Soil Pollutants: Transferring To Water, What's The Risk?

The transfer of pollutants from soil to water is a pressing issue, with far-reaching consequences for both ecosystems and human health. Sources of pollution include fertilizer runoff from farms, leaky septic tanks, and organic pesticides, all of which can contaminate nearby water bodies. This process introduces pollutants such as nitrates and phosphates, leading to eutrophication and oxygen depletion, which in turn harm aquatic life and reduce water quality. With such wide-ranging impacts, it is crucial to address these environmental concerns through effective management of agricultural practices and wastewater disposal.

Nitrates in wastewater

Nitrates are a form of nitrogen found in both terrestrial and aquatic ecosystems. They are essential plant nutrients, but excess nitrates in wastewater can cause significant water quality problems. Municipal wastewater includes household sewage, discharge from appliances, municipal wastes, and storm sewer runoff. The primary goal of municipal wastewater treatment is to remove soluble contaminants, notably nitrates.

Sources of nitrates in wastewater include runoff from fertilized lawns and cropland, failing on-site septic systems, and runoff from animal manure storage areas. Fertilizer in runoff is the most prevalent source of nitrate contamination. According to the U.S. Environmental Protection Agency, other sources include leakage from septic tanks and erosion of natural deposits. Animal manure, particularly cattle manure, is another significant contributor to nitrates in wastewater.

When excess fertilizer is applied to agricultural lands, it can run off into nearby water bodies or leach into groundwater, carrying nitrates into aquatic ecosystems. This leads to eutrophication, where the over-enrichment of water with nutrients promotes excessive growth of algae and other aquatic plants. As the algae and plants die and decompose, they consume oxygen in the water, resulting in oxygen depletion, which is harmful to aquatic life.

Nitrate removal from wastewater is challenging because it is a molecule that can migrate quickly into groundwater. Additionally, wastewater treatment plants often don't have strict removal requirements for nitrates, and when they do, the focus is typically on total nitrogen rather than nitrates specifically. Nitrate removal can be achieved through biological and chemical processes, as well as through the use of anaerobic bacteria, specifically Pseudomonas bacteria, which obtain oxygen by removing it from nitrates in wastewater. Another method for nitrate removal is cycling aeration on and off in an aeration basin, which reduces nitrate levels and increases the general heterotrophic removal of ammonia, limiting nitrate production.

Eutrophication

The most conspicuous effect of cultural eutrophication is the creation of dense blooms of noxious, foul-smelling phytoplankton that reduce water clarity and harm water quality. These algal blooms limit light penetration, reducing growth and causing die-offs of plants in littoral zones while also lowering the success of predators that need light to catch prey. Eutrophication can also deplete dissolved inorganic carbon and raise pH to extreme levels during the day. Elevated pH can, in turn, 'blind' organisms that rely on perception of dissolved chemical cues for their survival by impairing their chemosensory abilities.

When the dense algal blooms eventually die, microbial decomposition severely depletes dissolved oxygen, creating a hypoxic or anoxic 'dead zone' lacking sufficient oxygen to support most organisms. This process can be harmful to aquatic life, creating an environment where most organisms cannot survive.

Phosphates in detergents

Phosphorus is a naturally occurring nutrient that is essential for plant growth. In aquatic ecosystems, phosphorus, along with nitrogen, promotes the growth of algae and aquatic plants, which provide food and habitats for fish and other organisms. However, an excess of phosphorus in waterways can cause environmental issues, such as eutrophication, where the over-enrichment of water with nutrients leads to excessive growth of algae and other aquatic plants. This process can deplete oxygen levels in the water, creating "dead zones" where aquatic life cannot survive.

Phosphates, which contain phosphorus, have been commonly used in detergents due to their excellent cleaning properties. They were particularly effective in preventing 'hard-water' type limescale deposits. However, due to environmental concerns, there has been a growing trend to phase out the use of phosphates in detergents in many countries. As early as the 1980s, Italy began phasing out phosphates, and other countries, such as Canada, Australia, and the European Union, have followed suit with regulations and bans on phosphates in detergents.

In the United States, the discussion of banning phosphates in detergents arose due to the pollution of the Great Lakes. As of 2010, 17 states had implemented partial or full bans on the use of phosphates in dish detergents, and two states, Maryland and New York, extended the ban to commercial dishwashing. These bans have had a positive impact on reducing the contribution of phosphates to wastewater, but it is important to note that stormwater runoff and agricultural sources continue to be significant contributors to phosphate levels in waterways.

To address the environmental concerns associated with phosphates, manufacturers have been exploring alternative chemicals. For example, substitutes such as EDTA and other biodegradable chemicals are being used in detergent formulations. Additionally, liquid detergents have been found to be more environmentally friendly than powdered detergents, and consumers are encouraged to make the switch where possible.

Organic pesticides

Agroecosystems are affected by many pollutants, including agricultural pollutants, which are byproducts of farming practices. These agricultural sources of pollution include heavy metals, fertilizers, and pesticides.

Pesticides are designed to kill or repel unwanted species of insects, plants, or fungi. However, they can also have toxic effects on the environment and human health. For example, pesticides such as chlordane, dieldrin, hexachlorobenzene thiobencarb, and endrin resist degradation and remain in the environment as persistent organic pollutants. These pesticides can bioaccumulate, reaching high concentrations in the environment. This can have toxic effects on organisms such as honeybees and other beneficial pollinators.

The use of pesticides in agriculture can result in the contamination of soil and water sources. When pesticides are applied to fields, they can be absorbed by plant roots and transported through the vascular system, accumulating in the plant tissues. Excessive or moderate use of pesticides can result in persistent pesticide residues on plants, which can then be transferred to other parts of the environment, including nearby water bodies, through processes such as runoff.

Septic tanks

Nitrates are a significant concern, as they can lead to eutrophication. This process occurs when nutrients like nitrates and phosphates build up in water bodies, promoting excessive algae growth. While algae initially thrive on these nutrients, their subsequent death and decomposition consume oxygen in the water, leading to a lack of oxygen for aquatic life. This can result in dead zones where aquatic organisms cannot survive.

Nitrogen is another pollutant that can be released from septic tanks into the surrounding soil and eventually reach water bodies. It can enter groundwater and flow towards surface water bodies, potentially causing eutrophication. Saltwater is particularly vulnerable to nitrogen pollution, and high levels of nitrogen compounds in drinking water can pose health risks, such as reducing oxygen in the blood, a condition known as "blue baby" in infants.

Phosphorus pollution is also a concern, especially in freshwater. Phosphorus from wastewater can be absorbed and retained in the soil. However, if the soil cannot absorb all the phosphorus, the excess can travel in groundwater and contaminate water bodies. This increases algal growth, lowers dissolved oxygen levels, and contributes to eutrophication.

Additionally, failing septic systems can release pathogens, such as E. coli and Salmonella, into the soil and, ultimately, into surface waters. These harmful bacteria can cause flu-like symptoms and pose serious health risks to individuals exposed through swimming or wading in contaminated water.

To prevent water pollution from septic tanks, proper maintenance and usage are crucial. Homeowners should ensure their septic systems are well-maintained, properly designed, and not overloaded. Regular pumping and sludge removal are essential to maintaining the effectiveness of septic tanks in treating wastewater and preventing the release of pollutants into the soil and water.

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