Environmental Engineering Reference
In-Depth Information
Picture 9.3
Bioretention cell used for stormwater collection and pollutant retention by both the soil
and plants.
Courtesy US Department of Agriculture.
pollutants, and create time for infiltration. The addition of check dams to swales creates longer
retention times that encourage infiltration.
Excess water from swales can be released into retention ponds, detention ponds, bioreten-
tion cells, or constructed wetlands. A retention pond is a permanent pond designed to retain
and store water for long periods of time. What is more, water from retention ponds can serve
other purposes than just holding rainwater, such as landscape irrigation. Detention ponds are
ponds designed to hold water temporarily after a storm and let the water drain slowly and
evaporate.
Bioretention cells, also know as rain gardens, are collecting areas of stormwater that allow
pollutant retention by both the soil and plants and promote aquifer recharge through water
infiltration. Bioretention cells are shallow depressions in the ground lined with porous mate-
rial that is covered with soil and planted with native plants, shrubs, and trees. When the soil is
impervious so bioretention cells are not feasible, constructed wetlands can serve as final
on-site stormwater disposal devices.
SUMMARY
Water is constantly recirculated above and below ground as a result of the hydrologic cycle.
The portion of freshwater readily available for consumption is mainly limited to aquifers and
surface water. Surface water and rechargeable aquifers are restocked at a finite rate that
depends on precipitation activity and the rate of extraction; therefore, these two sources of
water are considered renewable. Other aquifers, however, are isolated from recharging points
and cannot be replenished by precipitation so are not renewable.
The increasing water extraction for economic activities and human consumption at a faster
rate than replenishment will lead to an eventual water crisis in the near future; thus, we need
to find ways to reduce water use to avoid a crisis.
