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The field of aquatic chemistry covers the reactions and processes which determine the speciation and distribution of chemical species in aquatic systems. Water is a unique solvent due to its various physical properties such as a high specific heat, attraction to polar molecules, and being less dense in solid-phase. Natural water systems vary from ocean waters to glacial streams to groundwater and can interact with many other systems such as sediments, organisms, and the atmosphere. Some of the most common chemical species in natural waters are gasses, organics, metals, and nutrients. Some of the major processes are reduction-oxidation reactions, photochemical reactions, particle-particle interactions, and mass transport of chemical species in the environment. The principles behind aquatic chemistry can determine where a chemical is found and in what form that chemical is found.

Water as a solvent
Water is a very strong solvent for many different solutes. It acts so strongly due to the high dielectric constant between molecules of water. This constant denotes the strong forces that water molecules generate with each other. If a solute has similar forces, it will be surrounded by water molucules and is refered to as hydrophilic. Hydrophilic substances are generally ionic or polar and include salts and short organic molecules. Substances that do not dissolve in water are hydrophobic and act that way due to their nonpolarity. These substances would include large organic molecules such as fats. Nonpolar substances will stay together instead of dissolving as that minimizes the interaction between the water and substance.

The ability of water to dissolve many substances contributes to the state that molecules are found in when measured in natural waters. Water may interact or react with solutes to hydrate them or react and change their structure.

Solutes found in natural waters
There are many different solutes found in natural waters. These chemical species can and do interact with each other and more chemicals. The solutes found may be fairly consistent across large scales in the case of carbon, nitrogen, and phosphorus found in the oceans (Redfield ratio). Solutes also may differ drastically within relatively small scales such as metals leaching from mineral-rich rock in one valley but not the next valley over (acid rock drainage).

In addition to what solutes are present, each solute can be present in different species as well. In the atmosphere, chemicals can be found over liquid, gas, and aerosol phases. Beyond the physical phase, solutes will vary in their redox state. One example is of the element sulfur. Sulfur in rivers and other freshwater systems is often found as sulfate (SO$4$$2-$) while in marine systems the major species of sulfur is bound to a single sodium ion (NaSO$4$$-$).

Organic material
Organic material occurs in most natural waters as a result of plant or animal material decomposing. Organic matter in water bodies can either originate within the water body (autochthonous) or from outside of the water body (allochthonous). Within the natural waters, a variety of different sources produce this organic material. Algae, leaves, woody material, and other biota result in different types of organic materials.

Organic material is commonly discussed as dissolved organic carbon (DOC) and dissolved organic material (DOM). As the names may suggest, DOM describes all elements that compose organic matter while DOC is the 'skeleton' of organic matter and only describes the carbon within organic matter. The dissolved description is functional and is often defined as 0.45 micrometers or smaller in size. DOC is the most common measurement that is meant to present the amount of organic matter present in natural waters but is often tied with total organic carbon (TOC).

Dissolved organic carbon can act in many different ways in natural systems. The structure of DOC allows it to complex with metals which can reduce the bioavailability of metals in aquatic systems. However, in many modern drinking water purification systems chlorine is used which can react with organic matter to produce trihalomethanes which are carcinogenic.

Dissolved organic matter is measured by acidifying the samples to remove non-organic carbon, oxidized to convert the remaining carbon to carbon dioxide, and then quantified by either combustion, catalytic oxidation, photo-oxidation, or ultraviolet oxidation. Typical values for DOC range from 1-3 mg/L for oligotrophic lakes up to 50+ mg/L for dystrophic systems.

Metals
Metals are ubiquitous in natural water sources. Metals leach and weather from surrounding watersheds, settle from volcanic activity, or can arrive from a variety of human actions including industry, mining, and increased wildfires. Metal speciation and bioavailability are complex processes and are mediated by a variety of factors including hydrodynamic energy within a lake, water temperature, stratification, biological activity, acidity, and organic carbon activity.



Some metals such as sodium, potassium, magnesium, iron, zinc, and calcium are extremely common and are the major constituents of seawater. These metals are essential for human and many other organisms. Other metals such as nickel, cadmium, arsenic, and mercury are not ubiquitous and are extremely deleterious to human and environmental health.

The toxicity, mobility, and risk of metals in natural systems depends on the speciation of those metals. Although all of the factors listed above influence speciation, some of the most important factors are acidity and availability of molecules which may bind with free metal ions such as sulfides. There are many ways to determine the total concentration of a metal in water. Atomic adsorption spectroscopy is one common method. For trace metals, ICP-MS is able to detect them at lower concentrations.

Organic pollutants
Organic pollutants can include herbicides, pesticides, or an excess of organic matter. Herbicides and pesticides are both persistent organic pollutants (POPs) and are resistant to degradation. Persistent organic pollutants are often used for agriculture and make their way into receiving waters. These chemicals can have severe effects on both human and environmental health. For POPs to degrade, they must undergo one or more of several types of processes. Microbial degradation can occur when microorganisms derive energy from degrading organic chemicals such as POPs. This can result in either a mineralized product which has been degraded to carbon dioxide or cometabolized in which the chemical is transformed to a similar chemical compound. Photochemical reactions occur from exposure to sunlight but may increase the toxicity of a chemical in certain scenarios. Chemical reactions such as hydrolysis, oxidation, and reduction can also degrade POPs.

An excess of organic matter can lead to eutrophication which is almost entirely biologically mediated. The resulting change in available solutes and altered microbial activity from eutrophication will impact the other reactions and processes going on in natural waters.

Common reactions and interactions
Due to the unique properties of water, the water molecules play important roles in mediating and acting in reactions. Water is amphoteric and therefore is able to act as both an acid and a base in chemical reactions. Water is a weak acid and base and will act as determined by the stronger acid or base. When reacting with a strong acid, water will act as a base. When reacting with a strong base, water will act as an acid.



In addition to acting as an acid or base, water can complex with metal ions. This varies from true complexation such as with perrhenic acid or simply adjacent to a metal complex such as with Iron(II) sulfate.

In organic reactions, water can readily hydrate in a hydration reaction or dehydrate in a dehydration reaction when water is the solvent. Hydrolysis can occur when reacting with proteins and fats.