The chemical compoition of a lake is a function of its climate and its basin geology. Each lake has an ion balance of the three major anions and four major cations, including HCO3-, SO4-2, Cl-, Ca+2, Mg+2, Na+,and K+. Ion balance means the sum of the negative ions equals the sum of the positive cations when express as equivalents. These ions are usually present at concentration expressed as mg/L (ppm) whereas other ions such as the nutrients phosphate, nitrate, and ammonium are present at ug/L (ppb) levels.

Humans can influence lake chenistry with excessive land disturbance around a lake which causes higher rates of leaching and erosion by removing vegetation, exposing soil, and increasing water runoff. Lawn fertilizers, and urban runoff add micronnutrients such as nitrogen and phosphorous, chloride and potassium, and where highways and parking lots are close to the lake, oils and heavy metals. Emissions from motor vehicles, fossil-burning utilities and industry, and other sources produce compounds attecting lake chemistry. One important consideration to this is the fact pollution can occur many miles away from your faviorite lake and can be blown by the winds or carried by streams.

The best understood ions are hydrogen (H+, which indicates acidity), sulfate (SO4-2) and nitrate (NO3-), which are associated with acid rains. Mercury (Hg) is a significant air pollutant affecting aquatic ecosystems by accumulation in the food web in the lake. Lakes with hight concentration of calcium (Ca+2) and magnesium (Mg+2) are labled "hardwater" lakes, while those with low levels of these ions are called "softwater" lakes. Concentration of other ions, especially bicarbonate, are highly correlated with the concentration of the hardness ions, especially Ca+2. The ionic concentration influence the lake's ability to assimilate pollutants and maintain nutrients in solution. Calcium carbonate in the form of "marl" can precipitate phosphate from the water and thereby remove this important nutrient from the water.

The total amount of ions in lake water is called TDS (total dissolved salt, or total dissolved solids concentration). Both the concentration of TDS and the relative amounts or ratios of different ions influence every living creature in a lake, as well as, affect important chemical reactions that occur normally in the water.