High Nitrate Levels: A Thermal Pollution Trigger?

Nitrates are essential for plant growth and are naturally present in the environment. However, human activities, such as agriculture, industry, and sewage, have led to an increase in nitrate levels in water bodies, causing water pollution. High nitrate levels in drinking water can have adverse health effects, especially on infants, and can also lead to ecological consequences, including oxygen depletion and excessive algae growth. This raises the question: do high nitrate levels also contribute to thermal pollution in water bodies?

Nitrate levels in water

Nitrogen is a crucial nutrient that helps plants and crops grow, but high concentrations are harmful to people and nature. Clean water is vital to human health and natural ecosystems. Excess nitrogen from agricultural sources is one of the main causes of water pollution in Europe. Nitrates and organic nitrogen compounds from fertilizer and manure enter groundwater through leaching and reach surface water through runoff from agricultural fields.

The U.S. Environmental Protection Agency (EPA) standard for nitrate in drinking water is 10 milligrams of nitrate (measured as nitrogen) per liter of drinking water (mg/L). The natural level of ammonia or nitrate in surface water is typically low (less than 1 mg/L); in the effluent of wastewater treatment plants, it can range up to 30 mg/L. Sources of nitrates include wastewater treatment plants, runoff from fertilized lawns and cropland, failing on-site septic systems, runoff from animal manure storage areas, and industrial discharges that contain corrosion inhibitors.

High levels of nitrate—over 10 mg/L in water—can result from runoff or leakage from fertilized soil, wastewater, landfills, animal feedlots, septic systems, or urban drainage. About 4% of private wells constructed in Minnesota since 1991 have nitrate levels above 3 mg/L. While 3 mg/L is less than the EPA standard, it suggests human-made sources of nitrate may have contaminated the water, and the level could increase over time. The Minnesota Department of Agriculture Township Testing Program found that over 10% of the private wells sampled in some townships in Minnesota have nitrate levels above 10 mg/L.

Consuming too much nitrate can affect how blood carries oxygen and cause methemoglobinemia (also known as "blue baby syndrome"). Bottle-fed babies under six months old are at the highest risk of getting this illness, which can result in serious illness or death. Symptoms will often resolve once the nitrate source is removed. People with glucose-6-phosphate-dehydrogenase deficiency or other metabolic conditions may be at higher risk of getting this illness.

To prevent nitrate pollution, the Nitrates Directive in Europe requires EU Member States to monitor the quality of waters and identify areas that drain into polluted waters or are at risk of pollution. These areas are defined as Nitrate Vulnerable Zones (NVZs). In the United States, the Minnesota Pollution Control Agency found that 27% of surface water samples had nitrate levels above 10 mg/L.

Sources of high nitrate levels

Nitrate is a compound that occurs naturally and has many human-made sources. It is found in some lakes, rivers, and groundwater. Nitrate can be introduced to water sources through runoff or leakage from fertilized soil, wastewater, landfills, animal feedlots, septic systems, or urban drainage. In Minnesota, the Minnesota Pollution Control Agency found that 27% of surface water samples had nitrate levels above 10 mg/L.

Nitrate is also commonly found in food sources, particularly in green leafy vegetables and root vegetables. Examples of vegetables with high levels of nitrate include spinach, lettuce, broccoli, carrots, and beetroot. Nitrates are also added to some foods, especially cured meats such as ham, deli meat, and bacon, to improve colour and taste and to prevent the growth of bacteria.

In addition to natural sources, human activities can also contribute to high nitrate levels in water. For instance, nitrate is found in fertilizers, manure, agricultural runoff, dairy lagoons, and liquid waste discharged from septic tanks. Rain or irrigation can carry nitrate from these sources into the soil and subsequently into groundwater, which may then be drawn into wells and drinking water supplies.

High nitrate levels in drinking water can pose health risks, particularly for infants under six months old who are at risk of developing methemoglobinemia, also known as "blue baby syndrome". This condition affects the ability of red blood cells to carry oxygen, causing the lips and skin to turn a bluish colour. Other potential health impacts of long-term exposure to high nitrate levels in drinking water include associations with thyroid problems, adverse pregnancy outcomes, and cancers, particularly colorectal cancer.

Nitrate pollution and agriculture

Nitrogen is a crucial nutrient that helps plants and crops grow, but high concentrations are harmful to people and nature. Pure, clean water is vital to human health and natural ecosystems. Excess nitrogen from agricultural sources is one of the main causes of water pollution in Europe. Nitrates and organic nitrogen compounds from fertiliser and manure enter groundwater through leaching and reach surface water through runoff from agricultural fields. A high level of nitrate makes water unsuitable as drinking water.

Nitrate pollution of ground and surface water bodies worldwide is generally linked with continually increasing global fertiliser nitrogen (N) use. After 1990, with more fertiliser N consumption in developing countries, especially in East and South Asia, nitrate pollution of freshwaters is now becoming a pervasive global problem. It is now evident that not more than 50% of the fertiliser N is directly used by the crops to which it is applied. While a small portion may directly leach down and reach groundwater and surface water bodies, a large proportion ends up in the soil organic N pool from where N is mineralised and is taken up by plants and/or lost via leaching during several decades. Present trends of nitrate pollution of freshwaters, therefore, reflect legacies of current and past applications of fertilisers and manures.

Sources of nitrates include wastewater treatment plants, runoff from fertilised lawns and cropland, failing on-site septic systems, runoff from animal manure storage areas, and industrial discharges that contain corrosion inhibitors. Nitrates from land sources end up in rivers and streams more quickly than other nutrients like phosphorus. This is because they dissolve in water more readily than phosphates, which have an attraction for soil particles. As a result, nitrates serve as a better indicator of the possibility of a source of sewage or manure pollution during dry weather.

The EU has been addressing the issue since the 1990s, and things have improved, but even in some of the richest countries in the world, nitrate levels are still too high. In Denmark, a national nitrate management plan, in place since the 1990s, offers farmers advice on efficient fertiliser use and imposes an annual nitrogen 'budget' for agriculture that mustn't be exceeded. Between 1985 and 2010, Denmark saw a 40% reduction in surplus nitrogen. The Nitrates Directive requires EU Member States to monitor the quality of waters and identify areas that drain into polluted waters or are at risk of pollution. These concern waters that due to agricultural activities are eutrophic or could contain a concentration of more than 50 mg/l of nitrates. Those areas are defined as Nitrate Vulnerable Zones (NVZs).

Nitrate pollution and health

Nitrate pollution in groundwater is a pressing issue, with human activities being the largest source of contamination. The primary human-induced causes of nitrate pollution are septic tanks, nitrogen-rich fertilizers used on turfgrass, and agricultural processes. Natural processes can also cause low levels of nitrate in drinking water, usually below 3 mg/L. However, when nitrate-nitrogen concentrations exceed safe levels, there can be detrimental biological consequences for organisms dependent on groundwater, including humans.

High nitrate levels in drinking water pose a significant health risk, particularly to infants. The United States Environmental Protection Agency (EPA) has established a drinking water standard and health advisory level of 10 mg/L nitrate-nitrogen, equivalent to 10 ppm nitrate-nitrogen or 45 ppm nitrate. This standard aims to safeguard against the potentially fatal condition methemoglobinemia, commonly known as "blue baby syndrome." The condition reduces the blood's ability to carry oxygen, leading to symptoms such as shortness of breath and blueness of the skin. Infants below six months old are most vulnerable and can succumb to the illness rapidly if they consume water with nitrate levels above the advisory limit.

The contamination of private wells with high nitrate levels is a significant concern. Wells constructed near septic systems or in areas with agricultural activities are particularly vulnerable. In rural regions, the incidence of methemoglobinemia has been linked to elevated nitrate concentrations in private wells. This underscores the necessity of implementing preventative measures and treatment methods to protect groundwater sources from nitrate pollution.

Additionally, nitrate pollution in water bodies can have broader ecological implications. Excessive nitrates in rivers, lakes, and marine waters can stimulate excessive algae growth, disrupting the natural ecosystem and leading to oxygen depletion in the water. This hypoxic condition, characterized by low dissolved oxygen levels, can become toxic to warm-blooded animals at higher nitrate concentrations.

Recent scientific studies have also suggested possible health impacts associated with long-term exposure to nitrate in drinking water, even at levels below the current regulatory standards. These potential health risks include thyroid problems, adverse pregnancy outcomes, and an increased incidence of cancers, particularly colorectal cancer. These findings highlight the importance of ongoing research and continuous evaluation of nitrate standards to ensure the protection of public health.

Reducing nitrate levels

Nitrates are not all bad, and tanks or aquariums need some level of nitrates to survive. However, elevated nitrates in a reef tank reduce water quality, interfere with coral health, and contribute to nuisance algae growth. High nitrate levels in drinking water can also be harmful to human health. Therefore, it is important to keep nitrate levels in check. Here are some ways to reduce nitrate levels:

Water Changes

One of the most common ways to reduce nitrate levels is by performing water changes. In an established system, changing 20% to 50% of the water every month will help reduce nitrate levels. The frequency of water changes depends on the type of fish, maintenance of the tank, biofiltration, and type of filtration media. Small, frequent water changes can be more effective than a large weekly water change. However, this method may not be suitable for saltwater aquariums as it requires adding more salt, and can cause a change in pH levels.

Aquatic Plants

Adding aquatic plants to your aquarium can help reduce nitrate levels as they use nitrates as fertilizer. In a freshwater aquarium, you can add plants such as macroalgae, which is a quick grower and takes up nitrates and phosphates. After the algae absorb the nitrates, they can be harvested and removed from the tank.

Natural Nitrate Reduction

Consider using a refugium with macroalgae, a chaeto-reactor, or a turf scrubber to naturally reduce nitrates. These methods are effective and have no ongoing maintenance costs. Algal filtrations use algae growth to remove nitrates, and as the algae grow, they absorb nitrates and phosphates from the water. Once the algae are harvested, the nitrates are effectively removed.

Limit Nitrogen Fertilizers

To prevent nitrate losses from leaching and runoff, limit the periods when nitrogen fertilizers are applied to the land. Target the application to periods when crops require nitrogen. Additionally, limit the conditions for fertilizer application, such as avoiding steeply sloping ground, frozen or snow-covered ground, or areas near watercourses.

Monitor Nitrate Levels

Regularly monitor nitrate levels in water systems to ensure they meet the Environmental Protection Agency (EPA) standard of 10 milligrams of nitrate per liter of drinking water (mg/L). The EPA standard protects infants against methemoglobinemia, which is caused by consuming too much nitrate and can lead to serious illness or death.

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