Freshwater acidification

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Diagram depicting the sources and cycles of acid rain precipitation. Origins of acid rain.svg
Diagram depicting the sources and cycles of acid rain precipitation.

Freshwater acidification occurs when acidic inputs enter a body of fresh water through the weathering of rocks, invasion of acidifying gas (e.g. carbon dioxide), or by the reduction of acid anions, like sulfate and nitrate within a lake, pond, or reservoir. [1] Freshwater acidification is primarily caused by sulfur oxides (SOx) and nitrogen oxides (NOx) entering the water from atmospheric depositions and soil leaching. [1] Carbonic acid and dissolved carbon dioxide can also enter freshwaters, in a similar manner associated with runoff, through carbon dioxide-rich soils. [1] Runoff that contains these compounds may incorporate acidifying hydrogen ions and inorganic aluminum, which can be toxic to marine organisms. [1] Acid rain also contributes to freshwater acidification. [2] A well-documented case of freshwater acidification in the Adirondack Lakes, New York, emerged in the 1970s, driven by acid rain from industrial sulfur dioxide (SO₂) and nitrogen oxide (NOₓ) emissions. [3]

Contents

Causes

Natural

CO2 from the atmosphere or the decomposition of organic matter affects freshwater acidity. [4] The CO2 dissolved in water to form carbonic acid. This carbonic acid dissociated into hydrogen ions (H⁺) and bicarbonate (HCO₃⁻), which increases the H+ ions and leads to decrease in pH level. [5]

CO₂ + H₂O → H₂CO₃; H₂CO₃ ⇌ H⁺ + HCO₃⁻

Microbial activity breaks down of organic matter releases organic acids such as humic and fulvic acids. These acids accumulate in water bodies, especially those surrounded by forests and wetlands. [6] Peatlands and wetlands often produce acidic waters because of the high levels of organic matter decomposition. [7] This creates naturally acidic conditions, which are common in boreal and subarctic regions.

Volcanic activity can release sulfur dioxide (SO₂) and other acidic oxides into the atmosphere. [8] In air, sulfur dioxide converts to sulfuric acid: [9] This sulfuric acid dissociates into sulfate ions (SO₄²⁻) and hydrogen ions (H⁺), increasing the acidic condition.

SO2  +  ½ O2  +  H2O  →  H2SO4; H2SO4 → 2H⁺ + SO42⁻

Anthropogenic

Rio Tinto in Spain presents an acid drainage of both natural and artificial origin (mining) Rio tinto river CarolStoker NASA Ames Research Center.jpg
Rio Tinto in Spain presents an acid drainage of both natural and artificial origin (mining)

Human activities can significantly accelerate freshwater acidification. In addition to carbon dioxide, the combustion of fossil fuels sulfur dioxide (SO₂) and nitrogen oxides (NOₓ). These gases react with water and air to form sulfuric acid (H₂SO₄) and nitric acid (HNO₃). [8] [10] [11] Similar to sulfuric acid, nitric acid also decrease the pH level by dissociates into hydrogen ions (H⁺) and nitrate ions (NO₃⁻).

NOₓ + H₂O + ½ O₂ → HNO₃; HNO₃ → H⁺ + NO₃⁻

This process is particularly harmful in areas where the natural buffering capacity of the water is low, as these ecosystems are less able to neutralize the added acidity.

Mining can significantly contribute to freshwater acidification through the process of acid mine drainage. When sulfide minerals such as pyrite (FeS₂) are exposed to air and water during mining operations, they oxidize to form sulfuric acid. [12]

Buffering Capacity

A map depicting Atlantic Canada. Atlantic Canada - Natural Earth.jpg
A map depicting Atlantic Canada.

The buffering capacity of ecosystems helps them resist changes in pH. The presence bicarbonate (HCO₃⁻) and carbonate (CO₃²⁻) ions in freshwater systems can neutralize the income hydrogen ions (H+).

HCO₃⁻ + H⁺ → CO₂ + H₂O

However, low-alkalinity regions (e.g., with silicate bedrock) lack the natural buffering capacity to neutralize incoming ions, leading to rapid pH drops. [13] For example, the Atlantic region of Canada has the lowest acid deposition rates in Eastern North America, yet it has the most acidic waters on the continent due to the low buffering capacity of the regional bedrock and the addition of natural organic acids produced from close by wetlands. In most of the Atlantic region, granite and shale bedrock are found, which contain very little buffering material. Soil formed from low-buffering materials and the waters that drain from them are, therefore, susceptible to acidification, even under low acid deposition. [14]

Effects on ecosystems

This pond shows an overabundance of Sphagnum Nationaal Park Drents-Friese Wold. Locatie Fochteloerveen. Waterveenmos (Sphagnum cuspidatum), Veenpluis (Eriophorum angustifolium) 03.JPG
This pond shows an overabundance of Sphagnum

Acidification of freshwater ecosystems can decrease native biodiversity and can alter ecosystem structure and function entirely. [9] Macro-invertebrates and large vertebrates exhibit higher mortality and lower reproductive rates under acidified conditions. Conversely, algae thrive in acidified environments, and may quickly dominate these habitats, outcompeting other species. In particular, it is common to see an increase in the abundance of the sphagnum. Sphagnum has a high capacity to exchange H+ for basic cations within freshwater. The thick layer of sphagnum restricts the exchange between surface water and sediment, further contributing to reduction in nutrient cycling in the ecosystem. [9] Aquatic biomonitoring can be used to examine the health of aquatic ecosystems.

Soil that undergoes acidification can negatively impact agriculture. [15] Some species are able to withstand low pH levels in their environment. For example, frogs and perches can withstand a pH level of 4. [16] This allows these species to be unaffected by the acid deposition in their aquatic environment, allowing them to survive in these conditions. [16] However, most aquatic species, such as clams and snails, are unable to withstand low pH levels which negatively impacts their growth and survival. The high acidic levels deteriorate their thick shells decreasing their protection from predators. [16]

Minimizing acidification

Agricultural runoff is a major source of nitrogen and phosphorus, which contribute to freshwater acidification. Implementing best management practices (BMPs) in agriculture, such as reducing the use of chemical fertilizers, improving manure management, and adopting precision agriculture techniques, can significantly reduce nutrient runoff into water bodies. [17] Establishing riparian buffer zones—strips of vegetation planted along water bodies—can also help to filter pollutants from agricultural fields before they reach freshwater systems. [18] These measures not only reduce acidification but also mitigate eutrophication and improve overall water quality.

Wetlands and peatlands serve as buffers for freshwater systems by absorbing pollutants regulating water flow. [19] Wetland restoration projects have been shown to increase the resilience of freshwater systems to acidification and other environmental stressors. [20]

Liming is one of the most common and best practices for remediating acidification. In this process calcium carbonate (CaCO3) is added to the system to increase pH levels. [21] By increasing pH levels, liming helps the habitat return to a similar condition to how it was before acidification.

Some techniques are used to mitigate the mining contribution of acidification, like passive treatment through natural biological processes and treatment of the drainage with alkaline materials. [22] Another important factor to consider when looking at reducing freshwater acidification are the choices people make to protect the environment everyday. Following a circular approach to reduce, reuse and recycle can reduce resource depletion and waste minimization, including decreasing water acidity. [23]

Regulations

Regulation of anthropogenic emissions, specifically SOx and NOx, can lead to large decreases of acid rain and acidic bodies of water. [24] For example, the Canada-United States Air Quality Agreement has greatly minimized acid rain and ozone levels by 78% in Canada and 92% in the United States, as of 2020. [25] Moreover, investing in scientists to monitor and collect data is essential to create a model used to establish successful policies. [26] For instance, a protocol can be implemented to mitigate the issue. [26] Also, governments could invest funds to subsidize companies to decrease their pollution and incentivize them to use innovative methods of production, to lower both greenhouse gas emissions and the amount of acidic substances created. Furthermore, government institutions across the globe can connect on the issue of acidification and work together to find a feasible solution through international agreements. [15] Some successful government implementations include the Acid Rain Program [27] established in the United States in 1995, and the most recent Gothenburg Protocol, established by the United Nations Economic Commission for Europe (UNECE) to reduce acidification. [28]

Case Study: Freshwater Acidification in the Adirondack Lakes, New York

The Adirondack Lakes in New York is one of the most well-documented case studies for freshwater acidification. As early as the 1970s, it was showing signs of acidification due to low values of acid ANC (Acid Neutralizing Capacity) industrial emissions of sulfur dioxide (SO₂) and nitrogen oxides (NOₓ), resulting in acid rain. [3] Winds carried the pollutant from Midwestern United States to the Adirondack region and decreasing the pH level of water bodies and surrounding soils. [29] The acidification of waters resulted in a significant decline in aquatic biodiversity, including the disappearance of fish and crustacean species. [30]

Several efforts were made to recover the environmental condition of Adirondack lakes by reducing SO2 and NOₓ emissions through the Clean Air Act 1990. [3] Monitoring data shows improvements in water quality, although many ecosystems remain vulnerable due to the long-lasting effects of acid deposition on soils and watersheds. [31] This case demonstrates how the Clean Air Act have played a role in addressing the anthropogenic causes of freshwater acidification. However, studies show that ecological recovery remains challenging due to the long-term impacts of acid deposition. [32]

Further reading

Related Research Articles

<span class="mw-page-title-main">Acid rain</span> Rain that is unusually acidic

Acid rain is rain or any other form of precipitation that is unusually acidic, meaning that it has elevated levels of hydrogen ions. Most water, including drinking water, has a neutral pH that exists between 6.5 and 8.5, but acid rain has a pH level lower than this and ranges from 4–5 on average. The more acidic the acid rain is, the lower its pH is. Acid rain can have harmful effects on plants, aquatic animals, and infrastructure. Acid rain is caused by emissions of sulfur dioxide and nitrogen oxide, which react with the water molecules in the atmosphere to produce acids.

<span class="mw-page-title-main">Nitrate</span> Polyatomic ion (NO₃, charge –1) found in explosives and fertilisers

Nitrate is a polyatomic ion with the chemical formula NO−
3. Salts containing this ion are called nitrates. Nitrates are common components of fertilizers and explosives. Almost all inorganic nitrates are soluble in water. An example of an insoluble nitrate is bismuth oxynitrate.

<span class="mw-page-title-main">Sulfur dioxide</span> Chemical compound of sulfur and oxygen

Sulfur dioxide or sulphur dioxide is the chemical compound with the formula SO
2. It is a colorless gas with a pungent smell that is responsible for the odor of burnt matches. It is released naturally by volcanic activity and is produced as a by-product of copper extraction and the burning of sulfur-bearing fossil fuels.

<span class="mw-page-title-main">Nitrogen cycle</span> Biogeochemical cycle by which nitrogen is converted into various chemical forms

The nitrogen cycle is the biogeochemical cycle by which nitrogen is converted into multiple chemical forms as it circulates among atmospheric, terrestrial, and marine ecosystems. The conversion of nitrogen can be carried out through both biological and physical processes. Important processes in the nitrogen cycle include fixation, ammonification, nitrification, and denitrification. The majority of Earth's atmosphere (78%) is atmospheric nitrogen, making it the largest source of nitrogen. However, atmospheric nitrogen has limited availability for biological use, leading to a scarcity of usable nitrogen in many types of ecosystems.

<span class="mw-page-title-main">Soil pH</span> Measure of how acidic or alkaline the soil is

Soil pH is a measure of the acidity or basicity (alkalinity) of a soil. Soil pH is a key characteristic that can be used to make informative analysis both qualitative and quantitatively regarding soil characteristics. pH is defined as the negative logarithm (base 10) of the activity of hydronium ions in a solution. In soils, it is measured in a slurry of soil mixed with water, and normally falls between 3 and 10, with 7 being neutral. Acid soils have a pH below 7 and alkaline soils have a pH above 7. Ultra-acidic soils and very strongly alkaline soils are rare.

<span class="mw-page-title-main">Fen</span> Type of wetland fed by mineral-rich ground or surface water

A fen is a type of peat-accumulating wetland fed by mineral-rich ground or surface water. It is one of the main types of wetland along with marshes, swamps, and bogs. Bogs and fens, both peat-forming ecosystems, are also known as mires. The unique water chemistry of fens is a result of the ground or surface water input. Typically, this input results in higher mineral concentrations and a more basic pH than found in bogs. As peat accumulates in a fen, groundwater input can be reduced or cut off, making the fen ombrotrophic rather than minerotrophic. In this way, fens can become more acidic and transition to bogs over time.

<span class="mw-page-title-main">Water pollution</span> Contamination of water bodies

Water pollution is the contamination of water bodies, with a negative impact on their uses. It is usually a result of human activities. Water bodies include lakes, rivers, oceans, aquifers, reservoirs and groundwater. Water pollution results when contaminants mix with these water bodies. Contaminants can come from one of four main sources. These are sewage discharges, industrial activities, agricultural activities, and urban runoff including stormwater. Water pollution may affect either surface water or groundwater. This form of pollution can lead to many problems. One is the degradation of aquatic ecosystems. Another is spreading water-borne diseases when people use polluted water for drinking or irrigation. Water pollution also reduces the ecosystem services such as drinking water provided by the water resource.

A hydrogen ion is created when a hydrogen atom loses an electron. A positively charged hydrogen ion (or proton) can readily combine with other particles and therefore is only seen isolated when it is in a gaseous state or a nearly particle-free space. Due to its extremely high charge density of approximately 2×1010 times that of a sodium ion, the bare hydrogen ion cannot exist freely in solution as it readily hydrates, i.e., bonds quickly. The hydrogen ion is recommended by IUPAC as a general term for all ions of hydrogen and its isotopes. Depending on the charge of the ion, two different classes can be distinguished: positively charged ions and negatively charged ions.

<span class="mw-page-title-main">Alkalinity</span> Capacity of water to resist changes in pH that would make the water more acidic

Alkalinity (from Arabic: القلوية, romanized: al-qaly, lit. 'ashes of the saltwort') is the capacity of water to resist acidification. It should not be confused with basicity, which is an absolute measurement on the pH scale. Alkalinity is the strength of a buffer solution composed of weak acids and their conjugate bases. It is measured by titrating the solution with an acid such as HCl until its pH changes abruptly, or it reaches a known endpoint where that happens. Alkalinity is expressed in units of concentration, such as meq/L (milliequivalents per liter), μeq/kg (microequivalents per kilogram), or mg/L CaCO3 (milligrams per liter of calcium carbonate). Each of these measurements corresponds to an amount of acid added as a titrant.

<span class="mw-page-title-main">Marine pollution</span> Pollution of oceans from substances discarded by humans

Marine pollution occurs when substances used or spread by humans, such as industrial, agricultural and residential waste, particles, noise, excess carbon dioxide or invasive organisms enter the ocean and cause harmful effects there. The majority of this waste (80%) comes from land-based activity, although marine transportation significantly contributes as well. It is a combination of chemicals and trash, most of which comes from land sources and is washed or blown into the ocean. This pollution results in damage to the environment, to the health of all organisms, and to economic structures worldwide. Since most inputs come from land, either via the rivers, sewage or the atmosphere, it means that continental shelves are more vulnerable to pollution. Air pollution is also a contributing factor by carrying off iron, carbonic acid, nitrogen, silicon, sulfur, pesticides or dust particles into the ocean. The pollution often comes from nonpoint sources such as agricultural runoff, wind-blown debris, and dust. These nonpoint sources are largely due to runoff that enters the ocean through rivers, but wind-blown debris and dust can also play a role, as these pollutants can settle into waterways and oceans. Pathways of pollution include direct discharge, land runoff, ship pollution, bilge pollution, atmospheric pollution and, potentially, deep sea mining.

<span class="mw-page-title-main">Hydrobiology</span> Science of life and life processes in water

Hydrobiology is the science of life and life processes in water. Much of modern hydrobiology can be viewed as a sub-discipline of ecology but the sphere of hydrobiology includes taxonomy, economic and industrial biology, morphology, and physiology. The one distinguishing aspect is that all fields relate to aquatic organisms. Most work is related to limnology and can be divided into lotic system ecology and lentic system ecology.

<span class="mw-page-title-main">Ocean acidification</span> Decrease of pH levels in the ocean

Ocean acidification is the ongoing decrease in the pH of the Earth's ocean. Between 1950 and 2020, the average pH of the ocean surface fell from approximately 8.15 to 8.05. Carbon dioxide emissions from human activities are the primary cause of ocean acidification, with atmospheric carbon dioxide levels exceeding 422 ppm. CO2 from the atmosphere is absorbed by the oceans. This chemical reaction produces carbonic acid which dissociates into a bicarbonate ion and a hydrogen ion. The presence of free hydrogen ions lowers the pH of the ocean, increasing acidity. Marine calcifying organisms, such as mollusks and corals, are especially vulnerable because they rely on calcium carbonate to build shells and skeletons.

Soil acidification is the buildup of hydrogen cations, which reduces the soil pH. Chemically, this happens when a proton donor gets added to the soil. The donor can be an acid, such as nitric acid, sulfuric acid, or carbonic acid. It can also be a compound such as aluminium sulfate, which reacts in the soil to release protons. Acidification also occurs when base cations such as calcium, magnesium, potassium and sodium are leached from the soil.

<span class="mw-page-title-main">Human impact on the nitrogen cycle</span>

Human impact on the nitrogen cycle is diverse. Agricultural and industrial nitrogen (N) inputs to the environment currently exceed inputs from natural N fixation. As a consequence of anthropogenic inputs, the global nitrogen cycle (Fig. 1) has been significantly altered over the past century. Global atmospheric nitrous oxide (N2O) mole fractions have increased from a pre-industrial value of ~270 nmol/mol to ~319 nmol/mol in 2005. Human activities account for over one-third of N2O emissions, most of which are due to the agricultural sector. This article is intended to give a brief review of the history of anthropogenic N inputs, and reported impacts of nitrogen inputs on selected terrestrial and aquatic ecosystems.

In the study of air pollution, a critical load is defined as "a quantitative estimate of an exposure to one or more pollutants below which significant harmful effects on specified sensitive elements of the environment do not occur according to present knowledge".

Marine chemistry, also known as ocean chemistry or chemical oceanography, is the study of the chemical composition and processes of the world’s oceans, including the interactions between seawater, the atmosphere, the seafloor, and marine organisms. This field encompasses a wide range of topics, such as the cycling of elements like carbon, nitrogen, and phosphorus, the behavior of trace metals, and the study of gases and nutrients in marine environments. Marine chemistry plays a crucial role in understanding global biogeochemical cycles, ocean circulation, and the effects of human activities, such as pollution and climate change, on oceanic systems. It is influenced by plate tectonics and seafloor spreading, turbidity, currents, sediments, pH levels, atmospheric constituents, metamorphic activity, and ecology.

Estuarine acidification happens when the pH balance of water in coastal marine ecosystems, specifically those of estuaries, decreases. Water, generally considered neutral on the pH scale, normally perfectly balanced between alkalinity and acidity. While ocean acidification occurs due to the ongoing decrease in the pH of the Earth's oceans, caused by the absorption of carbon dioxide (CO2) from the atmosphere, pH change in estuaries is more complicated than in the open ocean due to direct impacts from land run-off, human impact, and coastal current dynamics. In the ocean, wave and wind movement allows carbon dioxide (CO2) to mixes with water (H2O) forming carbonic acid (H2CO3). Through wave motion this chemical bond is mixed up, allowing for the further break of the bond, eventually becoming carbonate (CO3) which is basic and helps form shells for ocean creatures, and two hydron molecules. This creates the potential for acidic threat since hydron ions readily bond with any Lewis Structure to form an acidic bond. This is referred to as an oxidation-reduction reaction.

<span class="mw-page-title-main">Marine biogeochemical cycles</span>

Marine biogeochemical cycles are biogeochemical cycles that occur within marine environments, that is, in the saltwater of seas or oceans or the brackish water of coastal estuaries. These biogeochemical cycles are the pathways chemical substances and elements move through within the marine environment. In addition, substances and elements can be imported into or exported from the marine environment. These imports and exports can occur as exchanges with the atmosphere above, the ocean floor below, or as runoff from the land.

<span class="mw-page-title-main">Ocean acidification in the Arctic Ocean</span>

The Arctic Ocean covers an area of 14,056,000 square kilometers, and supports a diverse and important socioeconomic food web of organisms, despite its average water temperature being 32 degrees Fahrenheit. Over the last three decades, the Arctic Ocean has experienced drastic changes due to climate change. One of the changes is in the acidity levels of the ocean, which have been consistently increasing at twice the rate of the Pacific and Atlantic oceans. Arctic Ocean acidification is a result of feedback from climate system mechanisms, and is having negative impacts on Arctic Ocean ecosystems and the organisms that live within them.

<span class="mw-page-title-main">Ammonia pollution</span> Chemical contamination

Ammonia pollution is pollution by the chemical ammonia (NH3) – a compound of nitrogen and hydrogen which is a byproduct of agriculture and industry. Common forms include air pollution by the ammonia gas emitted by rotting agricultural slurry and fertilizer factories while natural sources include the burning coal mines of Jharia, the caustic Lake Natron and the guano of seabird colonies. Gaseous ammonia reacts with other pollutants in the air to form fine particles of ammonium salts, which affect human breathing. Ammonia gas can also affect the chemistry of the soil on which it settles and will, for example, degrade the conditions required by the sphagnum moss and heathers of peatland.

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