Catchment News

Reanimation of functional wetlands and application of the ‘Integrated Constructed Wetland’ (ICW) concept

River basins typically comprise a diversity of catchments that intercept precipitation. Each catchment is circumscribed by topographical boundaries involving a series or parallel set of functional ecosystems. These receive water that subsequently percolate to ground, flow to receiving waters, absorb, evaporate or transpire to the atmosphere. Together the geomorphology, pedology, topography, and associated biology of each ecosystem, especially that of vegetation, which play key roles in catchment hydrology, and subsequently effect associated receiving waters. More than any other European country, Ireland has suffered great diminutions of its two once dominant habitat types of primary importance to its catchment ecosystems: forests and wetlands. This tragic largely unnoticed integral loss persists, in spite of such habitats’ substantial social, economic and environmental importance being repeatedly alluded to by oversight bodies such as the UNEP/FAO and Ramsar.

In the first of three articles on the roles that forests and wetlands play in the hydrology of catchments (2016. EPA Catchments Newsletter Issue 4.), the authors presented an overview of their sustainable capacities to sequester carbon and to intercept, transform, recycle and retain water-vectored contaminants – both pollutants and nutrients. This second article focuses on reanimating functional-wetlands with special reference to the applying the Integrated Constructed Wetland (ICW) concept.

Lost habitats generally and forests and wetlands in particular, can be functionally reinstated by reanimating their basic structure, vegetation and supporting constituents – key facts needing to be acknowledged.

Reanimated wetland ecosystems, most particularly those that are shallow and have a marsh-fen structure, are best appreciated when appropriately landscaped and designed for intercepting known ‘contaminated’ water sources. As known by experience and experiment, these may include severely burdened sources such as animal slurry and acid-mine drainage and less challenging sources such as sewage (also combined with storm-water), industrial waste waters, and lightly contaminated land drainage, road and urban sources.

Manifold benefits of forestry have been made evident and accepted over the past 100 years in Ireland and other largely deforested European countries. On the other hand, and irrespective of replicated experiment and operational demonstrations over the past six decades, wetlands of various designs continue to struggle to gain wider sorely needed acceptance. This is mainly because ‘functional’ wetlands have mostly been designed solely to achieve maximum performance for the treatment of influent waters typically by minimizing the land area used. As might be expected, other implicit benefits such as comparative ease in operational management, aesthetics and biodiversity are generally acknowledged. But just like forests, wetlands can deliver many more benefits beyond their primary function. Moreover by adopting an explicit ecosystem/catchment based approach to water management – as done over the past 21 years with the ‘Integrated Constructed Wetland’ (ICW) concept implemented in the Dunhill/Annestown catchment of south County Waterford and elsewhere, a comprehensive range of additional benefits and ecosystem services beyond efficacious water management alone has been achieved. Furthermore this optimal approach, not attempting to maximize, yields a more robust infrastructure for coping with the unexpected – such as increased flows generated by severe weather events, as recently experienced in the northwest of the Country.

Endorsed by more than 80 science journal papers and presentations at relevant professional symposia including those by VESI Environmental, the results of this integrated wetland approach comprehensively demonstrate the intrinsic social, economic and environmental benefits of such an ecosystem based enterprise. Paradoxically, and for many perhaps counter-intuitively, whereas wetland reanimation may be considered the antithesis of drainage, it can be complementary to it by intercepting both point and diffuse drainage waters before they discharge to receiving waters such as streams, rivers, lakes and inshore waters. Such interception attenuates flow – a key factor in preventing flooding, and helps remove ‘emerging pollutants’ that might otherwise evade conventional water treatment. There is clear evidence that by reanimating wetlands designed for water treatment and flow purposes, many urgent social, economic and environmental needs can be met, particularly the chemical, physical and biological integrities of the Country’s surface and ground waters – Ireland’s river basins.

Marsh-fen type wetlands, typically shallow (less than 200mm deep) and densely vegetated by emergent/helophyte species, simultaneously support both aerobic and anaerobic aquatic environments. Consequently they nurture one of the widest ranges of biogeochemical processes for any habitat type. The transformative capacity of such dual simultaneously-existing microbial assemblages is truly vast, given their 4 billion year evolutionary background. They competitively and symbiotically ‘digest’ through-flowing substrates and are limited by few ambient conditions. The helophyte vegetation that dominate these habitats have specifically evolved to marsh-fen environments and feature many significant adaptive characteristics not found in terrestrial vegetation. Together microbial assemblages and vegetation have the combined capability of treating through-flowing water, sequestering nutrients and carbon, and volumetrically attenuating surface flow. When configured and sized to meet a specific landscape, they can be relied upon to provide long periods of service. This is well demonstrated by Integrated Constructed Wetlands (ICWs) constructed more than two decades ago which continue to function as designed and are, with minimal management, expected to have a functional life-span of more than 100 years. Such prolonged functional life-span has further economic and environmental significance due to the sustained capacity to sequester carbon at rates greater than that found in possibly any other ecosystem type due to their high primary productivity and phenol inhibited decay (typically 15 tonnes of organic matter (dry weight) per hectare per year).

As natural and reanimated wetlands tend to have well defined catchment boundaries, they lend themselves to detailed hydrological and ecological study (Society of Wetland Scientists – http://www.sws.org/europe-chapter). The ICW concept continues to provide further insights into the many benefits of wetland-intercepted water management and wider opportunities for their application and delivery.

Natural marsh-fen type wetlands may form wherever water slows on its passage to the sea. They, like most other natural habitats, are the product of geological happenstance.

Understanding the factors that contribute to their formation points to how they can be reanimated wherever conditions allow. The main factors fundamental to ICW systems are:

• Underlying soils and ground conditions limiting through-flow exfiltration and permitting construction.
• Influent toxicity limiting/influencing vegetation growth at the point of inflow.
• Hydraulic loading determining the area of wetland required and its configuration to treat the influent, including that from precipitation.
• Topography and the challenges of constructing level ground.
• Attenuation capacity of receiving waters, whereby the expected discharge concentration of specific effluent parameters from the wetland are achieved. Zero surface discharge is possible with the required wetland area and appropriate vegetation cover.
• Surrounding landscape and ecology – to meet the aesthetic potential of the surrounding area and habitats.
• (Additional information on the above six factors are to be found at: http://www. housing.gov.ie/sites/default/files/migrated-files/en/Publications/Environment/Water/ FileDownLoad%2C24931%2Cen.pdf )

Detailed studies show that for optimal function, ICW systems need to be divided into a ‘cascading sequence’ of separate wetland cells through which influent water may flow. This minimises priority flows and sequentially decreases eutrophic conditions for each subsequent cell, thus capturing, retaining and degrading influent contaminants with increasing effectiveness. The accumulated nutrients and associated organic matter of the most eutrophic wetland cells may be dewatered, composted and their contents used in various ways. The configuration of each wetland cell is optimal when curvilinear, and has a similar length to width ratio. The anatomical structure and transpiration capacity of the dense vegetation in each cell progressively reduces water through-flow, thereby achieving both concentration and mass reductions of influent contaminants, as well as contributing to flood attenuation (Figure 1). A schematic diagram of ICW layout showing how the flow-attenuating freeboard between wetland cells is achievable, along with their indicative hydraulics, are shown in Figure 2.

ICW systems have been effectively deployed to treat a wide, often challenging, range of effluent types (Figure 3). Their benefits are found in each location but excel when collectively considered within a catchment. Applied over 21 years in combination with the ecological reanimation of the Dunhill/Annestown stream in Co, Waterford, some 20 ICW systems have contributed both to the natural return of salmon and sea trout, and to the lessening of flooding – by about 1m at the lower reaches of the catchment. This work has also provided many other benefits for the local community, including multi-use riparian woodland corridors, infrastructure for the creation of a high-use amenity/exercise track, and infrastructure allowing wider appreciation of the aquatic environment.

Wetland economics is becoming ever more to the fore as water management including wastewater issues, become more demanding. These along with flood attenuation-needs, which were recently debated for Cork City, gain in importance. Water management is clearly a land use issue, the economics of which are, as shown in the many case studies undertaken by the authors and others, indicative of high economic return on both the investment needed in land, and in their construction, making in many instances the reanimation of wetlands one of optimal land use and profitability.

The more society becomes aware of what water vectors, the more relevant wetland functions become. Water from land drainage, built environments, and products of enterprise such as farming, industry, mining and forestry, requires more effective management than it is presently implemented. Experience shows that this can be achieved by acknowledging the role that functional wetland reanimation demonstrated by the Integrated Constructed Wetland (ICW) concept can have, and that this concept can play a marked positive role in the economy and environment of the Country for the long-term wellbeing of its people (Figures 4 and 5).

Aila Carty, Caolan Harrington, Rory Harrington, VESI Environmental Ltd. Little Island, Cork & Dunhill, Co. Waterford