold-climate application (Ferro et al., 2002). The pilot system, consisting of four cells, each dosed at a nominal ow rate of 5.4 m 3 /d, was operated between August and December 2002. The pilot tested the effects of wetland mulch and aeration on system performance. Areal rate constants (k A ) were calculated based on an assumed three tanks in series (3TIS). The presence of both wetland sod and aeration improved treatment perfor- mance. Mean k A values were 244 m/yr for cells without sod or aeration, and improved to 356 m/yr for cells with sod and aeration (Table 13.2). Based on the results of the pilot system, a full-scale wet- land system (capable of operating at 6,000 m 3 /d) was started up in May 2003 (Wallace and Kadlec, 2005). The full-scale system achieved permit compliance within one week of start-up. University of Edinburgh, Scotland Bench scale vertical ow wetlands were operated to demon- strate benzene removal (Eke and Scholz, 2006). The systems were operated in a ll-and-drain batch mode, cycling twice per week. Inuent concentrations of 1,000 mg/L benzene were removed to 37–87 mg/L in the outdoor environment. The presence of wetland media (gravel), fertilizer, and warm temperatures were noted in improving treatment. © 2009 by Taylor & Francis Group, LLC 520 Treatment Wetlands ALKANES Omari et al. (2001) determined removal efciencies in both vegetated and unvegetated horizontal-ow gravel beds for straight-chain alkanes of 10 to 26 carbon atoms. Removals were typically above 80% for a vegetated top-fed wetland mesocosm, and above 70% for a top-fed unvegetated meso- cosm, in eight hours’ detention. Removals were somewhat less for the lighter hydrocarbons, up to C15, but did not dif- fer for the heavier alkanes. Omari et al. concluded that con- structed wetlands planted with Typha latifolia were capable of treating oil-polluted water. Similarly, Hoffman (2003) found no pronounced effects of chain length on alkane removals in willow mesocosms, over the range C10–C32. Willows enhanced the removal of TPH, with the efuent alkane concentrations in vegetated mesocosms about half of that in unvegetated mesocosms. Mechanisms of removal were not elucidated by these studies. Salmon et al. (1998) evaluated removal of total hydro- carbons in FWS mesocosms in France. The bed was planted with Typha latifolia, and during the experiment, natural development of Lemna minor occurred. Constructed wet- lands removed 90% of total hydrocarbons. Descriptions of other petroleum industry treatment wet- land projects may be found in Knight et al. (1997). These include the earliest such project at Mandan, North Dakota (Litcheld and Schatz, 1989; Litcheld, 1990; Litcheld, 1993), as well as well-studied projects such as that at Bulwer Island, Australia (Simi and Mitchell, 1999; Simi, 2000). POLYCYCLIC AROMATIC HYDROCARBONS Polycyclic aromatic hydrocarbons (PAHs) are fused ring aromatic compounds formed during the incomplete combus- tion of almost any organic material, and are ubiquitous in the environment (Figure 13.1). Some of them are considered as dangerous substances as a function of their toxic and muta- genic or carcinogenic potentialities. The presence of PAHs in contaminated soils and sediments may pose a risk to the environment and human health. PAHs are hydrophobic com- pounds, whose persistence within ecosystems is chiey due to their low aqueous solubility. These materials are not vola- tile, and no loss to the atmosphere is anticipated under wet- land conditions. Creosote consists of a mixture of organic compounds, dominated by PAHs, many of which are individually desig- nated as hazardous wastes. Wood may be treated with creo- sote to enhance its resistance to decomposition. For instance, railroad ties are typically treated with 128 kg/m 3 . Many of the industrial sites that are or were used to treat ties are now TABLE 13.2 Casper, Wyoming, Mean Pilot System Areal Rate Constants k A , in m/yr Aeration No Aeration Aeration Compound Wetland Mulch No Mulch Wetland Mulch No Mulch Full Scale Benzene 518 (237) 456 (414) 317 (273) 226 (164) 240 BTEX 356 (218) 311 (136) 257 (151) 244 (151) 350 TPH 1,058 (1,141) 965 (722) 725 (558) 579 (506) 325 Note: Based on 3TIS; standard deviation in parenthesis. Source: Data from Wallace and Kadlec (2005) Water Science and Technology 51(9): 165–171. Naphthalene Anthracene Phenanthrene Pyrene Fluorene Benzo(a)pyreneChrysene Fluoranthene Acenaphthene FIGURE 13.1 Example polynuclear aromatic hydrocarbons. © 2009 by Taylor & Francis Group, LLC Organic Chemicals 521 classied as hazardous waste sites, due to spillage and the associated soil contamination. Conversely, there is little or no risk to aquatic and wetland environments from ties as they are normally placed (Brooks, 2000b), nor from bridge or dock pilings (Brooks, 2000a). Biodegradation Aromatics follow a pattern, with polyaromatic hydrocarbons (PAHs) degrading more slowly than benzene; those with more than three rings may not support microbial growth (Zander, 1980). The aerobic microbial degradation of PAHs having two and three rings is well documented. A large number of bacteria that metabolize PAHs have been isolated (Alcali- genes denitricans, Rhodococcus spp., Pseudomonas spp., Mycobacterium spp.; Giraud et al., 2001). A variety of bac- teria can degrade certain PAHs completely to CO 2 and meta- bolic intermediates. As the number of fused rings increases, the degree of degradation decreases (Cookson, 1995, as referenced in Walsh, 1999). Degradation of PAHs by anaero- bic organisms has not been very successful. However, some degradation has been achieved under denitrifying, sulfate reducing, and methanogenic conditions. Naphthalene and anthracene have been found to be slightly degraded anaero- bically under denitrifying conditions (Walsh, 1999). PAH-degrading bacteria were isolated from contami- nated estuarine sediment and salt marsh rhizosphere by enrichment using naphthalene, phenanthrene, or biphenyl as the sole source of carbon and energy (Daane et al., 2001). Identication of the isolates assigned them to three main bac- terial groups: gram-negative pseudomonads; gram-positive, non-spore-forming nocardioforms; and the gram-positive, spore-forming group, Paenibacillus (Table 13.3). This study indicated that the rhizosphere of salt marsh plants contains a diverse population of PAH-degrading bacteria, and the use of plant-associated microorganisms has the potential for bio- remediation of contaminated sediments. Contaminated sedi- ment was obtained from Newtown Creek in the New York Harbor, Brooklyn, New York. Chemical analysis showed the dredged sediment to contain 2 to 7 ppm of the PAHs naph- thalene, anthracene, and phenanthrene. An induction period of 15 days was observed for pyrene, after which degradation was complete in 10 to 15 days. It should be noted that both the pure culture and microbial slurry experiments were per- formed under highly oxygenated conditions and that the lim- ited diffusion of oxygen into organic-rich sediments has been found to restrict PAH biodegradation in the natural environ- ment (DeLaune et al., 1981). So r ption Peat soils adsorb PAH compounds quite effectively. These organics partition very strongly to carbonaceous soils (IT Corporation, 1997; Pardue and Shin, 2000), and are not read- ily desorbed (Pardue and Shin, 2000; Shin and Pardue, 2002). It is therefore expected that PAHs would be sorbed and stored in wetland peats. Partition coefcients in the range of 10 3 –10 4 L/kg were found for phenanthrene (Shin and Pardue, 2002). Plant Uptake Polycyclic aromatic hydrocarbons are not taken up by wet- land plants to any signicant extent. Studies at Duluth, Minnesota, have shown very low concentrations in above- ground plant parts, and only trace amounts associated with roots (IT Corporation, 1997). PAHs were found in dogwood (Cornus stolonifera) and cattail (Typha spp.) shoots at the Duluth site. Willows (Salix spp.) and bulrushes (Scirpus spp.) have been reported to access tightly bound phenanthrene (Gomez-Hermosillo et al., 2000; Gomez and Pardue, 2002). Most of the PAH uptake was to the roots, and was associated TABLE 13.3 Abilities of Several Isolates to Utilize a Variety of PAHs % PAH Remaining Isolate Naphthalene Biphenyl Fluorene Phenanthrene Pyrene Pseudomonads PR-N7 0 85 100 85 90 PR-N9 0 69 78 71 90 PR-N10 40 90 97 89 100 PS-P2 44 51 79 28 100 G r am-Positive Non-spore Formers PR-N15 0 84 80 68 100 PR-P3 0 84 42 1 100 S pore former PR-P1 0 0 29 8 100 Source: Data from Daane et al. (2001) Applied and Environmental Microbiology 67(6): 2683–2691. © 2009 by Taylor & Francis Group, LLC 522 Treatment Wetlands with sorption. Because PAHs are not required for plant metabolism and growth, up take is dependent on concentra- tions and supplies to the pore water in the root zone. Wetland System Studies Testing of wetlands for PAH removal has presented mixed results. Some tests show minimal removal, whereas others show excellent reductions. Anecdotal monitoring of a cypress swamp (forested peatland) receiving landll leachate in Florida indicated no PAH removal after about 15 years (Schwartz et al., 1999). However, other landll leachate con- structed wetland studies have shown no detectable PAHs in the efuent, including naphthalene at a Minnesota site; and naphthalene, uorene, and phenanthrene at a Michigan site (Kadlec, 2003c). Boving (2002) found virtually no retention or removal of ten frequently detected PAHs in a stormwater system comprised of ponds and wetlands. Boving showed that heavier PAHs were present in urban freeway runoff at concen- trations in excess of U.S. EPA benchmarks for chronic toxicity. In particular, benzo(a)pyrene was present at over 1.0 Mg/L. Subsurface ow constructed wetlands were used to treat coke plant wastewaters at Sollac, Fos sur Mer, France (Jardinier et al., 2001). Removal of 45% was achieved in 11 days’ deten- tion. Jardinier et al. concluded that reedbeds were a valid method to remove PAHs. In a German study, naphthalene was removed using hydroponic cultures of Carex gracilis and Juncus effusus and using sand-bed reactors planted with Carex gracilis and Juncus effusus, respectively, under batch and ow through conditions (Wand et al., 2002). Concentrations of about 30 mg/L naphthalene were efciently eliminated over peri- ods of up to six months. Vegetated cultures were found to achieve a better removal rate than systems without vegeta- tion. In the systems investigated, naphthalene was thought to be mainly degraded by bacteria in the rhizosphere. The behavior of the three-ring PAH phenanthrene was investigated in a VF wetland system in Munich, Germany (Machate et al., 1997), with overall removals of more than 99% at a detention time of 6.5 days. Intermediate degrada- tion involved formation of 1-hydroxy-2-naphthoic acid as a bacterial metabolite, which subsequently was also removed in the wetland. Phenanthrene-degrading bacteria were enu- merated, and found to be highest in the inlet zone of the sys- tem (10 4 per mL) in comparison to the outlet end (fewer than 10 per mL). Feed concentrations of 0.385 mg/L phenan- threne were reduced to less than 0.003 mg/L. The lava rock substrate had only a small partition coefcient, in the range 0.1 to 1.8 L/kg. An experimental subsurface ow constructed wetland was developed in Curienne (Savoie-France), and operated with a feed of wastewater dosed with uoranthene (Giraud et al., 2001). Two beds were operated in series, with a total detention time of three days. The inlet uoranthene concen- tration was set at 6,660 mg/L, and no PAHs were detected in the wetland outows. A total of 40 fungal species (24 gen- era) were isolated and identified from samples (gravel and sediments) from the test wetland and a control wetland. Fluor- anthene was degraded efciently by 33 species whereas only two species were able to remove anthracene by over 70%. Salmon et al. (1998) evaluated removal of total hydro- carbons in FWS mesocosms in France. The bed was planted with Typha latifolia, and during the experiment, natural development of Lemna minor occurred. Constructed wet- lands removed 90% of total hydrocarbons. 13.2 CHLORINATED HYDROCARBONS C HLORINATED BENZENES Chlorobenzenes were and are used in the production of phenol, aniline, and DDT. Mono-, di-, and trichloroben- zenes are used as solvents (Grayson, 1985). Various benzene hexachloride isomers are used as broad-spectrum insecti- cides, including Lindane™. Mono-, di-, and trichloroben- zenes are resistant to photo-oxidation, and to both aerobic and anaerobic degradation in purely aquatic environments, with estimated half-lives of up to 6 to 24 months (Howard et al., 1991). However, wetland environments are quite differ- ent, because of the close interactions with plants and soils. The major routes for removal of chlorobenzenes from wet- land waters are: biological (microbial) degradation, sorption, volatilization, and plant uptake. Biodegradation In general, chlorinated hydrocarbons may be dechlorinated under anaerobic conditions, and the responsible microbial consortia have been identied (van Eekert and Schraa, 2001). Reductive dechlorination of chlorobenzenes occurs via an anaerobic sequential pathway in wetland soils and sediments. Jackson and Pardue (2000) established that the dichloroben- zene (DCB) formed monochlorobenzene (MCB), which sub- sequently mineralized: DCB MCB CO H O Cl 2 ll 2 (13.1) Their study showed that MCB produced reached half of the initial DCB concentration in 30 days, and was accompa- nied by the formation of methane. In separate experiments, Jackson and Pardue (2000) recovered 13% of 14 C-MCB as 14 CO 2 in a surface sediment modulated decomposition. Sorption Chlorobenzenes sorb strongly to both organic and inorganic wetland substrates (Pardue et al., 1993). Shin and Pardue (2000; 2002) showed that there are both reversible and irre- versible portions of the overall sorption. For several sediment and soil samples, the reversible part ranged 1.88 a log 10 K OC a 2.88, while the irreversible part ranged 3.75 a log 10 K OC a 5.55. Thus, partitioning to organics is very strong, and domi- nated by irreversible binding. Suspended particulate matter forms a mobile sub- strate for partitioned chlorobenzenes (Shugui et al., 1994). © 2009 by Taylor & Francis Group, LLC Organic Chemicals 523 The FWS wetland biogeochemical cycle creates and pro- cesses very large quantities of total suspended solids (TSS), and thus can play an important role in organic chemical cycling and removal in these systems. The wetland environ- ment is further complicated by the presence of large mol- ecules of humic substances, which comprise a good share of dissolved organic carbon. Organic solutes can partition to these dissolved humic substances, thus creating two soluble forms with different chemical characteristics. Volatilization The chlorobenzenes are volatile, and therefore may be easily lost from water, especially shallow water bodies, such as free water surface wetlands. Of direct interest are the estimates, based on wetland data, of Keefe et al. (2004a) for a FWS wet- land in Arizona. Values were determined of K w 0.011 m/hr (100 m/yr) for chlorobenzene, and K w 0.008 m/hr (70 m/yr) for 1,4 dichlorobenzene. These are relatively high rates, com- pared to other pollutants treated in FWS wetlands. Plant Uptake Plants are capable of taking up organic chemicals (Trapp and Matthies, 1995), and processing them in a number of ways. They may be carried through the plant into the atmo- sphere via the transpiration ux, metabolized, or accumu- lated in plant tissues. For example, Leppich et al. (2000) found up to 100 mg/kg of various chlorobenzenes in black willow (Salix nigra) bark, and up to 25 mg/kg dw in leaves. The willows were growing in a contaminated swamp site, with large concentrations of di-, tri-, penta-, and hexa-chlo- robenzenes. Leppich et al. (2000) concluded that partition- ing of these organics to the plants formed an important part of the site model. Ove rall Removal Coefficients Despite the investigations detailed above, there is no reported treatment wetland that has specically been designed to tar- get chlorobenzenes. Therefore, results from several treatment technologies are examined here to gain some insight as to the anticipated rates of removal to be expected in wetlands. Wetlands Keefe et al. (2004a) calculated removal rate coefcients from a FWS dataset to be 135 m/yr for chlorobenzene, and 67 m/yr for 1,4 dichlorobenzene. The wetland was a 1.4-ha free water surface system of depth about two feet. The detention time was 3.9 days. These results correspond to a 67% reduction of 1,4 dichlorobenzene from inlet to outlet. Keefe et al. (2004a) attributed the reduction to volatilization, but concluded that theoretical predictions were only good for order-of-magni- tude estimation. Braeckevelt et al. (2006) studied monochlorobenzene reduction in a pilot-scale horizontal subsurface ow wetland constructed of local soil materials. Contaminated groundwater containing up to 22 mg/L of monocholorobenzene was added to the system at a hydraulic loading rate of 2.3 cm/d. Choro- benzene reductions of up to 77.1% were observed in the system. Monochlorobenzene reductions were highest in the upper soil layer, possibly due to volatilization, and decreased to 37.1% at the bottom of the wetland bed (0.5 m). Enrich- ment of 13 C and low dissolved oxygen concentrations suggest that reductive dehalogenation under anaerobic conditions was the dominant removal mechanism. CHLORINATED ETHENES Perchloroethylene (PCE) and trichloroethylene (TCE) are solvents that saw extensive use for metal cleaning and other applications in previous decades. At many locations, these materials found their way into groundwater, creating hazard- ous waste conditions, because of concern due to their carcino- genic properties. Wetlands provide environments for several mechanisms of removal of chlorinated aliphatic organics, including sorption, volatilization, reductive dechlorination, direct biological oxidation, co-metabolism, and plant uptake and metabolism (Pardue et al., 1993; Parkin, 1999; Pardue et al., 2000). Kassenga (2002) conducted continuous vertical ow col- umn and wetland microcosm studies to investigate the atten- uation potential of chlorinated volatile organic compounds. Calibrated simulations indicated that removal of TCE in con- structed wetland columns was controlled by biodegradation whereas both sorption and biodegradation were important natural wetland columns. Kassenga et al. (2003) evaluated the removal of cis-1,2-dichlorethene (cis-1,2-DCE) in upow wetland mesocosms planted with Scirpus americanus. The results conrmed that biodegradation was occurring in the system, and sand, peat, and Bion soil mixture had greater degradation rate than the sand and peat mixture. Lorah et al. (1997; 2002) observed complete removal of TCE and daughter products as contaminated water moved upward through peat to the surface. Reducing conditions were present in the peat, and both methanogenic and iron- reducing zones were identied. This important study pro- vided the impetus to examine the future role of natural and constructed wetlands in the remediation of TCE. A former TCE recycling plant is now the site of a city park in New Brighton, Minnesota. A TCE plume in a sur- cial aquifer discharges into the wetland of an adjacent lake (Bankston et al., 2002). Transect studies showed TCE in monitoring wells just upgradient from the wetland, and to a lesser degree in the fringe of the wetland. The anaerobic degradation product of TCE, cis-1,2-DCE, was detected in the aquifer and the wetland. It was hypothesized that the indigenous cattail (Typha latifolia) assisted in phytoreme- diation. Accordingly, microcosm studies were performed to determine the fate of the removed TCE. The recovery of 14 C totaled 94.1%, of which 46.8% was volatilized, most likely as [ 14 C] TCE because it was added to the microcosms by surface application. Plant tissue contained 38.2% of the 14 C; 5.3% was present as [ 14 C] CO 2 , and 3.7% was recovered from the © 2009 by Taylor & Francis Group, LLC 524 Treatment Wetlands soil and water. This data suggested that natural attenuation is a potential bioremediative strategy for TCE-contaminated wetlands. Given that natural wetlands contribute to reduction of TCE, the next logical step is the reconstruction of former wetlands that are positioned in the ow path of a contamina- tion plume. Richard et al. (2002; 2003) lled and planted a dredged channel that was conveying TCE from a contami- nated site to a lake. In this case, an aquatic feature was con- verted to wetland to aid in treatment. This Minnesota project was completed in 2000. In other cases, terrestrial landforms may be converted to constructed treatment wetlands to provide reductions in TCE. The Schilling Farm Project at Hillsdale, Michigan, intercepts a TCE plume for treatment in a constructed FWS wetland, built in a former corn eld. The wetland system is made up of four treatment cells in parallel (see Figures 4.21 and 4.26). The two large central cells were sited to intercept the plume, and the two small anking cells were added to ensure the full capture of that plume. Groundwater moves down a slight incline into a 4-m deep capture trench, designed to intercept approximately the top 2 m of the underground ow. This trench is lled with coarse rock to eliminate safety hazards and control rodent problems. The water ows upward and out across the FWS wetland cells, carrying TCE and DCE into the wetlands. During passage through the wetland, TCE undergoes reductive dechlorination to DCE and then to vinyl chloride (VC) in anoxic zones, and these are further degraded to carbon dioxide and water in aerobic zones. There is also volatilization of VC, and to a lesser extent DCE and TCE. Water is collected in rock-lled trenches, approximately 1 m deep, at the downstream end of all four cells. These four outows are metered, and merged to form a single project outow for compliance monitoring. Additionally, another independent rock-lled trench is positioned across the entire downstream end of the four cells, which is drained via perfo- rated pipe into the compliance outow. The purpose of this trench is to capture waters that may pass totally underneath the treatment wetland. Control of this wetland system is solely by means of set- ting water levels in each of the four cells, by use of the weir settings in the outlet structures. It is totally passive, with no pumps, and runs year-round in a cold climate. Therefore, water level control must compensate for ice formation. The correct operating strategy is the subject of ongoing investiga- tions. Herbivory and short-circuiting were created by musk- rats (Ondatra zibethicus), necessitating removal of both the animals and their habitat, by lling trenches with large rock, and by fencing the wetland. T a ble 13.4 shows the performance results for the sys- tem for 88 of the 95 months of operation, October 1998 through September 2005. Figure 13.2 shows performance for October 2003 through September 2005, respectively. The rst seven months were a dormant period for vegetation planted in autumn 1998, which remained sparse for that period. Efu- ent standards for TCE (limit 150 µg/L) were met in 84 of 88 months after startup. Exceedances for VC (limit 13 µg/L) tend to occur in late summer, when wetland surface ow con- tributes little to the outow. In summary, these projects all utilize the characteristics of wetlands to reduce TCE. Research work continues at the time of this writing at Wright–Patterson Air Force Base in Ohio (Entingh, 2002; Blalock, 2003). The addition of recycle in 2007 has eliminated VC from the outow. 13.3 ORGANIC CHEMICALS E XPLOSIVES It has been estimated that approximately 100 military bases and explosives manufacturing facilities have soil and/or groundwater contaminated with munitions (Medina et al., 2000). Studies have shown that plants are capable of trans- forming 2,4,6-trinitrotoluene (TNT) without microbial con- tribution, but very little accumulation of TNT has been found in plant material. Therefore, plant-enhanced degradation, or phytoremediation, of TNT by aquatic macrophytes has been proposed as a promising groundwater treatment process. TABLE 13.4 Performance of the Schilling Farm Constructed FWS Wetlands Annual Winter Spring Summer Autumn HLR (cm/d) 1.00 1.14 1.39 0.85 0.60 T (nC) 11.9 1.0 12.4 23.6 12.7 TCE C i (µg/L) 726 726 726 726 726 TCE C o (µg/L) 37 87 25 18 21 cis-1,2-DCE C i (µg/L) 472 472 472 472 472 cis-1,2-DCE C o (µg/L) 44 65 21 46 46 VC C o (µg/L) 5.8 5.4 1.6 8.5 7.6 Note: Inlet concentrations are average undiluted plume values. © 2009 by Taylor & Francis Group, LLC Organic Chemicals 525 TNT is a reactive molecule that biotransforms readily under both aerobic and anaerobic conditions to give amino- dinitrotoluenes (Brannon and Myers, 1997; Hawari et al., 2000; Xiang, 2001; Esteve-Nunez et al., 2001). The resulting amines biotransform to give several other products, including azo, azoxy, acetyl, and phenolic derivatives, leaving the aromatic ring intact. Little or no mineralization is encountered dur- ing bacterial or wetland treatment. The nonaromatic cyclic nitramine explosives hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX) and octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX) lack the electronic stability enjoyed by TNT or its transformed products. Therefore, an enzymatic change on one of the N-NO 2 or C-H bonds of the cyclic nitramine could lead to a ring cleavage, and subsequent mineralization. Medina et al. (2000) performed a series of batch-scale experiments to assess the engineering kinetics of phytodeg- radation of TNT under a variety of operational conditions. Parrotfeather (Myriophyllum aquaticum) was hydroponically grown in laboratory microcosms. TNT was degraded accord- ing to a near-rst order model in vegetated microcosms, but not in unvegetated systems (Figure 13.3). Measurements at varying plant densities indicated that rate constants increased with increasing plant abundance. Removal rate constants also increased with increasing temperature from 2 to 34°C, leveled off between 34 and 43°C, and at 54°C, no activity was found. This pattern is also found for enzyme kinetics, in which rates increase until the enzyme denatures. Plants appeared healthy up to 34°C, but wilted at 43°C. Plants incu- bated at 54°C were dead by the end of the experiment. The modied Arrhenius temperature coefcient was 1.093 for water temperatures between 2 and 30°C. Hughes et al. (1997) found that Eurasian water milfoil (Myriophyllum spicatum) degraded TNT to aminonitrotoluenes, whereas unvegetated controls did not. 0 100 200 300 400 500 0 30 60 90 120 150 180 210 240 270 300 330 360 Julian Day TCE or DCE (µg/L) TCE Out DCE Out TCE In DCE In FIGURE 13.2 Annual pattern of treatment of TCE and DCE in the Schilling Farm wetland system. The inlet concentrations have been adjusted for dilution with clean groundwater entering the ank cells. Winter conditions provide lesser treatment. There is a second period of lesser treatment in late summer and autumn, which is occasioned by very low surface ows and dominance of underground ows. 0.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0 0 5 10 15 20 25 Time (hr) TNT (mg/L) Parrotfeather Unvegetated FIGURE 13.3 Disappearance of TNT in batch microcosms. The vegetated system points represent the mean of ve replicates. The half-life of TNT was about seven hours. (Data from Medina et al. (2000) Water Research 34(10): 2713–2722.) © 2009 by Taylor & Francis Group, LLC 526 Treatment Wetlands Zoh and Horne (2000) also performed a series of batch- scale experiments. Utilizing straw, cattail, and bulrush lit- ter in water, they found no degradation without litter, and rst-order behavior in the presence of litter. Sorption and 14 C studies indicated that removal was due to initial sorption fol- lowed by degradation to aminonitrotoluenes. Volumetric rate coefcients (k V ) translate to areal rates (k A ) on the order of 25 m/yr for TNT, regardless of litter type. A modied Arrhe- nius temperature coefcient (Q) of 1.17 t the observed dif- ference in rates at 10 and 20°C. The ability of ten species of submerged aquatic to phy- toremediate explosives-contaminated groundwater was investigated by Best et al. (1997a). Phase I of this project provided for laboratory-scale plant screenings to evaluate locally adapted aquatic and wetland species for their dif- ferential ability to diminish levels of TNT and RDX. These were evaluated under hydroponic batch conditions. Analysis of the data according to a batch rst-order areal model shows remarkable similarity among species (Table 13.5). Best et al. (Best et al., 1999a,b) reported that per unit of mass, uptake of TNT was higher in submerged (Elodea canadensis, Potamogeton pectinatus, Heteranthera dubia, Myriophyllum aquaticum) rather than emergent species (Acorus calamus, Phalaris arundinacea, Scirpus cyperinus) and biotransformation of TNT had occurred in all plant treat- ments after a seven-day incubation in 1.6 to 3.4 mg/L TN. TNT declined less with substrates, and least in controls with- out plants. Mineralization to CO 2 was very low, and evolu- tion into C-volatile organics negligible. RDX disappeared less rapidly than TNT from groundwater. Cattails (Typha angustifolia) in FWS mesocosms were used to test treatment of mono-, di-, and trinitrotoluene mix- tures at the Volunteer Army Ammunition Plant, Tennessee (Best et al., 2000; 2001). Rate coefcients (Table 13.6) ranged from 16 to 45 m/yr. The potential contribution of photodeg- radation was determined by shielding nonplanted mesocosms from UV in sunlight. Radiation accounted for 30% of TNT, 60% of DNT, and 10% of NT removal in the absence of plants. TABLE 13.5 Batch Kinetics of TNT Reduction for Several Submersed Species with Data Including Two Different Plant Densities Scientific Name Common Name k A (m/yr) Myriophyllum aquaticum Parrotfeather 39 Myriophyllum spicatum Milfoil 48 Egeria densa Egeria 30 Eiodea canadensis Elodea 40 Vallisneria amerilcana Vallisneria 50 Potamogeton crispus Curly leaf pondweed 48 Potamogeton pectinatus Sago pondweed 40 Heteranthera dubia Star-grass 45 Eleocharis parvula Spikerush 49 Chara vulgaris Stonewort 49 Source: DatafromBest et al. (1997a) Screening aquatic and wetland plant species for phytore- mediation of explosives-contaminated groundwater from the Iowa Army Ammunition Plant. Technical Report EL-97-2, U.S. Army Corps of Engineers Waterways Experiment Station: Vicksburg, Mississippi. TABLE 13.6 Continuous-Flow Kinetics of Nitrotoluenes Reduction for Typha angustifolia Mesocosms Contaminant Inlet (mg/L) Outlet (mg/L) Percent Removal (season) Areal Rate Constant (m/yr) Trinitrotoluene 2.73 0.30 79% 31 2,4-dinitrotoluene 16.66 6.00 58% 16 2,6-dinitrotoluene 5.17 1.40 61% 17 2 nitrotoluene 42.64 2.20 — 45 Source: Data from Best et al. (2000) Explosives removal from groundwater at the Volunteer Army Ammunition Plant, Tennessee, in small-scale wetland mod- ules. Means and Hinchee (Eds.), Wetlands and Remediation: An International Conference; Battelle Press: Columbus, Ohio, pp. 365–374; and Best et al. (2001) Water Science and Technology 44(11–12): 515–521. Note: Season refers to June through October. © 2009 by Taylor & Francis Group, LLC [...]... most commonly used agricultural herbicide is atrazine, and it has therefore been the subject of many treatment wetland studies ATRAZINE Atrazine (2-chloro- 4-( ethylamino )-6 -( isopropylamino)-s-triazine) is a triazine herbicide used to control broadleaf weeds © 2009 by Taylor & Francis Group, LLC Treatment Wetlands in corn, sorghum, sugarcane, turfgrass, fruits, and vegetables (Kao et al., 2001b) Agricultural... those sediments according to a first-order rate law, with a half-life of 40 to 90 days However, degradation was faster on cattail litter, precluding a rate measurement The estimated half-life on cattail detritus was on the order of five days Organic Chemicals 533 TABLE 13. 9 Removal of Pesticides in Treatment Wetlands Pesticide Wetland Type Size Aldrin FWS Field Azinphos-methyl Chlorothalonil FWS FWS Chlorpyrifos... for the wetland are 5.3 and 2.4 days during 528 Treatment Wetlands average and high water discharges, respectively Annual wetland retention of TN varied in the range of 6 to 36% during 1998 to 2001 During airport de-icing, January through March 2001, 660 kg urea-N out of 2,600 kg applied urea-N reached the wetland, and approximately 40% of the incoming urea-N was eliminated in the wetland system at air... of 63% The full-scale facility had a phenol rate constant removal rate coefficient of 100 m/yr The treatment system at Amoco Mandan, North Dakota, is comprised of an API separator, lagoon, and a series-parallel set of FWS wetlands Approximately 5,700 m3/d of process water is directed to an API separator for primary treatment, and then passed through the oxidation lagoon for secondary treatment Process... 14C-labeled LAS Sorption and evolution of 14CO2 were followed with time LAS that was sorbed to the detritus was mineralized without a lag, with a half-life of 12.6 days This study showed that detritus represents a significant site of surfactant removal in detritus-rich systems Barber et al (2001) performed synoptic sampling of several existing treatment wetlands, including analysis for LAS (Table 13. 8)... research (Inaba, 1992) LAS was effectively reduced in a FWS Typha–Phragmites wetland The 474-m2 system treated gray water from a 100-inhabitant community The calculated plug flow volumet- TABLE 13. 8 Results of Synoptic Sampling of Linear Alkylbenzene Sulfonate (LAS) at Several Treatment Wetland Sites Site Hemet Arcata treatment Arcata enhancement Halsey Halsey Halsey Corvallis Wetland Type FWS FWS FWS HSSF... mesocosms had higher removal rates than unshielded unplanted systems (Best et al., 2001) The U.S Army Corps of Engineers constructed treatment wetlands with the purpose of containing and treating residual TNT (2,4,6-trinitrotoluene) and RDX (hexahydro1,3,5-trinitro-1,3,5-triazine) contamination at the Iowa Army Ammunition Plant (Thompson et al., 2003) Screening studies established the relative capabilities... of BOD5 (see Chapters 8 and 13) Degradation of urea follows patterns established for other species of nitrogenous compounds (see Chapter 9) However, pollutant loads occur at cold temperatures, which minimize some biological processes, notably those for nitrogen reduction The threat of bird–aircraft strikes indicates the use of SSF wetlands in many cases The applicability of treatment wetlands for reducing... Lopez-Flores et al (2003) Wilson et al (2001) 534 Treatment Wetlands TABLE 13. 10 Photolysis Half-Lives of Triazine Herbicides in Natural Waters (in days) Lake Water River Water Groundwater Distilled Water 53 65 51 43 53 52 26 29 28 35 33 32 Atrazine Propazine Prometryne Note: Dark blanks have been deducted Source: Data from Konstantinou et al (2001) Journal of Environmental Quality 30(1): 121 130 Outflows... into a 300,000 cubic meter detention facility, prior to the wetlands Treated water flows out of the area to surface receiving waters The cost to construct the wetlands was CDN $2 million A 0.24-ha HSSF wetland system was installed at the Westover Air Reserve Base in Chicopee, Massachusetts to demonstrate the treatment of stormwaters from on-site deicing operations (Karrh et al., 2002; ESTCP, 2004; Naval . encountered dur- ing bacterial or wetland treatment. The nonaromatic cyclic nitramine explosives hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX) and octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine. Pyrene Pseudomonads PR-N7 0 85 100 85 90 PR-N9 0 69 78 71 90 PR-N10 40 90 97 89 100 PS-P2 44 51 79 28 100 G r am-Positive Non-spore Formers PR-N15 0 84 80 68 100 PR-P3 0 84 42 1 100 S pore former PR-P1 0. and it has there- fore been the subject of many treatment wetland studies. ATRAZINE Atrazine (2-chloro- 4-( ethylamino )-6 -( isopropylamino)-s-tri- azine) is a triazine herbicide used to control