Atmospheric Deposition

Atmospheric deposition is the set of processes that remove gases and particulates from the atmosphere and incorporate them back into terrestrial and aquatic ecosystems [26]. These processes take three main forms: the deposition (and biouptake) of gases; dry deposition of particulates and associated adsorbed chemicals; and wet deposition through precipitation [27]. While atmospheric deposition is an important, natural means of distributing nutrients throughout the environment, it can also lead to widespread distribution of man-made contaminants.

The exact contribution of atmospheric deposition to water quality depends on the type of pollutant, the distance from the emissions source, and the physical, chemical, and biological characteristics of the waterbody in question. However, the pollutant load from atmospheric deposition can be substantial. For example, atmospheric deposition accounts for 10–40 percent of the nitrogen pollutant load in certain Gulf Coast estuaries [20] and nearly 84 percent of the annual mercury load to Lake Michigan comes from atmospheric deposition [28].

Pollutants from atmospheric deposition do not always remain “local” to emissions sources. Some pollutants, like elemental mercury (Hg⁰), can be transported for thousands of miles before being deposited into a waterbody. The type and chemical species of pollutant affects the distance a given contaminant will travel and how much it is transformed in the atmosphere [27].

Atmospheric deposition pollutants can be natural or manmade. Natural sources include microbial activity, lightning strikes, and natural burns (e.g., forest fires). Manmade pollutant sources include vehicle emissions, burning sulfur-bearing fossil fuels such as coal, metal smelting and other industrial emissions such as chloro-alkali plants, waste incinerators, and aerosolized contaminants from fertilizer application, animal feedlots, and waste lagoons. Certain categories of pollutants, such as polycyclic aromatic hydrocarbons (PAHs) are the result of incomplete combustion of plant biomass and fossil fuels. Other emissions, such as polychlorinated biphenyls (PCBs), are the result of aerosolization of contaminated soil [26]. As a result, atmospheric deposition ties together contaminant routes regulated by the CAA, CWA, and RCRA.

Major contaminants from atmospheric deposition:

• Sulphur dioxide
• Nitrogen oxides/ Ammonia/ Total Nitrogen
• Mercury and other heavy metals
• PCBs (poly-chlorinated biphenyls)
• PAHs (poly-cyclic aromatic hydrocarbons)
• Pesticides and herbicides

Wastewater and Sewage

Municipal wastewater treatment plants and sewage collection systems (sewers) are regulated as ‘point sources’ under the CWA. All wastewater treatment systems must receive a National Pollutant Discharge Elimination System (NPDES) permit and treat their wastewater to a particular standard, before releasing effluent to a downstream water body [1]. Most wastewater treatment plants in the United States treat water for turbidity, grease and oils, biological oxygen demand (BOD), and pathogens through a combination of physical, biological, and chemical treatment processes [29]. 

Most municipalities use wastewater treatment plants as a centralized treatment facility, routing all wastewater from industrial and domestic uses to the plant for final treatment. Because the contaminants in some industrial wastewater streams would be toxic for the bacteria that wastewater plants use in their treatment processes, the CWA mandates pre-treatment for certain types of waste streams before they can be discharged into a municipal sewer [2]. Although wastewater treatment plants do significantly reduce the concentration of certain types of pollutants (like BOD), not all plants effectively treat other pollutants. In highly urbanized areas, wastewater treatment plants are a major source of nutrient contamination to local waterbodies [10]. Other pollutants of concern in wastewater effluent include personal care products (e.g., detergents, soaps, and shampoos), pharmaceuticals, and hormones, which can function as endocrine disruptors for aquatic species in downstream waters [30].

In many older cities, stormwater and wastewater are routed into a single combined sewer, which then flows to a downstream wastewater treatment plant for treatment and eventual discharge. Combined sewers help to manage some of the water quality challenges associated with urban stormwater runoff. However, storm events often overwhelm older combined systems, which were not designed for the dense populations and large amount of paved surfaces that characterize modern cities. To avoid backups of raw sewage into citizen’s homes, combined systems relieve pressure on the system during a storm event through combined sewer overflows (CSOs): discharges of untreated wastewater into downstream bodies of water. CSOs are a significant risk to human health and the environment, introducing large amounts of fecal bacteria, BOD, sediments, nutrients, and other contaminants into local waterways. Since 1978, the USEPA has entered into over 50 consent decrees to cities across the country (including Chicago, Illinois, Washington, D.C., and Boston, Massachusetts) in an effort to combat CSOs [31]. Combined sewers also run the risk of peak flows in excess of what can be effectively treated by the biological processes in downstream wastewater treatment plants.

Other cities choose separate sewer systems for stormwater and wastewater. Stormwater is routed to a storm sewer network and discharged to a stream or river with little to no treatment, while wastewater is routed into a sanitary sewer for treatment at a wastewater treatment plant. Rarely, sanitary sewers can still suffer from sanitary sewer overflows (SSOs). Sanitary sewers which are not watertight (due to faulty seals, cracks in the pipes, or improper connections) can receive large amounts of additional water in the form of infiltration and inflow (I/I) during storm events. Pressure on the system from this excess water can lead to an SSO or sewer backup. Other reasons for SSOs include equipment failures, improper maintenance, broken pipes, or blockages caused by improperly-disposed materials entering the sanitary sewer system.

Major wastewater contaminants:

• Nitrogen
• Phosphorus
• Fecal bacteria
• BOD (organic pollutants)
• Sediments
• Pharmaceuticals and other emerging contaminants of concern

Stormwater Runoff

Stormwater runoff is a diffuse (non-point) pollutant source. Runoff is caused when rainfall does not infiltrate into the ground but instead is routed across the surface to the nearest waterbody, picking up any contaminants on the land surface as it travels. Stormwater runoff occurs in both urban and rural settings, with different challenges arising in both.

In urban areas, the rapid increase in impervious (paved) surfaces prevents water from infiltrating into the ground to recharge aquifers, be used by plants, or be slowly released to rivers and streams. Instead, urban runoff flows tend to be “flashy,” creating large, fast-moving peaks during rain events when stormwater is rapidly routed to local streams via roads, gutters, and sewers. The fast flows cause significant downstream bank erosion and increased sediments in the water, resulting in increased turbidity. Urban stormwater also carries loose sediments from construction sites and dirt from roads and streets into local waterways, increasing phosphorus and nitrogen loads and turbidity. Excess nutrients from lawn fertilizer, pet waste, and unraked leaves are transported by stormwater into local streams. Other contaminants include salts, pesticides, and motor oil [32]. In some rapidly developing areas, including the area upstream of the Chesapeake Bay, urban stormwater runoff is the fastest-growing pollutant source. According to the Chesapeake Bay Program, urban stormwater contributes 17 percent of the nitrogen load and 17 percent of the phosphorus load entering the Bay every year [33].

Agricultural areas face similar significant challenges. In 2000, the National Water Quality Inventory listed agricultural runoff as the primary source of water quality impacts on surveyed lakes and rivers [34]. Agricultural runoff impacts come from two main sources: soil erosion and nutrients. Rain can wash large amounts of unprotected topsoil from farm fields into rivers and streams. A 2006 study found that topsoil erosion costs U.S. farmers $37.6 billion every year in lost productivity [35]. At the same time, this excess sediment increases turbidity and phosphorus loads in local waterbodies. Agricultural stormwater carries fertilizers, sludge, or manure that have not been used by crops, significantly impacting nitrogen and phosphorus loads in streams and rivers. Water-soluble pesticides and herbicides like atrazine can also be washed by stormwater into local water bodies [36].

Major stormwater contaminants:

• Sediment (turbidity)
• Nitrogen
• Phosphorus
• Pesticides and herbicides
• Heavy metals
• Salt

Concentrated Animal Feeding Operations

Concentrated animal feeding operations (CAFOs) are industrial agricultural facilities where large densities of animals are raised for eggs, milk, and meat. CAFOs are classified by the number and type of animals they contain and the way they process waste [37]. In a CAFO, waste material comes into contact with a water supply in one of two ways: either a pipe carries manure or wastewater to surface water or animals come in direct contact with surface water that runs through their confined area [38].

Although CAFOs help provide low-cost meat, eggs, and dairy, they create environmental challenges. The vast quantities of manure and other waste produced by CAFOs can be staggering. A CAFO with 800,000 pigs can produce over 1.6 million tons of waste a year: an amount one and a half times the annual sanitary discharge of the city of Philadelphia, Pennsylvania [38]. Although sewage treatment plants are required to treat and dispose of human waste, no such requirement exists for animal waste.

CAFOs are considered "point sources" under the CWA and so are required to obtain a permit to discharge waste [2]. These permits limit the levels of manure discharge. CAFO operators must apply manure to fields as fertilizer; contain it in clay- or concrete-lined pits, treatment lagoons, or holding ponds; or truck it offsite. However, accidental spillage, leaks, or the loss of applied manure due to surface runoff or leaching all create serious risks to both surface water and groundwater quality [38].

A 2001 USEPA study found that states with high concentrations of CAFOs suffer on average twenty to thirty serious surface water quality issues per year as a result of manure management problems [39]. Storms can cause treatment lagoons to overrun, resulting in runoff of untreated waste into nearby waterbodies. They can also cause soil erosion, washing large amounts of sediment (and associated, adsorbed contaminants such as phosphorus) into waterways. Pollutants can also be transported to surface waters via drainage ditches or flushing systems or via groundwater [38]. This last route is a particular concern for pathogens, antibiotics and hormones, and water-soluble nitrates, as these groundwater systems can be important sources of drinking water for nearby communities.

Major CAFO contaminants:

• Nitrogen
• Phosphorus
• Antibiotics
• Hormones
• Heavy metals
• Fecal bacteria and other pathogens

Mining, Oil, and Gas

Water is used in many aspects of mining and oil and gas extraction: in quarrying, in the milling of mined materials, and in resource recovery in unconventional production methods such as hydraulic fracturing (“fracking”). 

Water quality issues can occur in abandoned mine sites that have not been properly decommissioned. The most common of these challenges is acid mine drainage. Water seeps into fissures in abandoned mines or into piles of waste rock (tailings), reacting with subsurface sulfur-bearing rocks and sediment to form sulfuric acid, which in turn dissolves heavy metals. The runoff from acid mine drainage is low in pH and often tinged red or yellow with dissolved iron that can precipitate along the bottom of streams, killing benthic macroinvertebrates. It also often contains high levels of mercury, copper, and lead [40]. Abandoned mine sites dating from before 1978 can also contain high levels of PCBs due to abandoned electrical equipment, particularly transformers [41].

The extraction of oil and gas through processes like fracking creates produced water as a byproduct. Produced water is a mixture of water, leftover hydrocarbons, any chemicals introduced during the fracking process, and elevated levels of contaminants naturally found in the geological feature being exploited. These contaminants can include salts, heavy metals, and radioactive isotopes. Over three trillion gallons of produced water are generated from U.S. oil and gas extraction operations every year. The most common method of disposal for produced water is via deep injection wells [42].

Major mining, oil, and gas contaminants:

• pH
• Salts
• Heavy metals
• PCBs
• Radioactive isotopes 

[1] R. Percival, C. Schroeder, A. Miller and J. Leape, Environmental Regulation: Law, Science, and Policy, 9th ed., Boston, MA: Aspen Publishing, 2021.

[2] American Bar Association, The Clean Water Act Handbook, 4th ed., M. A. Ryan, ed., Washington, D.C.: ABA Book Publishing, 2018.

[10] D. Sedlak, Water 4.0: The Past, Present, and Future of the World's Most Vital Resource, New Haven, Conn.: Yale University Press, 2015.

[20] H. Paerl, R. Dennis and D. Whitehall, “Atmospheric Deposition of Nitrogen: Implications for Nutrient Over-Enrichment of Coastal Waters,” Estuaries, vol. 25, no. 4, pp. 677–693, 2002.

[26] United States Environmental Protection Agency, “Frequently Asked Questions About Atmospheric Deposition: A Handbook for Watershed Managers,” Washington, D.C., 2001.

[27] J. Pacyna, “Atmospheric Deposition,” in Encyclopedia of Ecology, Amsterdam, The Netherlands: Elsevier B.V., 2008, pp. 275–285.

[28] M. Landis and G. Keeler, “Atmospheric Mercury Deposition to Lake Michigan during the Lake Michigan Mass Balance Study,” Environmental Science and Technology, vol. 36, no. 21, pp. 4518–4524, 2002.

[29] M. Davis, Water and Wastewater Engineering, 1st ed., Columbus, Ohio: McGraw-Hill Education, 2010.

[30] A. Ebele, M. Abdallah and S. Harrad, “Pharmaceuticals and personal care products (PPCPs) in the freshwater aquatic ecosystem,” Emerging Contaminants, vol. 3, pp. 1–16, 2017.

[31] Environmental Protection Agency National Enforcement Initiative, “Status of Civil Judicial Consent Decrees Addressing Combined Sewer Systems,” May 1, 2017. [Online]. Available: https://www.epa.gov/enforcement/status-civil-judicial-consent-decrees-addressing-combined-sewer-systems-csos. [Accessed June 6, 2022].

[32] D. Makepeace, D. Smith and S. Stanley, “Urban stormwater quality: Summary of contaminant data,” Critical Reviews in Environmental Science and Technology, vol. 25, no. 2, pp. 93–139, 1995.

[33] Chesapeake Bay Program, “Stormwater Runoff,” 2022. [Online]. Available: https://www.chesapeakebay.net/issues/stormwater_runoff. [Accessed 6 June 2022].

[34] United States Environmental Protection Agency, “National Water Quality Inventory,” USEPA Office of Water, Washington, D.C., 2002.

[35] D. Pimentel, “Soil Erosion: A Food and Environmental Threat,” Environment, Development and Sustainability, vol. 8, p. 119–137, 2006.

[36] United States Environmental Protection Agency, Protecting Water Quality from Agricultural Runoff, Washington, D.C.: USEPA Nonpoint Source Control Branch, 2005.

[37] United States Department of Agriculture and United States Environmental Protection Agency, Unified National AFO Strategy Executive Summary, Washington, D.C., 2015.

[38] C. Hribar, “Understanding Concentrated Animal Feeding Operations and Their Impact on Communities,” National Association of Local Boards of Health, Bowling Green, Ohio, 2010.

[39] United States Environmental Protection Agency, “Environmental Assessment of Proposed Revisions to the National Pollutant Discharge Elimination System Regulation and Effluent Limitations Guidelines for Concentrated Animal Feeding Operations,” USEPA Office of Water, Washington, D.C., 2001.

[40] United States Geologic Survey, “Mining and Water Quality,” June 8, 2018. [Online]. Available: https://www.usgs.gov/special-topics/water-science-school/science/mining-and-water-quality. [Accessed June 6, 2022].

[41] D. Bench, “PCBs, Mining, and Water Pollution,” in Mine Design, Operations & Closure Conference, Paulson, Mont., 2003.

[42] M. McLaughlin, et al., “Water quality assessment downstream of oil and gas produced water discharges intended for beneficial reuse in arid regions,” Science of the Total Environment, vol. 713, p. 136607, 2020.