Quantcast SAMPLING CONSIDERATIONS

 
  
 
noxious odors from the degassing of hydrogen sulfide are usually reported by fishermen, people at
tailwater recreation sites, or project personnel. High concentrations of iron or manganese cause
increased treatment costs for potable water suppliers who routinely monitor these constituents and often
report unusually high concentrations. Biological monitoring by state and federal agencies may detect
long term trends in biota (e.g., changes in community composition, irregular growth). Other forms of
problem identification include knowledge of episodic events such as chemical spills, noxious algal
blooms, or routine water quality monitoring (e.g., measurements of temperature or dissolved oxygen
concentrations).
Once the problem has been identified, objectives must be determined, clearly stated,
coordinated with other involved or interested parties, and evaluated for attainability. For example,
hydrogen sulfide odors generated by the degassing of hydrogen sulfide are obnoxious and potentially
harmful. The odors are indicative of severe anoxia in the upstream reservoir and a clear objective
would be to prevent the occurrence. However, in-lake measurements to avoid the development of
anoxia may not be plausible or could be extremely expensive. The objective to prevent the occurrence
may not be attainable since the enhancement technique may not be applicable.
Processes contributing to the identified problem must be determined and evaluated. Often
several processes are involved and an approach that considers interactions between processes is
required. The role of the hydrology and morphology of the tailwater must be determined. Often a
problem such as low dissolved oxygen concentrations is more pronounced during low flow (especially
at night) than at high flow. Conversely, resuspension of sediments is greater during high flow
(particularly on the leading edge of the hydrograph). This type of information assists in sample design,
identification of constituents of interest, and evaluation of applicable enhancement techniques.
Constituents of concern (excluding variable flow) most often include temperature, dissolved
oxygen, nutrients, metals, particulates, and algae. Most often the severity of water quality problems
associated with these constituents is related to the time scale or duration of abrupt or lasting changes.
For example, a sudden increase in water temperature is an acute impact on biota whereas a slower
increase in water temperature (e.g., seasonal warming) may have no impact or a chronic impact (e.g.,
timing of increased temperatures coincides with spawning seasons and too high temperatures inhibit
spawning or decrease survival rates). As described earlier, dissolved oxygen concentrations in
tailwaters follow daily and seasonal patterns and seasonal trends in nutrients and metals are often
apparent. Knowledge of these patterns is necessary for the efficient assessment of water quality
problems (sample design, data analysis, etc.).
Another important consideration is the application of remote monitoring and discrete sampling
for assessing water quality. Continuous monitoring of temperature, dissolved oxygen, pH, and specific
conductance in situ is the most common method of remote monitoring. Sensors for light, oxidation-
reduction potential, ammonia, and turbidity (and other constituents) are also available. Vorwerk et al.
(1996a) have evaluated some of these methods for monitoring releases from hydropower projects and
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