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refers to minerals, metals, salts, other that dissolve in water

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Non-point sources of pollution are often termed ‘diffuse’ pollution and refer to those inputs and impacts which occur over a wide area and are not easily attributed to a single source.
http://www.epa.vic.gov.au/your-environment/water/protecting-victorias-waters/point-and-nonpoint-sources-of-water-pollution

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A point source is a single, identifiable source of pollution.

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Pollutants enter the water environment from two main types of sources.

Point sources

A point source is a single, identifiable source of pollution (Wikipedia), such as a pipe or a drain. Industrial wastes are commonly discharged to rivers and the sea in this way. High risk point source waste discharges are regulated by EPA through the works approval and licensing system, and associated compliance and enforcement activities.

EPA's regulation of point source waste discharges has been important in improving the quality of Victoria's water environment over the past 40 years. No longer do we have raw sewage and abattoir waste continually flowing in our rivers as per the 1970s. While focus and effort is still needed to maintain and improve these further, some of the major threats to the health of Victoria's water quality are now from nonpoint sources.

Nonpoint sources

Non-point sources of pollution are often termed 'diffuse' pollution and refer to those inputs and impacts which occur over a wide area and are not easily attributed to a single source. They are often associated with particular land uses, as opposed to individual point source discharges.

Urban land use

In our urban areas rainfall run-off as stormwater is one of the major nonpoint sources of pollution impacting the water quality of our waterways and bays. Stormwater from street surfaces is often contaminated with car oil, dust and the faeces of animals and soil and sediment run-off from construction sites, and in industrial areas often contains more toxicants and chemicals.

In some outer-urban and urban fringe areas, a reticulated sewerage system is not available so sewage is discharged to onsite wastewater systems and septic tanks. Seepage and surface run-off of septic tank effluents may also be forms of non-point source pollution of streams in these areas.

Agricultural land use

In farming areas non-point sources of pollution include pesticides, fertilisers, animal manure and soil washed into streams in rainfall run-off. Where stock are given access to stream banks they may foul the water and accelerate erosion.

Forestry land use

Forestry operations may contribute to non-point source pollution of streams by increasing soil erosion and sediment run-off.

Non-point source pollution is often more difficult to control than point source pollution. In urban areas the provision of reticulated sewerage systems and adequate street cleaning are important measures, while in farming and forestry areas, soil conservation practices and the controlled application of pesticides and fertilisers are necessary if pollution of waterways is to be avoided.

While EPA does not directly licence or approve many of these non-point source activities through regulation, we often work with other partners from government, industry and community across different aspects of our operating model to help tackle these issues.

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Point and nonpoint sources of water pollution

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Pollutants enter the water environment from two main types of sources. A point source is a single, identifiable source of pollution (Wikipedia), such as a pipe or a drain. Industrial wastes are commonly discharged to rivers and the sea in this way.

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An overabundance of algae (called an algal bloom) may cause pH levels in an estuary to rise significantly, and this can be lethal to aquatic animals.
https://oceanservice.noaa.gov/education/kits/estuaries/media/supp_estuar10f_ph.html

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national oceanic and atmospheric administration

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NOAA's National Ocean Service Education: Estuaries

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Each number represents a 10-fold change in the acidity/basicness of the water. Water with a pH of five is ten times more acidic than water having a pH of six.
https://water.usgs.gov/edu/ph.html

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Erosion is the process of eroding or being eroded by wind, water, or other natural agents.
The pH can change through erosion of materials like limestone

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Runoff from fields with pesticides or fertilizers can cause the pH to change rapidly, harming the ecosystem.

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America's hugely productive food system is one of its success stories. The nation will export a projected $139.5 billion in agricultural products this fiscal year alone. It's an industry that supports "more than 1 million jobs," according to Agriculture Secretary Tom Vilsack.

But all that productivity has taken a toll on the environment, especially rivers and lakes: Agriculture is the nation's leading cause of impaired water quality, according to the U.S. Environmental Protection Agency.

Scientists want to get a better sense of how all that agricultural runoff is affecting water quality. So this summer, three dozen scientists from the EPA and U.S. Geological Survey are wading into some 100 streams, from Ohio to Nebraska. Their mission: Test for hundreds of pesticides and nutrients used in farming, and check for possible effects on what's living in the streams.

This is the first time scientists have tested for so many chemicals in a whole region's waters or considered the impact of agricultural runoff on fish, frogs, bugs and algae at this scale. The study is costing the USGS $6 million and the EPA $570,000.

"These aren't the kinds of studies that are done routinely, because they are pretty difficult to do," says USGS biologist Diana Papoulias. "But we know that some of these chemicals that we're finding in the runoff from the ag fields can affect [aquatic] reproduction and egg production. Whether they are at the concentrations that can do that or not - we don't know yet."

Some tests being conducted can measure even the tiniest amounts of mercury, livestock hormones and pesticides, including the weed killer glyphosate - better known by its trade name, Roundup.

Farm runoff has become an even more pressing concern this year because of the Midwest's extremely wet spring. In between the frequent storms, farmers had only brief stretches of weather dry enough to apply pesticides and fertilizers.

"Whether we want to blame it on climate change or just variability in the weather conditions, anytime you have these heavier rainfalls during the spring, after a drier period when you could have got something put in the field, you're going to see, in most cases, a large amount of runoff," says Bob Broz, a water quality specialist with the University of Missouri Extension.

"It's been happening for years," he says. "The problem is now we seem to be seeing more of these more intense rainfalls. And that, in turn, creates a huge amount of nutrient loss."

Nutrients like nitrogen and phosphorus are essential for growing crops, but they can also trigger algal growth in rivers, lakes and bays.

Because last year's drought prevented some parched fields from absorbing fertilizers, an extra load of nutrients is flowing into Midwest waterways and will end up in the Gulf of Mexico. Once there, the chemicals can threaten aquatic life, says Bob Lerch, a USDA soil scientist.

"There's the direct impact on the aquatic ecosystem," Lerch says. "And then there's the downstream impacts on say, drinking water, or a reservoir, or a recreational [body of water]."

Agricultural runoff flows into the lakes and rivers that hundreds of towns draw their water from. For example, herbicide runoff from a farm in Centralia, Mo., might end up in Goodwater Creek, which empties into the Salt River, which then flows into Mark Twain Lake. That lake provides drinking water for 70,000 residents. Water treatment plants spend millions on chemicals to clean up that surface water.

In northeast Missouri, for example, the Clarence Cannon Wholesale Water Commission treats 1.5 billion gallons of water each year. On a walk through the plant, Mark McNally, the commission's general manager, points out a massive, 900-pound bag of powdered activated carbon being funneled into untreated water.

The powder is used to remove atrazine, a herbicide widely applied to cornfields in the spring. The chemical alone costs roughly $130,000 a year, McNally says. The plant passes that bill on to customers.

"Aunt Agnes, you know, on Third Street, has to pay more for her water because we have to recoup our money," McNally says. "I mean, we're not in the business to make money. But we can't go broke."

In Iowa's Raccoon and Des Moines rivers, record-high levels of nitrate runoff this year are making it extremely difficult to meet the demand for clean drinking water. The general manager of Des Moines Water Works, Bill Stowe, fears long-term effects.

"Our concern, obviously, is that once you shake customers' faith in the safety of tap water, you turn them to other sources like bottled water, which is ... certainly a competitor," Stowe says. "It changes our business model and puts us at risk in the long term as a viable utility."

Scientists say they'll have results from their Midwest stream study after the fieldwork is completed in August. Over the next several years, the U.S. Geological Survey plans to replicate the study in other regions.

Abbie Fentress Swanson is a reporter for Harvest Public Media, a public radio reporting collaboration that focuses on agriculture and food production.

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What Is Farm Runoff Doing To The Water? Scientists Wade In

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Across the Midwest this summer, scientists are wading into 100 streams to collect water samples and check cages for fish eggs. It's part of a large study to understand how pesticides and agricultural chemicals from farms are affecting the nation's streams.

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Eutrophication is excessive richness of nutrients in a lake or other body of water, frequently due to runoff from the land, which causes a dense growth of plant life and death of animal life from lack of oxygen.

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Eutrophication: Causes, Consequences, and Controls in Aquatic Ecosystems | Learn Science at Scitable

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Eutrophication is a leading cause of impairment of many freshwater and coastal marine ecosystems in the world. Why should we worry about eutrophication and how is this problem managed?

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pH changes have a serious affect on the life of organisms

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What Is the Effect of PH on Living Organisms?

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Scientists measure the acidity of water by testing the pH level. The pH scale ranges from 0 to 14, with 7 being perfectly neutral. The low end of the scale represents high acidity, while the high end represents acid’s opposite, alkaline.

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Pollution can change the water's pH balance which can harm the biological life in the water

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Chapter Overview
What is pH?

pH is a determined value based on a defined scale, similar to temperature. This means that pH of water is not a physical parameter that can be measured as a concentration or in a quantity. Instead, it is a figure between 0 and 14 defining how acidic or basic a body of water is along a logarithmic scale ¹. The lower the number, the more acidic the water is. The higher the number, the more basic it is. A pH of 7 is considered neutral. The logarithmic scale means that each number below 7 is 10 times more acidic than the previous number when counting down. Likewise, when counting up above 7, each number is 10 times more basic than the previous number ².

pH stands for the "power of hydrogen" ³. The numerical value of pH is determined by the molar concentration of hydrogen ions (H+) ³. This is done by taking the negative logarithm of the H+ concentration (-log(H+)). For example, if a solution has a H+ concentration of 10⁻³ M, the pH of the solution will be -log(10⁻³), which equals 3.

This determination is due to the effect of hydrogen ions (H+) and hydroxyl ions (OH-) on pH. The higher the H+ concentration, the lower the pH, and the higher the OH- concentration, the higher the pH. At a neutral pH of 7 (pure water), the concentration of both H+ ions and OH- ions is 10⁻⁷ M. Thus the ions H+ and OH- are always paired - as the concentration of one increases, the other will decrease; regardless of pH, the sum of the ions will always equal 10⁻¹⁴ M ². Due to this influence, H+ and OH- are related to the basic definitions of acids and bases.

Acids and Bases

As an operational definition, an acid is a substance that will decrease pH when added to pure water. In the same manner, a base is a substance that will increase the pH of water ⁴. To further define these substances, Arrhenius determined in 1884 that an acid will release a hydrogen ion (H+) as it dissolves in water, and a base will release a hydroxyl ion (OH-) in water ⁴. However, there are some substances that fit the operational definition (altering pH), without fitting the Arrhenius definition (releasing an ion). To account for this, Bronsted and Lowry redefined acids and bases; an acid releases a hydrogen ion or proton (equivalent to H+) and a base accepts a hydrogen ion or proton ⁴. This means that acids and bases can cancel each other out, as shown in the water equation to the right.

Basic or Alkaline

The terms "alkaline" and "basic" mean approximately the same thing. By the Bronsted-Lowry definition, basic describes any substance that reduces the hydrogen ion concentration and increases the pH of water, or in other words, a base ⁴. Alkaline comes from alkali, which refers to ionic compounds (salts) containing alkali metal or alkaline earth metal elements that form hydroxide ions when dissolved in water ⁵. Alkali salts are very common and dissolve easily. Due to the hydroxide ions they produce (which increase pH), all alkalis are bases. Some sources define any soluble base as an alkali ⁵. As such, soluble bases can be described as "basic" or "alkaline". However, insoluble bases (such as copper oxide) should only be described as basic, not alkaline.

Alkalinity and the pH of Water

Alkalinity does not refer to alkalis as alkaline does ⁶. While alkalinity and pH are closely related, there are distinct differences. The alkalinity of water or a solution is the quantitative capacity of that solution to buffer or neutralize an acid. In other words, alkalinity is a measurement of water's ability to resist changes in pH. This term is used interchangeably with acid-neutralizing capacity (ANC) ⁷. If a body of water has a high alkalinity, it can limit pH changes due to acid rain, pollution or other factors ⁸. The alkalinity of a stream or other body of water is increased by carbonate-rich soils (carbonates and bicarbonates) such as limestone, and decreased by sewage outflow and aerobic respiration. Due to the presence of carbonates, alkalinity is more closely related to hardness than to pH (though there are still distinct differences). However, changes in pH can also affect alkalinity levels (as pH lowers, the buffering capacity of water lowers as well) ⁶. pH and alkalinity are directly related when water is at 100% air saturation ⁹.

The alkalinity of water also plays an important role in daily pH levels. The process of photosynthesis by algae and plants uses hydrogen, thus increasing pH levels ¹⁰. Likewise, respiration and decomposition can lower pH levels. Most bodies of water are able to buffer these changes due to their alkalinity, so small or localized fluctuations are quickly modified and may be difficult to detect ¹⁰.

pH and Alkalinity Units

pH values are reported as a number between 0 and 14 as a standard pH unit. This unit is equivalent to the negative logarithm of the hydrogen ion molar concentration (-log(H+)) in the solution. Depending on the accuracy of the measurement, the pH value can be carried out to one or two decimal places.

However, because the pH scale is logarithmic, attempting to average two pH values would be mathematically incorrect. If an average value is required, it can be reported as a median or a range, not as a simple calculation ¹⁰.

Alkalinity can be reported as mg/L or microequivalents per liter (meq/L). When in mg/L, it refers to carbonate (CO3 ²⁻), bicarbonate (HCO3 ⁻) or calcium carbonate (CaCO3) concentrations, though calcium carbonate is most common ¹¹.

1 mg/L alkalinity as CaCO3 = 0.01998 meg/L alkalinity
1 mg/L alkalinity as CaCO3 = 0.5995 mg/L alkalinity as CO3 ²⁻
1 mg/L alkalinity as CaCO3 = 1.2192 mg/L alkalinity as HCO3 ⁻

Why is pH Important?

If the pH of water is too high or too low, the aquatic organisms living within it will die. pH can also affect the solubility and toxicity of chemicals and heavy metals in the water ¹². The majority of aquatic creatures prefer a pH range of 6.5-9.0, though some can live in water with pH levels outside of this range.

As pH levels move away from this range (up or down) it can stress animal systems and reduce hatching and survival rates. The further outside of the optimum pH range a value is, the higher the mortality rates. The more sensitive a species, the more affected it is by changes in pH. In addition to biological effects, extreme pH levels usually increase the solubility of elements and compounds, making toxic chemicals more "mobile" and increasing the risk of absorption by aquatic life ¹³.

Aquatic species are not the only ones affected by pH. While humans have a higher tolerance for pH levels (drinkable levels range from 4-11 with minimal gastrointestinal irritation), there are still concerns ¹⁴. pH values greater than 11 can cause skin and eye irritations, as does a pH below 4. A pH value below 2.5 will cause irreversible damage to skin and organ linings ¹⁴. Lower pH levels increase the risk of mobilized toxic metals that can be absorbed, even by humans, and levels above 8.0 cannot be effectively disinfected with chlorine, causing other indirect risks ¹⁴. In addition, pH levels outside of 6.5-9.5 can damage and corrode pipes and other systems, further increasing heavy metal toxicity.

Even minor pH changes can have long-term effects. A slight change in the pH of water can increase the solubility of phosphorus and other nutrients - making them more accessible for plant growth ¹⁰. In an oligotrophic lake, or a lake low in plant nutrients and high in dissolved oxygen levels, this can cause a chain reaction. With more accessible nutrients, aquatic plants and algae thrive, increasing the demand for dissolved oxygen. This creates a eutrophic lake, rich in nutrients and plant life but low in dissolved oxygen concentrations. In a eutrophic lake, other organisms living in the water will become stressed, even if pH levels remained within the optimum range.

Factors that Influence the pH of Water

There are many factors that can affect pH in water, both natural and man-made. Most natural changes occur due to interactions with surrounding rock (particularly carbonate forms) and other materials. pH can also fluctuate with precipitation (especially acid rain) and wastewater or mining discharges ¹³. In addition, CO2 concentrations can influence pH levels.

Carbon Dioxide and pH

Carbon dioxide is the most common cause of acidity in water ¹⁵. Photosynthesis, respiration and decomposition all contribute to pH fluctuations due to their influences on CO2 levels. The extremity of these changes depends on the alkalinity of the water, but there are often noticeable diurnal (daily) variations ¹⁶. This influence is more measurable in bodies of water with high rates of respiration and decomposition.

While carbon dioxide exists in water in a dissolved state (like oxygen), it can also react with water to form carbonic acid:

CO2 + H2O <=> H2CO3

H2CO3 can then lose one or both of its hydrogen ions:

The released hydrogen ions decrease the pH of water¹⁵. However, this equation can operate in both directions depending on the current pH level, working as its own buffering system. At a higher pH, this bicarbonate system will shift to the left, and CO3 ²⁻ will pick up a free hydrogen ion.

This reaction is usually minimal as H2CO3 has a low solubility constant (Henry's Law) ¹⁵. However, as CO2 levels increase around the world, the amount of dissolved CO2 also increases, and the equation will be carried out from left to right. This increases H2CO3, which decreases pH. The effect is becoming more evident in oceanic pH studies over time.

The above equations also explain why rain has a pH of approximately 5.65 ¹⁵. As raindrops fall through the air, they interact with carbon dioxide molecules in the atmosphere. This creates H2CO3 in the raindrops, lowering the rain's pH value ¹⁷. A pH level of 5.65, though acidic, is not considered acid rain. Natural, unpolluted rain or snow is expected to have pH levels near 5.6, assuming a standard atmospheric CO2 concentration of 0.0355% ¹⁵. Acid rain requires a pH below 5.0 ²¹.

5.65 is also the pH of water that has equilibrated with the air and has not come in contact with carbonate materials or limestone.

Natural pH Influences

Carbonate materials and limestone are two elements that can buffer pH changes in water. Calcium carbonate (CaCO3) and other bicarbonates can combine with both hydrogen or hydroxyl ions to neutralize pH¹⁸. When carbonate minerals are present in the soil, the buffering capacity (alkalinity) of water is increased, keeping the pH of water close to neutral even when acids or bases are added. Additional carbonate materials beyond this can make neutral water slightly basic.

As mentioned earlier, unpolluted rain is slightly acidic (pH of 5.6). The pH of rain can also be lowered due to volcanic ash, sulfate-reducing bacteria in wetlands, airborne particulates from wildfires and even lightning ¹⁹. If rain falls on a poorly buffered water source, it can decrease the pH of nearby water through runoff.

Pine or fir needles can also decrease the pH of soil, and any water that runs over it, as they decompose ¹⁸. Intense photosynthesis increases the pH of water as it removes CO2, though this change is usually diurnal ²⁰.

Man-Made pH Influencers

Anthropogenic causes of pH fluctuations are usually related to pollution. Acid rain is one of the best known examples of human influence on the pH of water. Any form of precipitation with a pH level less than 5.0 is known as acid rain ²¹. This precipitation comes from the reaction of water with nitrogen oxides, sulfur oxides and other acidic compounds, lowering its already slightly acidic pH. These emissions usually come from mining and smelting operations or fossil fuel combustion (coal burning and automobiles) ¹⁸. Extremely high levels of CO2 can also further decrease the pH of rain ¹⁷.

Point source pollution is a common cause that can increase or decrease pH depending on the chemicals involved ¹⁸. These chemicals can come from agricultural runoff, wastewater discharge or industrial runoff. Mining operations (particularly coal) produce acid runoff and acidic groundwater seepage if the surrounding soil is poorly buffered ²². Wastewater discharge that contains detergents and soap-based products can cause a water source to become too basic.

Typical pH Levels

Typical pH levels vary due to environmental influences, particularly alkalinity. The alkalinity of water varies due to the presence of dissolved salts and carbonates, as well as the mineral composition of the surrounding soil. In general, the higher the alkalinity, the higher the pH; the lower the alkalinity, the lower the pH ⁶. The recommended pH range for most fish is between 6.0 and 9.0 with a minimum alkalinity of 20 mg/L, with ideal CaCO3 levels between 75 and 200 mg/L ²⁰.

Oceanic organisms like clownfish and coral require higher pH levels. pH levels below 7.6 will cause coral reefs to begin to collapse do to the lack of calcium carbonate ³⁹. Sensitive freshwater species such as salmon prefer pH levels between 7.0 and 8.0, becoming severely distressed and suffering physiological damage due to absorbed metals at levels below 6.0 ⁴⁰.

Environmental Considerations
Natural precipitation, both rain and snow, has a pH near 5.6 due to contact with CO2 and other atmospheric influences. Most grasses and legumes prefer soils with a pH of 4.5-7.0, so the slight acidity of rain can benefit carbonate soils ²³.

The acidity of the surrounding environment can also affect the pH of water. This is most obvious near mining areas, but the effect can also occur naturally. Acid runoff depletes the water's alkalinity and lowers pH below optimum levels. This may be tolerable for some aquatic species (such as frogs) but not for most fish. Some frogs and other amphibians can often tolerate pH levels as low as 4.0 ²⁴. Acidic soils in the Amazon cause many of the lakes and rivers to naturally have low pH values ³⁸. Due to the dissolved humic substances from runoff and uptake, "blackwater" sources can have a pH as low as 4.43. "Clearwater" sources will have a slightly higher, but still acidic, pH value ³⁸. That is why angel fish and discus from the Amazon River Basin can thrive quite happily in waters with a pH as low as 5.0 ²⁵.

Seawater has a pH around 8.2, though this can range between 7.5 to 8.5 depending on its local salinity. pH levels will increase with salinity until the water reaches calcium carbonate (CaCO3) saturation ¹⁶. The oceans generally have a higher alkalinity due to carbonate content and thus have a greater ability to buffer free hydrogen ions ²⁷.

Freshwater lakes, ponds and streams usually have a pH of 6-8 depending on the surrounding soil and bedrock ²¹. In deeper lakes where stratification (layering) occurs, the pH of water is generally higher (7.5-8.5) near the surface and lower (6.5-7.5) at greater depths ¹⁰. Some states, such as Alaska, are attempting to maintain a pH standard for water quality. The Alaska Water Quality Standard requires pH levels between 6.5 and 8.5 to protect the many salmon populations in the state ⁴⁰.

Stratification Considerations
Stratification is usually caused by temperature differences within a body of water, where each layer of water does not mix with the layers above or below ³⁷. These layers are separated by clines, known as thermoclines (temperature divides) or chemoclines (chemistry gradients). Chemoclines can be based on oxygen, salinity, or other chemical factors that do not cross the cline, such as carbon dioxide. Due to CO2's influence on the pH of water, stratification can cause pH levels to differ across a cline.

Differences in pH levels between water strata are due to increased CO2 from respiration and decomposition below the thermocline. In crater lakes such as Lake Nyos or Lake Monoun, the pH rapidly drops from a surface level around 7 to 5.5 below 60 m (at the thermocline and chemocline) ²⁶. This significant drop comes from the saturated CO2 that is stored up in the lower strata of the lake.

Adaptability
While ideal pH levels for fish are 7-8 (fish blood has a pH of 7.4) ²⁰, most fish can adapt to the pH level of their environment (6.0-9.0) as long as there are no dramatic fluctuations. A dramatic fluctuation is considered a shift in pH of 1.4 (up or down) ²². For saltwater fish, the pH of water should remain between 7.5 and 8.5 ⁹.

Unusual pH Levels and Consequences

Harmful effects become noticeable when the pH of water falls below 5.0 or rise above 9.6. Ill effects due to acidification are more pronounced in saltwater fish due to their adaptation to a higher pH. When pH is below optimal levels, fish become susceptible to fungal infections and other physical damage ¹⁶. As the pH of water falls, the solubility of calcium carbonate is reduced, inhibiting shell growth in aquatic organisms ¹⁶. In general, fish reproduction is affected at pH levels below 5.0 and many species (such as saltwater fish or sensitive freshwater fish like smallmouth bass) will leave the area ²¹. Fish begin to die when pH falls below 4.0 ¹².

Low pH levels can encourage the solubility of heavy metals ¹². As the level of hydrogen ions increases, metal cations such as aluminum, lead, copper and cadmium are released into the water instead of being absorbed into the sediment. As the concentrations of heavy metals increase, their toxicity also increases. Aluminum can limit growth and reproduction while increasing mortality rates at concentrations as low as 0.1-0.3 mg/L ²². In addition, mobilized metals can be taken in by organisms during respiration, causing physiological damage ²² . This is particularly detrimental to species such as rainbow trout ¹³.

On the other side of the spectrum, high pH levels can damage gills and skin of aquatic organisms and cause death at levels over 10.0. While some african cichlids thrive at high pH levels (up to 9.5), most fish cannot tolerate them. Death can occur even at typical levels (9.0) if ammonia is present in the water ²¹. At low and neutral pH levels, ammonia combines with water to produce an ammonium ion:

Ammonium, NH4⁺, is non-toxic and will not affect aquatic life. However, at pH levels over 9, the equation reverses and ammonia is released into the water ²². Ammonia, NH3, is extremely toxic to aquatic organisms, and as pH increases, the mortality rates rise with the NH3 concentration.

On the ecosystem side, mosses can begin invading a body of water as the pH of water falls below 5. In eutrophic lakes, pH-tolerant algae can dominate, driving the pH levels to diurnal high and low extremes, forming algae blooms that can kill the lake ¹⁶.

Alkaline and Acid Lakes

Spread across the world are a number of lakes with unusual pH levels. Alkaline lakes, also known as soda lakes, generally have a pH level between 9 and 12. This is often due to a high salt content (though not every salt lake has a high pH). These lakes have high concentrations of minerals, particularly dissolved salts: sodium, calcium, magnesium carbonates and bicarbonates ²⁸. Depending on the lake, borates, sulfates and other elements (usually strong base ions) can also be present ²⁹. Alkaline lakes are formed when the only outlet for water is evaporation, leaving the minerals behind to accumulate ³⁰. These minerals often form columns of mineral deposits, known as tufa columns. Many alkaline lakes are a commercial resource for soda ash and potash, while others are popular tourist destinations for their "magical" healing properties (due to the mineral content).

A notorious example of an alkaline lake is Lake Natron in Tanzania. Lake Natron has a pH up to 10.5 due to high concentrations of sodium carbonate decahydrate (soda ash) and sodium bicarbonate (baking soda) that enters the water from the surrounding soil ³¹. While the lake supports a thriving ecosystem, including flamingos, alkaline tilapia and pH-resistant algae, Nick Brant, a photographer, has created many haunting images of animals that died in this lake ³¹. The bodies of these animals are preserved by the sodium carbonate, much like the ancient Egyptian mummification process.

Acid lakes usually develop near volcanoes, where sulfuric acid, hydrogen sulfide, hydrofluoric acid, hydrochloric acid and carbon dioxide can leach into the water ³². In non-volcanic areas, acid lakes can also develop after acidic deposition from events such as acid rain, pollution or acid runoff from mining operations ³³. Much like their alkaline counterparts, acid lakes have no outlet except evaporation, concentrating the sulfates and acids. The acids can enter the water through atmospheric diffusion from coal burning, acid rain or after an eruption. In volcanic lakes, acids can enter the water through an active fumarole, or volcanic vent.

The acid lakes at Dallol in Ethiopia are the result of acid leaching from nearby volcanoes. The sulfur and iron in the water leave yellow and rust-colored deposits around the water's edge.

With a pH level below 5.0, few organisms can live in acid lakes. However, there is one notable exception: the Osorezan dace, or Japanese dace. This fish thrives in the acidic waters of Lake Osorezan, resting comfortably at a pH of 3.5, and swims into neutral pH waters only to spawn ³⁴.

Ocean Acidification

Ocean acidification is caused by an influx of dissolved carbon dioxide. As atmospheric CO2 levels increase due to anthropogenic causes, dissolved CO2 also increases, which in turn decreases the pH of water.

When water becomes saturated with CO2, it not only reduces the ocean's pH, but depletes the calcium carbonate sources as well ³⁵. Calcium carbonate, CaCO3, is a necessary ingredient in building corals, shells and exoskeletons for many aquatic creatures. As CO3²⁻ levels decrease, it becomes more difficult for marine creatures to build their shells.

As mentioned in the section "Carbon Dioxide and pH", additional CO2 increases the number of hydrogen ions in the water, reducing pH:

CO2 + H2O <=> H2CO3 ... H2CO3 <=> (H+) + HCO3⁻

At pH levels between 6.4 and 10.33, some of those hydrogen ions attach to carbonate ions ²²:

Thus as CO2 levels increase, the availability of carbonate, CO3 ²⁻ decreases, reducing the amount available for shell and coral building ³⁶.

CO2 + H2O + CO3²⁻ <=> 2HCO3⁻

High CO2 levels also make it more difficult to maintain current shells due to lower pH levels and competition for carbonate ³⁵.

Furthermore, the air saturation of water is based on partial pressures from Henry's law. As CO2 levels in the air increase, so too does their partial pressure. This reduces the partial pressure of oxygen, reducing its saturation levels and contributing to hypoxic (low O2) conditions ³⁵.

While the oceans will never become "acidic" (with a pH of less than 7), even decreasing pH a slight amount stresses saltwater organisms and increases mortality rates. pH is logarithmic, meaning that a decrease by 0.1 is equivalent to nearly a 30% increase in acidity ³⁵.

Cite this work:
Fondriest Environmental, Inc. "pH of Water." Fundamentals of Environmental Measurements. 19 Nov. 2013. Web. < http://www.fondriest.com/environmental-measurements/parameters/water-quality/ph/ >.

More Information:

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pH of Water - Environmental Measurement Systems

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Chapter Overview pH is a determined value based on a defined scale, similar to temperature. This means that pH of water is not a physical parameter that can be measured as a concentration or in a quantity. Instead, it is a figure between 0 and 14 defining how acidic or basic a body of water is along a logarithmic scale ¹.

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This pollution can be from humans and the burning of fossil fuels or natural causes like volcanic eruptions

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Volcanic Ash Impacts & Mitigation - Water Supply

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Volcanic Ash, what it can do and how to minimize damage. Water supply

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pH specifically is a measurement of the amount of free hydrogen and hydroxyl ions in the water.
https://water.usgs.gov/edu/ph.html

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determines the solubility of the water and the amount of chemical constituents that can be utilized in the biological life in the water of chemical constituents-nutrients or metals.

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pH: Water properties, from the USGS Water-Science School

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pH, acidic and alkaline water, from the USGS Water Science School.

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But acid rain is much lower, usually around 4.0 or lower.

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Acid rain, or acid deposition, is a broad term that includes any form of precipitation with acidic components, such as sulfuric or nitric acid that fall to the ground from the atmosphere in wet or dry forms. This can include rain, snow, fog, hail or even dust that is acidic.

Acid rain results when sulfur dioxide (SO ₂) and nitrogen oxides (NO X) are emitted into the atmosphere and transported by wind and air currents. The SO ₂ and NO X react with water, oxygen and other chemicals to form sulfuric and nitric acids. These then mix with water and other materials before falling to the ground.

While a small portion of the SO ₂ and NO X that cause acid rain is from natural sources such as volcanoes, most of it comes from the burning of fossil fuels. The major sources of SO ₂ and NO X in the atmosphere are:

Burning of fossil fuels to generate electricity. Two thirds of SO₂ and one fourth of NOX in the atmosphere come from electric power generators.

Vehicles and heavy equipment.

Manufacturing, oil refineries and other industries.

Winds can blow SO ₂ and NO X over long distances and across borders making acid rain a problem for everyone and not just those who live close to these sources.

Forms of Acid Deposition

Wet Deposition

Wet deposition is what we most commonly think of as acid rain. The sulfuric and nitric acids formed in the atmosphere fall to the ground mixed with rain, snow, fog, or hail.

Dry Deposition

Acidic particles and gases can also deposit from the atmosphere in the absence of moisture as dry deposition. The acidic particles and gases may deposit to surfaces (water bodies, vegetation, buildings) quickly or may react during atmospheric transport to form larger particles that can be harmful to human health. When the accumulated acids are washed off a surface by the next rain, this acidic water flows over and through the ground, and can harm plants and wildlife, such as insects and fish.

The amount of acidity in the atmosphere that deposits to earth through dry deposition depends on the amount of rainfall an area receives. For example, in desert areas the ratio of dry to wet deposition is higher than an area that receives several inches of rain each year.

Measuring Acid Rain

Acidity and alkalinity are measured using a pH scale for which 7.0 is neutral. The lower a substance's pH (less than 7), the more acidic it is; the higher a substance's pH (greater than 7), the more alkaline it is. Normal rain has a pH of about 5.6; it is slightly acidic because carbon dioxide (CO ₂) dissolves into it forming weak carbonic acid. Acid rain usually has a pH between 4.2 and 4.4.

Policymakers, research scientists, ecologists, and modelers rely on the National Atmospheric Deposition Program's (NADP) National Trends Network (NTN) for measurements of wet deposition. The NADP/NTN collects acid rain at more than 250 monitoring sites throughout the US, Canada, Alaska, Hawaii and the US Virgin Islands. Unlike wet deposition, dry deposition is difficult and expensive to measure. Dry deposition estimates for nitrogen and sulfur pollutants are provided by the Clean Air Status and Trends Network (CASTNET). Air concentrations are measured by CASTNET at more than 90 locations.

When acid deposition is washed into lakes and streams, it can cause some to turn acidic. The Long-Term Monitoring (LTM) Network measures and monitors surface water chemistry at over 280 sites to provide valuable information on aquatic ecosystem health and how water bodies respond to changes in acid-causing emissions and acid deposition.

Next, learn about the Effects of Acid Rain.

Or, learn more about:

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Introduction to acid rain including its causes and the different types of acid rain.

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pH is measured in a logarithmic scale ranging from 0-14, with 7 being neutral

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pH is measured in a logarithmic scale ranging from 0-14, with 7 being neutral

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the unit of measurement to determine how acidic or alkaline a substance is.
https://water.usgs.gov/edu/ph.html

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A high pH can cause a bitter taste, deposits in pipes, and decreases the effectiveness of chlorine. A low pH can cause the corrosion of metals and other substances.
https://water.usgs.gov/edu/ph.html