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Freshwater salinization is the process of salty runoff contaminating freshwater ecosystems, which can harm aquatic species in certain quantities and contaminate drinking water. It is often measured by the increased amount of dissolved minerals than what is considered usual for the area being observed.

Naturally occurring salinization is referred to as primary salinization; this includes rainfall, rock weathering, seawater intrusion, and aerosol deposits. Human-induced salinization is termed as secondary salinization, with the use of de-icing road salts as the most common form of runoff. Approximately 37% of the drainage in the United States has been effected by salinization in the past century. The EPA has defined two thresholds for healthy salinity levels in freshwater ecosystems: 230 mg/L Cl− for average salinity levels and 860 mg/L Cl− for acute inputs.

Primary salinization
Salinity plays a major role in a freshwater organism's attempts to maintain an osmotic balance between ion concentration and their internal fluids. Salinization increases osmotic pressure, thus negatively affecting the chance of an organism's fitness and survival. Higher levels of salinity present in freshwater environments can lead to declining species richness in general observations, though toxicity varies among freshwater species and the identity of the ions that are causing the salinization. Excluding an organism's death, excess salinity may also lead to a decrease in both individual and population fitness via stunted growth during adolescence, decreased feeding ability , oxidative stress , and overall bodily disfigurement.

Excess amounts of saline water in freshwater areas also play a significant role on larger population scales; they may alter trophic interactions within ecosystems and transform pre-existing biochemical cycles into 'new' ones by changing the flow of compound direction. The altercation of ecosystems may facilitate the intrusion of invasive species that are able to handle brackish to saline water conditions.

Secondary salinization
Human interaction accelerates rates of primary salinization. Land development, like construction and mining, causes compounds found in bedrock to be released from their tight locations and come to the surface, which are then exposed to accelerated rates of weathering, eventually leading to leaching ions in nearby water sources. Agricultural practices also generate highly saline irrigation that may enter freshwater through the introduction of various pesticides or husbandry-related runoff, and naturally saline groundwater can be brought to the surface via land clearing.

Chloride in the form of chlorine is recognized as the most common type of anthropogenic salts exposed to an environment. In agricultural practices, chlorine is mixed together with other compounds to produce an antibacterial solvent used to treat water. This treated water moves from fields into watersheds where it may remain present for long periods of time. Aggregation of chlorine is especially prevalent where improper irrigation occurs. Raised chloride levels may lead to acidification, movement of metalloid compounds via ion exchange with the stream bed, tampering with lake mixing schedules, and modifications of freshwater biotic relationships.

Effects of salinization on freshwater organisms
Due to body permeability, the salinity of the organism’s aquatic environment can have a huge influence on cellular stability. Organisms residing in freshwater ecosystems need to maintain an osmotic balance between their body fluids and the ion concentrations within their cells. Changes in osmotic pressure requires large amounts of energy and can result in cellular damage and cellular death within the organisms. Changes within salinity levels affect organisms within freshwater ecosystems both directly and indirectly. The toxic levels of salt ions can directly result in physiological changes in species which can cause harmful effects to not only the individual, but also the species population. The various effects on these organisms can then indirectly affect the overall freshwater ecosystem by modifying the aquatic community structure and function. As salinity increases within a freshwater ecosystem, often this results in a decrease of biota diversity and richness. The extinction rate for freshwater organisms are among the highest worldwide, and as salinity levels in these aquatic ecosystems continue to increase, more species and their environments will become threatened.

A study performed in Baltimore revealed that at low concentrations, increased levels of chloride hinders the denitrification process within lakes, which is crucial for removing nitrate, the byproduct of ammonia from fish and other aquatic organisms. Chloride levels in the Northeastern USA increase seasonally to around 5 grams a liter from street salt use in the winter. This vacillation causes freshwater communities closer to urban areas to have reduced biodiversity and trophic complexity.

Biomodification of salt toxicity
Due to numerous concurrent stressors present in freshwater communities, increased levels of salinization may have unforeseen effects caused by interactions with other compounds. Freshwater salinization syndrome (FSS) is cited to be a familiar threat to freshwater located in North America and Europe. The interactions between salt and pH, nutrients, metals, and base cations is not adequately known, though may exacerbate existing issues to negatively effect water quality, carbon dioxide concentrations, and biodiversity. The ion concentration of salt toxicity may change the level of reactivity a species will respond with. To be able to properly recognize the threat salinity plays requires the proper proportions of each ion present to be accounted for. Sensitivity also varies between species. Studies focusing on the abiotic interactions with freshwater organisms found that salinity had an additive effect on the detrimental compounds being observed for the majority of the time, but not always, which makes the prediction process difficult for scientists.

Salinization and alkalization have been linked through the study of arid regions across North America and have negatively effected 37% and 90% of freshwater drainage areas, respectively. Their interaction is best noted by the levels of rising pH in streams and rivers measured in 232 USGS sites in 2018. Among these sites, 66% have shown a significant escalation of pH, the most commonly affected area being heavily populated cities in the east and mid-west. Along with the usual salinization offenders of agricultural runoff and road ice, lime and concrete quickly weather down to contribute base ions and salts into water streams. Noticeable signs of FSS include infrastructure deterioration, lowered biodiversity, and the increased mobilization of pollutants within an aquatic system. In conjunction with photosynthetic organisms, basic levels of pH can enter a positive feedback loop via the deficiency of dissolved carbons in the water in relation to the amount of dissolved carbon dioxide, thus further exacerbating FSS.