Urbanization and the Disruption of Matter and Energy Flows within Watersheds - Urban Watersheds: Geology, Contamination, and Sustainable Development - page 426

• Simplification
• Contamination
• Overgrowth
Reduction is the loss of areal coverage of an ecosystem or community. Fragmentation
consists of breaking ecosystems into spatially separated units or fragments. Substitution
occurs when one or more organisms/species are replaced by others. Simplification is
the reduction in the number of species in an ecosystem or community. Contamination
is the introduction of harmful substances into an ecosystem, typically anthropocentric-
manufactured chemicals. Overgrowth refers to the importation of excess nutrients into
an ecosystem.
The necessity to harness water for use by large concentrations of population and to pro-
cess the wastewater produced has input massive quantities of energy and materials into
urban watersheds. Construction of water infrastructure has changed the course of rivers
(the Mississippi) and created a false sense of security about flood risk (Hurricane Katrina).
In addition, the discharge of millions of gallons of effluent by urban wastewater plants into
streams often constitutes a significant percentage of their total flow.
Due to their scope and permanence, many of the impacts resulting from these activities
can be readily identified. Here, a matrix is employed to accomplish this task (Table 12.2).
For the purposes of this analysis, water infrastructure is divided into five human-
engineered systems: water supply, sanitary sewers, stormwater, overland flow, and
navigation/flood control. The cells of the matrix contain one impact associated with each
human-engineered system. Impacts without broad significance are assigned “minimal.”
Water supply . This system includes water treatment facilities and the pipes comprising
the water distribution network. Also included are the land and infrastructure required
to accomplish groundwater recharge, such as the conjunctive use system practiced in the
Los Angeles basin (Faunt 2009). Dams, reservoirs, pipes, and pumps required for interba-
sin water transfers to urban areas are not included. Conventional landfills are included,
because one of the outputs from the water supply system is a fair quantity of nontoxic lime
sludge.
Result: Overall, the impacts from this system are minimal, but there must be land set aside
for treatment facilities. Proper siting of these structures avoids the interruption of habitat
corridors and the displacement of sensitive ecosystems, such as wetlands. If withdrawals
for the water supply are made from a local stream, care must be taken not to go beyond a
20% reduction below bankfull stage. Otherwise, temperatures can rise in the stream and
disrupt its ecosystem (Poole and Berman 2001). Since warmer water holds less dissolved
oxygen, many fish adapted to colder water (e.g., trout) and requiring a certain level of oxy-
gen may be threatened if water temperature rises.
Sanitary sewers . Includes wastewater treatment plants and the sanitary sewer pipe network
delivering sewage to them. Landfills for handling the sludge produced are also included,
since it is toxic and is disposed in sanitary landfills, incinerated, or processed into biosol-
ids for agricultural applications.
Result: The main threat here is from infiltration/inflow (I & I), which refers to clear water
from rain and snowmelt improperly draining into the sanitary sewer system. Much of
the inflow results from cracked pipes, with older urban areas being more prone to leaks.
During heavy rainstorms, I & I may cause the sanitary sewers to quickly fill with clear