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. (Debunk 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, with their large contribution to greenhouse gases in the atmosphere, it can be argued that they are doing more harm than good. The trapping of excess greenhouse gases in the atmosphere leads to global warming, which has an overall negative effect on the planet. (Waddington 78)

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. This experiment focusses mainly on the prevention of CH4 emissions because, in view of its fairly large GWP relative to the large quantities of the gas often released by wetlands, methane poses as the greatest environmental threat. (Glass&Gordon 2003)

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)