Transcript Retention Basins and Rain Gardens

Retention Basins and Rain Gardens
A Possible Solution to
Nutrient Pollution in
Natural Waters?
Population Growth & Urbanization
• Impervious surfaces lead to excessive storm water
• Because of urbanization hydrology is changed
Water Quality Decline
• Influx of pollutants
– Nitrogen (N2) and
phosphorous (P)
• Excessive quantities
can cause
eutrophication
– ↑ in algae
– ↓ dissolved O2 levels
– Leads to widespread
fish kills
Water Quality Decline
• Key problem is nutrient pollution,
particularly nitrogen (N) pollution
• About ½ half of N load to Barnegat Bay
comes from surface runoff (Kennish et al.
2007)
• Removing nutrients from surface
water before it enters the Bay is cost
effective way to improve Bay health
DIRECT ATM.
DEPOSITION
34%
DIRECT
GROUNDWATER
DISCHARGE
12%
Source: Hunchak-Kariouk and Nicholson, 2001
SURFACE
WATER
54%
N Concentrations in Bay Streams,
1987-1997
Nitrogen Cycle – a Key Player
• N2 makes up over
75% of air
– Contained in Amino
Acids, proteins &
nucleic acids of living
things
• Comes in various
forms
– Only ammonium
(NH4+) & nitrate(NO3-)
are useable by plants
& algae
Nitrogen Cycle – a Key Player
• Nitrogen-fixation
– anaerobic bacteria
convert atmospheric
into NH4+ & NO3-
• Vital part of Nitrogen
Cycle
–
–
–
–
Made of several parts
Nitrification
Denitrification
Ammonification
http://www.google.com/imgres?q=nitrogen+cycle&hl=en&gbv=2&biw=1024&bih=566&tbm=isch&tbnid=Oer5J0d51BtSrM:&img
refurl=http://www.h2ou.com/h2nitrogencycle.htm&docid=cnNVBqgcmbsX0M&imgurl=http://www.h2ou.com/h2images/Nitrog
enCycle-lgr-F.jpg&w=697&h=605&ei=qZKzT9DaBNCN6QHmobiQCQ&zoom=1
http://www.google.com/imgres?q=nitrogen+fixation&hl=en&biw=1024&bih=566&gbv=2&tbm=isch&tbnid=cl6dCdYpP4HkM:&imgrefurl=http://landscapeforlife.org/give_back/3c.php&docid=KXqSzFlwUhex0M&imgurl=http://landscapeforlife.org/images/nit
rogencycle.jpg&w=880&h=687&ei=qpGzT8b4C8Gbgwfk89T_Bg&zoom=1
You tube Video on Nitrogen Cycle
• Nitrogen Cycle Description
Rationale
• Human development increases impervious cover in watersheds
• Traditional rain gardens collect storm water from impervious
areas and infiltrate it through soil, slowing delivery to rivers
and bays, but don’t reliably remove nutrients, esp. Nitrogen
Source: Hunchak-Kariouk and Nicholson, 2001
INCREASING URBAN
Nutrient Pollution Standards
• US Federal Government changed standards in 2007
• Resulted in a call to lower nutrient pollution from
industry, commercial, and residential areas
• ↓ non-point pollutants
– ↓ N2 and P
Rationale
• 2008 National
Estuarine
Eutrophication
Assessment suggests
Barnegat Bay is in
pretty bad shape
• Problems are
expected to worsen
over time
Solution: Keep nutrients out of waterways
• Limit amounts of N- & Pcontaining nutrients
making their way into
aquatic ecosystems
• In turn, algal blooms
would decrease
• Also slow flow and
possibly divert storm
water away from rivers
and streams
• Reduce downstream
flooding
Prior Studies
• Many studies have
investigated ways to reduce
nutrient pollution in waters of
East Coast of United States e.g.
– Charles River in Boston
– North Carolina
• Aimed at decreasing excessive
nitrogen and phosphorous in
bodies of water
Bioretention hydrology & nutrient
Removal in North Carolina
• Bioretentions built in
proximity to Shopping
Center with proper
filtration can allow in
upwards of 40% of
nitrogen &
phosphorous removal
• Water outflow also
reduced, especially in
warmer seasons
Stormwater ponds & biofilters in
Charles River, Boston MA
• Reduction of
phosphorous from
nearby industries into
bodies of water can
be achieved
• Up to 65% removal
with the use of
detention pond and
filter system
Traditional “Infiltration Style” Rain Garden
• Designed to slow
water flow
• Absorbs excess water
flow from nearby
impervious surfaces
• Flood reduction by in
increased absorbency
• Decreases soil erosion
Traditional “Infiltration Style” Rain Garden
• Unreliable at nutrient
removal
• Maximum phosphorus
and nitrogen removal is
between 30 – 40%
• Inadequate at reducing
the amount of nutrients
making its way into
bodies of water
• Water generally flows
vertically through
traditional style rain
garden
Current Studies
• Univ. of New Hampshire Stormwater Center is a
leader in study of methods for nutrient removal
• Their proposed solution uses horizontal filtration of
water through a “Gravel Rain Garden”
• Dense plantings at surface means lots of plants to
take up nutrients through their root system
• More important, though, is the anaerobic
environment created in crushed stone that lies
beneath.
• Allows for denitrification
Nitrogen-Removing Rain Gardens
• Create extensive saturated zone
designed to maintain anoxic conditions
to promote denitrification through use
of impervious liner
• Longer retention of water, increases
nitrogen removal efficiency
• Base of gravel to provide high surface
area for microbial activity
• Top with soil and plant with typical rain
garden plants
Disadvantages
• More expensive
• New technology so not
yet commonly
constructed
• Replacement of
vegetation every three
years to ensure nitrogen
removal capabilities are
optimized
Advantages
• Existing ponds and detention
basins can be turned into
subsurface gravel rain gardens
• Works well in cold climates
• Much superior and more
reliable nutrient removal
than obtained with
traditional “infiltration style”
rain gardens
Questions?