Environmental Engineering Reference
In-Depth Information
All heavy metals exist in surface waters in colloidal, particulate, and dis-
solved phases, although dissolved concentrations are generally low. Colloidal
and particulate metals may be found in hydroxides, oxides, silicates, or sul-
fides or they may be adsorbed to clay, silica, or organic matter. The soluble
forms are generally ions or unionized organometallic chelates or complexes.
The solubility of trace metals in surface waters is predominately controlled
by water pH, the type and concentration of liquids on which the metal could
adsorb, and the oxidation state of the mineral components and the redox
environment of the system. The behavior of metals in natural waters is a
function of the substrate sediment composition, the suspended sediment
composition, and the water chemistry. Sediment composed of fine sand and
silt will generally have higher levels of adsorbed metal than will quartz, feld-
spar, and detrital carbonate-rich sediment.
The water chemistry of the system controls the rate of adsorption and desorp-
tion of metals to and from sediment. Adsorption removes the metal from the
water column and stores the metal in the substrate. Desorption returns the
metal to the water column, where recirculation and bioassimilation may take
place. Metals may be desorbed from the sediment if the water experiences
increases in salinity, decreases in redox potential, or decreases in pH.
Although heavy metals such as iron and manganese do not cause health
problems, they do impart a noticeable bitter taste to drinking water, even at
very low concentrations. These metals usually occur in groundwater in solu-
tion, and these and others may cause brown or black stains on laundry and
on plumbing fixtures.
Nutrients
Elements in water such as carbon, nitrogen, phosphorus, sulfur, calcium, iron,
potassium, manganese, cobalt, and boron—all essential to the growth and
reproduction of plants and animals—are called
nutrients
(or biostimulants).
The two nutrients that concern us in this text are nitrogen and phosphorus.
Nitrogen, an extremely stable gas, is the primary component (78%) of the
Earth's atmosphere. The nitrogen cycle is composed of four processes. Three
of the processes—fixation, ammonification, and nitrification—convert gas-
eous nitrogen into usable chemical forms. The fourth process, denitrifica-
tion, converts fixed nitrogen back to the unusable gaseous nitrogen state.
Nitrogen occurs in many forms in the environment and takes part in many
biochemical reactions. Major sources of nitrogen include runoff from ani-
mal feedlots, fertilizer runoff from agricultural fields, municipal wastewater
discharges, and certain bacteria and blue-green algae that obtain nitrogen
directly from the atmosphere. Certain forms of acid rain can also contribute
nitrogen to surface waters.
