Drought Fact Sheet #10
How Does Runoff Affect
Our Water Supply?
Contaminants in Runoff Can Taint Our Water Supply
What Do We Mean When We Talk About Runoff?
Runoff is surface water that flows across the land in
order to get to a body of water such as a stream,
lake, or wetland. Runoff often transports
contaminants to these water bodies, reducing their
usefulness as a source of water in the event of a
drought. Because drought preparedness is
concerned with decreased water quality and the loss
of groundwater reserves, there is interest in all kinds
of runoff, regardless of regulatory jurisdiction.
Consequently, for the purposes of this discussion,
runoff includes both stormwater and agricultural
runoff.
Stormwater runoff carries with it the contaminants
that accumulate on roadways, in green space and on
open storage areas. These can include sediment, oils
and greases, pet waste, trace metals, and fertilizers
and chemicals from lawns. Another potential source
of stormwater runoff is found at construction sites.
In areas where excavation has taken place, the
natural soil surface is disturbed and easily eroded by
a significant storm event (see Fact Sheet #6).
Agricultural runoff can contain sediment, chemicals,
and livestock waste. Possible sources include
agricultural tillage, which can reduce ground cover
and increase sediment release, livestock grazing,
which can bring livestock waste directly or
indirectly in contact with the waterway, and
irrigation practices that release contaminant-laden
water back into waterways.
How Does Runoff Impact Our Water Supply?
Believe it or not, increased runoff, can actually cause
a stream to become drier in the long run (see Fact
Sheet #8). Another key concern is that runoff
transports sediment and dissolved contaminants to
the waterway. The sediment itself is a seriously
degrading factor, but the chemicals, bacteria and
other contaminants that attach themselves to the
Stormwater runoff carries contaminants that accumulate
on roadways, in green space and on open storage areas.
sediment worsen the impact. There are also
dissolved contaminants in surface flow that are
independent of the sediment. These can include
salts as well as various residential and farm
chemicals. Any runoff that flows directly into a
waterway will carry with it some of these
chemicals. In addition, when runoff is diverted into
drainage systems that route water directly to the
nearest river or stream, the water does not soak into
the ground and is lost to groundwater recharge.
How Can We Mitigate the Effects of Runoff?
From the perspective of drought planning there are
two primary effects that need to be addressed. The
first is the impact of the pollutants on the surface
water and its degradation as a water source in the
event of a drought. The second is loss of recharge
water through the direct discharge of runoff.
One effective strategy is the development of
undisturbed vegetated buffers along the edges of
streams and waterways to control runoff. These
buffer areas can effectively intercept the runoff and
filter sediment and the attached contaminants from
the water through a number of natural processes that
include vegetative uptake, chemical breakdown,
uptake in the soil, and biodegradation. Additional
details about these processes and an introduction to
several helpful agencies and documents can be found
at www.epa.gov/nps/agriculture.html An excellent
paper that discusses agricultural runoff can be found
at www.forester.net/sw_0305_impact.html
The use of detention basins has gained widespread
use in providing a way to increase stormwater
recovery for groundwater recharge. The principle
used in these basins involves holding the water long
enough for sediment to settle out, and allowing a
substantial portion of the stormwater to infiltrate into
the groundwater supply after it is filtered and
cleaned by the soil profile. An excellent source of
additional stormwater information can be found at
http://cfpub1.epa.gov/npdes/pubs.cfm?program_id=6
An excellent text that compares the various options
for stormwater control can be found at
http://www.epa.gov/npdes/pubs/usw_c.pdf
direct flow of runoff into waterways and provide a
way for this detained water to be filtered through
the natural soil to recharge the groundwater will
have the greatest benefit to the prevention of
drought.
How does Runoff Impact Drought Planning?
Increased surface water runoff effectively short
circuits the natural processes that ensure water
treatment and groundwater recharge. Every
Drought Plan needs to carefully consider measures
for the restoration of these natural systems.
Encouraging local policy decisions that support
measures to minimize runoff and encourage natural
infiltration is an important step in the long-range
preparation for a drought event. Remember, the
groundwater beneath your community is your
“water savings account.” It is what recharges your
streams during the dry season. It is what you will
be counting on when a drought event occurs (see
Fact Sheet #2).
The NARC&DC would like to thank the USDA’s
Natural Resources Conservation Service for their
assistance in the development of this fact sheet.
Constructed wetlands provide another method to
detain and treat stormwater or agricultural runoff
before it continues through the hydrologic cycle.
Constructed wetlands are different than natural
wetlands in that they are designed with water
treatment and detention in mind. They rely on the
same treatment processes as the other methods
discussed above, and can include the use of aquatic
wildlife, and emergent vegetation to provide
additional treatment. Existing natural wetlands
(jurisdictional wetlands) cannot be used to treat
runoff.
Stormwater detention basins, grassed waterways,
riparian buffers, and constructed wetlands are all
effective ways to prevent runoff from entering and
degrading surface water that we depend on for help
in the event of a drought. Designs that prevent the
Constructed wetlands provide another method to
detain and treat stormwater or agricultural runoff.
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March, 2006