For example, multiple nonpoint sources may be contributing to eutrophication related problems in the
downstream reservoir and, if this has been an ongoing situation, the reservoir sediments may also be
contributing significantly to the problem. Assessment of processes in the watershed, reservoir, and
tailwater, as previously discussed, is absolutely necessary in identifying the source(s) of the problem.
These assessments can lead to some surprising results. In a recent study of a large multi-river
watershed in the southeast, the Apalachicola-Chattahoochee-Flint River Basin, urban and suburban
land use accounted for only 5% of the basin but was considered to have the most important effect on
stream-water quality (U.S. Geological Survey, 1998).
Other good sources for assessing water quality using indicators include bioassessment protocols
(Plafkin et al., 1989) and websites for decision support systems such as WATERSHEDDS (Osmond
et al., 1997). The URL for this website is http://h2osparc.wq.ncsu.edu.
2.1.4 OVERVIEW OF CONTAMINANTS, SPILLS, ETC.
Introduction. Chemicals associated with discharges from industry, agricultural, and mining
practices, and chemical spills are often considered as contaminants instead of being included in
eutrophication problems previously described. Contaminants may be grouped as heavy metals (e.g.,
cadmium, lead, zinc), specific elements of concern such as mercury, arsenic, and selenium,
hydrocarbons such as polychlorinated biphenyls (PCBs), an array of pesticides and herbicides, and
others. In general, these contaminants respond to water quality conditions such as anoxia and pH, may
be biologically magnified (increase in concentration in organisms from the lower to the upper levels of
the food chain), and remain in the environment for a wide range of time. These problems are often site
or event specific and are not subject to conventional enhancement techniques mentioned below.
However, a brief overview is provided with the recommendation for more detailed review of the
literature for those with greater interests.
2.1.4.1 HEAVY METALS
Most often, these contaminants are partitioned into a soluble phase and a particulate phase
associated primarily with clay minerals which are easily transported as suspended material (Figure
2.1.1). Cadmium, iron, and zinc are associated with particulate transport as colloids. Heavy metals are
also associated with lower molecular weight fractions of organic carbon and labile organic complexes
which are more electrochemically active and therefore better sites for metal sorption. Bacteria and
inorganic materials often provide the surfaces required for adsorption. Metal sorption is a function of
pH (sorption decreases as pH decreases, Figure 2.1.2), suspended solids concentrations (sorption
increases as suspended solids increase due to an increase in site availability), and particle composition
(sorption increases as organic surface area increases). Metals and non-labile organic compounds
adsorb to particles and desorb as pH decreases. Surface charge of particles is pH dependent - at low
pH a positively charged surface prevails at high pH a negatively charged surface prevails. Under
2.1-5
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