Team C: Project Web Page Template (For Display Only)
Note: This webpage is for instructional purposes only and the scenario described below is fictional.
A Comprehensive Overview of the Efficiency of Constructed Wetlands to Treat Agricultural Wastewater and Runoff
Katherine Zettel, Undergraduate in Plant Pathology
The Environmental Protection Agency (EPA) has implemented the use of constructed wetlands to filter runoff water before it is deposited in our lakes and streams. Having the desire to create a system that is the most beneficial to states that rely heavily on agricultural, the EPA has recruited a research task force to understand how constructed wetlands and agriculture interact, as well as analyze the relationship between the environment and the efficiency of constructed wetlands. Ultimately, how are constructed wetlands best utilized to achieve the most benefits and cost-effectiveness?
In the past, wetlands were filled in to maximize the usable space for more profitable returns. Within the last twenty-five years numerous researchers have demonstrated that wetlands provide many benefits to agriculture and have published findings on the ecosystem services wetlands provide. For example, integrated constructed wetlands (ICW) retain excess water, reducing the force of flooding (DeLaney, 1995) and retain nutrient rich organic matter (Brix, 1995).
The main feature of ICW, water filtration, has been found to remove nitrogen. In a comparison across climates, constructed wetlands were found to remove anywhere from 3 to 15 percent of nitrogen (Braskerud, 2002a) in some countries, to nearly 99% in others, depending on various factors (Diaz et al., 2012). A few studies conducted in Europe stated that ICWs were able to keep around 41 to 44% of phosphorus from escaping into lakes and streams (Braskerud, 2002b; Uusi-Kamppa et al. 2000). There are even instances of contaminates such as pesticides, heavy metals, and livestock wastes being alleviated from nearby water sources. However, through comparison with other best management practices, we show that ICW may not be the most efficient approach to accomplishing these ecosystem services.
Furthermore, this paper balances the economical and ecological costs with benefits of constructed wetlands. The costs can vary greatly because constructed wetlands need to be uniquely constructed for each environment and farmland. While they can be costly build and maintain, they are less expensive than livestock waste treatment facilities, and there can be government incentives available to help with the costs.
Agricultural efficiency is essential to the global economy. However, the current trend of highly industrialized farming has resulted in runoff witha high concentration of essential nutrients, sediment, and chemicals. These components, which are necessary to agriculture, are instead being deposited into public bodies of water where the high concentrations increase pollution and result in natural ecosystems being overrun by algae.
A range of best management practices (BMPs) utilizes natural water filtration through soil to reduce the nutrient and soil loss from farmland. To improve this system, constructed wetlands have been created to intercept crop and livestock waste streams from point and non-point sources. Since their implementation, published research on constructed wetlands claim that this practice can increase ecosystem services beyond managing water quality.
However, constructed wetlands need to be precisely designed in order to reach full potential. While this BMP can be a highly effective tool in the sustainability of agriculture, it can be costly to develop and properly maintain. This paper questions how constructed wetlands benefit agriculture; what ecosystem services they may provide, and how do these services make agriculture more efficient. We also examine the economic and ecological costs and trade-offs associated with constructed wetlands to conclude if constructed wetlands are the most efficient practice to achieve desired benefits.
Below is a video from The Leopold Center at Iowa State university produced for M&M Divide Resource Conservation & Development (RC&D) that further explains the benefits of constructed wetlands, introduce you to how they are constructed, and explain how they work.
We hypothesize that constructed wetlands and integrated systems of constructed wetland provide addition ecosystem service that make them worth the investment. To address this idea, we examine:
- How can constructed wetlands benefit agriculture?
- What ecosystem services do wetlands provide, and how do these services make agriculture more efficient?
- What are the economic and ecological costs and/or trade-offs associated with constructed wetlands?
- Are constructed wetlands the most efficient practice to achieve specific benefits (sediment and nutrient management, reduction of contaminated runoff)?
In order to compare both ecological and economical costs and benefits, we conducted a literary analysis of previously published material on this subject. We researched an expansive amount of peer-reviewed and academically published journals and articles that describe various results of the systematic impact of constructed wetlands. The studies were specifically chosen because they highlight two important factors:
Benefits and Impacts
The articles chosen provide a representative sample of the impacts of constructed wetlands. Several articles claiming similar benefits and detriments of wetlands were compiled and compared to one another in order to provide supporting proof of the studies achieving desired impacts.
Articles also demonstrate the differences in performance under various seasonal and spatial scenarios. We focus on studies in both warm and cold climates, and also identified results differentiating between the effects of irrigated versus non-irrigated field discharge nutrient concentrations.
To complete our comparative analysis of the costs and benefits considering these two factors we reference 40 peer-reviewed studies that identify several specific variables.
To explicitly reference the studies compiled for each variable above we have created a table that summarizes the number of the articles used and their primary authors.
Environmental Impact of Agriculture
Due to the use of manures and commercial fertilizers, agricultural runoff often contains high levels of nutrients. These nutrients are lost to streams and lakes via runoff. While the amount lost varies due to several variables, animal agriculture in particular is tied to excess nutrient pollution, which is widely known to contribute to degraded water quality and eutrophication (Cronk, 1996;Darwiche-Criado et al., 2017). North Carolina demonstrated a loss rate of up to 26 percent of the nitrogen and 3 percent of the phosphorus in a watershed with row crop and swine agriculture indicating that a large amount of the nitrogen applied fields ends up in nearby surface waters (Cronk, 1996).
Relationship Between Agriculture and Wetlands
Until recently, wetlands were viewed as wasted space. In order to convert this area into something with clear economic value, many wetlands have been drained and filled. (Brix, 1995;DeLaney, 1995) According to the NRCS (1995) wetlands were lost primarily to development, agriculture, and deep water habitat. Recently, scientific studies have increasingly emphasized the importance of wetlands, which can help control water flow during heavy rain and flooding. This is especially important in agricultural fields, where uncontrolled streaming can be detrimental yields (DeLaney, 1995). Wetlands can store excess water, organic matter, and nutrients (Brix, 1995). However, agriculture can be harmful to natural wetlands as excess nutrients and sediment can alter native vegetation and macroinvertebrates, which are crucial to systematic functions (DeLaney, 1995). Use of constructed wetlands could not only mitigate agricultural runoff: It can also protect natural wetlands and the services they provide, but the efficiency of constructed wetlands for agriculture depends on factors such as soil type, fertilization patterns, crops and wetland vegetation, and the local climate (Comin et al., 2014), and location in the watershed (DeLaney, 1995).
Other Best Management Practices
Filter Strips/Buffer Strips
While buffer zones are able to slow surface water runoff and allow for greater infiltration and sediment deposition, under high flow conditions, they are generally not able to remove sediments or dissolved phosphorus in the water column (Uusi-Kamppa et al., 2000).
Grassed waterways utilize dense perennial vegetation as an intercrop strip to drain what would be concentrated water flowage through fields. This disrupts the process of soil erosion into gullies, and filters nutrients from water that would otherwise run off into farmland streams. Grassland waterways can cost between $2,000 to $3,000 per acre to implement for shaping and seeding (Lewandowski et al., 2015).
Constructed Wetlands as a Best Management Practice
Wetlands serve a key function in connecting land and water ecosystems (Brix, 1995). They regulate high water conditions and excessive runoff, to weaken the effects these ecosystems can have on one another (DeLaney, 1995). They provide numerous services such as water filtration and storage; and wetlands provide habitat as well as aesthetically pleasing green space (Everard et al., 2012). Wetlands can also supply ecosystem services, the most obvious being water treatment. Constructed wetlands have been shown to remove suspended solids, stabilizing dissolved oxygen and biological oxygen demand (BOD) in the water, and reduce the nutrient levels in runoff (Brix, 1995;Cronk, 1996).
For the purpose of this paper we group together most types of wetlands, however there are multiple types. Palustrine emergent wetlands are generally considered the most effective at water treatment (Harrington & McInnes, 2009) as heavy vegetation and anaerobic conditions allow for greater nutrient uptake, denitrification, sediment deposit, and contaminant filtration. Some of the studies included contrast between in-stream and off-stream; surface flow and subsurface flow; and individual versus integrated systems of wetlands. Each type has different design standards, and varying levels of efficiency at removing nutrients, suspended solids, and other contaminants. Furthermore, this paper focuses on constructed wetlands that are “intentionally created from nonwetland sites to produce or replace natural habitat” (Brix, 1995). We focus here because natural wetlands are not suited for the concentrated runoff of agriculture fields and as a result may be altered or harmed. A constructed wetland system is designed for greater ability of water treatment, and simultaneously may protect existing natural wetlands by taking on water with higher concentrated runoff (Brix, 1995). The use of stand-alone constructed wetlands can result in sedimentation and the need for maintenance and dredging, so recent design efforts have focused on developing integrated constructed wetland (ICW) systems which providing numerous ecosystem services while also mitigating the impacts of wastewater on wetlands by allowing for filtration through multiple basins (Everard et al., 2012). This systems approach improves water treatment as well, since the use of multiple basins offer more avenues of contaminant removal.
Table 1: Adapted from Harrington, R. & McInnes, R. (2009) Relative magnitude (per unit area) of ecosystem services derived from sixteen ICWs in the Annestown stream catchment (based on the methodology used in Millennium Ecosystem Assessment (2005). (Scale: 1 low; 2 medium; 3 high.)
Source: Harrington, R. & McInnes, R. (2009). (See Table 3).
|Food||Production of fish, wild game, fruits, grains and rhizomes||2|
|Fresh water||Storage and retention of water for domestic, industrial, and agricultural use||3|
|Fuel and Fiber||Production of fuelwood, peat, fodder||2|
|Biochemical||Extraction of medicines and other materials from biota||1|
|Genetic Materials||Genes for resistance to plant pathogens, ornamental species, etc.||2|
|Climate regulation||Source of and sink for greenhouse gases; influence temperature, precipitation, and other climatic processes||3|
|Water Reguation||Impeding/regulating hydrological flows to surface water; groundwater recharge/discharge||2|
|Water purification and waste treatment||Retention, recovery, and removal of excess nutrients and other pollutants||3|
|Erosion Regulation||Retention of soils and sediments||3|
|Natural Hazard regulation||Flood control, storm protection||2|
|Pollinators||Habitat for pollinators||2|
|Soil Formation||Sediment retention and accumulation of organic matter||3|
|Nutrient Cycling||Storage, recycling, processing, and acquisition of nutrients||3|
Benefits of constructed wetlands
Surface Water Quality
According to Braskerud, 2002a, through sedimentation retention and denitrification constructed wetlands are able to remove nitrogen from the surface water. However, several studies indicate that that these processes are less efficient at colder temperatures.
In studies of Norway and Sweden, nitrogen removal rate was found to be between 3% and 15% (Braskerud, 2002a; Arheimer et al., 2004). Comparatively, research done by Beutel et al. in 2009 at constructed wetlands in the Yakima Basin found rates of 63% removal of nitrogen, and in the San Joaquin Valley Diaz et al., (2012) found removal rates ranging from 22% to an impressive 99%. While there is a noticeable positive relationship between temperature and nitrogen removal rates, other factors will affect the efficiency of nitrogen removal including wetland vegetation and design. Another factor in determining the possible benefit of constructed wetlands is inflow concentration of nitrogen as differing concentrations in runoff alters the efficiency of nitrate removal. In off-stream wetlands, the non-irrigated concentrations ranged from 18.45-59.35 mg/L, while the irrigated concentrations averaged 12.2-27.74 mg/L (Darwiche-Criado et al., 2017). Similarly, for in-stream wetlands, non-irrigated concentrations were 10.1-38.32 mg/L and irrigated concentrations of 13.91-49.38 mg/L. These findings indicate that wetlands provide diminishing returns with higher input concentrations of nitrogen.
Braskerud, 2002b found that constructed wetlands in Norway were able to remove 44% of phosphorus in agricultural runoff, and interestingly rate of removal increased with flow rate. This indicates that phosphorus removal rates are efficient even in high flow or high water conditions. These results confirmed findings from Uusi-Kamppa et al. 2000 who found that in Norway, Sweden, Finland, and Denmark field total phosphorus losses ranged from 0.0002 - 1.89 kg/ha; of which constructed wetlands were able to successfully absorb up to 41%.
Constructed wetlands are also able to remove less common pollutants such as heavy metals, hydrocarbons, bacteria, and small amounts of pesticides and herbicides which enter surface water through erosion, runoff, spray drift, leaching, and atmospheric deposition (Brix, 1995; Vymazal & Brezinova, 2015).
Mitigating Livestock Waste
Constructed wetlands can be specially designed to meet the needs of wastewater treatment in animal agriculture. While amount and makeup of waste can vary, ICWs have already demonstrated their ability to remove nutrients and fecal contaminants from the water (Harrington & McInnes, 2009). However, in order to properly treat the wastewater, it cannot contain toxic levels of contaminants, the wetland has to be both large enough to accommodate the amount of wastewater produced, and deep enough to ensure anaerobic conditions. (Harrington & McInnes, 2009).
Due to the high levels nutrients, solids, and organic content in livestock wastewater, constructed wetlands used to filter this liquid will need additional maintenance in the form of harvesting vegetation to protect against the decline of nutrient uptake (Healy et al., 2007). Also, constructed wetlands may not be able to fully replace manure storage facilities, as it seems pretreatment is still a necessary factor of manure management in the ICW process (Cronk, 1996). Furthermore, it may be necessary to install sub-surface constructed wetlands, which are slightly less effective at removing nitrogen, but can still filter organic matter, suspended solids, and bacteria. (Healy et al., 2007).
In dairy operations, the wastewater produced by cleaning milking parlors has high levels of organic content (fat and sugar) and biological oxygen demand (BOD) (Cronk, 1996). High levels of BOD can result in depleted levels of dissolved oxygen (DO), which needs to be treated to prevent anoxic conditions that could have adverse effects on aquatic communities. Research on the efficiency of constructed wetlands for dairy wastewater in New Zealand demonstrated that the removal of BOD was approximately 80 percent, fecal coliforms were reduced by more than 99 percent, and 75-85 percent of suspended solids were successfully removed from the wastewater at the outflow of the wetland (Tanner et al., 1995).
In swine operations, the manure has high concentrations of nutrients, resulting a higher level of treatment to improve the water quality (Hammer, 1992). Zhang et al. (2017) measured the input of nitrogen flow into an ICW system in Hunan Province, China and found an average concentration of 635.4 mg/L of total nitrogen.
Because constructed wetlands are often used to treat wastewater, they may not be able to offer the same ecosystem services as natural wetlands. Habitat maps and flora/fauna surveys near constructed wetlands in Southern Italy showed a reduction in ecosystem services and degradation of habitat (Semeraro et al., 2015).
One of the most beneficial ecosystem services provided by wetlands, aside from nutrient and sediment removal, is flood attenuation (DeLaney, 1995;Harrington & McInnes, 2009). ICW systems have the potential to increase water storage. A review of wetland systems in Brazil have demonstrated that major flooding through the summer and fall, is absorbed by the system of natural wetlands, which then slowly release clean, filtered water into smaller streams throughout the year (Hammer, 1992).
Wetlands promote greater biodiversity, and increase species richness, which increases the resilience of ecosystems (Harrington & McInnes, 2009; Strand & Weisner, 2013). In addition to the species that thrive within the wetland itself, constructed wetlands improve the habitat and population of macroinvertebrates near intensively agricultural ecosystems (Becerra-Jurado et al., 2012). Also, in a study constructed wetlands were home to 31 species of breeding bird populations, including 9 threatened species. In Swedens, 80 percent of the wetlands studied provided habitat to at least one of the six notable amphibian species, some of which were removed from the red list or moved to a lower threat category (Strand & Weisner, 2013).
Costs and Tradeoffs
Costs of constructed wetlands vary based on environment, incentives, and the characteristics of the farmland, and if the amount of land that needs to be purchased. Arheimer et al., (2004) indicated that the average cost of implementing constructed wetlands in Sweden was approximately $1.7 million (SEK) in the early 2000s. Based on the resulting 5% nitrogen reduction, the cost-effectiveness of constructed wetlands was low. In comparison, using other best management practices were over half a million dollars less expensive to implement and removed six times more nitrogen (86 tons per year) than the wetlands (Arheimer et al., 2004). Constructed and restored wetlands in the Flumen River watershed cost an average of $5000 US per hectare (Comin et al., 2014). However when it comes to treatment facilities for livestock manure, constructed wetlands could treat the waste at a fraction of the cost. Regarding a study of ICWs in Anne Valley, most farmers, business owners, and local residents support the use of wetlands, do to the lower cost of treatment services, do to funding from outside sources. If the government were to provide funding for ICWs, most residents believed that even greater economic and environmental benefits would be seen (Everard et al., 2012).
While constructed wetlands can have obvious benefits, some preventative BMPs may be more efficient at certain services. For instance, Arheimer et al., (2004) found that a combination of BMPs were far more efficient at removing nitrates than constructed wetlands. No-till agricultural practices prevent soil loss which itself can reduce nutrient pollution and sedimentation (DeLaney, 1995). Compare this to sedimentation removal from wetlands, which if they trap too much sediment can have impaired flood attenuation and water quality services (DeLaney, 1995). To avoid this, wetlands need to be maintained far more than BMPs that keep soil on the fields. Wetlands also take up more land than some other BMPs. Comin et al., (2014) claims the land needed to treat wastewater for nitrates is proportional to the amount of nitrates coming in. Loss of farming land is a difficult tradeoff for farmers struggling to increase yields and make a living.
Greenhouse Gas Emissions
Due to the microbial process that facilitates denitrification, constructed wetlands release methane as a result of organic matter decay. Methane is a greenhouse gas with approximately 23 times the warming potential of CO2. Globally, agricultural wetlands are estimated to release approximately 11% of the total methane emissions, or twice the methane released through livestock waste (Tanner et al., 1997). Pilot studies of methane released from constructed wetlands in New Zealand revealed wide variability in emissions, ranging from 45 to 526 m2/day (Tanner et al., 1997).
Constructed wetlands are versatile, and can be manipulated to work in different climates, to treat different impacts of agriculture, across many disciplines of agriculture. Because of this diversity there are several factors that affect the efficiency and benefits that constructed wetlands have to offer. Therefore, in our research we were unable to single out variables in a way that they could be easily compared across all platforms. For this reason, we present each of the costs and benefits of constructed wetlands separately.
Furthermore, we were unable to find research that alluded to any future effects that removing nutrients and sediment from runoff could have on ecosystem services such as potential reductions in green house gas emissions, or any of the benefits of expanding ecosystem services that are measured qualitatively.
Finally, while we found multiple studies that came to similar conclusions on the benefits and efficiency of constructed wetlands, the studies did not include algorithms or relationships in the literature. For instance, we know that the rate of denitrification slows down at colder temperatures, but we could not identify a rate coefficient that would allow us to predict the rate of denitrification because there are so many other factors acting on each wetland system. For this reason, we can provide only generalized findings for our benefits and costs.
Our research has lead us to believe that creating constructed wetlands are able to provide agricultural runoff treatment as well as provide some of the ecosystem services that have been depleted by filling in natural wetlands. Constructed wetlands can be modified to work at their maximum efficiency depending on the shape and needs of each agricultural production space. It is imperative to realize that while constructed wetlands can reduce many types of water pollutants there is no singular design or system that can be utilized for maximal results in all conditions. For example, the system to remove phosphorus from run off (sedimentation) requires different conditions than denitrification, and by manipulating the wetland for one environment or the other you may reduce the effectiveness of other key services such as flood attenuation.
Arheimer, B., Torstensson, G. & Wittgren, H. B. (2004). Landscape planning to reduce coastal eutrophication: Agricultural practices and constructed wetlands. Landscape and Urban Planning, 67, 205-215
Becerra-Jurado, G., Harrington, R. & Kelly-Quinn, M. Hydrobiologia (2012) 692: 121. https://doi-org.ezproxy.library.wisc.edu/10.1007/s10750-011-0866-2
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Comin, F. A., Sorando, R., Darwiche-Criado, N., Garcia, M. & Masip, A. (2014). A protocol to prioritize wetland restortion and creation for water quality improvement in agricultural watersheds. Ecological Engineering, 66, 10-18.
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Darwiche-Criado, N., Comin, F. A., Masip, A., Garcia, M., Eismann, S. G. & Sorando, R. (2017). Effects of wetland restoration on nitrate removal in an irrigated agricultural area: The role of in-stream and off-stream wetlands. Ecological Engineering, 103, 426-435..
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Healy, M. G., Rodgers, M. & Mulqueen, J. (2007). Treatment of dairy wastewater using constructed wetlands and intermittent sand filters. Bioresource Technology, 98, 2268-2281.
Lewandoski et al., (2015). Managing sediment and water. Fields to streams: Managing water in rural landscapes; University of Minnesota Extension. 16-37.
Semeraro, T., Giannuzzi, C., Beccarisi, L., Aretano, R., De Marco, A., Pasimeni, M. R., Zurlini, G. & Petrosillo, I. (2015). A constructed treatment wetland as an opportunity to enhance biodiversity and ecosystem services. Ecological Engineering, 82, 517-526.
Strand, J. A. & Weisner, S. E. B. (2013). Effects of wetland construction on nitrogen transport and species richness in the agricultural landscape – Experiences from Sweden. Ecological Engineering, 56, 14-25.
Tanner, C. C., Adams, D. D. & Downes, M. T. (1997). Wetlands and aquatic processes: Methane emissions from constructed wetlands treating agricultural wastewaters. Journal of Environmental Quality, 26(4), 1056-1071.
Tanner, C. C., Clayton, J. S. & Upsdell, M. P. (1995). Effect of loading rate and plantings on treatment of dairy farm wastewaters in constructed wetlands – I. Removal of oxygen demand, suspended solids and faecal coliforms. Water Resources, 29(1), 17-26.
Tournebize, J., Chaumont, C. & Mander, U. (2017). Implication for constructed wetlands to mitigate nitrate and pesticide pollution in agricultural drained watersheds. Ecological Engineering, 103, 415-425.
Uusi-Kamppa, J., Braskerud, B., Jansson, H., Syversen, N. & Uusitalo, R. (2000). Buffer zones and constructed wetlands as filters for agricultural phosphorus. Journal of Environmental Quality, 29(1), 151-158.
Vymazal, J. & Brezinova, T. (2015). The use of constructed wetlands for removal of pesticides from agricultural runoff and drainage: A review. Environmental International, 75, 11-20.
Zhang, M., Luo, P., Liu, F., Li, H., Zhang, S., Xiao, R., Yin, L., Zhou, J. & Wu, J. (2017). Nitrogen removal and distribution of ammonia-oxidizing and denitrifying genes in an integrated constructed wetland for swine wastewater treatment. Ecological Engineering, 104, 30-38.
This project would not have been successful without the contributions of the outstanding students in our Food Systems, Sustainability, and Climate Change class. We would particularly like to acknowledge the wonderful and challenging questions, and the specific knowledge that students with different areas of expertise provided.