Abstract

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. However, it is a problem that needs to be addressed before constructed wetlands are ready for widespread implementation.

While wetlands generally emit methane, carbon dioxide and nitrous oxide, the quantity of methane released is usually much greater than nitrous oxide, and methane is a much stronger greenhouse gas than carbon dioxide. The exact amount of each gas released is dependent on several factors affecting the wetland environment. The purpose of this experiment was to reduce the amount of methane released from a constructed wetland model by controlling the factors affecting trace gas exchange.

Based on the factors affecting wetland gas exchange, the following hypotheses were formed:

1. The wetland model with the greatest water volume, Model E, would release the largest quantity of methane, and the smallest quantity of carbon dioxide. The models with the smallest water volume, C and D, would release the smallest quantity of methane and the largest quantity of carbon dioxide.

2. Models C and D would have the lowest Global Warming Potential, and Model E will have the highest.

Five wetland models with varied water volumes were built. Models A and B held 6L of distilled water; Models C and D held 4L, and Model E held 8L. 20 Arrowhead plants were planted in each. After allowing the plants to grow for several months, gas samples were taken. The models were each covered with plastic and secured to produce airtight chambers. Gas samples were taken using syringes. The gas produced by each model was allowed to accumulate over thirty minutes. Samples were taken from each model at the following intervals: 0, 10, and 30 minutes. These samples were analysed within 48 hours on a gas chromatograph.

The gas chromatograph provided a ppm measurement of the accumulation of CO2 and CH4 over 30 minutes. The fluxes of each gas were calculated in gCO2/m2/day or mgCH4/m2/day. Using this information and surface area of each chamber, and the daily gas uptake by plants in each model, the Global Warming Potential of each model was calculated.

The first hypothesis, that Model E would release the largest quantity of methane, and Models C and D would release the least, was correct. The second hypothesis, regarding the Global Warming Potentials, was incorrect. Additional decomposition took place in Models A and B, giving them the highest CO2 emissions, and the largest GWPs. The plants in Model E were very healthy and took in large quantities of CO2, giving it the lowest GWP. Therefore, it can be concluded that large water volumes in constructed wetlands will not affect their Global Warming Potential on a large scale as long as plants are kept in good health and decomposition is controlled.