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Animal Ag Water Pollution Sources

The primary sources of nitrogen and phosphorus are:
 Concentrated amounts of manure on factory farms.
 Excess use of chemical fertilizers on feed crops.
 Crop fixation of nitrogen by soybeans and alfalfa/hay.

In a large regional study covering the Mississippi Basin and all or parts of 31 states, USGS scientists reported:
 For Nitrogen – Manure was the largest pollution source, followed by chemical fertilizers and then fixation.
 For Phosphorus – Chemical fertilizers were the largest anthropogenic source, followed by manure.[1]

 

  1. Robertson, D. M. & Saad, D. A. (2021). Nitrogen and phosphorus sources and delivery from the Mississippi/Atchafalaya River basin: An update using 2012 SPARROW models. JAWRA Journal of the American Water Resources Association, 57(3), 406-429. Table 2; Table 4; Figure 5.

Manure and its nutrients – nitrogen and phosphorus – has almost no value in the factory farming system.[1] For most factory farms, it is simply a logistical question of how to get rid of it. There are only 3 places for those nutrients to go:
 Temporarily held in manure storage systems.
 Dispersed into the environment.
Effectively incorporated into the soil and eventually into crops.[2]

Despite decades of research, subsidies, incentives, and weak regulations, almost all the nutrients in manure are eventually released into the environment.[3]

The EPA has understood the severity of the problem for many years, saying in 2004, “Improperly managed manure has caused serious acute and chronic water quality problems throughout the United States.”[4] And yet there is no solution anywhere on the horizon.[5-7]

 

  1. See, The Manure Problem [question: “Why is manure now viewed as having little value?”]
  2. Note that there are many minor uses for manure that may temporarily embody nutrients in some other form, though according to our understanding, none have had a significant impact on nutrient dispersal. See, Lim, T., et al., (2023) Increasing the Value of Animal Manure for Farmers, USDA Economic Research Service, AP-109
  3. See, Total Nutrient Volume in Manure [question: “How much of the nitrogen in manure from factory farmed animals is applied to cropland?”]
  4. EPA 40 CFR Parts 9, 122, 123 and 68 Fed Reg. 7176 (February 2003). [This is the “final rule” that “establishes a mandatory duty for all CAFOs to apply for an NPDES permit and to develop and implement a nutrient management plan.” This rule was overturned by the courts.]
  5. Ribaudo, M. & Shortle, J.S. (2019). Reflections on 40 Years of Applied Economics Research on Agriculture and Water Quality. Agricultural and Resource Economics Review, 48(3), 519–530, p. 520. [See comments from a USDA researcher who spent 35 years at the agency trying to address nutrient pollution from agriculture, acknowledging that, “Nearly 40 years later, agriculture remains a major cause of unmet water goals in the U.S.”]
  6. Aillery, M. P., et al., (2005). Managing manure to improve air and water quality. USDA ERS Rpt 9., p. 6. [“…manure management practices with strictly public benefits (benefits realized off the farm) will be little used unless economic and regulatory conditions change.”]
  7. Ribaudo, M., et al., (2003). Manure management for water quality costs to animal feeding operations of applying manure nutrients to land. USDA-ERS Agricultural Economic Report 824, p. 82. [Note: USDA sources from 20 years ago display an optimism about potential solutions that has been overtaken by the realities of agricultural exceptionalism and ineffective environmental protections. “We assume here, for presentation sake, that up to 40 percent of cropland would receive manure after nutrient application standards are implemented.” at p. 82. In 2022, manure was applied to ~22 million acres, or less than 10% of the 237 million acres that received chemical fertilizers. See, Total Nutrient Volume in Manure [question: “On how much cropland is manure applied?”]

Immediate Dispersal – Portions of the nitrogen in manure are dispersed into the air and soil as soon as it is excreted and continue to be lost during every stage of “manure management” regardless of management systems.[1]

 Nutrient Loss from Storage Sites – Manure can leach or overflow from storage piles, enclosed manure pits, and open manure lagoons and ponds. This can be due to regular expected occurrences (like leaching), intentional releases, structural failures, or severe weather events.[2] Large amounts of nitrogen are lost throughout the storage process.[3] For example, dairy flush barns and uncovered hog lagoons lose ~80% of total excreted nitrogen prior to field application; poultry and feed cattle operations lose about 30 to 40% of manure N.[4] Phosphorus tends to sink to the bottom of storage systems where it can remain for years, eventually reaching soil or water.[5]

Manure Land-Applications – When manure is over-applied on surrounding farmland, the plants and soil cannot absorb the excess nutrients which are then carried into surface waterbodies. Manure is regularly over-applied, presumably as a method for dispersing a waste product.[6,7]

Atmospheric Deposition from Manure – Ammonia escaped into the atmosphere (NH3) comes primarily from manure on factory farms (both livestock and poultry) and is eventually deposited back in the soil or in waterways.[8,9]

 

  1. Rotz, C. A. (2004). Management to reduce nitrogen losses in animal production. Journal of animal science, 82 (suppl_13), E119-E137, Table 2. [“Volatile loss begins soon after excretion, and it continues through all manure handling processes until the manure nutrients are incorporated into soil.” “The primary pathways of N loss are volatile emissions into the atmosphere and leaching and runoff losses to ground and surface waters.” pp. E-119 and 120]
  2. Burkholder, J., et al., (2007) Impacts of Waste from Concentrated Animal Feeding Operations on Water Quality. Environmental Health Perspectives, 115(2), 308–312, p. 308 [“Contaminants from animal wastes can enter the environment through pathways such as through leakage from poorly constructed manure lagoons, or during major precipitation events resulting in either overflow of lagoons and runoff from recent applications of waste to farm fields, or atmospheric deposition followed by dry or wet fallout.”]
  3. U.S. EPA (2002) Non-Water Quality Impact Estimates for Animal Feeding Operations, pp. 1-2. [“Nitrogen losses from animal manure as ammonia can easily exceed 50 percent.”]
  4. Aillery, M. P., et al., (2005). Managing manure to improve air and water quality. USDA ERS Rpt 9, Table 2-1, p. 12.
  5. Lim, T., et al., (2023) Increasing the Value of Animal Manure for Farmers, USDA Economic Research Service, AP-109, p. 13. [“Nitrogen can volatilize before being land-applied, and phosphorus can settle to the bottom of lagoons where it may remain for years.”]
  6. Lim, T., et al., (2023), Table 2, Figure 4. [The great majority of manure was applied to corn crops; more than half of corn producers that land-applied manure did not reduce their normal usage of chemical fertilizer.]
  7. Ribaudo, M., et al., (2017). The potential role for a nitrogen compliance policy in mitigating Gulf hypoxia. Applied Economic Perspectives and Policy, 39(3), 458-478, p. 12. [USDA scientists acknowledge that, “Farms with confined animals generally have inadequate cropland to assimilate nutrients produced, and are characterized by excess nutrient applications on cropland they control.”]
  8. U.S. EPA (2024) 2020 NEI Supporting Data and Summaries – Data Queries for Sector Summaries. [Query: National/Ammonia NH3/Livestock Waste (49.2%), fertilizer application (33.5%), agricultural field burning (2.7%)]
  9. Robertson, D. M. & Saad, D. A. (2021). Nitrogen and phosphorus sources and delivery from the Mississippi/Atchafalaya River basin: An update using 2012 SPARROW models. JAWRA, 57(3), 406-429, Figure 5, p. 18. [Atmospheric manure nutrients are the source of ~13% of nitrogen in waterways.]

Fertilizer on Feed Crops – The applications of chemical nitrogen and phosphorus fertilizers on feed crops, including corn and soybeans, have consistently far exceeded crop requirements.[1] Chemical fertilizers applied in excess of crop needs contaminate surface waters through runoff or by leaching into groundwater.[2,3]

Landscape Modifications – Artificial drainage techniques, including ditches and subsurface drains, are used on ~25% of the nation’s cropland, including feed crops. Runoff water moves quickly through these drains carrying nutrients and other contaminants from the fields directly to nearby streams.[4,5]

 

  1. Hellerstein, D., et al., (2019) Agricultural Resources and Environmental Indicators, USDA Economic Research Service, Bulletin 208, pp. 45-46. [“Nutrient recovery is the ratio of the amount of nutrient in the harvested crop to the amount of nutrient applied.” Corn N recovery = 76%. Soybeans N = 46%. Corn P = 65%. Soybeans P = 58%.]
  2. Capel, P., et al. (2018) Agriculture – A River Runs Through It – the Connections between Agriculture and Water Quality. U.S. Geological Survey, National Water-Quality Assessment Project, Circular 1433, p. 1. [“Typically, 5 to 50 percent of applied nitrogen moves from fields to streams
    through runoff and through groundwater discharge.”]
  3. Sharpley, A., et al., (2013). Phosphorus legacy: overcoming the effects of past management practices to mitigate future water quality impairment. Journal of environmental quality, 42(5), 1308-1326, p. 1308. [“Accumulated P can be remobilized or recycled, acting as a continuing source to downstream water bodies for years, decades, or even centuries.”]
  4. Capel, P., et al., (2018), p. 6. [“About 25 percent of the cropland in the country has artificial drainage—surface (ditches) and subsurface (tiles)… Nitrogen export in streams with substantial subsurface drainage in their watersheds is slightly more than three times larger than in other agricultural streams in the Nation.”]
  5. Chris Torres (Ed.) (May 9, 2023) Tiling and nutrient runoff: An uneasy relationship, American Agriculturist Farm Progress. https://www.farmprogress.com/crops/tiling-and-nutrient-runoff-an-uneasy-relationship

Feed crop production – especially corn and soybeans – has expanded onto lands that are naturally too wet, too dry, or have low soil fertility.[1] Croplands that require irrigation, drainage, or extra fertilization, tend to accelerate nutrient pollution.[2] Irrigation taken from nearby waterways can deplete water flows and thereby increase nutrient concentrations in lower water volumes leading to high levels of eutrophication.[3]

According to the USDA’s 2023 Irrigation and Water Management Survey, corn, soybeans, and alfalfa accounted for ~60% of all irrigated acres harvested.[4]

 

  1. Lark, T. J., et al., (2020) Cropland expansion in the United States produces marginal yields at high costs to wildlife. Nature communications. 11 (1), 4295. [“We found that croplands are moving onto lower-quality land in less-suitable regions—a dual setback to production gains from cropland expansion.”]
  2. Capel, P., et al., (2018) Agriculture – A River Runs Through It – the Connections between
    Agriculture and Water Quality, National Water-Quality Assessment Project, Circular 1433, U.S. Geological Survey, p. 165. [“Agricultural modifications such as drainage and irrigation, substantially change the water flowpaths compared to the natural hydrologic system, and can quickly move agricultural chemicals to the stream.”]
  3. Richter, B., et al., (2020). Water scarcity and fish imperilment driven by beef production. Nature Sustainability, 3(4), 319–328, p. 320. [“…human-induced flow reductions can concentrate problematic nutrient and chemical conditions, leading to eutrophication or depleted oxygen levels that are hazardous to fish and other aquatic organisms.”]
  4. USDA NASS (2024) Census of Agriculture, 2023 Irrigation and Water Management Survey, Vol 3, AC-22-SS-1, Table 38. [See, Water Usage by Feed Crops [question: “How many acres of crops primarily used for animal feed are irrigated?” These crops accounted for ~61% of all harvested irrigated acres in 2023.]

Water Pollution