Abstract

Constructed wetlands have been proven to be remediative for several decades, yet their potential contribution to global warming has yet to be explored in depth. Factors such as availability of nutrients, position of the water table, saturation, and temperature control the amount and type of gas released from a wetland environment. Scientists have begun to explore the use of constructed wetlands in the filtration of rivers and streams with high nitrate concentrations. These water bodies are often polluted as a result of runoff of agricultural wastes (e.g. fertilizers) from their original dumping sites. Unfortunately, the input of nitrates into a wetland system stimulates the production of nitrous oxide, a harmful greenhouse gas. Although greenhouse gas emissions are a small side effect when compared with the benefits of such a system, they must be addressed before nitrate-filtering constructed wetlands are implemented on a large scale.

The purpose of this experiment was to develop a constructed wetland model that filters the maximum ammonium nitrate concentration, while maintaining a minimum Global Warming Potential, by controlling the factors that affect nitrous oxide emissions.
Based on the factors affecting wetland gas exchange, the following hypotheses were formed:

1. The models with the largest water volumes will demonstrate the largest methane (CH4) flux, and the smallest carbon dioxide (CO2) flux.
2. Following the addition of ammonium nitrate (NH4 NO3), there will be a decrease in the methane and carbon dioxide emissions in all of the models.
3. Following the addition of NH4 NO3, the models with the highest saturation will release the largest nitrous oxide (N2O) quantities.
4. The models with the largest water volumes will demonstrate the largest reduction in nitrates in their water over time.

Three replicas of four wetland models were constructed, with varying water volumes. Twenty arrowhead bulbs were planted in each. After allowing the plants to grow, the models were covered to form airtight chambers. Gas samples were taken using syringes at 0, 10, and 30-minute intervals. These samples were analysed for CH4 and CO2 concentrations. A controlled quantity of NH4 NO3 was added to nine of the models. Another set of gas samples was taken, at 0 minutes and 2 hours. Water samples were also taken from the substrate of each model at these intervals, using pipettes. Six days following the contamination of the models, a final set of gas samples was taken and analysed for CH4 and CO2. All gas samples were analysed using a gas chromatograph, while NH4 NO3 concentrations in the water samples were measured with a spectrophotometer.

The results showed that the models with the greatest water volumes had the largest CH4 fluxes, and surprisingly the largest CO2 fluxes. They also showed the greatest reduction in nitrate concentrations. All the carbon dioxide fluxes dropped with the addition of ammonium nitrate to the models. The models with the highest saturation resulted in the largest nitrous oxide fluxes. Therefore, the saturation in a nitrate-filtering constructed wetland system has a measurable effect on its overall Global Warming Potential.The water volumes of the models in this experiment affected their CO2, CH4, N2O, and plant uptake fluxes. However, other variables, such as the health of the plants and the amount of decomposition that took place in each model, created differences in the GWPs of models with identical water volumes. Thus, it is possible to construct a highly saturated nitrate-filtering model with a maximum biomass consisting of hardy macrophytes that filters a large quantity of nitrate, while maintaining a low Global Warming Potential.