Chapter 12 Wetlands.
Wetlands are sources, sinks, and transformers of a multitude of chemical, biological, and genetic materials, making them important ecosystems. They have been described as the "kidneys of the landscape" because of their ability to cleanse polluted waters. The capabilities of natural wetlands have been widely recognized and, taking a lesson from nature, artificial or constructed wetlands are being designed to treat a wide range of waste streams.
Approximately 6 percent of the land surface of the world (8.6 million [km.sup.2]) is wetland (Mitsch & Gosselink, 2000). During the nineteenth and twentieth centuries, large portions of the world's wetlands were artificially drained for agricultural or urban uses. Over 50 percent of U.S. wetlands were drained during this period (Dahl, 1990). However, since approximately 1970, the widespread destruction of wetland ecosystems has been curtailed. The Convention on Wetlands of International Importance, also referred to as the Ramsar Convention, held in 1971, developed a global treaty to protect wetlands as habitats for migratory birds (Blasko, 1997). The treaty requests countries to designate and protect their wetland resources. As of 2003, 136 nations have agreed to the Ramsar Convention by protecting 1288 wetland sites, totaling 108.9 million hectares.
The definition of a wetland would seem to be a fairly simple task; however, no widely accepted definition exists. All definitions refer to the combined influences of hydrology, soils, and vegetation. The identification or delineation of a wetland requires the evaluation of each of these components. Generally, a difference in elevation of 0.1 m can separate an area from being classified as a wetland versus an upland site (Figure 12-1) (Lyon, 1993). Unfortunately these transitional areas can be classified as a wetland under one set of criteria and upland under another (Skaggs et al., 1994).
Much emphasis has been placed on the development of criteria for delineating wetlands (Lyon, 1993; Mitsch & Gosselink, 2000). According to the U.S. Army Corps of Engineers, Section 404 of the 1977 Clean Water Act Amendments, wetlands are defined thus (Mitsch & Gosselink, 2000):
The term "wetlands" means those areas that are inundated or saturated by surface or groundwater at a frequency and duration sufficient to support, and under normal circumstances do support, a prevalence of vegetation typically adapted for life in saturated soil conditions. Wetlands generally include swamps, marshes, bogs, and similar areas.
While developing a classification system for wetlands, the U.S. Fish and Wildlife Service adopted the following more detailed description (Cowardin et al., 1979):
Wetlands must have one or more of the following three attributes: (1) at least periodically, the land supports predominantly hydrophytes; (2) the substrate is predominantly undrained hydric soil; and (3) the substrate is nonsoil and is saturated with water or covered by shallow water at some time during the growing season.
Hydrophytes are water-loving plants found where water is at or near the surface. Hydric soils are saturated, flooded, or ponded long enough during the growing season to develop anaerobic conditions near the surface. Nonsoils are the rocky areas along shorelines near oceans and lakes.
The Swampbuster Provision of the 1985 Food Security Act enhanced the Fish and Wildlife Service definition to produce the following:
The term "wetland" except when such term is part of the term "converted wetland" means land that (A) has a predominance of hydric soils; (B) is inundated or saturated by surface or ground water at a frequency and duration sufficient to support a prevalence of hydrophytic vegetation typically adapted for life in saturated soil conditions; and (C) under normal circumstances does support a prevalence of such vegetation.
[FIGURE 12-1 OMITTED]
A converted wetland is an area that was drained, dredged, filled, leveled, or otherwise manipulated before the Swampbuster Provision was enacted (December 23, 1985) for the purpose of, or to have the effect of, making the production of an agricultural commodity possible. Under the Swampbuster Provision, there are no restrictions on either drainage maintenance or additional drainage on these prior converted wetlands, which are estimated to total more than 20 million ha. Wetlands falling under the description of Section 404 of the 1977 Clean Water Act definition or the Swampbuster Provision are called jurisdictional wetlands and represent legally defined wetlands in the United States.
The international definition developed at the Ramsar Convention, extends the definition of wetlands to include (Blasko, 1997):
... areas of marsh, fen, peatland or water, whether natural or artificial, permanent or temporary, with water that is static or flowing, fresh, brackish or salt, including areas of marine water the depth of which at low tide does not exceed six meters.
The international definition includes a wide variety of habitats such as rivers, lakes, and coastal areas that are not typically described as wetlands by other definitions. The logic for extension to areas with water depths up to 6 m was used to include all habitats of migratory birds.
Landscape position, along with hydrology and vegetation, are used to classify the various types of wetlands. Wetlands are found on landscapes ranging from alpine slopes to ocean coastlines and in temperature regimes from the polar regions to the tropics. In 1974, the U.S. Fish and Wildlife Service began an inventory of wetland resources and, in the process, developed a wetland classification system that classifies natural wetlands into five major systems (Cowardin et al., 1979). These are
* Marine (coastal wetlands, including coastal lagoons, rocky shores, and coral reefs);
* Estuarine (including deltas, tidal marshes, and mangrove swamps);
* Lacustrine (wetlands associated with lakes);
* Riverine (wetlands along rivers and streams); and
* Palustrine (meaning "marshy"-marshes, swamps, fens, and bogs).
Outside of Alaska (43 percent of surface area is wetland), most of the wetlands in the United States are in the upper Midwest and eastern coastal regions (Figure 12-2). In addition to these natural wetland systems, there are numerous human-influenced wetland environments, including farm ponds and reservoirs, paddy fields, gravel pits, and constructed wetlands.
[FIGURE 12-2 OMITTED]
12.1 Marine Wetlands
The marine wetland system extends from the outer edge of the continental shelf (water depths less than 6 m) to shoreline features that are impacted by the high-energy waves produced by the ocean. The most recognizable aspects of marine wetlands are the salinity content greater than 30 ppt (parts per thousand) and the action of waves, which greatly reduces the presence of vegetation. Marine wetlands are nourished only by ocean water and the minerals and nutrients contained therein. Subsystems of marine wetlands include subtidal (where the substrate is continuously submerged) and intertidal (where the substrate is alternately exposed and flooded by tides). Typical examples include the rocky cliffs and islands adjacent to the ocean, ocean beaches, coral reefs, and tide-influenced lagoons. Aquatic beds of algae, sea lettuce, and kelp are also classified as marine wetlands. Examples include Monterey Bay and Bolinas Lagoon sites on the northern coast of California and the 62 000-ha Baie du Mont St. Michel site on the western coast of France.
12.2 Estuarine Wetlands
Estuarine wetlands occur at the interface between saline and freshwater environments with salinity ranging between 0.5 and 30 ppt. The absence of high-energy wave action distinguishes the estuarine system from the marine system and allows salt marshes to develop in the middle to high latitudes and mangrove swamps to develop in tropical and subtropical regions (Mitsch & Gosselink, 2000). Mangroves are salt-tolerant trees that have specially adapted roots and leaves that enable them to occupy the saline coastal waters where most plants cannot survive. Many estuarine wetlands are located in areas where freshwater rivers merge into the oceans. As with the marine system, estuarine wetlands are further classified as subtidal and intertidal subsystems. Estuarine systems described as salt marshes can be found along the Atlantic and Gulf coasts of the United States, including the 45 000-ha Chesapeake Bay Estuarine Complex and the 51 000-ha Delaware Bay. Mangrove swamps are located along the southern tip of Florida and the tropical coasts of Africa, Central and South America, and Asia.
12.3 Lacustrine Wetlands
The lacustrine system, as delimited by Cowardin et al. (1979), includes deep freshwater habitats and wetlands with a total area greater than 8 ha situated on landscape depressions or dammed rivers. These wetlands lack trees, shrubs, persistent emergent plants, emergent mosses, or lichens with greater than 30 percent areal coverage. In effect, lacustrine wetlands are essentially shallow lakes with limited "rooted" vegetation that are classified with regard to water depth. Areas with water depths between 2 m and 6 m are considered limnetic, and areas between the shoreline and a depth of 2 m are considered littoral. Distinctions between lacustrine and palustrine wetlands are made based on the total area and vegetation characteristics. Lacustrine wetlands are the second most prevalent type of wetland in the United States.
12.4 Riverine Wetlands
Riverine systems include rivers and streams characterized by water flowing from an upstream position to a downstream position through a defined channel. These systems extend from near-ocean locations with ocean-derived salt contents less than 0.5 ppt to the river source. Additional terminal points of riverine systems are where a channel enters or leaves a lake. Other than the streambank, riverine systems do not include the land adjacent to the stream that may contain wetland-supported trees, shrubs, persistent emergent plants, emergent mosses, or lichens. The Cowardin classification system labels these wetland areas adjacent to the stream as palustrine wetlands.
There are four subsystems of riverine wetlands. Riverine systems close to the oceans with low gradients and fluctuating flow velocity influenced by the tides are classified as tidal. Continuously flowing streams without tidal influence are either lower perennial or upper perennial depending on having either a low gradient or high gradient. Rivers and streams that do not flow for part of the year are classified as intermittent systems.
12.5 Palustrine Wetlands
The palustrine system includes all nontidal wetlands dominated by trees, shrubs, persistent emergent plants, emergent mosses, or lichens and similar tidal wetlands that have less than 0.5 ppt ocean-derived salt contents. The dominant feature of palustrine wetlands is the presence of persistent wetland vegetation. The palustrine system can include nonvegetated wetlands if they are less than 8 ha in area, lack a wave-formed or bedrock shoreline, water depth is less than 2 m at the deepest point, and salinity due to ocean-derived salts is less than 0.5 ppt. Most inland wetlands are classified as palustrine and include marshes, swamps, bogs, fens, tundra, potholes, and floodplains.
As described earlier, wetlands are sources, sinks, and transformers of a multitude of chemical, biological, and genetic materials. The two primary processes that occur in wetland environments are sedimentation and nutrient cycling. The unique characteristics of a wetland allows for the hydrology, soils, and vegetation to interact to create an ideal environment for chemical and biological changes to take place.
The presence of water is a key ingredient for delineating a wetland. A method to track the water flows within a wetland is the water budget consisting of a mass balance of inflows and outflows of water:
P + SWI + GWI = ET + SWO + GWO + [DELTA]S [12.1]
where P = precipitation (L),
SWI = surface water inflow (L),
GWI = ground water inflow (L),
ET = evapotranspiration (L),
SWO = surface water outflow (L),
GWO = ground water outflow (L),
[DELTA]S = change in storage (L).
The relative importance of each component varies both spatially and temporally. A particular lacustrine wetland may be influenced by surface inflows and outflows, whereas a nearby lacustrine wetland may be more influenced by ground water inflows. All the components interact to define the hydrology of an individual wetland and must be included in the mass balance. Factors that influence the water budget are landscape position, vegetation, soil, and climate.
An identifiable aspect of the water storage in a wetland is the water depth. During some periods, inflows are greater than outflows and water becomes ponded on the soil surface. During other times, outflows may be greater than inflows and water may disappear from the surface as happens during low tide or during extended dry periods. The water depth may actually fall just below the soil surface forming a shallow water table. The water table is defined as the upper surface of a saturated zone below the soil surface where the water is at atmospheric pressure (ASAE, 2001). The water table remains close enough to the soil surface that the soil profile is nearly saturated or can become saturated for a sufficient time to allow hydrophytic (water-tolerant) vegetation to survive.
The daily, monthly, or seasonal pattern of water level changes of a few wetland systems are shown in Figure 12-3. These patterns describe the hydroperiod and represent the hydrologic signature of the wetland (Mitsch & Gosselink, 2000). Coastal wetlands exhibit relatively uniform hydroperiods influenced by the regular tidal fluctuations. Inland wetlands may be influenced by climatic factors such as dry or wet years or seasonal effects related to evapotranspiration (ET) demands. Riverine wetlands or palustrine wetlands near rivers often show hydroperiods associated with seasonal floods. For wetlands that encounter random changes in water level, the amount of time the wetland is flooded is called the flood duration and the average number of flood events within a given period is called the flood frequency. The wetland hydroperiod can last for weeks, months, or even years. A depth-duration curve (Figure 12-3) can be constructed for any wetland to summarize the water depth and hydroperiod.
[FIGURE 12-3 OMITTED]
Another key hydrologic parameter for wetlands is the residence or retention time [R.sub.t], which describes the average amount of time water is retained in the wetland. An estimate of [R.sub.t] is
[R.sub.t] = wetland volume/[summation]inflows [12.2]
where the wetland volume is a function of the surface area of the wetland and the water depth. Note that this calculation generally overestimates the actual residence time as plug flow is assumed. In reality, short-circuiting can occur in most natural wetlands, allowing water to bypass regions within the wetland. Constructed wetlands are designed with baffles or berms to control the flow pattern and to reduce the amount of short-circuiting that can occur.
The following data were obtained for a coastal marsh. Precipitation was 965 mm, and evapotranspiration was 680 mm for the year. Surface inflows to the marsh totaled 87 300 [m.sup.3] and surface outflows were 101 700 [m.sup.3]. Ground water inflows were 13 270 [m.sup.3]. Determine the change in storage for this 1.3-ha wetland. If the depth of water in the wetland was 0.3 m at the start of the year, what was the depth at the end of the year? Estimate the residence time.
Solution. In order to compute a mass balance, all quantities must be converted to the same units. Precipitation and ET can be converted to units of volume.
P = 965 mm x 1.3 ha = 12 545 [m.sup.3]
ET = 680 mm x 1.3 ha = 8840 [m.sup.3]
Now apply Equation 12.1.
Inflows = Outflows + [DELTA]Storage
12 545 + 87 300 + 13 270 = 8840 + 101 700 + [DELTA]Storage
[DELTA]Storage = 2575 [m.sup.3] = 0.2 m added depth over the 1.3-ha wetland
Depth at the end of the year was
0.3 m + 0.2 m = 0.5 m
Use the average depth over the year and Equation 12.2 to estimate the residence time.
[R.sub.t] = 0.5(0.3 + 0.5)13 000/(12 545 + 13 270 + 87 300) = 0.023 yr = 8.4 day
Sediments, nutrients, trace metals, microorganisms, and organic materials are transported to wetlands by ground water and surface water. Wetlands can trap, precipitate, transform, recycle, and export many of these constituents, causing the quality of outflow water from a wetland to differ markedly from that of the entering water (Mitsch & Gosselink, 2000).
The hydroperiod and residence time significantly affect the capability of a wetland to alter water quality. Short, pulsing hydroperiods resulting from tidal influences force the wetland area to cycle through periods of wetting and drying. As the water level rises, sediment and other solutes are transported into the wetland where they may settle out; however, the short residence time limits the amount of nutrient transfer that can occur. Long hydroperiods found in the Everglades or swamps in the southeastern United States allow for significant sedimentation and nutrient cycling to occur as water moves slowly through these systems.
Water-quality enhancement from wetlands can affect an entire drainage basin. Water chemistry in basins that contain a large proportion of wetlands is usually different from that in basins with fewer wetlands. Basins with more wetlands tend to have water with lower specific conductance and lower concentrations of chloride, lead, inorganic nitrogen, suspended solids, and phosphorus (total and dissolved). Wetlands may change water chemistry sequentially; that is, upstream wetlands may serve as the source of materials that are transformed in downstream wetlands. Estuaries and tidal rivers depend on the flow of freshwater, sediments, nutrients, and other constituents from upstream sources.
Soils that are subject to flooding or ponding of water for extended periods will exhibit hydric or waterlogged characteristics. An important aspect of a saturated soil profile is the lack of oxygen (anaerobic). As water fills the pores, air is driven out. Air, most importantly oxygen, cannot be replaced because it diffuses very slowly through water (Lyon, 1993). The resulting anaerobic conditions that exist greatly affect the chemical and biological transformations that can occur in wetlands. Depending on the microbial population and the availability of organic material, the change from aerobic to anaerobic conditions can occur on the order of a few hours to a few days (Mitsch & Gosselink, 2000). After the soil profile becomes anaerobic, microbial activity and organic decomposition are decreased, which leads to the accumulation of organic matter in wetlands.
The redox potential or oxidation-reduction potential is the measure of the tendency of a wetland soil solution to exchange electrons. Since there are no free electrons in the soil solution, every oxidation reaction is accompanied by a simultaneous reduction reaction. Oxidation occurs during the uptake of oxygen or the removal of hydrogen. In essence, it is the giving up of electrons. Reduction is the release of oxygen, the gaining of hydrogen, or the gaining of electrons (Mitsch & Gosselink, 2000). Once oxygen is removed from the system, reduction of other compounds occurs in the order nitrogen, manganese, iron, sulfur, and carbon.
A platinum electrode can be constructed and used to measure the redox potential relative to a hydrogen electrode in units of millivolts (mV) (Mitsch & Gosselink, 2000). When dissolved oxygen is available, the redox potential is in the range of 400 to 700 mV. As dissolved oxygen is depleted and other compounds in the soil subsequently release oxygen (electrons), the redox potential becomes lower and eventually negative. The redox potential provides information about compounds that are or have undergone change in the wetland environment (Table 12-1). If a redox potential of--2150 mV is measured in a wetland, then it can be assumed that no dissolved oxygen is present and that nitrate, manganese, and iron have all been reduced. Since sulfur is reduced at--150 mV, there is probably a slight odor of hydrogen sulfide.
Nitrogen. Nitrogen occurs in many forms in a wetland depending on the oxidation-reduction state. The transformation from one form to another involves several microbial processes, including ammonification, nitrification, and denitrification.
Ammonification or N mineralization is the biological transformation (decomposition and degradation) of organic nitrogen to the ammonium ion (N[H.sub.4.sup.+]) that occurs in aerobic (oxidized) and anaerobic (reduced) environments. Although ammonification occurs more rapidly in aerobic environments, ammonium is more likely to accumulate in anaerobic environments because nitrification is inhibited (Kadlec & Knight, 1996). Once the ammonium ion is formed it can be absorbed by plants, adsorbed onto negatively charged soil particles, or nitrified into nitrate.
Nitrification is the two-step process of converting the ammonium ion into nitrate. Since nitrification is the process of removing hydrogen ions and adding oxygen ions, it must occur in an aerobic environment. Nitrosonomas bacteria conduct the first step of the process by slowly converting ammonium to nitrite (N[O.sub.2.sup.-]). Nitrobacter bacteria rapidly convert the nitrite to nitrate with addition of more oxygen. Since the nitrate ion is negatively charged, it cannot be immobilized by attaching to soil particles and is highly mobile in solution. The mobility of nitrate makes it a potential ground and surface water contaminant.
Denitrification represents a significant path for nitrogen removal in a wetland since it occurs in anaerobic environments. In the absence of free oxygen, several microorganisms such as Bacillus, Enterobacter, Micrococcus, Pseudonomas, and Spirillum bacteria use oxygen from the nitrate ion for metabolism. These organisms are called denitrifying bacteria. Nitrate is transformed to nitrogen gas ([N.sub.2]), nitrous oxide ([N.sub.2]O), or nitric oxide (NO) in the process (Kadlec & Knight, 1996). Denitrification, however, can be inhibited if a carbon source is not available for the bacteria. The denitrifying bacteria increase in population based on the quantity of nitrate and carbon available. Bacterial populations are greater in treatment wetlands because of the consistent and higher inputs of organic materials in these wetlands than generally occur in natural wetlands.
Phosphorus. Phosphorus retention is an important attribute in natural and constructed wetlands (Mitsch & Gosselink, 2000). The retention occurs via sedimentation, sorption, and uptake. If residence time is sufficiently long, sedimentation will occur whereby phosphorus sorbed to soil particles can settle out of the water and be deposited in the wetland. Riparian wetlands or other wetland environments that are exposed to short-term high flows can actually be phosphorus sources if erosional forces remove significant quantities of phosphorus-rich sediment that had previously been deposited in the wetland. Phosphates also form complexes with iron, calcium, and aluminum. Since phosphorus is an important nutrient for plant growth, plant uptake represents an important removal mechanism in wetlands. Unfortunately, the phosphorus is retained in the plant tissues and can be returned to the soil profile when the plant dies, causing the soil-plant system to be saturated with phosphorus with no net decrease. Biomass removal or harvest may be an important maintenance requirement for some treatment wetlands to remove the phosphorus and keep the wetland functional.
The types of plants growing in a wetland depend to a large degree on the hydroperiod or amount of time the wetland is inundated with water (Table 12-2). For cases with continual flooding of water 1 m or deeper, free-floating plants will dominate. In cases where the water level is less than a meter in depth or the area is occasionally flooded, emergent herbaceous plants and woody species become more dominant.
Although most plants require aerobic soil for normal root development, wetland plants are adapted to saturated soil conditions. One of these adaptations is the development of aerenchymous tissues that transport air to the roots (Kadlec & Knight, 1996). Air, including oxygen, can move out of the roots to develop a small, aerated zone around each root. Other plants develop adventitious roots that extend from the plant stem into the water. Adventitious roots have the capability of absorbing dissolved oxygen and nutrients from the water profile. The hydroperiod, water depth, water chemistry, and dissolved oxygen content of the water create the environmental conditions that allow individual species to thrive in wetlands.
Wetland Creation and/or Restoration Techniques
As stated earlier, significant wetland areas have been removed from the environment over time through drainage and filling. Restoration of these impaired/abolished wetlands may be easier than the process of creating a wetland in an upland area. Since ditches or subsurface drains have been used to drain many of the wetlands in the midwestern and coastal plain regions of the United States, the simple "plugging" of the ditches or drains is thought to be a viable method of restoring wetland functions. Unfortunately, agricultural or silvicultural activities, in addition to land-forming or -leveling procedures have removed most if not all of the original wetland topography or surface roughness. In areas with high soil organic carbon content, land subsidence may have occurred after the area was drained. The combination of a lack of surface roughness and inundation promotes a monoculture of wetland plants and may encourage a different population of vegetation to inhabit the area than existed prior to drainage.
Since the restoration of wetlands on prior drained sites is a relatively new practice, no design criteria are available concerning the amount of surface roughness necessary for restoring the appropriate hydrologic function. Saturation will slowly restore the anaerobic environment necessary for wetland function, but some surface roughness should be created. If possible, minimum and maximum water levels should be measured in nearby functional wetlands. Topographic "islands" and "depressions" can be created with a bulldozer prior to blocking the drain outlets. The development of surface roughness is an important part of the restoration process for promoting diverse communities of plants and wildlife.
The surface roughness can be used to promote mixing in the restored or created wetland. It is advisable to construct berms or barriers to force inflows to spread out over the wetland area. Serpentine flow patterns tend to maximize the residence time of a wetland.
In areas with minimal topographic relief, the blocking of a drainage ditch could flood many square kilometers with significant loss of prime farmland, commercial land, or housing sites. In Ohio, researchers have promoted a Wetland Reservoir Subirrigation System that allows drainage water to flow through a wetland before being stored in a small reservoir. Water from the reservoir is subsequently used during the growing season to irrigate a crop. In North Carolina, researchers have removed soil from the sides of a drainage ditch to create an instream wetland. In both cases, soil is removed to create a depression for the wetland. The advantages of these types of systems are the ability to maintain productive agricultural lands and to remediate water quality problems without having to flood large areas.
The restoration of the wetland hydrology to a site and a little patience (3 to 5 years) are enough to promote the formation of a wetland (Hammer, 1997; Mitsch & Gosselink, 2000). The seed bank not present in the soil will be transported over time to the site via wildlife. However, the development of a forested swamp, for instance, will take many more years (50-75) to develop than a simple marsh because the growth processes are much different for an oak or cypress tree compared to a cattail or bulrush. The types and diversity of plants colonizing a wetland will depend to a great extent on the water depth and hydroperiod of the wetland.
For those cases where quick establishment is important, seeding and planting techniques may be required. Water levels must be carefully managed to produce a wet, but not saturated, soil surface, because few seeds will germinate while completely submerged. Planting times and methods depend on the species and materials, such as seeds, tubers, rootstocks, plants, cuttings, or seedlings. Herbaceous vegetation and seeds are generally planted in the spring, whereas tubers, rootstalks, and seedlings are generally planted in the fall (Hammer, 1997). Ideally, plant materials can be retrieved from a nearby functional wetland and used as a plant and seed source for the new wetland. After planting, the hydrologic conditions must be carefully controlled to assure proper plant growth, because flooding too quickly can cause the death of many species.
Constructed wetlands are typically built to treat wastewater. Sources are as diverse as milkhouse wastes, animal wastes, agricultural runoff, home and municipal sewage, and industrial waste streams. These sources can have high biochemical oxygen demand (BOD), total suspended solids (TSS), nitrogen, phosphorus, fecal coliform bacteria, etc. Initial filtration or primary treatment techniques may be required to treat the waste stream prior to passing it through the constructed wetland. Unlike natural wetlands, constructed wetlands are heavily controlled by inlet and outlet conditions. Flows are monitored and cells are created to maximize the retention time. Detailed design procedures based on climate and waste characteristics should be followed (Kadlec & Knight, 1996).
Constructed wetlands (Figure 12-4) are classified as surface flow (SF) or subsurface flow (SSF) wetlands. Subsurface flow wetlands utilize large gravel as the porous media and as a result of the anoxic conditions are generally more efficient than surface flow wetlands. The primary disadvantage of subsurface flow wetlands is the inability to treat waste streams high in total solids because of clogging. Sizing of SF or SSF wetlands is based on either the assurance of a specified retention time or on the capacity of the wetland to make a specified reduction in a particular constituent in the waste stream.
[FIGURE 12-4 OMITTED]
Intuitively, the amount of time a waste stream is held in a wetland influences the quality of water exiting the wetland. Total suspended solids and BOD have been successfully reduced in SF wetlands with retention times of 7 to 10 days and in SSF wetlands after 2 to 4 days (Kadlec & Knight, 1996). Wetland area and water depth are the controlling factors for determining retention time. Equation 12.2 can be rewritten in terms of area, depth, and media porosity as
[R.sub.t] = [eta]Ab/Q [12.3]
where [R.sub.t] = retention time (T),
[eta] = porosity ([L.sup.3]/[L.sup.3]),
A = area ([L.sup.2]),
b = depth (L),
Q = inflow rate ([L.sup.3]/T).
For SF wetlands most of the flow is above the soil surface, with typical depths of 0.15-0.45 m. A portion of the flow is below the soil surface or around plants in the wetland giving a typical porosity in the range of 0.9 to 0.95. Either increasing the wetland area or increasing the depth of flow can increase retention time. However, at full inundation deeper water provides no significant increase in pollutant removal (Kadlec & Knight, 1996). For this reason, the term "hydraulic loading rate" q is often used in surface flow wetlands, which is the inflow rate Q divided by the area A.
Pollutant removal in subsurface flow wetlands is controlled by the development of biofilms on the porous media in addition to the anoxic environment. SSF wetlands have depths of 0.3 to 0.6 m and porosities of 0.3 to 0.5 [m.sup.3]/[m.sup.3].
Another approach used to size treatment wetlands is the mass balance design model or Field Test Method (Kadlec & Knight, 1996). In this model, the wetland area is a function of the desired inlet and outlet pollutant concentrations along with a rate constant describing the pollutant decay relationship. For a particular pollutant, the equation is
A = - Q/k [log.sub.e]([C.sub.2] - [C.sup.*]/[C.sub.1] - [C.sup.*]) [12.4]
where A = area ([L.sup.2]),
Q = inflow ([L.sup.3]/T),
k = rate constant (L/T),
[C.sub.1] = inflow concentration (M/[L.sup.3]),
[C.sub.2] = outflow concentration (M/[L.sup.3]),
[C.sup.*] = background concentration (M/[L.sup.3]).
The background concentration ([C.sup.*]) represents the concentration of the pollutant that resides in the wetland before inflow is introduced. Values for the rate constant have been derived by Kadlec & Knight (1996) for treatment wetlands, and by CH2M Hill & Payne Engineering (1997) for animal waste treatment wetlands (Table 12-3). Because the rate constant varies with temperature and maturity of the wetland, site-specific or regional rate constants should be developed and used in the design process.
Compute the wetland area required to reduce ammonia concentrations of an inflow stream (47 450 [m.sup.3]/yr) by 50 percent. The inflow concentration to the surface flow wetland was 200 mg/L and the background concentration of ammonia in the wetland was 25 mg/L. Estimate the retention time.
Solution. The outflow concentration is 50 percent of 200 mg/L or 100 mg/L. The rate constant from Table 12-3 for ammonia and SF wetlands is 18 m/yr.
A = - 47 450/18 In(100 - 25/200 - 25) = 2233 [m.sup.2]
If a wetland depth of 0.4 m and a porosity of 0.9 are assumed, the retention time can be computed (Equation 12.3)
[R.sub.t] = 0.9(2233)0.4/(47 450/365) = 6.2 days
Listing of wetland sites agreeing to the Ramsar Convention
An indication of the wetlands systems in each state of the United States can be found at
American Society of Agricultural Engineers (ASAE). (2001). Soil and Water Terminology. S526.1. ASAE Standards. St. Joseph, MI: Author.
Blasko, D. (1997). The Ramsar Convention Manual: A Guide to the Convention on Wetlands, 2nd ed. Gland, Switzerland: Ramsar Convention Bureau.
CH2M Hill & Payne Engineering. (1997). Constructed Wetlands for Livestock Wastewater Management: Literature Review, Database, and Research Synthesis. Prepared for the EPA's Gulf of Mexico Program, Alabama Soil and Water Conservation Committee, Montgomery, AL.
Cowardin, L. M., V. Carter, F. Golet, & E. T. LaRoe. (1979). Classification of Wetlands and Deepwater Habitats of the United States. FWS/OBS-79/31. Office of Biological Services, U.S. Fish and Wildlife Service.
Dahl, T. E. (1990). Wetlands losses in the United States 1780's to 1980's. U.S. Department of the Interior, Fish and Wildlife Service, Washington, DC. Jamestown, ND: Northern Prairie Wildlife Research Center Home Page. Online at http://www.npwrc.usgs.gov/ resource/othrdata/wetloss/wetloss.htm.
Hammer, D. A. (1997). Creating Freshwater Wetlands, 2nd ed. Boca Raton, FL: Lewis Publishers.
Kadlec, R. H., & R. L. Knight. (1996). Treatment Wetlands. Boca Raton, FL: Lewis Publishers.
Lyon, J. G. (1993). Practical Handbook for Wetland Identification and Delineation. Boca Raton, FL: Lewis Publishers.
Mitsch, W. J., & J. G. Gosselink. (2000). Wetlands, 3rd ed. New York: Wiley.
Skaggs, R. W., D. Amatya, R. O. Evans, & J. E. Parsons. (1994). Characterization and evaluation of proposed hydrologic criteria for wetlands. Journal of Soil and Water Conservation, 49(5), 501-510.
12.1 A subsurface flow wetland with a porosity of 0.43 [m.sup.3]/[m.sup.3] will be used in a waste treatment process to treat a waste stream of 28 [m.sup.3]/d. How large of a treatment wetland is required if the flow depth is 0.5 m and the required retention time is 6 days?
12.2 Nitrate nitrogen will be reduced from a concentration of 48 mg/L to a concentration of 8 mg/L in a surface flow wetland. The inflow stream is 33 400 [m.sup.3]/yr. The background concentration in the wetland is 4 mg/L. Compute the required size of the surface flow wetland.
TABLE 12-1 Typical Redox Potential Values for Chemical Reactions in a Wetland Chemical Reaction Redox Potential (mV) [O.sub.2] available 400 to 700 N[O.sup.-.sub.3] converted to [N.sub.2] 250 [Mn.sup.4+] converted to [Mn.sup.2+] 225 [Fe.sup.3+] converted to [Fe.sup.2+] 100 to -100 S[O.sup.2-.sub.4] converted to [H.sub.2]S -100 to -200 C[O.sub.2] converted to C[H.sub.4] < -200 TABLE 12-2 Typical Species Found in Wetlands Common Name Scientific Name Submerged Aquatic Species (0.1-3 m water depth) Water mosses Fontinalis spp. Brazilian waterweed Egeria densa Elodea Elodea spp. Hydrilla Hydrilla verticillata Eelgrasses Vallisneria spp. Naiads Najas spp. Water milfols Myriophyllum spp. Bladderworts Utricularia spp. Fanwort Cabomba caroliniana Floating Aquatic Species Mosquito fern Azolla caroliniana Water fern Salvinia rotundifolia Water lettuce Pistia stratiotes Duckweeds Lemna spp. Giant duckweeds Spirodela spp. Watermeals Wolffia spp. Bog mats Wolffiella spp. Water hyacinth Eichhornia crassipes Floating Rooted Aquatic (0.1-3 m water depth) Frog's bit Limnobium spongia Pondweeds Potamogeton spp. Water shield Brasenia schreberi Floating hearts Nymphoides spp. Lotuses Nelumbo spp. Spatterdocks Nuphar spp. Water lilies Nymphaea spp. Pennyworts Hydrocotyle spp. Woody Shrubs and Trees (continuous inundation) Cypresses Taxodium spp. Water-willow Decodon verticillatus Tupelo gums Nyssa spp. Buttonbush Cephalantus occidentalis Willows Salix spp. Red mangrove Rhizophora mangle Loblolly bay Gordonia lasianthus Woody Shrubs and Trees (less than continuous inundation) Pond pine Pinus serotina Cabbage palm Sabal palmetto Maples Acer spp. Hollies Ilex ssp. Black mangrove Avicennia germinans Hazel alder Alnus serrulata River birch Betula nigra Swamp dogwood Cornus foemina Titi Cyrilla racemiflora Fetterbush Lyonia lucida Oaks Quercus spp. St. John's wort Hypericum spp. Emergent, Herbaceous Plants (continuous inundation) Sphagnum mosses Sphagnum spp. Arrowheads Sagittaria spp. Wild taro Colocasia esculenta Spoon flowers Peltandra spp. Canna lilies Canna spp. Sedges Carex spp. Sawgrass Cladium jamaicense Spikerushes Eleocharis spp. Beak rush Rhynchospora spp. Bulrush Scirpus spp. Watergrass Hydrocloa Maidencane Panicum hemitomon Torpedograss Panicum repens Common reed Phragmites spp. Wild rice Zizania aquatica Southern wild rice Zizaniopsis milacea Blue flag iris Iris spp. Rushes Juncus spp. Arrowroot Thalia geniculata Pickerelweeds Pontederia spp. Bur reed Sparganium americanum Cattails Typha spp. Alligator weed Alternanthera philoxeroides Water primroses Ludwigia spp. Smartweeds Polygonum spp. Lizard's tail Saururus cernuus Reed canary grass Phalaris arundinacea Mannagrass Glyceria spp. Emergent, Herbaceous Plants (less than continuous inundation) Swamp fern Blechnum serrulatum Chain ferns Woodwardia spp. Royal ferns Osmunda spp. Giant cane Arundinaria gigantea Knotgrass Paspalum distichum Cordgrasses Spartina spp. TABLE 12-3 Rate Constants for the Mass Balance Design Model (m/yr) Animal Waste SF SSF Treatment Pollutant Wetland Wetland Wetland BOD 34 180 22 Total nitrogen 22 27 14 Nitrate nitrogen 35 50 Not reported Ammonia nitrogen 18 34 10 Total phosphorus 12 12 8 Fecal coliform 75 95 Not reported Total suspended Not 21 solids 1000 recommended
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|Publication:||Soil and Water Conservation Engineering, 5th ed.|
|Date:||Jan 1, 2006|
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