Eutrophication and Water Pollution
Water quality continues to remain one of the top concerns for ensuring environmental integrity and public health. Poor water quality can disrupt ecosystem balance, reduce biodiversity, and place stress on water treatment facilities, giving rise to several public health issues. Pollution occurs through the release of harmful particles into the environment. Harmful particles may include chemicals, nutrients, microorganisms, or energy (radioactive or thermal).
Eutrophication refers to a process where excess nutrients, such as nitrogen and phosphorus, overwhelm a waterbody and cause the proliferation of algae and other aquatic plants. When algae for example, grows too much, it smothers other organisms and blocks sunlight for them. An overgrowth of one organism over another causes an imbalance in the ecosystem.
When this excess algae dies, microorganisms use more oxygen to decompose it. Decomposing algae depletes oxygen from the water, impairing the survival of all aerobic organisms. Low-oxygen conditions, known as Hypoxia, occur every summer in parts of waterbodies such as Lake Erie, Chesapeake Bay and the Gulf of Mexico.
In some cases, algal blooms can be poisonous to humans, fishes, birds and other organisms. Harmful Algal Blooms (HAB) refer to a type of algae, mainly Cyanobacteria or Blue-green algae, that produces toxins and causes neurological, respiratory or liver damage. Humans and animals can be exposed to HABs by ingesting or inhaling the toxins during swimming, boating, or drinking contaminated water.
Plant growth depends on nutrients such as nitrogen and phosphorus, among other sources like light, water and carbon. Similarly, algae, being a plant, feeds on nutrients that may enter the waterbody through natural or anthropogenic means.
HOW DO NUTRIENTS ENTER WATERBODIES?
Nutrients can enter waterbodies through soil erosion, stormwater runoff, sewage systems or atmospheric deposition.
Fertilizers, pesticides, and manure contain nitrogen which can runoff into waterbodies from farms and lawns. Some sewage system overflows may also add nutrient-laden waste or soaps into waterbodies. Nitrogen may also enter the atmosphere (e.g. nitrogen oxide, ammonia) through vehicle emissions and industrial sources such as power plants, industrial boilers, cement kilns and turbines.
Several factors can enhance eutrophication in local waterbodies. The next section will elaborate on different practices or aspects that can increase runoff, emissions, or the need for excess nutrient application.
FACTORS THAT INFLUENCE EUTROPHICATION
Pesticide and Fertilizer Application
The United States uses about 1 billion pounds of pesticides per year to promote plant growth on farms, residential lawns and other green spaces. Pesticides and fertilizers both contain nutrients such as nitrogen and phosphorus that can runoff and end up in waterbodies. Runoff depends on soil characteristics, topography, climate and soil mismanagement.
More than 100,000 miles of U.S. rivers and 2.5 million acres of lakes and ponds suffer poor water quality because of nitrogen and phosphorus pollution. Depending on soil characteristics and rainfall intensity, nutrients can also seep through the soil and into groundwater. Groundwater provides the bulk of drinking water to millions of Americans.
The need for excessive fertilizer and pesticide application depends largely on soil and land management. Various soil characteristics govern the soil’s ability to store nutrients and make them available for plant uptake. Essentially, the need for excessive fertilizer application may decrease if plants readily absorb soil nutrients, rather than having these nutrients runoff into water bodies.
Soil Characteristics
Soil naturally contains several nutrients and minerals for plant growth. Soils with poor drainage and infiltration cause waterlogging on the surface and induce runoff during rainstorms, snowmelt, or irrigation. Compaction and low porosity hinder the soil’s ability to absorb and store water, leading to a smoother surface for water to flow as runoff.
Soils with high salt content can also decrease porosity, making the soil crusty, dry, weak and detachable. Salt pollution can occur through the application of road salts during the snowy weather. Excessive use of fertilizers may also increase soil salinity. When soil erodes quicker or water runs off easier, both mediums carry nutrients towards lowlands and waterbodies.
Excessive tillage is another detrimental agricultural practice that can weaken soil function and cause erosion. While occasional tillage may remain necessary to prepare farm beds and promote soil aeration, too much tillage destroys soil structure and redistributes nutrient-rich topsoil.
Logging and deforestation also increase soil erosion. Plant roots hold soil in place, stabilize soil structure and maintain soil organic matter (SOM). SOM refers to plant and animal residue in the process of being decomposed by microorganisms. SOM promotes particle binding and strengthens soil aggregates, reducing runoff and having a positive impact on water pollution.
Finally, creating cities and infrastructure also requires smoothing out surfaces and removing vegetation for constructing roads and buildings. As mentioned before, smoother surfaces allow for runoff to flow easier. Sloping landscapes with smooth surfaces accelerate runoff even more. Vegetation and trees act as an obstruction and filtration mechanism for runoff and erosion.
Tile Drainage Systems
A tile drainage system consists of perforated pipes installed underneath cropland in order to help drain excess water from a field. The underground pipes or “tiles” absorb excess water from soil and transport it to the nearest waterbody. As the tiles convey water from fields to waterbodies, they may also end up carrying soluble nutrients.
Drainage tiles act as a crutch for soil with poor structure and drainage ability. While tile drainage systems offer benefits for crop yield, they also enhance runoff and nutrient export during wet weather. The United States has about 22.48 million hectares of tile-drained cropland.
Combined Sewer Systems
Combined sewer systems collect stormwater runoff and wastewater in the same pipe. During normal conditions, the system conveys stormwater into a waterbody and transports sewage towards a wastewater treatment plant. Heavy rainfall however places strain on the capacity of this system, causing some untreated sewage and stormwater to overflow into a waterbody. Combined sewer systems typically exist in older communities with outdated infrastructure. In the United States, approximately 700 communities experience combined sewer overflow (CSO) discharges.
More than 1.2 trillion gallons of untreated sewage, stormwater and industrial waste flow into rivers across the United States. Sewage contains high levels of nutrients to feed algae, including nitrogen and phosphorus. Domestic detergents may also contain phosphorus. Combined sewer overflows can directly lead to eutrophication and water pollution. Outside of eutrophication, untreated sewage in waterbodies also poses a serious public health problem. We will discuss different controls for combined sewer systems later in this post.
CONTROLLING NUTRIENT LOADS IN WATERBODIES
Lowering Fertilizer and Pesticide Use
Farmers may apply fertilizers and pesticides to enhance crop yield and meet the persistent demands of a growing population. Fertilizers provide essential nutrients for plants, especially in areas lacking fertile soil. Simply stopping the use of fertilizers and pesticides may not be feasible without first ensuring soil health and plant uptake of nutrients.
Sustainable agricultural practices can be implemented to maintain fertile topsoil and lower the need for fertilizer and pesticide application. Practices such as crop rotations, cover crops, and agroforestry can help promote soil health, increase infiltration, control runoff, and prevent eutrophication.
Crop rotations involve growing different crops on the same field in alternate seasons or years. Crop rotations stabilize soil erosion, improve water and nutrient retention and interrupt pest cycles, reducing the need for applying excess fertilizers and pesticides.
Cover crops include canopying, tall, or deep-rooted plants that are grown amidst other crops. Cover crops enrich topsoil and improve soil structure. Agroforestry combines trees with crops on the same field.
Both cover crops and agroforestry improve soil porosity, enhance hydraulic conductivity and promote nutrient uptake, hence curbing runoff and eutrophication. More vegetation would also increase the amount of residue available for producing soil organic matter. Both these management systems provide a great habitat for pollinators and wildlife while controlling environmental pollution.
These sustainable practices may require additional costs, training, management and maintenance. For example, some plants may compete with primary crops for nutrients and water. Compatibility issues may arise when assessing which tree or cover crop would actually increase primary crop yield. Although these practices may have long-term benefits, crop yield may also decrease in the short-run in some cases.
Finally, fertilizers should be applied at the right time, when plant growth and nutrient uptake reaches its peak. Do not apply fertilizers during the rainy or summer season for example. Use fertilizers in mid-May or early fall so that plants can uptake the nutrients without them getting washed away in a rainstorm.
Pollutant Filtration
In addition to sustainable agriculture, management practices such as conservation buffers, bioretention systems, constructed wetlands, dry ponds and infiltration basins can also help filter pollutants before they reach waterbodies.
Conservation buffers include strips of vegetation that trap and uptake runoff nutrients, thus preventing them from flowing into a waterbody. Bioretention systems and rain gardens take buffers a step further as they contain special soil mixtures and a variety of plants to uptake nutrients and filter stormwater.
Constructed wetlands refer to man-made wetlands consisting of soil, water, plants, and microorganisms that help filter pollutants from agricultural, urban or industrial sources. The wetlands collect runoff while internal microbial processes and plants help remove excess nutrients from the water. In order to treat water laden with nitrates, microorganisms help facilitate a process called denitrification, where they convert nitrates into a less harmful and less mobile nitrogen gas.
Dry ponds temporarily store stormwater runoff in small excavated ditches. Particles settle out at the bottom and allow cleaner water to flow through. Detention basins can slow runoff and control floods, however, proper water quality efforts may need other management practices. Dry ponds may also promote mosquito breeding and carry the risk of disease and contamination. Infiltration basins are vegetated depressions that store runoff. The vegetation and microorganisms in the soil help the runoff infiltrate through the ground, naturally going dry in a few days and preventing mosquito proliferation.
Separated Sewer Systems
Separated sewer systems collect rainwater and wastewater in separate pipes. One pipe directs rainwater to a waterbody and the other pipe sends wastewater to a treatment facility. Separating the pipes prevents untreated sewage from entering waterbodies during heavy rainfall, thus reducing the nutrient load for eutrophication.
Separated sewer systems pose limitations when it comes to nutrient pollution from farms, urban areas and other non-point sources. These systems only address sewage overflows while allowing stormwater runoff to flow directly into waterbodies. To treat and clean stormwater runoff, separated sewer systems should also include a stormwater treatment vault.
A stormwater treatment vault refers to an underground system designed to remove pollutants from stormwater runoff before they reach waterbodies. The vault attaches to stormwater pipes and includes chambers, baffles and filtration media in order to trap and treat polluted stormwater. The chambers and baffles allow larger sediments to settle out of the flow. Filters could include sand, peat or other media that work via screening, trapping, or ion-exchange.
Stormwater treatment vaults require regular maintenance. Filtration media may only trap specific particles and may not remove all types of pollutants and nutrients. To be completely effective against eutrophication, separated sewer systems must be accompanied by other management practices such as buffers, bioretention areas, wetlands and stormwater basins.
EUTROPHIC WATER REMEDIATION
Some physical and chemical techniques can help remediate waterbodies already experiencing eutrophication. Physical strategies may incorporate dredging and aeration, while chemical methodologies can include coagulation/flocculation and adsorption.
Dredging involves excavating the bottom of a waterbody to potentially remove nutrient-laden sediments. Dredging however deepens the waterbody, increases turbidity, and destroys aquatic habitat. Aeration increases oxygen levels in the waterbody in order to abate eutrophication and maintain the survival of aerobic organisms.
Chemical coagulants such as ferric sulphate, magnesium sulphate and lime can help clump smaller particles together, making them easier to filter or settle out. These larger particles are known as “floc”. An adsorbent contains a solid material that attracts materials to collect on its surface. Adsorbents based on iron, aluminum, calcium and magnesium can effectively attract phosphates and lower eutrophication.
Regardless of the effectiveness of remediation techniques, prevention should always trump reaction. Remediation methods increase costs, require maintenance, and have the potential to further disrupt the environment.
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