Freshwater
Freshwater is chemically defined as containing a concentration of less than two parts per thousand (<0.2%) of dissolved salts.
Freshwater can occur in many parts of the environment. Surface freshwaters occur in lakes, ponds, rivers, and streams. Subsurface freshwater occurs in pores in soil and in subterranean aquifers in deep geological formations. Freshwater also occurs in snow and glacial ice, and in atmospheric vapors, clouds, and precipitation.
Most of the dissolved, inorganic chemicals in freshwater occur as ions. The most important of the positively charged ions (or cations) in typical freshwaters are calcium (Ca2+), magnesium (Mg2+), sodium (Na+), ammonium (NH4+), and hydrogen ion (H+). This hydrogen ion is only present if the solution is acidic; otherwise a hydroxy ion (OH−) occurs. The most important of the negatively charged ions (or anions) are sulfate (SO42−), chloride (Cl−), and nitrate (NO3−). Other ions are also present, but in relatively small concentrations. Some freshwaters can have large concentrations of dissolved organic compounds, known as humic substances. These can stain the water a deep-brown, in contrast to the transparent color of most freshwaters.
At the dilute end of the chemical spectrum of surface waters are lakes in watersheds with hard, slowly weathering bedrock and soils. Such lakes can have a total concentration of salts of less than 0.002% (equivalent to 20 mg/L, or parts per million, ppm). For example, Beaverskin Lake in Nova Scotia has very clear, dilute water, with the most important dissolved chemicals being: chloride (4.4 mg/L), sodium (2.9 mg/L), sulfate (2.8 mg/L), calcium (0.41 mg/L), magnesium (0.39 mg/L), and potassium (0.30 mg/L). A nearby body of water, Big Red Lake, has similar concentrations of these inorganic ions. However, this lake also receives drainage from a nearby bog, and its chemistry includes a large concentration of dissolved organic compounds (23 mg/L), which stain the water the color of dark tea.
More typical concentrations of major inorganic ions in freshwater are somewhat larger: calcium 15 mg/L; sulfate 11 mg/L; chloride 7 mg/L; silica 7 mg/L; sodium 6 mg/L; magnesium 4 mg/L; and potassium 3 mg/L.
The freshwater of precipitation is considerably more dilute than that of surface waters. For example, precipitation falling on the Nova Scotia lakes is dominated by sulfate (1.6 mg/L), chloride (1.3 mg/L), sodium (0.8 mg/L), nitrate (0.7 mg/L), calcium (0.13 mg/L), ammonium (0.08 mg/L), magnesium (0.08 mg/L), and potassium (0.08 mg/L). Because the sampling site is within 31 mi (50 km) of the Atlantic Ocean, its precipitation is significantly influenced by sodium and chloride originating with sea sprays. More continental locations have much smaller concentrations of these ions in their precipitation water. For example, precipitation at a remote place in northern Ontario has a sodium concentration of 0.09 mg/L and chloride 0.15 mg/L, compared with 0.75 mg/L and 1.3 mg/L, respectively, at the maritime Nova Scotia site.
Clouds and Cloud Types; Drought; Estuary; Floods; Glaciers; Groundwater; Humidity; Hydrologic Cycle; Rapids and Waterfalls; Stream Capacity and Competence; Stream Valleys, Channels, and Floodplains
This is the complete article, containing 487 words
(approx. 2 pages at 300 words per page).
