Will Wastewater Treatment Stop Eutrophication
of Impoundments?
THOMAS J. GRIZZARD, Graduate Teaching Assistant
ERNEST M. JENNELLE, Associate Professor
Virginia Polytechnic Institute and State University
Blacksburg, Virginia
INTRODUCTION
The eutrophication of surface waters in the United States has become a problem of
significant importance in the area of water quality management. The increased concern
over the degradation of the nation's waters is indeed desirable, for our surface waters
constitute a vital natural resource that must be carefully protected to insure its continued
suitability for a myriad of uses.
The aging rate of an impoundment is accepted to be a function of the nutrient and
organic loadings to which is is subjected. While the eutrophication process is a natural
phenomena, the presence of man and his accompanying activities may drastically affect the
factors relating to the rate of maturation. As the drainage basin above an impoundment is
altered by agricultural development, industrial expansion, or the growth of urban areas,
there is an increase in the quantity and diversity of materials being introduced into the
water-courses. These materials generally contribute to an acceleration of the eutrophication
process.
A thorough understanding of the effect of cultural activity in a watershed on the
temporal and spatial distribution and transport of organic and nutrient loadings to an
impoundment is necessary in order to arrive at a complete description of the lake
fertilization process. Each watershed of interest should be evaluated on the basis of its own
characteristics, and not on those of similar basins. Generalizations can lead to erroneous
conclusions about the processes related to the fertilization of a particular impoundment.
Attempts to arrive at some understanding of the process of eutrophication have often
been marked by an effort to ascribe the cause of the phenomenon to some single component
rather than to a synergism of interrelated components. Thus, the concept of the limiting
nutrient in accordance with Von Leibig's Law of the Minimum (1) has often been pursued in
studies on the cause-effect relationship of lake enrichment.
Historically, phosphorus has generally been considered to be the so-called limiting factor in the eutrophication of impounded waters. Being relatively rare in natural waters
because of the insolubility of its inorganic compounds, phosphorus seemed to be the limiting nutrient most consistent with the application of the Law of the Minimum to algal
metabolism. The vital role played by phosphate in the structure of nucleo-proteins and the
intra-cellular transfer of energy makes it essential for biological growth. Sawyer (2) states
that nuisance growth conditions within a lake can be expected when the inorganic phosphorus concentration reached 0.01 mg/1. The actual availability of inorganic phosphorus
for algal metabolism in an impoundment is dependent upon the yelds in the tributary watershed.
The transformations of nitrogen and its compounds in impounded waters is also of
particular interest with respect to the proliferation of undesirable algal species. Both nitrate
nitrogen (NO3-N) (3) and ammonia nitrogen (NH3-N) (4) are utilized by plants and algae
which convert it to organic nitrogen (Org-N). There are also genera of algae and bacteria
that are able to fix elemental nitrogen to satisfy their metabolic needs. This fact raises some
questions about whether or not a nitrogen-limiting situation is possible in lakes since
aquatic organisms have access to a limitless supply of the element from the atmosphere.
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