TREATMENT WETLANDS - CHAPTER 13 doc

The major routes for removal of hydrocarbons from wetland waters include: volatilization, photochemical oxidation, sedimenta-tion, sorpsedimenta-tion, biological microbial degradasedimen

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 specific individual or groups of organic chemicals These

chemicals pose a new and somewhat more difficult 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, sorpsedimenta-tion, biological (microbial) degradasedimenta-tion, 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 affinity 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 specific

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 paraffinic and aromatic constituents

BTEX

Biodegradation

Biodegradation of BTEX in a wetland environment is

compli-cated by the existence of biofilms on submerged plant parts,

plant litter, and gravel Although small in terms of mass per

unit volume, these biofilms are very active in tion, and consequently serve as important sinks for organics

biodegrada-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 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

inocu-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 first-order loss

rate constants for BTEX constituents exhibit a fixed 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 Kw 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

field vapor collection in a peatland contaminated with BTEX Losses averaged 2,500 mg/m2·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

BTEX components are expected to partition strongly to

organic wetland substrates, although a definitive wetland

study is lacking Wetland sediments typically have high

organic content, and therefore sorption may be an important

first step in overall removal Benzene sorption onto peat was

found to be strong, with a linear sorption coefficient of KOC

5.6 L/kg (Moore et al., 2002), whereas binding to clay was

much weaker, with KOC 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 landfills, army

ammunition plants, and former oil refinery 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 landfill leachate

are typically lower than those associated with refineries 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 Influent

BTEX concentrations ranged from 4.5–12.1 mg/L, and

influ-ent flows 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 five subsurface flow wetlands in series, each being 2.5 m wide, 4.0 m long, and 0.8 m deep Beds 1 and 2 were vertical flow, with passive aeration to the underdrains Beds 3, 4, and 5 were horizontal flow Each bed was filled with a stratified 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 gravel, 10 cm of sharp sand, and topped with

2 cm of organic soil Water is intermittently dosed to the face, where it spreads and infiltrates, ultimately reaching the (10 cm) under-drains in the second layer The passive air is admitted to the first layer via perforated plastic pipe Details

sur-of this type sur-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 influent BTEX in August was 3.8 mg/L (1.5 mg/L benzene), and the outflow contained no detectable BTEX, with all constituents being below 0.01 mg/L The flow of 5 m3/d corresponded to about 5 cm/d hydraulic loading, or about five days’ detention Although these results are encouraging, they do not answer the question of how much of the five-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 first-order areal TH

model yields k 66 m/yr, with no evidence of temperature dependence This is consistent with the benzene removal rate coefficient, 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

hydrocarbons was reported (Wallace, 2002), despite high

influent concentrations (CBOD5  16,000 mg/L, NH4-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

flow 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 Landfill, 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 surficial and

increasingly deeper aquifers with toxic organic compounds

and heavy metals Extraction wells permit pumping of

leach-ate to the top surface of the landfill 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 landfill 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 five seasons of treatment,

oper-ating results indicated that the system efficiency 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 outflow

Saginaw Township, Michigan, Leachate Treatment System

Saginaw Charter Township’s Center Road Landfill 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

filters, which provide filtration and nitrification Underdrains

then convey the water to two parallel free water surface

wet-lands BTEX and its constituents were monitored for five 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 tion time was 3.9 days An 80% reduction of toluene from

deten-inlet to outlet was measured Keefe et al (2004a)

attrib-uted the reduction to volatilization, but concluded that retical predictions were only good for order-of-magnitude estimation

theo-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 first wetlands at the site were SSF wetlands, which were abandoned in favor of FWS because of continual

operational difficulties.

Casper, Wyoming, Groundwater Remediation

A pilot scale subsurface vertical flow wetland system was structed at the former British Petroleum Refinery in Casper, Wyoming, in order to determine benzene, toluene, ethylben-zene, and xylene (BTEX) degradation rates in a cold-climate

con-application (Ferro et al., 2002) The pilot system, consisting

of four cells, each dosed at a nominal flow rate of 5.4 m3/d, was operated between August and December 2002 The pilot tested the effects of wetland mulch and aeration on system

performance Areal rate constants (kA) were calculated based

on an assumed three tanks in series (3TIS) The presence of both wetland sod and aeration improved treatment perfor-

mance Mean kA 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 land system (capable of operating at 6,000 m3/d) was started

wet-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 flow wetlands were operated to strate benzene removal (Eke and Scholz, 2006) The systems were operated in a fill-and-drain batch mode, cycling twice per week Influent 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

demon-A LKANES

Omari et al (2001) determined removal efficiencies in both

vegetated and unvegetated horizontal-flow 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 effluent 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 (Litchfield and Schatz, 1989; Litchfield, 1990; Litchfield, 1993), as well as well-studied projects such as that at Bulwer Island, Australia (Simi and Mitchell, 1999; Simi, 2000)

P OLYCYCLIC A ROMATIC H YDROCARBONS

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 chiefly due

to their low aqueous solubility These materials are not tile, and no loss to the atmosphere is anticipated under wet-land conditions

vola-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/m3 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 kA , in m/yr

Compound Wetland Mulch No Mulch Wetland Mulch No Mulch Full Scale

Note: Based on 3TIS; standard deviation in parenthesis.

Source: Data from Wallace and Kadlec (2005) Water Science and Technology 51(9): 165–171.

Benzo(a)pyrene

FIGURE 13.1 Example polynuclear aromatic hydrocarbons.

classified 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 denitrificans, Rhodococcus spp., Pseudomonas spp.,

Mycobacterium spp.; Giraud et al., 2001) A variety of

bac-teria can degrade certain PAHs completely to CO2 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)

Identification 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 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

bio-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).

Sorption

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 coefficients in the range of 103–104L/kg were found for phenanthrene (Shin and Pardue, 2002)

Plant Uptake

Polycyclic aromatic hydrocarbons are not taken up by land plants to any significant 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

wet-(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

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 landfill leachate in

Florida indicated no PAH removal after about 15 years

(Schwartz et al., 1999) However, other landfill leachate

con-structed wetland studies have shown no detectable PAHs in

the effluent, including naphthalene at a Minnesota site; and

naphthalene, fluorene, 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 flow 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 flow through

conditions (Wand et al., 2002) Concentrations of about

30 mg/L naphthalene were efficiently 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 formadegrada-tion 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 (104 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 coefficient, in the range

0.1 to 1.8 L/kg

An experimental subsurface flow constructed wetland

was developed in Curienne (Savoie-France), and operated

with a feed of wastewater dosed with fluoranthene (Giraud

et al., 2001) Two beds were operated in series, with a total

detention time of three days The inlet fluoranthene

concen-tration was set at 6,660 mg/L, and no PAHs were detected in

the wetland outflows 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 anthene was degraded efficiently by 33 species whereas only two species were able to remove anthracene by over 70%

Fluor-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 B ENZENES

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 identified (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:

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 14C-MCB as

14CO2 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 versible portions of the overall sorption For several sediment and soil samples, the reversible part ranged 1.88 a log10KOC

irre-a 2.88, while the irreversible pirre-art rirre-anged 3.75 irre-a log10KOCa5.55 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)

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 Kw 0.011 m/hr

(100 m/yr) for chlorobenzene, and Kw 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 flux, 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

Overall Removal Coefficients

Despite the investigations detailed above, there is no reported

treatment wetland that has specifically 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 coefficients 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 flow 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 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

Choro-to 37.1% at the botChoro-tom of the wetland bed (0.5 m) ment of 13C and low dissolved oxygen concentrations suggest that reductive dehalogenation under anaerobic conditions was the dominant removal mechanism

Enrich-C HLORINATED E THENES

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 flow 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

col-natural wetland columns Kassenga et al (2003) evaluated the removal of cis-1,2-dichlorethene (cis-1,2-DCE) in upflow wetland mesocosms planted with Scirpus americanus The

results confirmed 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 identified 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 surfi-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 14Ctotaled 94.1%, of which 46.8% was volatilized, most likely as [14C] TCE because it was added to the microcosms by surface application Plant tissue contained 38.2% of the 14C; 5.3% was present as [14C] CO2, and 3.7% was recovered from the

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 flow path of a

contamina-tion plume Richard et al (2002; 2003) filled 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 field 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 flanking 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 flow This

trench is filled with coarse rock to eliminate safety hazards

and control rodent problems The water flows 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-filled trenches, approximately 1 m

deep, at the downstream end of all four cells These four

outflows are metered, and merged to form a single project

outflow for compliance monitoring Additionally, another

independent rock-filled trench is positioned across the entire

downstream end of the four cells, which is drained via

perfo-rated pipe into the compliance outflow 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 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-

set-rats (Ondatra zibethicus), necessitating removal of both the

animals and their habitat, by filling trenches with large rock, and by fencing the wetland

Table 13.4 shows the performance results for the 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 first seven months were a dormant period for vegetation planted in autumn 1998, which remained sparse for that period Efflu-ent standards for TCE (limit 150 µg/L) were met in 84 of

sys-88 months after startup Exceedances for VC (limit 13 µg/L) tend to occur in late summer, when wetland surface flow con-tributes little to the outflow

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 outflow

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 forming 2,4,6-trinitrotoluene (TNT) without microbial con-tribution, but very little accumulation of TNT has been found

trans-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

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-NO2 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 ing to a near-first 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,

accord-in which rates accord-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 modified Arrhenius temperature coefficient 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

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 flank 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 flows and dominance of underground flows.

0.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0

Time (hr)

Parrotfeather Unvegetated

FIGURE 13.3 Disappearance of TNT in batch microcosms The vegetated system points represent the mean of five replicates The half-life

of TNT was about seven hours (Data from Medina et al (2000) Water Research 34(10): 2713–2722.)

Zoh and Horne (2000) also performed a series of

batch-scale experiments Utilizing straw, cattail, and bulrush

lit-ter in walit-ter, they found no degradation without litlit-ter, and

first-order behavior in the presence of litter Sorption and 14C

studies indicated that removal was due to initial sorption

fol-lowed by degradation to aminonitrotoluenes Volumetric rate

coefficients (kV) translate to areal rates (kA) on the order of

25 m/yr for TNT, regardless of litter type A modified

Arrhe-nius temperature coefficient (Q) of 1.17 fit 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 first-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 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 CO2 was very low, and evolu-tion into C-volatile organics negligible RDX disappeared less rapidly than TNT from groundwater

treat-Cattails (Typha angustifolia) in FWS mesocosms were

used to test treatment of mono-, di-, and trinitrotoluene tures at the Volunteer Army Ammunition Plant, Tennessee

mix-(Best et al., 2000; 2001) Rate coefficients (Table 13.6) ranged

from 16 to 45 m/yr The potential contribution of 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

photodeg-TABLE 13.5 Batch Kinetics of TNT Reduction for Several Submersed Species with Data Including Two Different Plant Densities

Source: Data from Best et al (1997a) Screening aquatic and wetland plant species for mediation of explosives-contaminated groundwater from the Iowa Army Ammunition Plant

phytore-Technical Report EL-97-2, U.S Army Corps of Engineers Waterways Experiment Station:

Outlet (mg/L)

Percent Removal (season)

Areal Rate Constant (m/yr)

mod-Note: Season refers to June through October.