Invention Journal of Research Technology in Engineering & Management (IJRTEM) www.ijrtem.com ǁ Volume 1 ǁ Issue 11 ǁ
ISSN: 2455-3689
Engineered biosystem treatment trains: A review of agricultural nutrient sequestration Elizabeth Lien &Joe Magner Department of Bioproducts and Biosystems Engineering, University of Minnesota 1390 Eckles Ave, St. Paul, MN 55108
ABSTRACT: Nutrient pollution is a problem across the globe. Excess nitrogen(N) and phosphorus(P) are impacting lakes, rivers, and oceans with algal blooms, hypoxia, and fish kills. As such, there are many opportunities for intervening to protect receiving ecosystems from excess nutrients. Historic treatment options have failed to control nonpoint source pollution. New options for trapping and treating intensively managed cropland runoff (IMCR) are presented; with a wealth of wastewater treatment experience in removing N and P, innovation is spilling over into the IMCR world. Agricultural producers can use technology to increase productivity and decrease nutrient runoff to streams and lakes using trap and treat biosystems engineering technology. In-field cover crops and mycorrhizae can be employed to increase nutrient use efficiency. At fieldedge and beyond, riparian buffers (surface and subsurface), wetlands (natural and constructed), and varying forms of carbon bioreactors can be utilized for nutrient consumption and sequestration. Options to mitigate IMCR nutrient pollution occur best with landscape treatment trains. The treatment train approach is possible and needed for ecosystem health; however, the key issues are 1) pathway and process awareness, and 2) balancing who pays the cost for best management practices and who reaps the benefits. KEYWORDS: nitrogen, phosphorus, nonpoint source pollution, treatment train
INTRODUCTION Intensively managed cropland runoff (IMCR) occurs throughout much of the upper Midwestern region of the United States where, typically, more than 75% of the land is allocated to intensive corn and soybean production [1]. In altered landscapes with extensive subsurface drainage due to dense underlying soils, 90% of streamflow can come from IMCR [2]. Upper Midwestern United States (UMUS) landscapes typically have impaired aquatic life that is directly linked back to IMCR [3]. Influxes of the nutrients into aquatic ecosystems have deleterious impacts, as nitrogen and phosphorus are limiting factors for primary production in these systems. Thus, increases in nitrogen and phosphorus lead to increases in primary production, often in the form of blooms of algae and cyanobacteria. Algal blooms cover the surface of water bodies, reducing sunlight penetration through the water and potentially killing submerged aquatic vegetation (SAV). Without photosynthesis from SAV, supplies of dissolved oxygen in water bodies are reduced. In these nutrient rich ecosystems, oxygen levels are further depleted through the process of decomposition of dead SAV, algae, and bacteria. If dissolved oxygen levels fall too low (creating conditions of hypoxia), the water body will no longer be able to support life, creating a dead zone [4]. Farmers in the Upper Mississippi River Basin have learned about Gulf hypoxia, but it is not a rare, unique phenomenon to North America. The Baltic Sea has the largest dead zone in the world. Originally an oligotrophic sea, the Baltic is now a eutrophic marine environment. Agriculture has been acknowledged as the largest contributor of nutrients to the water body, thus the leading contributor to Baltic hypoxia [5]. Today, hypoxia in the Gulf of Mexico occurs where the dissolved oxygen levels seasonally fall below life supporting levels, creating a dead zone approximately the size of Connecticut, United States, due to the 1.6 million metric tons of nitrogen and 0.14 million metric tons of phosphorus that the area receives from the Mississippi River every year [6][7].Marine based enterprise has been adversely affected and the United States federal government has called for action [6].
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Based on the breakdown of nutrient sources to the Gulf of Mexico, more than 65% of N and 40% of P comes of IMCR [8]. Water Quality Assessment Reports submitted by states in the United States to the Environmental Protection Agency (EPA), 22% of bay/estuary areas, 16% of river miles, and 25% of lake areas assessed thus far in the United States are nutrient impaired, with nutrient-related impairment defined to include algal growth, ammonia, noxious aquatic plants, and organic enrichment/oxygen depletion. These percentages are likely underestimates, as state-assessed water bodies are not always evaluated for nutrient impairment specifically [9]. While action plans have been developed for reducing nutrient impacts on water bodies, most nutrient best management practices (BMPs) for farmers are not mandatory. Clean Water Act (CWA) regulation is not present in agricultural land runoff. BMPs need to be low cost, require little high value land, and necessitate little time input to make them attractive for adoption within the agricultural community. But first, it is important to step back and ask how nutrient imbalances develop over time in IMCR regions. In the early 20th century, German chemist Fritz Haber discovered a process for converting atmospheric nitrogen into ammonia. By 1914, the Haber-Bosch process was utilized to produce ammonium on an industrial scale [10].A century later, humans are responsible for converting 121 million tons of N2 from the atmosphere each year, more than all other terrestrial processes on earth combined [11]. Clearly, synthetic fertilizers have been pivotal for meeting global food demands. However, only about half of the agriculturally applied nitrogen is harvested with the intended crops; the other half of the agriculturally applied nitrogen is lost through leaching, erosion, and emissions [12]. Like nitrogen, phosphorus is an essential component of life. Early sources of phosphorus fertilizer, used to enhance crop production, included bone, guano, and manure. Today, phosphorus is mined from apatite and usually undergoes acidification so that the phosphorus can be converted to water soluble phosphate salts to be used as fertilizer [13]. Phosphate rock is a finite resource and reserves are speculated to run out in 50-100 years [14]. Nutrient pollution reduction from fertilizers could be achieved by applying the “4R’s” of nutrient stewardship: Right source of nutrients at the Right rate, Right time, and Right place [15]. However, this practice is easier said than done, and the 4R recommendation has been suggested for a long time [1].Why does the 4R recommendation fall short? The right source over time needs to be sustainable; this implies recycling of nutrients, especially phosphorus given the limited amount of apatite. Further, the wrong sources over the wrong places can have long-lasting impacts. Landscapes with karst features underlain by carbonate bedrock will typically be susceptible to nutrient movement into groundwater, springs, and streams. Crawford and Lee [16] show that karst-derived groundwater can be aged by chemical signatures linked to the onset of intensive use of nitrogen fertilizer. Spring, instead of fall, application of nitrogen reduces nitrate leaching by 14%. However, for various reasons, such as avoidance of unpredictable spring conditions and greater availability of labor, 25% of nitrogen is fall applied in the UMUS [15]. There will always be an upper limit to crop yield due to some limiting factor. Ideally, fertilizer application would be applied to achieve maximum efficiency: a rate that would optimize yield as well as profit by requiring the least amount of fertilizer, maximizing profits and minimizing environmental impact. This practice is not always espoused because over application of fertilizers is viewed by many as a relatively low-cost insurance policy for producers to ensure high yields regardless of external circumstances. However, for realistic nutrient management strategies, there must be many options available so that solutions can be tailored to specific sets of parameters. We will explore practices that seek nutrient sequestration beyond the practice of the 4Rs. NUTRIENT SEQUESTRATION AND TREATMENT Concentrated Nutrient Treatment: In the point source world, nutrient concentrations can be very high and
cause severe in-stream damage and loss of aquatic life [1]. A variety of engineered systems have been developed over time to provide advanced treatment of influent waters. These include processes of chemical precipitation with aluminum, a phosphorus sorbing material that provides a metal cation for the phosphorus to bind to and form an insoluble compound [17] or Zeolites. Zeolites are hydrated aluminosilicates with the structure of three dimensional honeycombs. They contain large cavities that can trap ions and molecules. Examples of phosphorus sorbing materials include iron fillings, steel wool, native iron rich soils, Drinking Water Treatment Residuals (WTR) that contain aluminum and iron hydroxide, and sorptive media [18].
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Biological removal can occur with both phosphorus and nitrogen; the process is driven by environmental conditions that cycle from anaerobic to aerobic for phosphorus, and vice-versa for nitrogen. However, these processes, given the lack of CWA regulation, are too costly to be used in IMCR lands. A treatment train is a sequence of conservation practices that cumulatively utilize, trap and/or treat nutrients along a hydrologic flow path for a given agricultural landscape. In IMCR landscapes the drainage catena can be defined as upland (where crop production occurs), edge-of-field (a transition zone) and riparian (non-cropped land adjacent to a stream, lake, or wetland). The treatment train approach goes beyond a single targeted practice at a given location on the landscape. Hydrologic dynamics drive a treatment train design; it is important to take a “systems thinking” approach to IMCR landscapes. The treatment train will vary; steep rolling terrain will be more prone to surface erosion and sediment attached phosphorus transport, whereas, flat poorly drained land may be developed with drainage pipe. In landscapes with advanced drainage, nitrogen leaching becomes a key environmental concern. Surface buffers do little to trap and treat a subsurface flow path. Conservation practices must be tailored to be effective and show incremental pollutant load reductions [19].This paper offers a suite of BMPs that can be placed in a series to form a treatment train beginning with in-field options to a waterbody of concern. Treatment Train: a less costly approach for watersheds:
IN-FIELD: NUTRIENT RUNOFF PREVENTATIVE MEASURES
Cover Crops: Employment of cover crops is a well-recognized agricultural BMP in the UMUS; however, more research is needed in cold climates to develop successful cover crops in states like Minnesota [20]. Cover crops have many potential benefits including prevention of wind and rain erosion, weed suppression, and soil fertility improvements. Cover crops are cover crops planted after harvest to reduce nutrient leaching [20]. Most of the crops grown in the UMUS are corn and soybeans, warm weather crops with a typical growing season from May through September. When the harvest of these commodities is completed, there is no longer a plant demand for residual and generated nutrients in the soil, allowing mobile nutrients to be leached from the root zone. By planting a cold weather crop such as rye, more nutrients are consumed rather than transported, in many cases reducing nitrate flux from fields by one third [20]. Given the many benefits of cover crops, they were only applied to 3% of UMUS farm acreage in 2012 [21]. In the Cover Crop Survey conducted by Sustainable Agricultural Research and Education, the main barrier to the adoption of cover crops was the perception of cost. Other concerns include cover crops making planting cash crop more difficult and cover crops reducing yields of cash crops. Given these qualms, the survey found from those currently utilizing cover crops that the average yield of corn increased 3.1% and the average yield of soybeans increased 4.3%after the employment of cover crops. Many producers achieved a return on their investment [22], not to mention the improvement of soil health. Possible considerations for choosing cover crops include root depth, carbon to nitrogen ratio, and plant genus. Plants with deep roots (such as radish), can sequester nutrients deeper in soil strata that may be used by the cash crop. The carbon to nitrogen ratio dictates the speed at which nitrogen will be released after crop termination. Finally, it is important to pick a cover crop that is a different genus than the cash crop so that there will be less concern about disease and pest carryover[23]. A cover crop is an essential component of a nutrient treatment train; without a cover crop, too much treatment pressure is placed on edge-of-field and riparian practices. Mycorrhizae: Mycorrhizae are soil fungi that form a symbiotic relationship with plant root systems. The plant provides the fungus with carbohydrates while the fungus increases the root surface area of the plant, allowing increased water, nutrient, and mineral uptake [24]. The most common type of mycorrhizae is arbuscular mycorrhizae (AM) which forms mutualistic relationships with 80% of all vascular plants, giving AM great potential for agricultural applications [25]. AM increases the nitrogen and phosphorus use efficiency of plants by increasing root surface area, helping the plants to “find” more nutrients; by increasing root zone, allowing plants to reach more nutrients; and by converting nutrients into usable forms. Plants are only able to uptake soluble phosphorus, but mycorrhizae secrete extracellular enzymes to dissolve phosphorus and make it available to the plant [26]. Less nutrient | Volume 1 | Issue 10 |
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could be applied to crops with a larger proportion being consumed by the plant; high yields can be maintained without impacting the producer, yet aiding the environment [25].While AM are naturally occurring in the soil, over time, IMCR has diminished the presence of mycorrhizae in many fields. Reviving mycorrhizae populations may require inoculated with AM fungi. These fungi respond best with no-till practices, as tilling disturbs soil aggregates and lessens soil fertility and nutrient cycling capabilities [25]. Aside from decreased nutrient demand and nutrient leaching, integrating mycorrhizae as a BMP into agricultural row crop management could provide numerous benefits. With AM, crops are less susceptible to damages from pathogens; this is accomplished through a combination of AM altering the root structures, changing microbial populations, and providing competition to harmful fungi [25]. With a healthy presence of mycorrhizae, water needs of plants are reduced 30%, increasing crop drought tolerance. Finally, with more extensive AM enhanced root systems, overall field soil structure is improved. A healthy soil is the best defense for IMCR pollution. Nevertheless, climatic conditions are changing and will lead to nutrient movement from field, thus a second line of defense is necessary and represents the next portion of the treatment train. RIPARIAN TRAP AND TREATMENT
Edge-of-field Bioreactors: Bioreactors in general can provide about a 47% nitrate reduction depending on contact time with the carbon source as water flows through 3-8 cm sized wood chips [27]. Christianson and others [28] found that a 20 mg/l nitrate concentration could be reduced by half with 13 to 14 hours of wood chip contact time. Wood chip bioreactors reduce high nitrate concentrations typically found in southern Minnesota and northern Iowa drained landscapes. However, based on current designs (50 to 80-m2) only small fields (10-20 Ha) can be treated effectively. Further, during spring and summer storm flow, more than 70% of the water runoff will bypass the bioreactor and discharge directly to a ditch or stream. The largest constraint is the cost; the cost per unit volume of water treated with a bioreactor is higher than any other nitrate reduction BMP. Another concern is sustainability; woodchips break down over time and would need to be replaced within 12-18 years depending on the type of wood used. Zhang and Magner [29] tested a caramelized hardwood chip and found similar yet slightly lower nitrate reduction in a lab column study. Mixing biochar into a woodchip bioreactor could extend the life of a system, but long-term performance studies are needed to adjust design criteria. At this point in time, bioreactors hold less nitrogen treatment promise compared to other denitrification options due to installation and future maintenance costs. Riparian Buffers: A riparian buffer is perennial vegetated landscape adjacent to a row-crop field. Along with nutrient removal, riparian buffers can provide benefits of flood mitigation, wildlife habitat, stream bank stabilization, and river shading. The vegetation is highly influenced by the adjacent water connectivity and often consists of sedges, native grasses, forbs, trees, and shrubs. Riparian buffers should be 10-m to 30-m wide on either side of the stream depending on the surrounding landscape [30]. For riparian buffers to be effective, flow rates within the buffer must be slow enough for sediment to settle, so it is important that flow does not become channelized [6]. Wood dominated buffer systems remove subsurface dissolved nitrogen flow when the groundwater flows past active plant roots [31], allowing the nitrate to be removed by denitrifying microbes living in the plant roots and, to a lesser extent, by plant uptake [32]. Riparian buffers have a typical nitrogen removal rate of 2 to 6-g Nm-2 year-1 [6]. Riparian buffers are however, less effective at nitrogen sequestration than treatment wetlands. For wetlands to have a significant impact on watershed nitrogen removal, 3 to 6-million Ha (0.7%-1.8% of the Mississippi River Basin) would need to be converted to wetlands, resulting in a removal of 300,000 - 800,000 metric tons of N/year from the watershed. To achieve the same nitrogen removal with riparian buffers, 9 to 22-million Ha (2.7%-6.6%) of the Mississippi River basin would be required [6]. We discuss wetlands in more detail below. Grass riparian buffers are effective at removing as much as 90-95% of sediment from overland runoff flow. Phosphorus is generally transported attached to sediment in runoff; by catching sediment in riparian buffers,~50% sequestration of total phosphorus--including dissolved phosphorus and sediment-attached phosphorus—is possible.
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Riparian buffers are not as effective at removing dissolved phosphorus because the concentration of phosphorus in runoff is often less than that dissolved in the sediment trapped by the buffer so frequently concentrations of dissolved phosphorus increase after passing through a buffer. Furthermore, for a grass buffer to be able to maintain sediment removal capabilities, and thus phosphorus removal, it must be regularly maintained [32]. Saturated Buffers: Recently, new concepts have emerged regarding drain-pipe outlet location and elevation with respect to a stream. If upland drainage occurs in a landscape with sufficient slope and marginal land exists between the edge of a field and the stream, drain-pipe water can be redirected into the riparian zone. Not every landscape will work for this practice; steeply sloped stream banks run the risk of geotechnical failure from added ground water pore pressure. With the diversion of a portion of the drain-pipe water into riparian soil, the water table may rise too high near the streambank to prevent bank erosion. However, if the riparian area is wide enough, rich in organic carbon, and the hydraulic conductivity of the soil allows for percolation into and through floodplain deposits allowing for reaction time, then nitrate reduction can be at or near 100% [33]. If the soil hydraulic conductivity does not allow for complete infiltration of drain-pipe water, then the surface landscape must have depressions or a created trench/swale to hold un-infiltrated water. This type of system is a hybrid between a surface wetland and subsurface soil treatment system being explored by the second author based on field observations. Selecting the perfect site for a subsurface treatment system may be difficult in some landscapes. Adding a measure of surface earth work to retain wetland storage, offers optimal flexibility in site selection and nutrient treatment. Treatment Wetlands: Wetlands produce many ecosystem services including habitat, groundwater recharge, flood control, nutrient cycling, and water quality improvement [34]. Natural wetlands are defined by their hydrology, soils, and plants; to be classified as a wetland, for a portion of the year, water must be at or near the land surface, creating conditions of saturated soils and fostering populations of aquatic vegetation. Treatment wetlands capitalize on the water quality improvement aspects of wetlands by discharging wastewater into them and allowing the naturally elevated rate of biological activity of wetlands to remove nutrients [35]. Natural wetlands are not ideal for use as a treatment system as they are classified as waters of the United States, and discharges into the wetlands are subject to state, local, and federal standards. Nutrient inflows are regulated because a high inflow of nutrients would inherently change the wetland system. Because 80% of the wetlands in the Midwest have been destroyed in the past century, it is desirable to preserve the remaining natural wetlands [36] [6]. Thus, a constructed wetland that can be engineered to meet the specific treatment needs of the site is preferred. The three types of treatment wetlands are vertical flow (VF), free water surface wetland (FWS), and horizontal subsurface flow (HSSF). In vertical flow wetlands, water is evenly distributed over the media, typically sand or gravel, which is planted with wetland vegetation. Free surface wetlands are areas of open water with plant life resembling that of a natural marsh. Treatment takes place as the water percolates through the root zone. In a horizontal subsurface flow wetland, the water travels horizontally underground through a gravel bed that is planted with wetland vegetation [35]. Given economic constraints of constructing VF and HSSF systems in agricultural landscapes, we will only focus FWS wetlands. Bulrush and cattails are the dominant species in most constructed wetlands. Other prevalent species include reeds, rushes, and sedges. Harvesting of these plants is one pathway for removing excess nutrients from the wetland system. However, this pathway is minor compared to nitrogen removal caused by other biological processes within the wetland; up to 79% of nitrogen can be removed in wetlands [36].FWS wetlands have limited nitrification capabilities due to low oxygen transfer capabilities. However, after a few years, nitrogen removal improves due to the establishment of root systems, which create aerobic micro-sites in otherwise anaerobic conditions [36]. Nevertheless, dissolved phosphorus removal in FWS treatment wetlands is minimal due to the limited contact opportunity with the soil [36], yet shoot removal shows promise if a market can be created [37]. Floating Treatment Wetlands: Generally employed in water detention basins or nutrient polluted ponds, floating treatment wetlands, or floating islands, are composed of recycled plastic bottles planted with native species on top with roots extending down into the water. | Volume 1 | Issue 10 |
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Like other treatment wetlands, these islands host microbes, which break down the polluting nutrients. However, floating islands are less susceptible to fluctuations in water level than systems that are reliant on connection to the bed sediment [38]. Additionally, they provide habitat and aid in erosion control and wave damping. A floating island was installed in a secondary treatment lagoon for a wastewater treatment plant in Pennsylvania. Occupying 24-m2, the island covered approximately 3% of the lagoon surface area. The island was equipped with a solar powered pump for regularly dousing the floating island vegetation. The island was found to reduce lagoon effluent total nitrogen concentration by 1.7 mg/l, when compared to pre-island lagoon effluent. However, no significant improvements were made in effluent total phosphorus concentration [39]. Deering suggests nutrient sequestrations possible if the systems are designed to meet influent characteristics [40]. Oxbow Wetlands: Oxbow wetlands are wet riparian areas that typically sit at a higher elevation than an adjacent stream channel. Historically, the stream channel flowed through the oxbow, but some climatic or land use changed triggered the stream channel to down cut; often via a shoot cutoff leaving a meandering stream bend disconnected from the flowing water. Oxbows can evolve into wetlands because vegetation grows and expands compared to the active floodplain of the channel which is often scoured by event flow. Oxbows offer the potential to store and treat frequent event floods (1 up to the 5-year recurrence interval) and they can intercept pipe-drain water before entering the active stream [41]. Over time with an accumulation of organic carbon, these systems can provide better than 50-80% nitrate removal [42]. However, if these systems become dominated with reed canary grass, they will export dissolved orthophosphorus [41]. The location of these systems, typically between cropland and streams, make them ideal for nitrogen management. Phosphorus management is almost always more challenging because of desorption from sediment or vegetation; nevertheless, if the vegetation can be removed cost effectively, then phosphorus can be removed from the site. CONCLUSION
Nutrients have dramatic impacts on aquatic ecosystems due to their role as limiting factors in aquatic system health. Many water bodies have become nutrient impaired due to nonpoint source nitrogen and phosphorus pollution, largely from the agricultural sector. Due to the pervasive nature of nutrient pollution, a variety of solutions will need to be adopted to ensure the health of water bodies around the world. A treatment train of options may be the most cost effective approach to nutrient driven surface water pollution in IMCR. In-field measures such as cover crops and mycorrhizae offer a critical first line of defense. But we know in-field practices alone will not be enough to prevent eutrophication. A second line of defense is needed to intercept IMCR to trap and treat nutrients. Depending on the landscape terrain, varying degrees of biosystems engineering will be required to optimize nutrients sequestration. The right mix of soil, carbon, plants, and microbes must be implemented in the right location(s) to achieve the goal of keeping intensively managed landscapes sustainable. From chemical processes like phosphoric precipitation to biological processes like bacterial denitrification in treatment wetlands, many methods have demonstrated effectiveness in counteracting nutrient pollution. These and future technologies can aid in the improvement of the health of aquatic ecosystems. Can intensively-managed, nutrient-rich cropland runoff be sustainably sequestered? Yes, but only if society is willing to make a financial investment to help off-set the costs of implementing the treatment train concept at the local scale. ACKNOWLEDGEMENTS: This work was funded by several grants; Minnesota Corn Growers Association, USEPA Section 319 and the Minnesota Pollution Control Agency. Master thesis by Lu Zhang, Emily Deering and Michele Rorer represented in this review to provide support of new technology used to make treatment trains possible.
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Engineered biosystem treatment trains‌ 31. S. Komor, J. Magner, Nitrate in ground water and water sources used by riparian trees in an agricultural watershed: A chemical an isotopic investigation in Southern Minnesota. Water Resources Research, 32:1039-1050. 1996. 32. D. Osmund, J. Gilliam, R. Evans, Riparian buffers and controlled drainage to reduce agricultural nonpoint source pollution. (Technical Bulletin 318: North Carolina State University: Bio & Ag Engineering Extension 2002) http://content.ces.ncsu.edu/riparian-buffers-and-controlled-drainage-to-reduce-agricultural-nonpoint-source-pollution.pdf. 33. D. Jaynes, T. Isenhart, Reconnecting Tile Drainage to Riparian Buffer Hydrology for Enhanced Nitrate Removal. Journal of Environmental Quality, 43:631-638. 2014. 34. M. Ancell, C. Fedler, N. Parker, Constructed wetland nitrogen removal from cattle feedlot wastewater. (ASABE Paper No. 984123. Orlando, FL: ASABE, 1998) http://www.webpages.ttu.edu/cfedler/downloads/publications/recww/nitremoval.pdf. 35. R. Kadlec, S. Wallace, Treatment Wetlands (2nd ed.) (Boca Raton, FL: CRC Press. 2009) 36. S. Reed, Natural Systems for Waste Management and Treatment. (2nd ed.). (New York City, NY: McGraw-Hill 1995). 37. H. Cui, H. Zhang, Preliminary study on the thermal performance of cattail. (Shandong Textile Economy, 2013) www.cnki.com.cn 38. N. Chang, K. Islam, Z. Marimon, et al., Assessing biological and chemical signatures related to nutrient removal by floating islands in stormwater mesocosms. Chemosphere, 88:736-743. 2012. 39. A. Abukir, S. Zeller, D. Klinger, Project examines floating islands for tertiary nutrient removal: case study. WaterWorld, 27:62. 2011. 40. E. Deering, Exploring stormwater N and P reduction in Floating Islands. MS Thesis, University of Minnesota, St. Paul, MN. 2016 41. J. Magner, S. Alexander, Drainage and nutrient attenuation in a flat terrain agricultural runoff interception riparian wetland: southern Minnesota, USA. Environmental Geology, 54:1367-1376. doi 10.1007/s 00254-007-0918-0, 2008. 42. M. Rorer, Nutrient Sequestration Effectiveness of a South-central Minnesota Saturated Buffer Trench. MS Thesis, University of Minnesota, St. Paul, MN. 2016.
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