Constructed Wetlands

A wetland is an area where the flow of drainage and/or runoff is slowed, and so the soil is very wet and there is often ponding. Plants growing in wetlands need to be tolerant of these conditions. Wetlands can be either natural i.e. they have developed naturally, or they can be purpose built or constructed. During this time of relatively slow movement of drainage and runoff through the wetland, contaminants are removed from the water. 

The plants will uptake nutrients in the water such as nitrate and inorganic phosphorus, the wet conditions will encourage denitrification (the conversion of nitrate to dinitrogen gas), the larger sediments will settle, and the finer sediments are likely to be trapped on biofilms that form on the litter and plants under water. Therefore, wetlands can effectively improve water quality as it leaves the farm and before it arrives in streams and rivers. A constructed wetland is often the best edge-of-field mitigation where there are large volumes of drainage and/or runoff with smaller concentrations of contaminants. 

In-depth overview of a wetland 

Nathan Burkepile from New Zealand Landcare Trust describes the role of a wetland, along with the processes that occur in the wetland. 

Construction of a wetland 

Watch the video below if you would like to see how a wetland is constructed. This one was built at Massey University’s Dairy Farm 4 in the Manawatū