TREATMENT WETLANDS - CHAPTER 22 ppt

Monitoring and adjustment of flows, water levels, water quality, and biological parameters are the principal day-to-day activities required to achieve successful performance of these low

As discussed in the preceding chapters, most treatment

wetlands are designed and constructed to require infrequent

operational control or maintenance Through conservative

design and simple, low-maintenance mechanical controls, many

free-water surface (FWS) wetland treatment systems will

expe-rience minimal ecological changes and will continue to meet

final effluent limits for a long period of time The situation is

more problematic for horizontal subsurface flow (HSSF)

wet-lands, which will require periodic bed maintenance throughout

the lifetime of the system, and for vertical flow (VF) wetlands,

which are dependent on loading-and-resting regimes to

main-tain hydraulic conductivity, unless lightly loaded

Monitoring and adjustment of flows, water levels, water

quality, and biological parameters are the principal day-to-day

activities required to achieve successful performance of these

low-tech treatment wetlands Other operations and

mainte-nance activities in treatment wetlands, such as repair of pumps,

levees, and water control structures; vegetation management;

pest control; and removal of accumulated mineral solids,

typi-cally must be attended to at much less frequent intervals

This chapter provides guidance on the appropriate level

of operational monitoring necessary for the control of

wet-land treatment systems Typically, these are natural systems

with low hydraulic and constituent loadings Ecological

integrity therefore requires minimal attention, unless a

par-ticular preconceived plant mix has been mandated

Hydrau-lic balance and hydropattern control may require infrequent

attention Compliance monitoring is driven by the permit

conditions, but often a modest amount of extra monitoring is

useful in optimizing operations Emphasis should be placed

on the avoidance of unnecessary operational and

mainte-nance activities, including the management of nuisance

spe-cies when this achieves no measurable compliance goals

22.1 START-UP

It should be fairly obvious that a newly built wetland is not

in a condition to perform the functions that will ultimately

prevail in the system Yet, there have been many

misunder-standings resulting from start-up conditions that do not meet

long-term expectations Start-up transients are not

necessar-ily brief, because it can take considerable time for the wetland

biology to develop Several processes occur during start-up,

importantly including:

Plants increase in density and areal coverage

Senesced plant parts form a litter layer

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Newly placed soils either release constituents if initially loaded or absorb constituents until they are fully loaded

Because plants form part of the “biomachine” that accretes phosphorus and other chemicals, and because plant detritus forms the sites for periphyton and bacterial assemblages, it is expected that removal will be enhanced by grow-in One pat-tern of start-up is typified by the rapid vegetation of a bare soil wetland after an initial planting If that planting is sparse, the start-up period will be characterized by the time for plant fill-

in plus the time for litter development That can be relatively brief, especially in warm climates At the Orlando Easterly Wetland in Florida, the start-up duration was approximately

24 months, during which considerable change in the tem took place Figure 10.21 shows two of the principal vari-ables: vegetation density and phosphorus rate constant Note the close parallel between the amount of vegetation and the rate constant during the first two years After that period, the vegetation density levels off, and the rate constant drops

ecosys-to its long-term average value of 13.5 m/yr The phosphorus requirement for building the new standing crop of biomass leads to a peak uptake rate constant, that is, roughly double the long-term average value

An interesting feature of the unvegetated wetlands is the absence of a carbon source to fuel the conventional hetero-trophic denitrification That process would conventionally require a supply of carbon of approximately equal mass to that of nitrogen lost (Ingersoll and Baker, 1998) A newly constructed wetland does not have the carbon source cre-ated by litter decomposition, unless the system is mulched initially with carbonaceous material

Algae are an opportunistic first-colonizer in new lands, because the canopy is not yet extensive, and sunlight is available, perhaps accompanied by copious nutrient supplies (see Figure 3.1) Although this initial vegetative cover devel-ops rapidly, it utilizes only a small fraction of the nutrients in the system and temporarily creates a pond environment

wet-ANTECEDENT CONDITIONS

The initial soils and sediments may contain extra nutrients

or contaminants, or they may possess an initially high tion capacity The antecedent load of pollutants should be assessed to determine if there is treatment liability that must

sorp-be accommodated sorp-before the incoming water can sorp-be treated The same processes that will eventually provide treatment

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FIGURE 22.1 Start-up of a FWS wetland near Orlando, Florida The antecedent soils were laden with labile phosphorus, which was

released to overlying water during start-up This initial load was absorbed and flushed over the first 200 days of operation.

0 200 400 600 800 1,000 1,200 1,400

Time (days)

In Out Linear (In) Linear (Out)

for incoming water can also remove and immobilize such

antecedent pollutants However, if the water flow is

immedi-ately commenced, then the wetland can temporarily serve as

a “source” of contaminants If unanticipated, such releases

can create regulatory problems For instance, a wetland

treat-ment system in Orlando, Florida, released phosphorus and

nitrogen from antecedent land uses and conditions, during

a start-up in the flow through mode (Figure 22.1) Despite

the improvements to performance that ultimately resulted

in phosphorus removal, the regulatory expectation was that

removal should occur “from day one.” Very large fines were

levied just as the wetland came out of start-up, causing

clo-sure of the facility

The keys to avoiding such start-up problems are an

understanding of potential treatment liability and a plan to

avoid discharges until the wetland has “settled down” to a

post-start-up condition However, it is difficult to forecast the

direction or magnitude of potential start-up releases

Accord-ingly, a regulatory framework has been developed in South

Florida for start-up of FWS wetland systems Three periods

in the (early) life of a treatment wetland are identified:

Start-up phase The permittee shall manage water

depths in the treatment cells to facilitate the

recruitment of marsh vegetation in accordance

with an Operations Plan, which may include

recir-culating waters within the system The start-up

test for an individual flow-way begins when the

previously mentioned samples demonstrate, over

a four-week period, a net reduction in the

tar-get pollutant (phosphorus for the STAs) occurs

Discharge/flow through operations, from an

indi-vidual flow-way that has passed the start-up test,

may commence once documentation and all

sup-porting data and analyses are submitted The intent

of this phase is to prevent flow through operation

until it can be demonstrated that the project is

pro-ducing net benefit

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Stabilization phase Once flow through discharges

from a flow-way begin, the permittee shall initiate water quality monitoring for that flow-way consis-tent with the monitoring program Performance during this period is not expected to be optimal because the wetland is not yet fully developed The stabilization test is met when the long-term criteria for the facility are met, i.e., the concentra-tion limits are being met If, after the two years

of full flow through operation, the facility has not met this stabilization test, the permittee is to sub-mit a report evaluating reasons for not meeting the stabilization test

Routine operations phase During the routine

operations phase, the wetland is deemed in pliance if the permit effluent limitation is being achieved

com-In general, the South Florida projects have achieved

start-up and stabilization in periods ranging from a few weeks to more than two years, which is typical of warm climates In cold climates, a grow-in period of approximately two grow-ing seasons may be anticipated, depending of the planting density and rate of vegetation propagation (see Figure 3.12)

It is generally a bad idea to initiate the development of a wetland with large flows of high-strength wastewater Newly planted vegetation is susceptible to destruction if exposed to high-oxygen stress or high concentrations of some chemicals such as sulfur compounds Therefore, a “ramp up” from fairly good quality water to the strong influent is often used For instance, the Saginaw, Michigan landfill leachate treatment wetland anticipated source water of several hundred mg/L

of ammonia nitrogen The wetland was developed, including litter and plants, using rainwater and a potable water supple-ments The system was placed on large recycle and the introduction of leachate commenced The raw leachate was thus diluted with treated water This start-up strat-egy was successful As a counterexample, the potato water

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treatment wetland at Connell, Washington, was started-up

with an abrupt transition from clean irrigation water to

full-strength process water (anaerobic) Large populations of

purple sulfur bacteria developed, and the wetland developed

sulfides in excess of 40 mg/L (Burgoon et al., 1999) Very

strong odors were generated and inlet vegetation suffered

These were fortunately only start-up problems, which have

not continued beyond the commission phase

VEGETATION START-UP

Design specifications for vegetation establishment in

con-structed wetlands should clearly delegate responsibility for

plant maintenance from the time of planting until system

start-up This task may be the responsibility of the

contrac-tor or planting subcontraccontrac-tor, the engineer, the owner, or

some combination of these parties and should be carefully

described in the planting specifications (see Table 22.1)

The key requirements for healthy wetland plant propagules

to succeed are water, soil, nutrients, and light The first two

components must be controlled to some extent by the

engi-neer and the contractor, and nature generally provides the

other requirements for plant growth The establishment of

vegetation is usually a task of a planting contractor and not

the facility operator

Water level control is of primary importance during the

plant establishment phase Therefore, clear lines of

responsi-bility need to be spelled out: who is required to supply start-up

irrigation water, and who is responsible for watering the new

plants? Failure to supply water in appropriate amounts can

result in planting failure (see Figure 18.35) Generally, there

are three phases to water level adjustment during this phase:

Phase 1: Decreasing the water level to expose the

rooting media (mud flat) for the initial planting or

for replanting

Phase 2: A gradual increase of the water depth as

emergent plants become established The emergent

B Inspection

Sixty days after the completion of the planting, the Owner or Engineer will make an inspection to determine if satisfactory stands of cattail and bulrush, have been produced A satisfactory stand is defined as one in which: (1) spacing between planted seedlings averages 3 feet or less; (2) the seedling survival averages at least 80 percent; and (3) no areas of greater than 100 contiguous square feet with a seedling survival rate of less than

50 percent If satisfactory stands have not been established, another inspection will be made after the planting contractor has corrected any deficiencies and provided the Owner or Engineer with written notice that the stands are ready for inspection.

shoots should be above the water surface at all times to give the plants access to sunlight and oxy-gen Water level adjustments should be less than

2 cm/d during this time Typically, the higher the oxygen stress on the plants (high organic and/or nutrient loads), the smaller the incremental adjust-ment should be

Phase 3: Setting the water level at the design depth

after plants are established

Basic procedures for establishment of wetland plants for SSF wetlands have already been provided in Chapter 21

If there is significant failure during the initial planting, these steps may need to be repeated Initial plant inspec-tion should examine the viability of the planted propagules, whether seeds, seedlings, or field-harvested mature plants

If plants were planted in rows on specified centers, the viving plants in a subset of these rows can be counted to determine the survival rate When counting surviving plants, each of the original seedlings or plant clumps should only be counted once, and any daughter plants that may have arisen from rhizomes should not be counted If seeds were used, then random plots can be used to estimate germination suc-cess Planting success can be compared to planting specifica-tions based on these plant counts In some cases, a contractor may have to fill in some areas with new plants to comply with the requirements of the planting specifications

sur-Inexperienced contractors and inexperienced engineers writing specifications and providing construction supervi-sion have often killed their wetland plants through insuf-ficient soil moisture, excessive water depths, inadequate soil preparation, damaged plant material, inadequate plant spacing, inappropriate planting methods, and bad timing Success is highly probable as long as a contractor under-stands the growth requirements of the desired wetland plants and uses information available to most horticulturists.Once emergent plants are well established, replanting is usually not necessary unless the system is overwhelmed by

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rodents or waterfowl, which can destroy the emergent plant

community if left unchecked In cold climates, grazing

pres-sure in the winter and early spring is a significant operational

concern during the plant establishment phase, and wetland

designers should anticipate replanting and/or exclusion

activ-ities (Wallace et al., 2001).

22.2 MONITORING

All wetland treatment systems should be monitored for at

least inflow and outflow water quality, water levels, and

indi-cators of biological condition Following proper design and

construction, monitoring is the next most important factor

in successful operation of treatment wetlands Monitoring

information is essential to the ability to anticipate the need

for operational changes and should be collected accurately

and consistently, and frequently reviewed by a knowledgeable

operator Changing the character or function of the wetland

ecosystem takes considerable time and operator investment

The appearance of the wetland is not necessarily

representa-tive of what it is doing for water quality, because many

wet-land functions do not tie strongly to the abovewater canopy

Consequently, it is water quality and quantity data that are

the primary indicators of wetland operational status There

are few controls for Type A wetlands (the ones that are

pas-sively designed) but more for Type B systems (the ones with

added controllable features) Water level control is possible

for all types Added possibilities may include flow control

(fill and drain), recycle, bypass, added streams with

supple-mental constituents, and aeration, as discussed in Chapter 24

Alteration of the ecosystem is also possible for all types, but

usually is a drastic step, to be avoided if possible Dredging

of sediments or soils, bed media replacement, new plantings,

or removal of plants by mechanical or chemical means can

all be accomplished but at considerable expenditure of time,

money, and loss of treatment potential In all cases,

appro-priate diagnostic information is a prerequisite to initiating

maintenance or control steps

Water Quality

Continuous flow wetlands will require compliance

monitor-ing of a specified list of chemical constituents and inflow

rate, because most operate under some kind of discharge

per-mit The sampling locations can vary from project to project,

and usually include some or all of the wetland inflow and

outflow points However, some projects may require

inter-nal monitoring points in the treatment wetland, and in some

cases, monitoring of the receiving water body The locations,

types, and frequency of compliance sampling are spelled out

in the individual permits, as are the averaging periods and

limits for the various regulated constituents Laboratory

pro-tocols for chemical analysis are also officially prescribed and

include a variety of established methods, as defined in

Stan-dard Methods (APHA, 2005) or by U.S EPA, for example

However, the methods for acquiring wetland water

samples are neither well defined nor standard There is no

apparent difficulty with obtaining samples at inlet and outlet structures, yet there can be serious anomalies related to sam-pling methodology For instance, either autosamplers or grab sampling may be employed The autosampler can be set to flow-proportion, and thus (hopefully) produce flow-weighted concentrations for use in mass balances It is instructive to see what happens when both procedures are used over a long period of record (Figure 22.2) Not only do the sampling methods yield much different results, there is an apparent bias of the autosamplers to give higher results Speculatively, the reason is that autosampler intake tubing may “suck mud”

if the intake strays from its optimal position

Acquiring representative internal wetland water samples

in a FWS environment is very difficult Floating materials,

such as Lemna, can often be excluded by careful sampling

Water depths may be quite shallow, and there can be large amounts of readily suspended materials in the vicinity (see

will not be representative of the particulates that move with water in the undisturbed environment The local velocities created by the sampling activity resuspend an abnormally high amount of total suspended solids (TSS) That solid material can bias the lab analyses for total amounts of con-taminants such as total phosphorus or total Kjeldahl nitro-gen (TKN) The intense vertical gradients in water quality near the wetland bottom can also create anomalous results

In very shallow water, the sample can contain some or all of pore water, which usually has much different (higher) con-centrations from surface waters (see Figure 10.5, for exam-ple) All of these factors create a need to avoid disturbance

in the act of sampling and to avoid very shallow depths that may involve pore water In the various Everglades sampling

0 100 200 300 400 500

Grab Samples

FIGURE 22.2 Autosampler versus grab samples for total

phos-phorus for one of the inlet structures of STA2 of the Everglades Protection project Data span eight years of operation Note that the autosampler results are generally higher (mean = 223 µgP/L) than grab sample results (mean = 149 µgP/L).

activities, depths less than 20 cm are not bottle-dip sampled

for these reasons However, carefully aspirated samples may

be acquired at smaller depths

Similar problems exist in sampling SSF wetlands,

espe-cially because dedicated sample ports must be used to extract

in situ samples, which may or may not representative of the

flow path Figure 7.1 illustrates a typical sample port

assem-bly for SSF wetland systems

Peristaltic pumps are often a preferred sampling device

for SSF systems, as shown in Figure 22.3 Considerable care

must be taken in the placement of the sampler tubing to

obtain a representative sample; poor tubing placement will

disturb biofilms and/or suck sludge from the bottom of

con-trol structures or sample ports Dedicated sample tubing that

is permanently installed is often used at facilities where tine sampling is conducted

rou-S AMPLING AND A NALYSES IN S UPPORT OF M ASS B ALANCES

Compliance monitoring may not be enough to gain an standing of wetland function, because it is focused upon measuring the variables that may affect the receiving waters, and is commonly restricted to outflows A fuller understand-ing of wetland performance rests upon constituent mass bal-ances, which in turn depend upon the water mass balance for the wetland

Flow measurement can be accomplished using any one

of several techniques For pumped inflows, either hours plus the pump-discharge curve may be used or a tur-bine meter can be installed For small flows, a calibrated weir, possibly with continuously recording head, is a com-mon choice (Figure 22.4) Most large inlet and outlet devices that operate with a measurable head loss can be calibrated to determine flow from headwater and tailwater elevations For large inflows without a calibratable structure, devices such

pump-as Doppler flow meters or ultrpump-asonic velocity meters have been employed It is noteworthy that virtually every type of

FIGURE 22.3 Effluent grab sampling using a portable

peristal-tic pump at a HSSF wetland in Scandia, Minnesota Considerable

care must be taken with the sample tubing to obtain representative

samples.

FIGURE 22.4 A V-notch weir, with recorded head, is a user-friendly method of measuring small inflows and outflows Photo from the Duck

Spring wetland system near Alcoa, Tennessee.

flow measurement relies upon clean conditions and periodic

recalibration Any of the measurements are prone to being

somewhat in error with field accuracies on the order of ±15%

For any of the devices that include sensors and data logging,

frequent manual cross-checks are recommended

Low flows in small systems will be zero (no flow) This

has several design implications Most notably, there will be

an insufficient flow to create scouring velocities in household

plumbing and gravity sewer pipes During intermittent flow,

solids will “hop” down the pipe, moving only when the flow

and depth exceed certain limits Solid particles accumulate

into dams, partially obstructing the invert of the pipe When

flow pulses occur, water accumulates behind the dam and

pushes the solids along the pipe (Littlewood and Butler, 2003)

This “sliding dam” mechanism of movement is the dominant

form of solids transport in small treatment systems

Conventional weirs or upflow-splitting devices are

gener-ally not used as primary flow control elements for small

wet-land systems unless the solid material has been removed in a

primary treatment process Due to the low flow, toilet paper

and other solids will inevitably get caught on one of the

over-flow weirs because there is insufficient velocity to remove

the material All flow will go to the other outlet, until it, too,

clogs with solid material Flow will switch back and forth

between the outlets in an unpredictable manner and create

problems for the operations staff

Accurate flow measurement for a small collection network

is difficult to obtain for many reasons Use of an overflow weir

as the primary control element (combined with an ultrasonic

head or other sensing device) can present an ongoing problem

for maintenance personnel due to solids accumulation

Mag-netic flow meters have a minimum flow velocity threshold of

about 6 cm/s for accurate flow determination For small-scale

applications, this generally requires “necking down” the

pip-ing to 25 or 50 mm to get the necessary low-flow accuracy

This is generally not a viable option unless solids have been

removed from the wastewater in a settling tank

Small flumes, such as H or HS types (as shown in Figure 22.5), offer acceptable accuracy at low-flow rates (Grant and Dawson, 1997) but may not achieve scour veloc-ity, leaving the flumes susceptible to solids accumulation If a flume is used after a solids removal device (such as a settling tank), the velocity at low flow will allow the growth of a bio-film on the flume surface For this reason, bubbler units are not recommended as a sensing device because biofilm growth will rapidly plug the air tube Noncontact devices, such as ultrasonic heads, are recommended for use with flumes as shown in Figure 22.5 Tipping buckets offer very accurate low-flow measurements, but the corrosive atmosphere pres-ent in sewage systems precludes the use of mechanical coun-ters Also, tipping buckets can become “pinned” under high flows, so it is important to determine the suitability of the tipping bucket assembly at peak flow

Due to the reasons cited in the preceding text, elapsed time meters are often used as a means of flow measurement

on systems that receive a pumped influent, even though this may result in lower accuracy flow determinations

In pumped systems, the flow will be stagnant (zero ity) in pressure mains most of the time To prevent freezing problems in cold climates, pressure mains must be installed below the frost line If this is not possible, the pipe should be sloped to drain and provisions for a drainage outlet should be made at the pump end as well as at the discharge point

veloc-In the long term, water storage changes in the wetland are almost always negligible in comparison to other flows How-ever, water depth is a very useful parameter for other purposes than the water budget, and hence water stage in each cell is

a desirable data set Staff gages should be installed in each cell of the wetland and surveyed for vertical control at the time of start-up (Figure 22.6) When combined with a top-ographic survey of the wetland, stage measurements provide

a quantitative tool for assessing the average, maximum, and minimum water depths in the wetland and the frequency with which these depths occur These data are essential for

FIGURE 22.5 This small HS flume is used in conjunction with an ultrasonic head to record influent flows for a HSSF wetland in Delft,

Minnesota.

interpreting tracer measurements of hydraulic residence time

and for assessing any detrimental hydroperiod effects on biota

A variety of water level measuring instruments (e.g., analog

chart recorders, ultrasonic, pressure-sensor, and

capacitance-sensor units) are available for continuously recording water

levels at various key points within the wetland

Constituent Concentrations

Table 22.2 summarizes an example of monitoring program

in support of operation of a municipal wastewater-polishing wetland treatment system This list includes measurements of the water quality of all major inflows and outflows associated with the treatment wetland Inflows include the sources of pretreated wastewater entering the wetland as well as natural inflow streams that may have a significant effect on the water quality or the hydrologic budget of the natural wetland treat-ment system As described in Table 22.2, the parameter list

to be tested at all major inflows and outflows at least monthly includes all regulated pollutants and integrative measures such

as BOD5, TSS, pH, dissolved oxygen, water temperature, ductivity, NO2-N + NO3-N, ammonia, nitrogen, TKN, total phosphorus, chloride, and sulfate The frequency of opera-tional monitoring for system control is dictated by the size and capacity of the system, the sophistication of the owner’s staff and sampling equipment, and site-specific factors related

con-to influent quality variability and climatic faccon-tors

Inflow and outflow stations may also be monitored less frequently for selected heavy metals or organics that might

be present in the wastewater and for whole effluent acute and chronic toxicity If water quality characteristics are highly variable for any of the inflow or outflow locations, or if there are weekly or monthly permit limits, sampling should be more frequent than monthly or quarterly

These water quality data as well as water-budget eters should be organized and recorded in computerized spreadsheets for visual analysis of variability and trends The constituent loadings may be compared to design criteria to ascertain if the system is being operated according to design The constituent mass balances are easily constructed from system data on such a spreadsheet and may be used to deter-mine if design conditions are being achieved, and whether the system is performing as expected

param-Table 22.2 reflects the example of municipal wastewater treatment, but constructed wetland systems are used for a wide

FIGURE 22.6 A staff gage provides valuable information on

the water depth and volume in the wetland Photo from the Lake

Nebagamon, Wisconsin, treatment wetland system, in the exit deep

zone The screen box covers the intake for the effluent siphon.

TABLE 22.2 Example of Monitoring Requirements for Operation of a Large FWS Wetland Treatment System

Rainfall, pan evaporation, and air temperature Adjacent to wetland Daily

variety of other purposes, including animal waste treatment,

groundwater remediation, and urban and agricultural

storm-water Each of these applications generates a similar

require-ment for a strategy to promote understanding All are based

upon knowledge of the water budget, but the suite of chemicals

of interest may change, depending upon the application

B IOLOGICAL M ONITORING

Biological monitoring within a wetland treatment system

provides information, concerning the structural integrity

(health) of the vegetation and fauna Protection of this

bio-logical integrity is important from an environmental

habi-tat perspective and because of the biota’s control of wetland

operational performance It has been suggested that every

component of the wetland ecosystem should be monitored,

including sediments, water, vegetation, benthic invertebrates,

fish, amphibians, reptiles, birds, and mammals (Ecological

Services Group, 1996; Wren et al., 1997) This concept

origi-nates from the fact that some of these ecological components

have been routinely found to be above pristine background

for some contaminants, as expected for a treatment system

However, there have been no ecological disasters

associ-ated with the thousands of treatment wetlands over the

thirty-plus years of the technology, and thus the question involves

(possibly) impaired wetland ecology, not acute problems for

wildlife Therefore, biological studies fall in two categories:

monitoring needed to foster the design water quality

func-tions, and research needed to more fully define impacts of

less-than-perfect water on various organisms that may inhabit

the treatment wetland There is usually an important

distinc-tion between constructed wetlands and natural wetlands as

recipients of wastewater: the former are usually not subject to

intensive biological monitoring but the latter may be in some

states (e.g., Florida)

A compromise is typically put in place with some facet

of biological monitoring or research being conducted in

addi-tion to the requisite compliance and mass balance

monitor-ing Typical categories are bird, mammal, and fish studies

Broad-scale potential for ecological problems is commonly

assessed by examining or forecasting the concentrations of

specific chemicals or toxins For instance, great care is used

in the arid regions of the United States because of the known

acute effects of selenium that has impacted wildlife wetlands

(e.g., Kesterson, California)

Wetlands designed for ancillary benefits can have

non-treatment goals and requirements Trends in use of the

treat-ment wetlands by plant and wildlife species may indicate

needed changes in system management to support those

non-treatment goals Surveys for rare or threatened species may

also be conducted when appropriate

M ONITORING V EGETATION

Cover estimates and observations concerning plant health

should be a routine part of operational monitoring in a

con-structed wetland treatment system Because plants grow slowly

and are important for maintaining the performance of wetland

treatment systems used for water quality treatment, problems must be anticipated and prevented before they are serious or have progressed too far Reestablishing a healthy plant com-munity in a constructed wetland is a slow process when the plants have been harmed because of operator neglect

Plant growth is monitored by estimating percent cover and average plant height These nondestructive techniques are used to ascertain the status of plant development before and during wetland treatment system operation Plant cover

is an estimate of the percentage of the total ground area ered by stems and leaves This parameter can be estimated by walking through or next to a plant stand and visually deter-mining a cover category for the plants Typically, seven cover categories are sufficient for this visual estimation method: (1) less than 1%, (2) 1–5%, (3) 6–10%, (4) 11–25%, (5) 26–50%, (6) 51–75%, and (7) 76–100%

cov-Cover estimates can be made on a finer scale by using a frame to delineate specific areas Cover estimates should be made at enough locations in the constructed wetland to pro-vide reasonable statistical averages for comparison between cells and between dates

Ground-level photography may be used to provide val information about vegetation Photos at identified specific locations give a visual time series of potential succession in the system For instance, a rodent eatout may be documented (see Figure 19.6 for example)

archi-Aerial photography is an excellent supporting tool that can provide estimates of the type and percent cover of veg-etative communities in treatment wetlands Figure 22.7

illustrates the vegetative patterns for one of the constructed treatment wetlands at the Des Plaines River site near Wad-sworth, Illinois The various areas of different vegetation are clearly visible, but ground-truthing is required to establish which plants are growing in the zones

22.3 WATER LEVEL AND FLOW MANAGEMENT

There are not many controls on the operation of a treatment

wetland, especially Type A systems that mimic passive,

natu-ral wetlands Water depth and water flow rate are the two principal controllable features

storm-Flow Balancing

In the case of multiple parallel paths, the operator has the choice of how much of the incoming flow to apportion to each flow path Often, the adjustment of the proportion is

not physically easy but rather involves opening and closing

of multiple valves or structures The performance of each

flow path depends upon the amount of water it receives,

with the optimum occurring when the hydraulic loadings to

the various flow paths are equal, so that no one flow path is

underworked or overworked It is informative to examine the

sensitivity of the over all system performance to the flow split

or potential imbalanced loading To do so, consider a

hypo-thetical example of a system with two flow paths of equal

area in parallel, both of which perform according to the

P-k-C* model.

For the conditions indicated in Figure 22.8, if the equal

fractions of the flow are sent to each of the two paths, the

inlet concentration is reduced by about 80%, from 100 to 19.3

mg/L As more water is diverted to path 2, its performance

starts to decline, until the extreme when all the water goes to

path 2, doubling its hydraulic loading compared to design

At that extreme, the outlet concentration is about double the design value at 39.3 mg/L (about 60% reduction) Of course, path 1 becomes underloaded, and its performance increases The combined outflow from the system shows that imbalance

as well But interestingly, a modest amount of imbalance is not particularly harmful For the hypothetical example, a 60–40 split is forecasted to raise the outlet concentration to 20.1 mg/L, about 5% higher than design

The extreme of shutting down one flow path may be necessitated by a need for unusual maintenance activities For instance, flow paths in STA1W of the Everglades pro-tection project have suffered hurricane damage, requiring shut down of a parallel path for vegetation reestablishment

To the extent that such events may be anticipated, allowance can be made in design Water may be diverted elsewhere for

FIGURE 22.7 (A color version of this figure follows page 550) Wetland EW3 at the Des Plaines River site near Wadsworth, Illinois The

fringe zone is vegetated by cattails and bulrushes, whereas the interior has submerged aquatic vegetation and floating-leaved plants Open water shows as the dark areas on this false color infra-red photo.

0 5 10 15 20 25 30 35 40 45

Fraction to Path 1

Path 1 Path 2 Combined Outlet

FIGURE 22.8 A hypothetical example of flow imbalance between two parallel paths in a treatment wetland, for the conditions shown in the

corresponding table Path 1 is overloaded at the expense of Path 2.

C

k 0 0 m/yr Wetland Area  200,000 m 2 P  3 TIS

treatment if such alternatives exist, or extra area and paths

may be included in anticipation of path shutdowns

Systems with Storage

Flow management is an important part of those systems that

involve seasonal or event storage because the stored water

may be discharged according to any of several schedules (see

bleed down the stored water in a timely fashion to make room

for the next event or for the next storage season

In small-scale systems with winter storage, cost

consider-ations will usually lead to storage that does not much exceed

the volume of inflow over the cold season In spring, the

oper-ator is faced with selecting the program of withdrawals from

storage to be sent to the treatment wetland Such

withdraw-als may be limited by pump capacities, which would withdraw-also be

constrained by cost considerations, or if gravity discharge is

possible, the withdrawal rates may be much less constrained

For instance, the Houghton Lake, Michigan, system has a

wetland discharge season extending from May 1 to October

15 (170 days), but the transfer pump is capable of transferring

the water over about 100 days Therefore, the operator has

discretion over which days there is inflow to the wetland, just

so long as the storage is at its minimum level at the end of

the wetland-operating season Strategies ranging from paced

uniform discharge to fast emptying followed by minimum

pond level maintenance have been used (see Chapter 17)

The discharge of municipal wastewater treatment lagoons

is in some circumstances constrained by the ability of

receiv-ing waters to maintain acceptable water quality Such cases

occur when the volume of the discharge is significant

com-pared to a receiving stream flow In that situation, the stream

may become dominated by the effluent, especially at times

of low recipient flow It may then be desirable to vary the

dis-charge of treated effluent to avoid unacceptable water

qual-ity in the combined flow downstream of the discharge The

ability to vary discharges is in turn conditioned on the ability

to store water during periods of low acceptable discharge

The flows in the recipient dictate the magnitude of allowable

discharge, and hence this strategy is termed hydrograph

con-trolled release (HCR; Kadlec and Pries, 2004).

Stormwater systems may be preceded by flow-equalization

reservoirs to prevent the low degree of treatment that

accom-panies large event flows Examples of this are currently under

design and construction in South Florida, where vast (5–10,000

ha) reservoirs will collect stormwater runoff before passing it

through the comparably vast stormwater treatment areas

(con-structed wetlands), many of which are already built These are

likely to be HCR systems, serving the water-quantity needs of

the downstream-receiving systems, which are the Florida

Ever-glades Operating protocols are currently under development

Bypassing

It is often not feasible, for hydraulic reasons, to treat large

event flows in stormwater wetlands If the hydraulics allow,

treating all the water would generally result in the greater

load removal, but will pass higher concentrations to receiving waters for the large events Hydraulic integrity often requires that extreme flows be avoided or else damage to the wetland may result Therefore, the design of such systems typically allows for bypass However, this is usually accomplished by the design of control structures rather than by any overt oper-ator action Exceptions are the STAs of South Florida, most

of which have operator-controlled bypass structures

to manage water depth for the health and vigor of the plant community, which is typically sensitive to water depth

As an illustration, consider the two hypothetical FWS wetlands for which different level scenarios are calculated in

rate of 5 cm/d, and both are presumed to have the same (high) vegetation resistance to flow First consider a modest size 1-ha

wetland with L:W = 4 If operation is at an exit weir setting

of 0.5 m, then there is essentially a zero gradient in the water surface (level pool) If the exit weir is then lowered to 0.15 m, the water level goes down everywhere, with only a few cen-timeters of head loss forecast (Figure 22.9; 18.9 cm down to 15.0 cm) The average depth is lowered to 17.1 cm Next con-

sider the long, narrow wetland with L:W = 25 When the outlet

weir elevation is 0.5 m, the inlet elevation is 3.9 cm higher and the mean depth is 0.52 m For this system, when the outlet elevation is lowered to 0.15 m, there is not a drawdown of the entire wetland (Figure 22.10) The inlet depth drops only to 0.48 m, and the mean depth is reduced only to 36.5 cm For the specified flow, the vegetation is holding back the water and lowering the outlet level cannot change that

Design should be such that water depth control is at the let structure or else the operator has no option for depth control except to lower the flow rate For HSSF wetlands, the hydraulic profile will be governed by both Darcy’s Law and the outlet weir setting Again, outlet control is the preferred method of regulating the water level (see Figure 21.11) This will require a conservative bed design with a high hydraulic conductivity

out-W ATER DEPTH, PLANTS, AND NUTRIENT LOADINGS

Water depth is an important variable in the survival of

wet-land vegetation In mitigation and habitat wetwet-lands, it is

coupled with a hydropattern to provide the desired growth

conditions for any particular species The depth that may be

tolerated for continuous inundation is less than that which

can be withstood for brief periods of time (Table 22.3) Some

of the more robust plants, such as Typha, can withstand

pro-long periods of drought, lasting many months, and can porarily be completely submerged At continuous flooding water depths of 1 m or more, emergent plants in treatment wetlands can no longer survive

tem-0.00 0.10 0.20 0.30 0.40 0.50 0.60

Distance (m)

High Weir Low Weir Bottom Elevation

FIGURE 22.9 Effect of lowering the outlet weir setting on the water depth in a small, moderate aspect ratio wetland.

0.00 0.10 0.20 0.30 0.40 0.50 0.60 0.70

Distance (m)

High Weir Low Weir Bottom Elevation

FIGURE 22.10 Effect of lowering the outlet weir setting on the water depth in a large, high-aspect ratio wetland.

2 m Low Weir Inlet Depth 0.189 m d

c B//d x  0 00001 0 m/m Low Weir Outlet Depth  0 15 0 0 m

W Q

a 1b) /d $x  250 m High Weir Outlet Depth  0 50 0 m

m Low Weir Inlet Depth 0.

For SSF wetlands, the situation is different because there

is an unsaturated region within the wetland bed For HSSF

wetlands, this unsaturated region is a result of the water level

control regime and the desire to keep flows within the

wet-land bed If insulating materials such as mulch are used, this

will increase the thickness of the unsaturated rooting zone

In VF wetlands, unsaturated conditions will occur

through-out the wetland bed on a periodic basis due to

pulse-feed-ing and the load-and-rest operatpulse-feed-ing regimes typically used in

these systems

The net result of such unsaturated conditions is to shift

the competitive advantage away from plants that require

submerged root zones SSF wetlands are more likely to be

dominated by plants that enjoy wet but not submerged root

zones Plants in the genera of Phragmites, Schoenoplectus,

and Phalaris are commonly used in such systems (Vymazal

et al., 1998; Wallace and Knight, 2006; Stein et al., 2006a).

There are many sources of recommendations for water

depths and associated hydroperiods for the common wetland

plants (for example, Hotchkiss, 1972; Niering, 1985; Dressler

et al., 1987; Boon and Groe, 1990; Thurnhorst, 1993;

Egg-ers and Reed, 1997; Pineywoods RC&D, 2001; Shaw and

Schmidt, 2003) Hydrologic tolerances of wetland plants are

further discussed in Chapter 3 The treatment wetland

opera-tor should understand the allowable depth and hydroperiod

ranges for the plants at a particular site

As noted in Chapter 3, floating mats are prone to form

under excess depth conditions in FWS wetlands For instance,

if a good growth of cattails (Typha spp.) develops under

shal-low water conditions, it will likely have a shalshal-low, interlocked

rhizosphere This root mat has an inherent buoyancy created

by various gas-generating processes (such as

methanogen-esis) If water depths are then raised, the entire mat may tear

loose from the rooting media and form a floating mat If the

mat covers the entire wetland cell, then it can function

effec-tively in treatment But if floating islands develop, they are

subject to wind-driven motion and severe disruption of the wetland This happened in Cell 2 of the Everglades Nutri-ent Removal Project (ENRP), with disastrous impairment of treatment Removal of the plants was necessary

S EASONAL D EPTH A DJUSTMENTS

Although shallow water fosters plant growth and is effective

in promoting treatment in FWS wetlands, it can be atic in winter due to ice formation In densely vegetated wet-lands, the plant stems hold the ice layer in place If water levels subsequently drop, the ice layer generally drops with

problem-it, although an air gap can form under the ice at some ties within the wetland if the ice sheet is held up by plant stems However, this anchoring of the ice layer can create severe damage if water levels are subsequently raised The ice layer floats and tears out embedded vegetation as it moves upward If the ice layer becomes thick enough, under-ice flow is impeded, which can lead to over-ice flow (Kadlec, 1987) Water flows upward through holes or cracks, emerges, and flows downgradient over the ice and under the snow For example, this mode of flow is experienced each winter at the Frank Lake, Alberta, treatment wetland

locali-It is possible to provide more space for flow under the ice

by raising water levels prior to ice formation However, care must be taken that such increased water depths do not entirely submerge the plants, because the oxygen transfer necessary for their survival is primarily obtained via the standing dead stalks and their airways (see Chapter 5) Indeed, an effective method

of destroying cattails is to cut them and overflood the stubble This procedure of water level increase was used effectively at the Listowel, Ontario, FWS wetlands (Herskowitz, 1986) and

is currently employed at the Brighton, Ontario, wetland.For HSSF wetlands, there is a treatment penalty with deeper beds (see, for example, Table 8.13) If shallower beds (0.3 m) are selected, there is the potential that some bed vol-ume will be lost due to ice formation in the winter For very cold situations, mulch insulation of the SSF wetland may be required, as discussed in Chapter 4

22.4 CONTROL OF NUISANCE ANIMALS

Experience has shown that a large variety of creatures may interfere, in some way, with the desired operational condi-tions treatment wetlands The list of animals that have caused some degree of concern includes deer, elk, cattle, wild hogs, gophers, monkeys, coyotes, feral cats, manatees, and turtles However, most problematic are some birds, rodents, some fish, and mosquitoes

B IRDS

Blackbirds are attracted to treatment wetlands, both as a breeding location and as a place of refuge Redwing black-

birds (Agelaius phoeniceus) love to nest in cattails and do

so in great numbers in treatment wetlands of temperate

North America Yellowheaded blackbirds (Xanthocephalus

TABLE 22.3

Water Depths for Common Treatment Wetland Plants

Best Depth (cm)

Flood Depth (cm)

Bulrush Scirpus californicus 30–60 120

Soft-stem bulrush Scirpus validus 15–30 60

Broadleaf cattail Typha latifolia 30 60

Narrowleaf cattail Typha angustifolia 30 60

Common reed Phragmites australis 30–60 90

Yellow flag Iris pseudacorus 3–8 45

Pickerel weed Pontederia cordata 8–15 30

Woolgrass Scirpus cyperinus 3–15 30

Source: Adapted from Pineywoods RC&D (2001) Constructed

Wet-lands in East Texas (4 volumes) Pineywoods Resource Conservation

and Development Council, Nagodoches, Texas.

xanthocephalus) also nest and find nighttime refuge in

treatment wetlands (see Figure 19.5) Although their large

numbers may contribute to pathogens in the wetland, no

management techniques are typically used for blackbirds

Waterfowl may be more problematic because of their large

size and numbers during migration Canada geese (Branta

canadensis), ducks, and other waterfowl are known to cause

significant depredation through grazing and uprooting of plant

materials in FWS wetlands There are a number of threatened

and endangered birds that make use of wetlands in general

and may be attracted to treatment wetlands According to

fed-eral regulations in the United States, their presence requires

that the area in question be managed on their behalf

Wetlands in an urban setting are very attractive to geese,

which congregate in large numbers at such isolated pockets

of habitat Even if their grazing does not seriously harm the

wetland vegetation, they can deposit large quantities of fecal

matter on walking paths and grassy margins of the wetlands

(Figure 22.11) It is herbivory that can cause considerable grief

for operators, especially during the vegetation establishment

phase (see Figure 18.37) Young plants and tubers are preferred

food for geese, swans, and some ducks If the wetland planting

happens to coincide with either the spring or autumn

migra-tion, the wetland can be essentially denuded in a day or two

Under the Endangered Species Act, the U.S Fish and

Wildlife Service (USFWS) is mandated to identify and list

plant and animal species that are threatened or endangered

by extinction, or that are considered likely to be threatened

in the future Threatened and endangered species cannot be

harmed, killed, or otherwise negatively affected by human

activities The USFWS reviews potential impacts to federally

listed species and can veto a discharge permit by issuing a

“jeopardy” opinion The potential for occurrence of

threat-ened or endangered species inhabiting a project site should be

considered during project planning

Experience indicates that the presence of federally

pro-tected species does not always necessitate substantial changes

to wetland treatment project siting, design, or operation

In a number of cases, the discharge of treated wastewater effluents to constructed wetlands has enhanced the popula-tion of threatened or endangered wetland-dependent spe-cies by creating additional habitat and food resources For

example, populations of bald eagles (Haliaeetus alus), wood storks (Mycteria americana), and snail kites (Rostrhamus sociabilis), all federally protected bird species,

leucoceph-have increased in and around the 485-ha Orlando Easterly constructed wetland treatment system in Florida Conversely, the presence of protected species may require modifications

of operations For instance, the STAs of South Florida are attractive to burrowing owls and stilts, which nest in or on the levees Therefore, flooding depths that endanger their nests are precluded The treatment wetland operator should be aware that operations may have to be modified if threatened

or endangered species begin using the wetland

presum-A similar phenomenon occurred at the Tarrant County constructed wetlands in Texas, where floodwaters from the

source river brought carp (Cyprinus carpio), shad (Dorosoma cepedianum), and bullheads (Ictalurus spp.) into the system

The activity of the fish caused severe bioturbation of the ments during feeding and spawning activities (APAI, 1994)

sedi-FIGURE 22.11 Waterfowl at the Commerce Township, Michigan, municipal wastewater-polishing wetland.