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the ionic content of fresh water. Other ions, including those of N, phosphorus (P), and iron, are biologically important but make a minor contribution to total ions. Both the total and the concentration of individual constituents vary considerably from place to place, due to variability in natural and anthropogenic inputs. However, the vast majority of the world's rivers have TDS that contain more than 50% HCO", and from 10% to 30% (Cl" + SO4"). This reflects the dominance of sedimentary rock weathering, and especially of carbonate minerals (Berner and Berner 1987). Just under one fourth of the earth's land surface is covered by igneous and metamorphic rocks, versus three fourths covered by sedimentary rocks. Combining this information with the known differences in ionic concentrations associated with different geologies, it is evident that dissolved materials from sedimentary rocks contribute by far the greatest amount (more than 80%, Berner and Berner 1987) of the total dissolved load of rivers, and thus dominate the composition of "average" river water.

Salinity refers to the sum of the concentrations of all dissolved ions, and so is a more inclusive term than TDS, although for all practical purposes it is the same quantity. TDS is measured by evaporation and weighing of the residue. In addition, major ions can be measured directly by a number of ways, including colorimetric methods in which specific ions react with specific chemicals to form colored compounds, ion chromatography, and by means of ion-specific probes. Dissolved constituents are reported as units of mass, milligrams per liter (equal to parts per million [ppm]), or as chemical equivalents. In the latter case, milliequivalents per liter are calculated from milligrams per liter, by dividing by the equivalent weight of the ion (its ionic weight divided by its ionic charge).

Conductivity is a measure of electrical conductance of water, and an approximate measure of total dissolved ions. Distilled water has a very high resistance to electron flow, and the pres ence of ions in the water reduces that resistance. The relationship between TDS and specific conductance (SC) typically is linear (TDS = k x SC) with a value of k between 0.55 and 0.75. However, the value of the constant varies with location and must be established empirically (Walling 1984). Differences in conductivity result mainly from the concentration of the charged ions in solution, and to a lesser degree from ionic composition and temperature. Values are reported as microSiemens per centimeter (|S cm"1) at 20 or 25°C, and in the older literature as the reciprocal of ohms (|imho cm"1) (Golterman et al. 1978).

The ionic concentration of rainwater (Table 4.3) is much more dilute than most river water, with an average value of a few milligrams per liter (Berner and Berner 1987). Na+, K+, Ca2+, Mg2+, and Cl" are derived primarily from particles in the air, whereas SO4", NH^, and NO" are derived mainly from atmospheric gases. Marine salts (NaCl) are especially important near the oceans, and a transition to CaSO4 or Ca(HCO3)2 dominated rain occurs as one proceeds inland. The relative importance of these various inputs can vary seasonally and over quite short distances, as Sutcliffe and Carrick (1983) document for streams of the English Lake District.

TABLE 4.3 Concentrations of major ions in continental and marine coastal rainfall (mg L1). (From Berner and Berner 1987.)

Continental rain

Marine coastal rain

TABLE 4.3 Concentrations of major ions in continental and marine coastal rainfall (mg L1). (From Berner and Berner 1987.)

Continental rain

Marine coastal rain

Ca2+

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