Global Occurrence of Major Elements in Rivers
Abstract
Major dissolved ions (Ca2+, Mg2+, Na+, K+, Cl-, SO42-, HCO3-, and CO32-) and dissolved silica (SiO2) in rivers have been studied for more than a hundred years for multiple reasons: (i) geochemists focus on the origins of elements and control processes, and on the partitioning between dissolved and particulate forms; (ii) physical geographers use river chemistry to determine chemical denudation rates and their spatial distribution; (iii) biogeochemists are concerned with the use of carbon, nitrogen, phosphorus, silica species, and other nutrients by terrestrial and aquatic biota; (iv) oceanographers need to know the dissolved inputs to the coastal zones, for which rivers play the dominant role; (v) hydrobiologists and ecologists are interested in the temporal and spatial distribution of ions, nutrients, organic carbon, and pH in various water bodies; (vi) water users need to know if waters comply with their standards for potable water, irrigation, and industrial uses.The concentrations of the major ions are commonly expressed in mg L-1; they are also reported in meq L-1 or μeq L-1, which permits a check of the ionic balance of an analysis: the sum of cations (Σ+ in eq L-1) should equal the sum of anions (Σ- in eq L-1). Dissolved silica is generally not ionized at pH values commonly found in rivers; its concentration is usually expressed in mg L-1 or in μmol L-1. Ionic contents can also be expressed as percent of Σ+ or Σ- (%Ci), which simplifies the determination of ionic types. Ionic ratios (Ci/Cj) in eq eq-1 are also often tabulated (Na+/Cl-, Ca2+/Mg2+, Cl-/SO42-, etc.). As a significant fraction of sodium can be derived from atmospheric sea salt and from sedimentary halite, a chloride-corrected sodium concentration is commonly reported (Na#=Na+-Cl- (in meq L-1)). The export rate of ions and silica, or the yield (YCa2+, YSiO2) at a given station is the average mass transported per year divided by the drainage area: it is expressed in units of t km-2 yr-1 (equal to g m-2 y-1) or in eq m-2 yr-1.This chapter covers the distribution of riverine major ions, carbon species, both organic and inorganic, and silica over the continents, including internal regions such as Central Asia, and also the major factors such as lithology and climate that control their distribution and yields.Based on an unpublished compilation of water analyses in 1,200 pristine and subpristine basins, I am presenting here an idealized model of global river chemistry. It is somewhat different from the model proposed by Gibbs (1970), in that it includes a dozen major ionic types. I also illustrate the enormous range of the chemical composition of rivers - over three orders of magnitude for concentrations and yields - and provide two global average river compositions: the median composition and the discharge-weighted composition for both internally and externally draining regions of the world.A final section draws attention to the human alteration of river chemistry during the past hundred years, particularly for Na+, K+, Cl-, and SO42-; it is important to differentiate anthropogenic from natural inputs. Trace element occurrence is covered by Gaillardet (see Chapter 5.09).
- Publication:
-
Treatise on Geochemistry
- Pub Date:
- December 2003
- DOI:
- 10.1016/B0-08-043751-6/05164-1
- Bibcode:
- 2003TrGeo...5..207M