Environmental Engineering Reference
In-Depth Information
The total number of ions is estimated variously as
total dissolved solids
(the total mass of material dissolved in water),
salinity,
or
conductivity
.
Conductivity and salinity are commonly used because they are easy to mea-
sure. Conductivity is simply the relative amount of electricity that can be
conducted by water. The more dissolved ions present, the higher the con-
ductivity. Conductivity can be correlated approximately to system produc-
tivity because high nutrient waters have high conductivity, but other fac-
tors including concentration of nonnutrient salts also influence conductivity.
The value for total dissolved solids does not always exactly correlate to
that of conductivity. Only ionic compounds are included in conductivity,
whereas uncharged molecules (such as many dissolved organic compounds)
also contribute to total dissolved solids. Salinity is the mass of dissolved
salts per unit volume. Complete chemical analysis is necessary to determine
true salinity, but conductivity is generally used as a surrogate of salinity.
As water moves through terrestrial ecosystems, materials are dissolved
or
weathered
from the land. Chemical weathering releases dissolved mat-
ter, whereas mechanical weathering releases particulate matter that may re-
act to form dissolved matter at some point. Thus, the total concentration
of dissolved matter is related inversely to the amount of runoff because the
higher the runoff, the less time water has to dissolve ions. However, the re-
lationship between runoff and total dissolved solids is variable because of
differences in geomorphology, geology of the parent material (e.g., the rel-
ative abundance and solubility of the ions in the native sedimentary and
igneous rocks and soils), and area of runoff. The amount of dissolved ma-
terials associated with a set amount of precipitation can vary over an or-
der of magnitude as a result of differences in geomorphology or the com-
position of the parent material (Fig. 11.3). Furthermore, some ions, such
as nitrate, can decrease in concentration as they are assimilated (incorpo-
rated into biomass) by terrestrial and aquatic biota. Therefore, the relative
abundance of ions in water flowing from terrestrial habitats also depends
on chemical interactions with biota. Much of the material that enters wa-
tersheds probably does so through smaller rivers and streams (Alexander
et al.,
2000).
Solubility and relative abundance of elements in the earth's crust lead
to some general patterns of average relative abundance of dissolved ions in
river waters (Fig. 11.4). For example, aluminum (abundant but not solu-
ble) and manganese (low abundance and solubility) are found at low con-
centrations relative to calcium or sulfate. The concentrations of some ions
are constant across many rivers (e.g., silicate), but concentrations of oth-
ers (e.g., sodium and iron) vary over many orders of magnitude, depend-
ing on regional geology (Fig. 11.4). Concentrations of nitrate and phos-
phate vary over several orders of magnitude because plants have high
affinity for these nutrients when they limit primary production, but agri-
cultural fertilization can lead to very high concentrations.
The concentration of hydrogen ions (protons) is also very important
biologically as well as chemically. This concentration, or acidity, is ex-
pressed as
pH
. pH is a logarithmic scale, corresponding to the following
equation:
log
10
{H
}
pH
Search WWH ::
Custom Search