Greenhouse Gases

Constructed wetlands have emerged as a promising technology over the past 15 years for treatment of municipal, industrial discharges as well as urban, and agricultural runoff (Hammer, 1989; Moshiri, 1993; Kadlec and Knight, 1996; Vymazal et al., 1998; DuBowy and Reaves, 1998; Longhurts et al., 2000; Kadlec et al., 2000; Hunt and Poach, 2001). Recent studies have revealed that the levels of greenhouse gases released from constructed wetlands during the treatment of wastewater could become significant.  It is therefore important to monitor gaseous emissions from the constructed wetlands in order to estimate any potential environmental hazards that could occur from the application of this technology.

Wastewater remediation operations can be a source of air-transported pollutants, such as ammonia (via volatilization), or greenhouse gas (GHG) emissions such as nitrous oxide (N₂O) and methane (CH₄), which contribute to ozone depletion and global warming (USEPA, 2003).  Nitrous oxide has a lifespan in the atmosphere of approximately 120 years, and over a one hundred year span it has a 320 times stronger global warming potential than carbon dioxide (CO₂) (Johansson et al, 2003). In contrast, CH₄ has a life span between 12 and 17 years in the atmosphere, and over a one hundred year span it has only 24.5 times greater global warming potential than CO₂ (Mander et al, 2003).  However, CH₄ is currently expected to account for 25% of global warming while N₂O is only expected to contribute to 5% of global warming (Mander et al, 2003). The Intergovernmental Panel on Climate Change (IPCC), which calculated these values and forecasts global warming, has also calculated that N₂O emission factors from wetlands are approximately 0.75%.  Calculations at the Nykvarn constructed wetland by Johansson et al, however, showed that N₂O emission factors were consistently lower, between 0.02-0.27%, thus showing it is important to calculate the emission factors for constructed wetlands as they can differ from the standard given for wetlands in general (Johansson et al, 2003).

Mesuring gas emisions from a wetland.

Constructed wetlands have primarily been developed as a technology for wastewater effluent treatment. Despite their use for over two decades, constructed wetlands are still considered to be in the development phase (Hunt and Poach, 2001; USEPA, 2001b; LCBP, 2003). The design of Constructed Wetland systems has evolved over time, and the need for improved nutrient removal has been recognized. Scientists have recently begun to measure the potential greenhouse gas fluxes from constructed wetlands treating wastewater runoff.

Although wetlands have traditionally been viewed as major carbon sinks due to their dense vegetation, a growing concern about greenhouse gas emissions has generated considerable debate regarding the role of wetlands in relation to greenhouse gases such as N₂O and CH₄. With only a few studies having been conducted to assess potential greenhouse gas emissions from those systems (Gordon et al, 1998; Gui et al., 2001; Mander et al, 2003) it will become a critical area of investigation in the near future.

It has been recognized that greenhouse gases are generated through both nitrification and denitrification processes in aquatic systems (Williams et al., 1992; Groffman et al., 1998; Wrage et al., 2001), however, the relative contribution of these processes to gas emissions is still unclear.  In general, denitrification and N₂O emissions are regulated by the availability of oxygen, nitrate (NO₃^-) and organic matter (Martikainen and De Boer, 1993). Additionally, the spatial variability in production of nitrogen gas (N₂) and N₂O from wetland soils will be influenced by temperature, pH, moisture content and availability of carbon (Knowles, 1982; Wrage, 2001).

Di-methyl Sulfide

Natural wetlands play a vital role in restoring di-methyl sulfide (DMS) to the atmosphere, a critical compound that both provides condensate nuclei for cloud formation and reflects solar radiation.  The potential of a constructed wetland to alter or contribute to the production of this greenhouse gas is also important to consider.