RAINWATER AS THE RESOURCE - Low Impact Development and Sustainable Stormwater Management - page 27

water source. Trace pollutants that are soluble in the raw water are more diffi-
cult to remove. The disinfection processes that are a part of this treatment are
also capable of removing or killing all of the disease-producing microorganisms
found in rivers and lakes. Where only disinfection is included in the treatment
process (such as the City of New York), the potential for infectious protozoans,
such as Cryptosporidium parvum and Giardia lamblia to survive the disinfec-
tion treatment is very real and can pose a significant health threat, especially to
a compromised population (e.g., HIV-infected citizens). The initial sources of
these protozoans are wild and domesticated animals, but infected human wastes
can also transmit them with runoff.
Perhaps the greatest impact of degraded streams on potable water supply
sources is the subtle taste and odor impacts produced in a water supply, especially
during warm summer months when raw water reservoirs may themselves be
heavily laden with algae. Again, it is the phosphorus conveyed with stormwater
runoff that creates the eutrophic environment in the raw water reservoir. In some
developed watersheds, surface supply intakes are suspended during periods of
heavy rain, but this is not always possible in many supply systems.
Although most modern water treatment plants are very efficient, they do not
remove every pollutant conveyed in stormwater runoff, and an increasing number
of pollutants are being found in our water supplies. The long-term solution, of
course, is to prevent or eliminate their use in the environment, but many are
decomposition or end products of common and widely used materials that have
become a part of our living environment and cannot be eliminated easily.
1.6 THE HISTORIC APPROACH: DETENTION SYSTEM DESIGN
In the early 1970s, as we began to recognize the hydrologic impacts of land
development, the immediate concern was the condition of natural drainage imme-
diately downstream of the building development, and this impact was perceived
as a flow-rate problem. We took the technology developed several decades pre-
viously for small earthen dams built in agricultural sites (the farm pond) and
tailored it to serve as a rate reduction system for the increased runoff volume
produced by new impervious surfaces and soil alteration. The detention basin
quickly became the design standard, in both a technical and regulatory sense, for
stormwater management at every new development site across the country, with
significant site impacts (Figure 1-22).
The design criterion became flow-rate control , and the stated goal of every
stormwater management system was to assure that the rate of stormwater runoff
generated from a building site was no greater following development than
occurred prior to development. Initially, detention structures were designed
primarily to control the extreme runoff conditions of the most severe rainfalls,
once again applying the river structure guideline of the 100-year-frequency
storm to a small earthen basin built on the upland or in some small drainage
swale. With experience, it was recognized that smaller rainfall events could also