Agricultural Water Pollution: Causes And Impacts

Agriculture is the single largest user of freshwater resources, using a global average of 70% of all surface water supplies. It is also a major cause of water pollution. The pressure to produce enough food has led to expansion into marginal lands, the intensification of farming practices, and the increased use of fertilisers and pesticides. These activities have resulted in soil erosion, salinisation, and sediment loads in water, as well as the excessive use and misuse of agricultural inputs. Pesticides, fertilisers, livestock waste, antibiotics, and processing wastes from plantation crops can contaminate both groundwater and surface water, posing risks to aquatic life, wildlife, and drinking water supplies. Additionally, the improper disposal of animal waste in factory farming and the use of wastewater in agriculture can further contribute to water pollution.

Fertilizer and pesticide runoff

Fertilizers and pesticides are essential in agriculture to enhance crop growth and protect against pests. However, they can have detrimental effects on the environment if not properly managed. When fertilizers and pesticides are over-applied, improperly timed, or not incorporated into the soil, they can be washed away from fields by rainfall or irrigation water, leading to water pollution.

Impact on Aquatic Life

Pesticides, herbicides, and fertilizers are toxic to aquatic life, including fish, amphibians, and invertebrates. They can cause direct mortality and sub-lethal effects such as reproductive failure and developmental abnormalities. These chemicals can also bioaccumulate in the tissues of aquatic organisms, leading to higher concentrations as they move up the food chain, a process known as biomagnification. This poses risks not only to aquatic species but also to predators, including birds and humans, who consume contaminated fish.

Impact on Inland and Coastal Waters

Inland and coastal waters are vulnerable to eutrophication due to agricultural runoff. Eutrophication occurs when excessive nutrients, primarily nitrogen and phosphorus, enter water bodies, promoting the overgrowth of algae. This leads to algal blooms, which reduce light penetration and, as the algae and other aquatic plants die and decompose, oxygen in the water is consumed, creating hypoxic conditions or "dead zones" that cannot support marine life.

One well-known example of this is the Gulf of Mexico dead zone, which forms annually due to nutrient runoff from the Mississippi River Basin. Spanning 6,000 square miles, this dead zone disrupts marine ecosystems, leading to fish kills and a loss of biodiversity. It also affects local fisheries, causing economic hardship for coastal communities dependent on fishing.

Impact on Human Health

Agricultural runoff can significantly impact drinking water quality, posing serious health risks to humans. One of the primary concerns is the leaching of nitrates from fertilizers into groundwater and surface water sources. High concentrations of nitrates in drinking water are dangerous, especially for infants and young children, as they interfere with the blood's ability to carry oxygen, potentially leading to methemoglobinemia, commonly known as "blue baby syndrome," which can be fatal if not treated promptly.

In addition to nitrates, agricultural runoff introduces pesticides and herbicides into water sources. These chemicals, such as glyphosate and atrazine, are persistent and can contaminate drinking water, leading to various health issues in humans with chronic exposure, including cancer, reproductive disorders, endocrine disruption, and neurological impairments.

Pathogens, such as E. coli, Salmonella, and Cryptosporidium, can also enter water supplies through runoff contaminated with animal waste or improperly treated sewage. For example, in 2000, the town of Walkerton in Ontario, Canada, experienced a devastating water crisis when E. coli bacteria from cattle manure runoff contaminated the municipal water supply, affecting 2,300 people and resulting in seven fatalities.

Manure management

Nutrient Management:

• Nitrogen and Phosphorus: Manure contains high levels of nitrogen (N) and phosphorus (P), which are essential for plant growth. However, when applied in excess, these nutrients can contaminate water bodies. Nitrogen, in the form of nitrate, is of particular concern in groundwater, as it is the primary source of drinking water in many areas. High nitrate levels in drinking water can cause methemoglobinemia (blue-baby syndrome) in infants and interfere with oxygen uptake in the circulatory system of humans and livestock. To prevent this, farmers should follow guidelines for nutrient application, such as those provided by the University of Minnesota and the Minnesota Pollution Control Agency.
• Eutrophication: An excess of nutrients in water bodies can lead to eutrophication, where aquatic plants and algae grow rapidly and deplete oxygen levels during their decomposition. This results in hypoxic zones, which are detrimental to fish, shellfish, and other aquatic life. Certain types of algae can also release toxins harmful to people, pets, and livestock.

Pathogen Management:

Manure contains pathogens, such as harmful bacteria and viruses, which can cause diseases in humans and animals. Examples include E. coli, Cryptosporidium, Salmonella, Campylobacter spp., and Listeria. When manure is improperly managed, these pathogens can enter waterways and groundwater, posing a significant health risk.

Manure Storage and Handling:

• Manure storage structures require careful engineering, construction, operation, and maintenance. Regular inspections and timely removal of manure for land application are essential to prevent overflow and contamination. Covered storage structures are recommended to prevent rainwater and snowmelt from diluting the manure.
• Uncontained runoff from open lots is a primary risk factor for manure-related water pollution. Implementing containment systems, such as settling basins and holding ponds, can help manage this risk.
• When applying manure to fields, it is crucial to follow correct techniques. Manure should be incorporated into the soil to reduce the risk of surface runoff. Avoid applying manure on saturated or frozen soils, as this increases runoff potential.

Other Considerations:

• Manure Marketing and Transportation: With the consolidation of animal agriculture sectors, larger farms are producing more manure in smaller areas. This makes it challenging to find suitable fields for manure application. Farmers are exploring manure marketing and technologies that can add value to manure or transform it into a more economical transport form.
• Setbacks and Buffer Zones: Many states have implemented setbacks, which are minimum distance requirements between manure storage structures and water bodies, wells, and other environmentally sensitive areas. These setbacks help reduce the risk of contamination. Additionally, establishing grass buffer strips can help capture and filter manure runoff before it reaches waterways.

Antibiotic resistance

Antibiotics are used in agriculture to prevent and treat diseases in animals, and in sub-therapeutic levels in concentrated animal feed for growth promotion, improved feed conversion efficiency, and disease prevention. However, the use of antibiotics in agriculture is not limited to therapeutic purposes. The use of antibiotics in agriculture can lead to antibiotic residues in animal-derived products, including meat, milk, eggs, and edible tissues. When consumed by humans, these residues can cause direct toxicity, the development and emergence of antibiotic-resistant strains of bacteria, as well as therapeutic failure in clinical cases.

The use of antibiotics in agriculture can also lead to antibiotic pollution, resulting in antibiotic residues in animal-derived products, including meat, milk, eggs, and edible tissues. These residues can have a huge and negative impact on public health and food safety with regards to drug toxicity, immunopathological diseases, carcinogenicity, allergic reactions, and drug sensitization, among others.

The use of antibiotics in agriculture can also lead to the development of antibiotic resistance in bacteria. Antibiotics can create selective pressure for the development of resistance in bacteria, and resistant bacteria associated with animals may be pathogenic to humans, easily transmitted to humans via food chains, and widely disseminated in the environment via animal wastes.

Soil erosion

The effects of soil erosion go beyond the loss of fertile land. It leads to increased pollution and sedimentation in streams and rivers, clogging these waterways and causing declines in fish and other species. Degraded lands are also less able to absorb water, which can worsen flooding. Additionally, soil erosion can alter how water flows through the landscape, potentially making flooding more common.

The impact of soil erosion on water quality is profound. Eroded soil particles enter water bodies and settle at the bottom, leading to sedimentation. This process can disrupt aquatic habitats, reduce water clarity, and hinder the natural flow of water. Soil often contains nutrients like nitrogen and phosphorus, which are essential for plant growth. However, when these nutrients are washed into water bodies in large amounts due to erosion, they can cause nutrient pollution. This excess of nutrients can lead to excessive algae growth, depleting the water's oxygen supply and harming or killing aquatic organisms.

Pesticides and other contaminants commonly found in agricultural topsoil can also be carried into water bodies during soil erosion, posing risks to aquatic life and potentially contaminating drinking water sources. The presence of fine soil particles in the water increases turbidity, blocking sunlight from reaching aquatic plants and impairing the breathing of fish and other aquatic animals.

To protect water quality and meet environmental standards, controlling soil erosion is crucial. Conservation tillage, residue management, and cultural management practices can help increase the efficiency of nutrient management and erosion control. By adopting sustainable land use practices, it is possible to reduce the impacts of agriculture and livestock, preventing soil degradation, erosion, and the loss of valuable land to desertification.

Groundwater contamination

Excessive Use of Fertilizers and Pesticides

The excessive or improper use of fertilizers and pesticides in agriculture is a major contributor to groundwater contamination. Fertilizers, particularly those rich in nitrogen, can increase nitrate levels in groundwater, which can be harmful to human health, especially for infants and pregnant women. Pesticides, including herbicides, insecticides, and fungicides, can seep into the soil and contaminate groundwater sources. These chemicals can remain in the groundwater for extended periods, even if efforts are made to reduce their use above ground.

Agricultural Runoff

Agricultural runoff occurs when water from irrigation, rain, or snowmelt fails to soak into the soil and instead flows over the land, carrying contaminants with it. This runoff can transport pesticides, fertilizers, and other pollutants into nearby streams and water sources, ultimately affecting groundwater reserves.

Animal Waste from Confined Animal Feeding Operations (CAFOs)

CAFOs, where animals are raised in confined spaces, generate significant amounts of animal waste. This waste can contain high levels of nutrients, such as nitrogen and phosphorus, as well as pathogens and bacteria. If not properly managed, this waste can contaminate groundwater sources through runoff or leakage into wells.

Inadequate Well Construction and Maintenance

Improperly constructed or maintained wells can provide a direct pathway for contaminants to enter groundwater sources. Spills or leaks of fuels, solvents, or pesticides near wells can result in groundwater contamination.

Use of Wastewater or Treated Sewage (Biosolids)

In some cases, farms use treated wastewater or sewage sludge (biosolids) as a source of irrigation or fertilizer. While this practice can be beneficial for water reuse, it can also introduce contaminants into the soil and eventually reach groundwater sources.

Heavy Metal Contamination in Fertilizers and Pesticides

Certain fertilizers and pesticides have been found to contain heavy metals such as arsenic, cadmium, lead, and lithium. When these substances are applied to fields, the heavy metals can leach into the soil and contaminate groundwater sources.

To prevent groundwater contamination from agricultural activities, it is essential to implement sustainable farming practices, improve wastewater treatment processes, and promote proper well construction and maintenance. By adopting more sustainable and environmentally friendly approaches, we can minimize the impact of agriculture on groundwater quality.

Frequently asked questions