Environmental Engineering Reference
In-Depth Information
B.4 MODELS AND WATERSHED STUDIES
By the 1960s it was apparent that the nation's water resources were experiencing
a significant reduction in quality. The root causes of that diminution in both rivers
and lakes was attributed to nutrients produced by the thousands of wastewater
treatment facilities then in place, serving most major cities and towns, and the
inadequacy of the treatment processes used. After lengthy debate it was concluded
that a major investment was needed to upgrade the efficiency of wastewater
treatment process, a process that could be accomplished only with significant
help from the federal government for municipalities and stringent requirements
for industries. This public investment took the form of PL 92-500, passed in
1992, and initiated a massive effort that carried through the next two decades.
The issue of pollutants conveyed to surface waters from the land surface,
especially agriculture, was recognized, but the resulting problems were first
observed in lakes, where the influx of nutrients, especially phosphorus, was
changing the trophic state of every impoundment. Pioneering research in the
Tennessee Valley Authority system of lakes, built to produce hydroelectric
energy for a portion of the country not well served by the national power
grid, demonstrated clearly that this change in water quality could be attributed
to the sediment and nutrients carried with every significant rainfall from
the surrounding watershed. Studies in Lake Tahoe, situated on the border of
California and Nevada, resulted in the recognition that any solution depended on
better managing the land use of the surrounding drainage basin, and a planning
process was initiated that continues today.
This research was followed by the passage of PL 92-500, and included in that
act was specific wording on the issue of pollutant generation from land runoff
and the impact on lakes and human-made impoundments. The center of attention
was now Lake Erie, the shallowest of the Great Lakes, which had shown signs
of early pollution and changes in trophic state, again attributed to the influx
of phosphorus from major cities that ring the lake, such as Detroit, Cleveland,
Toledo, Buffalo, and Sandusky. It was not clear from the available data exactly
how much phosphorus was coming from these treatment facilities and how much
was coming from the surrounding 23,000-m
2
drainage area, much of which was
in active cultivation.
Following these studies, national attention shifted to major estuary systems,
from the Sacramento Estuary to Chesapeake Bay. On the local and regional levels,
numerous states and communities concluded that the only way to understand the
division of responsibility between point sources of wastewater treatment plants
and land runoff pollution, now labeled
non-point sources
, was to study an entire
watershed or basin. Of course, a few watersheds, rivers, and basins had been
studied during the late 1960s and early 1970s, and this was very informative as
to pollutant transport in rivers. We then began to consider the concept of mass
transport, rather than the concentration of a given pollutant, as a better measure
of water quality impact. In this section we offer a sampling of such studies and
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