Note: This webpage is for instructional purposes only and the scenario described below is fictional.
The Benefits and Tradeoffs in Treating Agricultural Runoff
Katherine Zettel, Undergraduate in Plant Pathology
The Environmental Protection Agency (EPA) is recommending the implementation of constructed wetlands to filter runoff water before it is deposited in our lakes and streams. Wanting to create systems that are the most beneficial to states that rely heavily on agricultural, but experience different climates, 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?
Our research focuses of providing information and understanding about constructed wetlands and how they can reduce the environmental footprint of agriculture. This is a well studied topic, however most of this research is focused on nutrient removal, and gloss over additional factors such as social and ecological benefits. Our findings highlight multiple other benefits such as pesticide removal, increased habitat and biodiversity, and spiritual and aesthetic value. Furthermore, those who implement constructed wetlands may experience setbacks such as monetary construction and opportunity costs, greenhouse gas emissions, and relative sustainability of these wetlands. By analyzing a greater range of costs and benefits provided by wetlands and assessing how they perform in various ecoregions, we hope to provide an overview of considerations that can aid farmers in decision-making.
Agricultural efficiency is essential to the global economy. However, the current trend of highly industrialized farming has resulted in runoff with a 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) utilize 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, they can be costly to develop and properly maintain. This paper questions: What are the benefits and costs associated with constructed wetlands?
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 wetlands provide additional ecosystem services that make them worth the investment. To address this idea, we ask what are the social, economic, and ecological costs and benefits associated with constructed wetlands?
Environmental Impact of Agriculture
After years of manure and fertilizer application to our soils, the nutrients that we try to incorporate into our agriculture is washed away by runoff into streams and lakes. While the amount lost varies due to several variables, animal agriculture is known to cause excess nutrient pollution, causing degraded water quality and eutrophication.
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. 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. 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, and location in the watershed (Comin et al., 2014).
Other Best Management Practices
There are many management practices that save farmers time and money while trying to avoid these externalities. Many practices aim to prevent soil erosion, protect water quality, preserve topsoil, and nutrients for future crops.
While these practices focus on holding soil in place, other management practices aim to capture soil once erosion has occurred. There are three main management practices that are designed to reduce sedimentation. Constructed wetlands, the focus of this paper, are one of the recommended methods of capturing sediment, nutrients, and other contaminants. Filter strips and grassed waterways perform many of the same functions as constructed wetlands, so they will be used here for comparison.
Filter Strips/Buffer Strips
Buffer strips and filter strips are used to slow surface water runoff in order to allow for greater infiltration and sediment deposition However, under high flow conditions, they are generally not able to remove sediments or dissolved phosphorus (Lewandowski et al., 2015, Uusi-Kamppa et al., 2000).
Grassed waterways utilize dense perennial vegetation grass to route, slow, and drain what would be concentrated water flowage through fields. This disrupts the process of soil eroding into gullies, and filters nutrients from water that would otherwise run off into farmland streams (Lewandowski et al., 2015).
Wetlands serve a key function in connecting land and water ecosystems. They regulate high water conditions and excessive runoff, to weaken the effects these ecosystems can have on one another. They provide numerous services such as water filtration and storage, provide habitat for native species, and an aesthetically pleasing green space( Everard et al., 2012, Brix, 1995, DeLaney, 1995). 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 (Cronk, 1996).
This paper focuses on constructed wetlands that are “intentionally created from nonwetland sites to produce or replace natural habitat” (Brix, 1995). Natural wetlands are not suited for the concentrated runoff of agriculture fields. A constructed wetland system is designed for greater ability of water treatment and to protect existing natural wetlands by taking on water with higher concentrated runoff. On their own, due to high levels of sedimentation, constructed wetlands can need maintenance and dredging, which can be costly. So, recent designs have focused on developing integrated constructed wetland (ICW) systems which could provide numerous ecosystem services while also mitigating the impacts of wastewater on singular constructed wetlands by allowing for filtration through multiple basins.
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, and performance.
Benefits and Impacts
The articles chosen provide a representative sample of the impacts of constructed wetlands. Multiple studies claiming a variety of benefits and detriments of wetlands were compiled and compared to one another in order to provide potential supporting proof of the studies achieving desired impacts.
The studies also demonstrate the differences in performance under various seasonal and spatial scenarios. We focus on studies in multiple different ecoregions, 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 20 peer-reviewed studies that identify several specific variables.
We aim to describe provide general benefits that could be seen across different types of communities and ecoregions shown in Table 2.
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).
|Evaluation from the Mississippi Alluvial Valley
|Food||Production of fish, wild game, fruits, grains and rhizomes||2||$1435-$1486*/ha/yr.|
|Fresh water||Storage and retention of water for domestic, industrial, and agricultural use||3||$1435-$1486*/ha/yr.|
|Fuel and Fiber||Production of fuelwood, peat, fodder||2||n/a|
|Biochemical||Extraction of medicines and other materials from biota||1||n/a|
|Genetic Materials||Genes for resistance to plant pathogens, ornamental species, etc.||2||n/a|
|Climate regulation||Source of and sink for greenhouse gases; influence temperature, precipitation, and other climatic processes||3||$171-$222/ha/yr|
|Water Reguation||Impeding/regulating hydrological flows to surface water; groundwater recharge/discharge||2||$1435-$1486*/ha/yr.|
|Water purification and waste treatment||Retention, recovery, and removal of excess nutrients and other pollutants||3||n/a|
|Erosion Regulation||Retention of soils and sediments||3||$1435-$1486*/ha/yr.|
|Natural Hazard regulation||Flood control, storm protection||2||$1435-$1486*/ha/yr.|
|Pollinators||Habitat for pollinators||2||n/a|
|Spiritual & Inspiration||Source of inspiration, spiritual & religious values||3||$16/ha/yr.|
|Recreational||Opportunities for recreational activities||3||n/a|
|Aesthetic||Source of beauty & aesthetic value||3||n/a|
|Educational||Formal & information educational opportunities||3||n/a|
|Soil Formation||Sediment retention and accumulation of organic matter||3||$1435-$1486*/ha/yr.|
|Nutrient Cycling||Storage, recycling, processing, and acquisition of nutrients||3||$1127-$5240/ha/yr.|
Benefits of constructed wetlands
Surface Water Quality
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). 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).
As referenced in the table above wetlands can provide numerous social benefits. Several peer reviewed journals find key social benefits including: Spiritual, recreational, aesthetic, educational, cultural. (Harrington, R. & McInnes, R.,2009; Ma, S. & Swinton, S. M., 2011 ). Further, studies in the Mississippi Alluvial Valley considered multiple social and ecological services together. These included waterfowl habitat and other wildlife habitat services, along with flood attenuation, water storage, and sediment and contaminant retention
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.
(Image provided by NASA)
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, design, and inflow concentration of nitrogen as differing concentrations in runoff alters the efficiency of nitrate removal.
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 constructed wetlands were able to successfully absorb up to 41% of phosphorus runoff.
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. 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.
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 (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, due 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. 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
Constructed wetlands can be a source or a sink for greenhouse gases such and methane, carbon dioxide, and nitrous oxide. 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, which is twice the amount released from livestock waste (Tanner et al., 1997). Studies in New Zealand revealed wide variability in the methane emissions, from constructed wetlands 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 constructed wetlands are able to restore 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 runoff 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. Additional benefits could be realized if these practices were combined with other practices to hold soil in place, including conservation tillage and crop rotation, and strategic manure and nutrient applications.For more information on methods and research data please see:
French, F. & Zettel, K. (2018). Constructed Wetlands: The Benefits and Trade-offs of Agricultural Runoff. Dairy Sciences 471.
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
Beutel, M. W., Newton, C. D., Brouillard, E. S. & Watts, R. J. (2009). Nitrate removal in surface-flow constructed wetlands treating dilute agricultural runoff in the lower Yakima Basin, Washington. Ecological Engineering, 35, 1538-1546.
Braskerud, B. C. (2002a). Factors affecting nitrogen retention in small constructed wetlands treating agricultural non-point source pollution. Ecological Engineering, 18, 351-370.
Braskerud, B. C. (2002b). Factors affecting nitrogen retention in small constructed wetlands treating agricultural non-point source pollution. Ecological Engineering, 19, 41-61.
Brix, H. (1995). Use of constructed wetlands in water pollution control: Historical development, present status, and future perspectives. Water Science & Technology, 30(8), 209-223.
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.
Cronk, J. K. (1996). Constructed wetlands to treat wastewater from dairy and swine operation: A review. Agriculture, Ecosystems & Environment, 58, 97-114.
DeLaney T. A. (1995). Benefits to downstream flood attenuation and water quality as a result of constructed wetlands in agricultural landscapes. Journal of Soil and Water Conservation, 50(6), 620-626.
Diaz, F. J., O’Geen, A. T. & Dahlgren, R. A. (2012). Agricultural pollutant removal by constructed wetlands: Implications for water management and design. Agricultural Water Management, 104, 171-183.
Everard, M., Harrington, R. & McInnes, R. J. (2012). Facilitating implementation of landscape-scale water management: The integrated constructed wetland concept. Ecosystem Services, 2, 27-37.
Hammer, D. A. (1992). Designing constructed wetlands systems to treat agricultural nonpoint source pollution. Ecological Engineering, 1, 49-82.
Harrington, R. & McInnes, R. (2009). Integrated constructed wetlands (ICW) for livestock wastewater management. Bioresource Technology, 100, 5498-5505.
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.
Ma, S. & Swinton, S. M. (2011).Valuation of ecosystem services from rural landscapes using agricultural land prices. Ecological Economics, 70, 1649-1659.
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.
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.
Katie Zettel is an undergraduate at the University of Wisconsin-Madison pursuing a degree in Plant Pathology. In the future Katie hopes to pursue a career in plant heath and crop sustainability.