Civil Engineering Reference
In-Depth Information
Formation of insoluble compounds may remove metal ions from solution
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Complexation also may solubilize metal ions from otherwise insoluble metal com-
pounds
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Strong complexation may shift oxidation-reduction potentials
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Little has been reported about levels of complexing agents and stable metal com-
plexes in natural waters. Complexing agents are not normally determined in water
analyses.
Humic and fulvic acids probably are the most important naturally occurring com-
plexing agents. These acids are rather loosely defined; they refer to a family of com-
pounds, similar in structure and chemical properties, formed during the decomposition
of vegetation. They can strongly bind metal ions, and they are found in both water
and soil. Synthetic complexing agents, such as sodium tripolyphosphate, sodium ethy-
lenediaminetetraacetate (EDTA), sodium citrate, and sodium nitrilotriacetate (NTA),
are produced in large quantities and almost certainly find their way into streams
through waste discharges. NTA may also solubilize heavy metals from sediments on
stream bottoms, depending on pH, bicarbonate concentration, calcium concentration,
and the nature of the sediments.
Effects of Clay Minerals and Other Suspended Material
Clay minerals are
among the most common suspended matter found in natural waters. In many streams,
clays might be considered the most important mineral solids present in colloidal sus-
pension or as sediments, for these reasons:
Clay minerals can fix dissolved chemicals in water and thus exert a purifying
action. The ability of clays to exchange cations is an important phenomenon
having an impact on the availability of trace-level metal nutrients in water.
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Because of their high surface area and other properties, clays also may sorb
organic compounds, such as pesticides and herbicides, and are important in the
transport and removal of organic pollutants in streams.
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It is also believed that some microbiological processes occur at clay mineral sur-
faces, so that clays may participate in the degradation of organic materials.
FACTORS IN THE LOCATION OF WATER INTAKES
There are a great many factors to be considered regarding intake locations. Raw-water
quality may vary greatly from stream to stream; it is assumed that a preliminary
selection among various streams has been made on the basis of best available raw-
water quality from a public health standpoint, as well as adequate quantity. The prob-
lem then becomes one of locating the intake along a given stream.
For the moment, the question of upstream wastewater discharge is set aside, and
the other factors are considered. This is done in order to help place the potential
hazards of wastewater contaminants in perspective with the risks involved in all water
systems, even when there is no pollution from wastewater.
Important items in the location of intakes, which take into account reliability, safety,
and cost, include:
