Do Created Wetlands Replace the

Wetlands that are Destroyed?

by Randall J. Hunt

U.S. Department of the Interior—U.S. Geological Survey Fact Sheet FS-246-96

Introduction

Wetlands, once perceived as worthless land, are

now recognized as a necessary component of a vital

landscape. However, due to draining and filling we

have lost many of our wetlands. The loss of wetlands

can have undesirable effects on the landscape, such

as erosion, flooding, habitat loss and deterioration

of water quality. While natural wetland systems are

being destroyed nationwide, the wetlands restored

or created to compensate for these losses are commonly not evaluated or contain large percentages of

non-wetland acreage. At the present time we do not

have established methodology that can uniformly

evaluate a wetland’s function, or that is useful for

providing guidelines that enhance wetland restoration/creation success.

Why should we care about

wetland loss?

Wetlands are often considered “kidneys of the

landscape” because of their role in filtering the

effects of surrounding land use, and have widely

recognized functions that include storm/flood water

retention, shoreline protection, water-quality improvement, and wildlife habitat. In fact, more than

one-third of our endangered species are associated

with wetlands even though wetlands comprise less

than five percent of the landscape! We have lost vast

areas of the pre-settlement wetland acreage—more

than 50 percent nationally and more than 95 percent

in some states. Increasing population, development,

farming and landowner’s rights have resulted in

increasing amounts of our wetland resource being

destroyed and have increased the pressure on the

wetlands that remain. As demonstrated by the floods

of 1993, the loss of wetland functions is becoming

increasingly recognized. The effects of wetland

loss, however, are poorly understood and wetland

research is still considered to be immature.

What is wetland mitigation?

In the broadest sense, mitigation is a process that

focuses on: 1) avoiding wetland loss, 2) minimizing

the effect of wetland loss, and 3) compensating for

unavoidable wetland loss. In general usage, however, mitigation has become synonymous with number 3 and now refers to replacing the function and

structure of a destroyed wetland by creating, restoring or enhancing a wetland somewhere else. This

mitigation of wetland loss has been mandated by

federal law, and there have been numerous large and

small wetland mitigation projects in every part of

the nation.

What are the challenges associated with wetland mitigation?

Wetland ecosystems span a large environmental

gradient—between occasionally wet uplands to shallow lakes. As might be expected over such a large

range, no “universal truths” apply to all wetlands, or

to wetland mitigation projects. It has become apparent that we are lacking basic wetland research techniques that can easily assess: 1) the functions occurring within the wetlands, 2) the role that destroyed

wetlands played in the greater watershed/ecosystem

health, and 3) the extent to which mitigation wetlands compensate for lost wetland systems. In the

midst of the pursuit to create and restore, wetland

scientists are becoming aware that the many unknowns make it virtually impossible to provide

definitive guidelines for successful wetland assessment and design.

What are some of the issues

surrounding wetland mitigation?

It is not widely accepted that mitigation projects

are successful. Although the current wetland permit

programs assume that wetland loss is being ameliorated, no long-term, interdisciplinary research shows

unequivocally that a created wetland has fully replaced the lost function resulting from a wetland’s

destruction. Secondly, there is a concern that created

wetlands do not provide in-kind compensation. That

is, many hard-to-create wetland types (such as fens,

bogs and sedge meadows) are being replaced with

common, easy-to-create wetland types (cattail

marsh), or the “quality” of the resulting mitigation

wetland is not equal to the wetland that was destroyed. A third concern is that placing mitigation

projects in areas distant from the destroyed wetland

will result in the wetland functions being replaced in

areas away from where they are needed and/or in

areas that are not wetland deficient. Finally, there is

great interest in mitigation “banks”—large wetland Wetlands have many uses, including that as an

outdoor classroom.

Wetland studies require intensive instrumentation

and labor to properly characterize the hydrology,

soils and vegetation.

Wetlands span a large range of “wetness”—from occasionally wet meadows and prairies to shallow lakes.
restoration or creation projects that can serve as

compensation credit for wetland losses elsewhere in

a given region. While many people agree that large,

intact wetland acreage is desirable, there is some

concern that mitigation banking projects will not

provide meaningful mitigation of the cumulative

effects of widely distributed, small-acreage wetland

loss.

Evaluation of Wetland Creation:

A case study

The U.S. Geological Survey and the Wisconsin

Department of Transportation have cooperatively

funded an eight-year study that has focused on both

evaluation and design of wetland creation projects.

Our work focused on 1) the appropriateness of

traditional techniques in wetland investigations, and

2) interdisciplinary evaluations of how the constructed wetland compares to the adjacent natural

analogue.

1) Do traditional techniques for

investigating hydrologic problems

work in wetlands?

Scientists commonly investigate hydrologic questions by determining how much water is moving

through a system, and what that water is carrying.

This understanding is then used to characterize how

the system functions, and how it interacts with the

surrounding landscape. Our work focused on evaluating how well these traditional methods work in

wetland investigations.

Measuring water flows: Traditionally, a relation called Darcy’s law has been successfully applied to ground-water problems in non-wetland areas. Darcy’s law relates the flow of ground-water to

the strength of the pressure driving the system (the

gradient) and how easily the water can flow through

the material (the material’s hydraulic conductivity).

Our work has demonstrated that in many cases this

simple relation underestimates the amount of groundwater flow in wetlands (fig. 2) because of the uncertainty in characterizing the hydraulic conductivity

of the sediments. The innovative techniques used in

our work included an isotope mass balance, a model

of heat and water flow, and a numerical water

balance model; these techniques are described in the

article referenced at the end of this fact sheet. This

level of understanding will likely be needed elsewhere, especially to answer those questions that

require knowledge of ground-water-wetland interaction.

Measuring wetland water quality: A wetland’s

ability to retain and transform potential contaminants is often cited as an important wetland function

to preserve. In most studies, a well with a 1- to 3-foot

long open interval is dug into the wetland and

pumped to obtain a water sample. We compared

traditional sampling from such a well to in-situ

sampling profiles that divided the well’s 3-foot long

open interval into 6-inch and 0.6-inch intervals

Site description: The site is located in the unglaciated region of

Wisconsin that is characterized by steep slopes and narrow valleys

that promote localized ground-water discharge and the formation of

river bottom wetlands (fig.1). The natural wetland consists of a

natural shrub-scrub/sedge meadow wetland dominated by sedges,

willow and alder and a riparian wetland dominated by alder, american

elm and black ash. During the summer of 1991, an adjoining upland

agricultural field was excavated to compensate for a wetland being

filled by a road construction project. A sedge meadow was the target

for the wetland creation, and the field was excavated to depths that

were specified on the basis of pre-construction water levels in 72

wells on the site. Salvaged marsh surface (wetland topsoil from a

destroyed wetland) was obtained from the on-site project and from

a highway project off-site. During the growing season, the groundwater level is generally 0.5 to 1.5 feet below ground surface. As a

result of their landscape setting, surface water is not important to

either the natural or constructed wetlands.

Figure 1. Aerial view of the Wisconsin Department of Transportation/U.S. Geological Survey wetland creation site near Wilton, Wisconsin one year after construction.

Within the created wetland are experimental plots where design parameters (for

example, depth to ground water) were varied. The locations of intensively instrumented sites in the natural and created wetland are also shown.

Figure 2. The blue bars represent the values of

ground-water flow estimated by using traditional

Darcy’s law calculations. The green bars represent

the inflow measured by innovative methods and

demonstrate that traditional approaches can significantly underestimate inflow to wetlands.

Figure 3. Three different sampling scales used to

investigate water quality in the natural and created

wetlands showed that very large differences in constituent concentration can be measured depending

on the amount of the subsurface sampled. Again,

traditional methods appropriate in other hydrologic

investigations may not be appropriate for work in

wetlands.

Natural riparian wetland

Experimental plot W2 Created wetland

F1

Natural

shrub-scrub/sedge meadow

wetland

W1

F2

0.00

0.05

0.10

0.15

0.20

0.25

0.30

0.35

0.40

W1 W2 F2

Natural Wetland Created Wetland

GROUND-WATER INFLOW (INCHES PER DAY)

Traditional Darcy’s law calculation

Average of non-traditional methods

0.22

12.6

41

0

5

10

15

20

25

30

35

40

45

CONCENTRATION (ppm as N)

Water table

well

Profile

average

(6-inch

interval)

Maximum

in profile

(6-inch

interval) 

Nitrate + nitrite

at created wetland

site F1

Traditional

sampling methods

(water table well

with 3 foot screen)

0.6-inch

sampling

interval

Peat

Sand

6-inch sampling

interval
(fig.3). As shown in Figure 3, concentrations of

dissolved chemical constituents (in this case nitrate

+ nitrite) measured in the samples from a water table

well can be more than 50 times lower than the

average concentration present in the subsurface.

This difference is a result of water entering the well

from preferential flow zones rather than uniformly

from the entire interval sampled by the well’s screen.

The 0.6-inch sampling interval also showed dramatic geochemical changes vertically—concentrations of some constituents in the root-zone soil water

differed by a factor of more than 1000 in water

samples collected just 3 inches apart. Variability

was present to some degree in each of the constituents measured. Clearly, our evaluations of wetlands—be they for wetland function analysis or

assessing the effectiveness of wetlands for wastewater treatment—would be fundamentally flawed if

this small-scale variability is ignored.

2) Is the created wetland similar to

the natural wetland next to it?

The simple answer is “in some ways yes, but in

other ways no”. We looked at the system from the

perspective of the essential components of wetlands—the water, the soils, and the vegetation.

Water: We used water tracers (naturally occurring stable isotopes of water) to identify sources of

water to the wetlands. In the natural wetland (fig. 4a)

and in some areas of the created wetland (fig. 4b),

ground water is the predominant source of water;

this represents a successful creation of the natural

wetland hydrology at the site. In other areas of the

created wetland, however, the major source of water

is rain (fig. 4c). Because the timing and availability

of these two water sources is very different, we can

expect that the two areas in the created wetland will

respond differently to environmental stresses such

as drought. This difference in water source also

indicates that wetland hydrology can vary significantly over small distances, and that the hydrology

may be as variable as the associated vegetation

community. Finally, the hydrologic results of this

study demonstrate that even a high density network

Figure 4. Assessing whether you’ve created the appropriate hydrology at a created wetland can be difficult. By

analyzing the water molecule we can identify the sources

of water at different depths in the wetlands. This approach

shows that areas of the created wetland have the same

ground-water source (a and b). Other areas of the created

wetland (c) depend on rain water to maintain water levels

and are expected to be drier in times of drought.

Salvage marsh surface (SMS) is excavated from the wetland that is to be filled

and is stockpiled for

application over the

created wetland.

SMS is a critical element for providing the

appropriate hydrological and chemical

environment for wetland plant establishment.

While aesthetically pleasing, significant differences remain between the created wetland and the adjacent

natural wetland five years after construction.

74

100

100

100

100

100

100

26

22

45

38

43

59

64

55

62

57

36

41

100

78

79

100

100

100

100

100

100

0.5 21

1.0

1.5

2.0

2.5

3.0

3.5

DEPTH BELOW GROUND SURFACE (FT)

a) Natural Wetland Site

W2

b) Created Wetland Site

F2

c) Created Wetland Site

F1

percent ground water percent rain-derived water
of wells and long-term pre- and post-construction

monitoring cannot guarantee that we will have sufficient understanding of the system to create a hydrologic regime needed for a sedge meadow wetland. This difficult-to-obtain hydrologic knowledge

is crucial for developing in-kind compensation, and

needs to be considered when assessing the appropriate mitigation strategy for destruction of hard-toreplace wetland types.

Soils: The organic salvaged marsh surface (SMS)

was only thinly spread on top of the created wetland

mineral soil, therefore the soils on the created wetland are more mineral (therefore more dense) than

those in the natural wetlands. This difference resulted in a 4° C increase in the root-zone temperatures of the created wetland. This has large implications for wetland seed germination and survival of

certain wetland plants. The addition of off-site SMS

also caused large deviations from water chemistry

seen elsewhere on the site. These deviations demonstrate that SMS provides a suitable chemical substrate for wetland seed germination and survival, as

well as a moist physical substrate. Areas that contain

the off-site salvaged marsh surface, however, may

never have the same wetland plant composition as

the on-site natural wetlands due to these large differences in soil water chemistry.

Vegetation: After five years, it appears that the

vegetation communities present in the created wetland are not any more similar to the natural wetlands

than those observed initially after wetland construction. The areas of the created wetland where on-site

salvaged marsh surface was applied were distinctly

different not only from the natural wetland, but also

from the areas of the created wetland that had

salvaged marsh surface obtained from off-site. Recently, the two areas of the created wetland have

become more compositionally similar, but are still

very different from the natural wetland. These results demonstrate that the adage “get the water right

and the wetland will follow” may not always hold,

and that many factors may come into play in wetland

development and persistence.

Study Schedule, Products and Future Work

This study was initiated in 1989 and is planned

for completion in September 1997. One scientific

journal article detailing the use of innovative methods for investigating natural and constructed wetland hydrology has been published and two others

are planned as this fact sheet goes to press. Additional work focusing on quantifying evapotranspiration and trace metal cycling in wetlands are ongoing.

Published Article:

Hunt, R.J., D.P. Krabbenhoft, and M.P. Anderson.

(1996). “Groundwater inflow measurements in

wetland systems.” Water Resources Research.

32(3): 495–507.

Titles of Planned Articles:

“Using stable isotopes of water and strontium to

investigate a natural and a constructed wetland”

“Assessing hydrogeochemical heterogeneity in natural and constructed wetlands”

Figure 5. The type of wetland targeted for a creation or restoration can have a large effect on the success

of the project. The Wisconsin Department of Transportation/U.S. Geological Survey restoration site targeted

a shallow-water marsh (a relatively easy wetland type to restore). This resulted in a higher wetland success

rate at the restoration site than at the creation site when evaluated as percent of site acreage.

Wetland creation involves

costly earth moving, making

it a more expensive alternative than wetland restoration.

For more information, please contact:

Randy Hunt

U.S. Geological Survey

6417 Normandy Lane

Madison, WI 53719

(608) 276-3847

www: http://wwwdwimdn.er.usgs.gov

U.S. Department of the Interior

U.S. Geological Survey

Fact Sheet FS-246-96

How does the wetland creation

compare to wetland restoration?

We also investigated how wetland restoration (restoring a wetland that was once

drained or filled so that it once again functions as a wetland) compares to wetland

creation in this same area of Wisconsin.

The wetland restoration consisted of converting a drained corn field to shallow water marsh (fig. 5) and wet meadow—the

types of wetlands found in the area. Shallow water marshes are considered easier

to construct than the sedge meadow attempted at the created wetland site. This

fact notwithstanding, the main conclusions

of our comparison show that:

1) The construction cost for the restored

site was one-fifteenth the cost of the created wetland. The high cost of earth moving required to create a wetland where one

has never existed makes it likely that the

costs of wetland creation will always be

higher than wetland restoration.

2) Restoration implementation time was

much shorter (two weeks) than wetland

creation (six months) due primarily to the

larger scope of work required for wetland

creation.

3) In a 1993 delineation of the wetland

creation and restoration sites, 60% of the

created site would have been delineated

as wetland and 100% of the restored site

was wetland.