In both natural and constructed wetlands, plants work with water, soils, and microbes to achieve desired treatment processes. These treatment processes can be found in some parts of natural wetlands, and if targeted properly, can be augmented and accelerated in constructed treatment wetlands. In all locations where we work, from the Yukon to Kyrgyzstan, proper plant selection is a key aspect of designing an effective treatment wetland.
Accretion is the naturally occurring process of wetland sedimentary material accumulation (soil, minerals, decaying plant material, etc.) over time. Once an accreting constructed treatment wetland is established and mature, targeted constituents are sequestered into the sediment and covered over time by newly generated sediments and detritus. This essentially seals away the treated constituents under layers of sediment, rendering them less bioavailable and less susceptible to re-entry into the water column, thus decreasing potential exposure to these constituents
All plants have specific attributes that allow them to aid in water treatment. These include (in no particular order): rate of accretion, generation of biomass, transpiration, structural strength, filtration, creation of microbial habitat, radial oxygen loss, and sorption.
Plant attributes differ greatly between species, and even by local ecotype. Therefore, the best-suited plants for a treatment wetland are often those found at or near the location where treatment is needed, but the selection should made on a site-specific basis and guided by the treatment objectives.
The following is a list of six important considerations for selecting plants for use in constructed treatment wetlands.
Root structure not only provides physical filtration, but also structural stability to soils, preventing resuspension or erosion during high flow events.
Plants can alter the hydrology of a wetland to encourage precipitation and settling of suspended solids. There are various structural attributes that allow plants to withstand different ranges of flow velocities and water depths, and different properties that contribute to filtration, settling, and sorption. Moreover, hydrology is altered by transpiration and root depth.
Plants influence the area that can be used for water treatment through transpiration, a process through which water is transported throughout the plant and the surrounding soil and air. Through transpiration, plants affect the rate at which water is lost to the atmosphere, but also influence the depth of water treatment activity. The treatment zone is dependent upon the root depth, as aside from the water/sediment interface, this is where the microbes are most active. While an increased treatment zone is beneficial, transpiration must also be viewed with caution, as together with evaporation, it can lead to a concentration of constituents within the water.
4.Habitat and food for microbial populations
Microbes can be considered the driving force of many pathways that are found in constructed treatment wetlands focused on using biogeochemical processes to treat constituents of concern. Plants affect the function of a treatment wetland by influencing the microbial community and therefore microbially catalyzed biogeochemical processes. Additionally,decomposing plant mater provides a source of nutrients and energy that s upport microbial metabolism, allowing for electrons to be generated which ultimately result in the transformation of metals and other compounds into less bioavailable forms which can then be incorporated into the soil.
5.Interaction with other wetland components
Figure 1A - Cattail roots with light colour indicating oxygen rich conditions.
Figure 1B - Dark Cattail roots indicating reducing and oxygen-deficient conditions.
As one example, depending on the associated soils and flow rates, the same plant ecotype can create either oxygen-rich or oxygen-deficient environments around their roots. This depends on factors such as soil characteristics, dead root decomposition, and the diffusion of oxygen from live roots (radial oxygen loss). These conditions can sometimes even be visually observed through the colour of plant roots. Often, when plant roots are light in colour, it indicates oxygen rich conditions (Figure 1A), while black colouration indicates oxygen-deficient (and reducing) conditions (Figure 1B). These inferences must of course be confirmed with in situ measurements and appropriate complimentary off-site testing.
Decaying plant matter also contributes to soil development through accretion, whereby decomposed plant material accumulates on the soil surface, essentially increasing the depth of the wetland soil over time.
Because of each of these attributes varies greatly between plant species (or even ecotypes), in order to have predictable and effective treatment in a constructed treatment wetland, it is imperative that a monoculture is used and that plant species are not intermixed. Not only does wetland plant species diversity serve as an attractant to wildlife (which is obviously undesirable when water is requiring treatment), but due to differences in plant characteristics and associated microbes, mixing of plant species within a treatment wetland can create competing biogeochemical processes, impeding treatment efficiency and predictability. Note, this is with the exception of aquatic bryophytes (mosses), which can be strategically incorporated into certain designs along with emergent or submerged macrophytes.
For further information about why plants are used in Constructed Wetland Treatment Systems (CWTSs), please consult the following sources which inform this blog post.
Haakensen, M., Pittet, V., Spacil, M.M., Castle, J.W., & Rodgers, J. H. (2015). Key aspects for successful design and implementation of passive water treatment systems. Oil, Gas, and Mining. February 27, 2015.