Background

Constructed Wetlands

Until recently, natural wetlands were not valued as ecosystems and the habitats of countless organisms they are known for today. Consequently, only a small number of swamps, fens, bogs, and marshes have survived the growth of industry. When a great amount of pollutant flows through a wetland consistently for a long period of time, the pollutant begins to accumulate, and the wetland reaches a point where it cannot consume any more. This results in the exhaustion of the wetland. It ceases to filter contaminants from water, and, while becoming clogged with sediment and pollutants, plants die. An exhausted constructed wetland can be replaced, cleaned up, etc., while damage to natural wetlands is more permanent. In addition to this, constructed wetlands can be engineered specifically to the needs of the site, while natural wetlands cannot. Thus, using constructing wetlands rather than using natural wetlands is both more efficient, and environmentally friendly. (Pries 104)

In constructed wetlands, physical, chemical, and biological processes are combined to remove pollutants from water. Physical removal processes involve sediment trapping, where sediments either settle on the floor surface of the wetland or are trapped in plant roots. Chemical removal processes include adsorption, the attachment of ions to soil particles by cation exchange or chemisorption, and vocalization, the diffusion of a dissolved compound from water into the atmosphere. Biological removal processes consist of plant uptake, the consumption of pollutants by plant species, and microbial decomposition, the breaking down of pollutant matter by bacteria within soil. (Debusk 32)

Constructed wetlands have proven very efficient in almost every aspect of water filtration, removing all types of contaminants from sewage to heavy metals. They are also more cost effective and aesthetically pleasing than current large scale water filtration methods. Greenhouse gas production is only a small side effect when compared to the benefits of such a system. (Kennedy 2002) It is also the main focus of this experiment.

Wetland Greenhouse Gas Emissions

The remediative properties of wetlands have been proven for several decades. However, when their large contribution to greenhouse gases in the atmosphere is considered, it can be argued that they are doing more harm than good. The trapping of excess greenhouse gases in the atmosphere results in global warming, which has an overall negative effect on the planet. (Waddington 2003)

Of the three main greenhouse gases generally produced by wetlands, carbon dioxide has the lowest global warming potential (GWP), that is, it contributes to global warming on a much smaller scale than both methane and nitrous oxide. Of the three, N2O has the greatest GWP, but is not generally released in large quantities from wetlands.(Glass&Gordon 2003) However, in the case of wetlands carrying nitrate-polluted water, the presence of nitrogen would cause the release of a significant quantity of N2O, depending on the quantity of nitrate present in the water.

Variables Affecting Type of Gas Released

Factors such as availability of nutrients, position of the water table, and temperature control the amount and type of gas released. The pH of the wetland and whether it is aerobic are also contributing factors. However, ultimately it is the bacteria within the wetland environment that determine the type of gas released.
(Schlesinger 1997) From each of the gases that could potentially be released from the wetland, the bacteria produces whichever the present conditions allow. If conditions in the wetland are not favourable for the production of a certain gas by one type of bacteria (i.e; no carbon source for the production of carbon dioxide), the next most easily-produced gas is formed, and becomes the dominant type of gas released.(Waddington 2004)

Nitrate Filtration in Wetlands

Nitrate is a natural compound that is naturally-occurring in all ecosystems. It is essential to plant life in minimal quantities. However, large concentrations have the potential to cause severe harm to an aquatic ecosystem, causing the water to become toxic. (Jamieson 2005)

Rivers and streams with high nitrate concentrations are often polluted as a result of the gradual runoff of agricultural wastes, such as fertilizers, from their original dumping sites The increase in nitrogen levels in an environment which exceeds its natural ecological balance is referred to as eutrophication. Eutrophication has a visible negative effect on aquatic ecosystems. Wetlands filter particulate forms of nitrogen through their settling and burial beneath the sediment layer. Liquid forms of nitrogen are removed through either storage within the wetland, or removal through the nitrogen cycle. (Debusk 2003)

Storage refers to the uptake of nitrate by the wetland’s biomass or its absorption to the substrate. Although it is an efficient removal method, absorption to the substrate is only temporary. In other words, it will eventually seep back into the water flowing through the wetland. In order for nitrate to be removed entirely from the wetland system, either the nitrate must be taken up by the wetland’s biomass; or, the nitrogen cycle must cause the various forms of nitrogen to be converted into several gaseous compounds composed of nitrogen. (“Wetland Conservation”)

Nitrification and denitrification reactions are the dominant nitrogen removal mechanisms in constructed wetlands. Nitrification refers to the biological formation of nitrite or nitrate from NH4. It occurs in the aerobic regions of the water column, soil-water interface, and root zone. Denitrification is the reduction of nitrate and nitrite into nitrogen gas, nitrous oxide, or nitric oxide. It cannot occur unless nitrate is in adequate supply. (Jamieson 2005)

One of the gases produced by wetlands as a result of denitrification is nitrous oxide. Nitrous oxide has a Global Warming Potential of 296 (Waddington 2006), meaning it has a warming effect approximately 270 times greater than CO2. Fortunately, it is not one of largest contributors to the greenhouse effect at this time, due to the fact that it is not being released consistently in large quantities. (Kennedy 2002) However, if a high concentration of nitrate was added to a wetland, nitrous oxide would be produced consistently until this quantity was consumed. Thus, if constructed wetlands became more widely used for the filtration of pollutants containing nitrate, nitrous oxide would have a definite impact on the greenhouse effect. Therefore it is necessary to control the quantities of nitrous oxide released by nitrate-filtering wetlands.