The Chemistry of Natural Waters

Natural waters have a broad range of total dissolved solids (TDS). Some fresh mountain streams might have TDS concentrations less than 250mg/kg. Seawater, on average, has TDS concentrations of nearly 35g/kg. Extreme TDS values are found in highly evaporated lake or isolated seawater basins and in the deep subsurface (so-called "formation waters"), with TDS of nearly 350g/kg (35% salt solution!). We will focus here briefly on the compositions of potential drinking water sources (rivers and lakes) and the origins of the dissolved species.

Flowing water, whether in aquifers or streams, interacts with rocks and soils and slowly dissolves some of their chemical constituents. The pH (hydrogen ion activity) of the water determines the rate of dissolution and solubility of many chemical species. However, we will not discuss chemical processes in any detail here. Some chemical substances, particularly redox-sensitive trace metals (e.g. Fe, Mn, Pb, As and others), are more soluble when natural waters are depleted in dissolved oxygen (see the section called Contaminant Example 2 below). Most chemical species in natural waters have both natural and pollutant sources of many types (Table 1).

Table 1: Most common inorganic substances found in natural waters on land and their dominant sources (Berner and Berner, 1996)
Ion (molecule)Natural SourcePollutant Source
Sodium (Na+)1, 28
Magnesium (Mg+)1, 28
Potassium (K+)1, 2, 38, 14
Calcium (Ca+)1, 28, 9, 10
Hydrogen (H+)1310
Chloride (Cl-)115
Sulfate (SO42-)1, 2, 5, 68, 10
Nitrate (NO32-)4, 58, 10, 11, 14
Ammonium (NH4+)514, 5
Phosphate (PO43-)2, 3, 58, 14
Bicarbonate (HCO3-)77 (5, 8, 9, 10, 11, 12)
SiO2, Al, Fe212

Key for Table Above

  1. wind-blown sea salt
  2. soil dust
  3. biogenic aerosols
  4. lightning and N2 in atmosphere
  5. biological decay
  6. volcanic activity
  7. carbon dioxide in air
  8. biomass burning
  9. cement manufacture
  10. fuel combustion
  11. automobile emissions
  12. land clearing
  13. gas reactions
  14. fertilizers
  15. industrial chemicals

Natural waters also contained dissolved gasses. For example, carbon dioxide from the atmosphere is dissolved in water, and, through a series of chemical reactions, contributes to the total dissolved carbon in waters—primarily bicarbonate (HCO32-). Gas solubility is inversely proportional to temperature and TDS. For example, dissolved oxygen solubility is shown as a function of temperature and salinity in Figure 1. Note that the amount of oxygen that can be held in fresh water decreases nearly 50% from near freezing temperature to 35°C. These are maximum concentrations, but natural waters can have lower dissolved oxygen concentrations as the result of biological activity such as the metabolism of water inhabitants, including bacteria. Photosynthesis of algae and aqueous plants can add oxygen to the water in which these primary producers grow. However, the breakdown of organic material by bacteria consumes dissolved oxygen. Thus, in waters below the surface wind-mixed layer (usually tens of meters or more) or in stably stratified lakes or bays, for which rates of oxygen replenishment to deeper depths are slow, deficiencies in dissolved oxygen can develop, with anoxia (total depletion of dissolved oxygen) at the extreme. Excess nutrient supply can have the same impact on a water body (eutrophication: see Module 1 and Contaminant Example 2: "Dead Zones" and Excess Nutrient Runoff) with deleterious effects on the aquatic biota.

Dissolved O2 starts between 7-11 ml/L least salinity has the greatest amount. All decrease as temperature increases 2 between 4-5 ml/L.
Figure1. Dissolved oxygen (ml/L, or ppm) solubility (maximum expected concentration) in waters of different salinity (parts per thousand). Seawater typically has a salinity of 35 parts per thousand, whereas fresh water is near 0 parts per thousand.
Source: Michael Arthur, Penn State

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