www.epa.gov
science BRIEF
BUILDING A SCIENTIFIC FOUNDATION FOR SOUND ENVIRONMENTAL DECISIONS
National Risk Management
Research Laboratory
www.epa.gov/nrmrl/
Green Infrastructure Research Program
Providing Research Solutions to Manage Wet-Weather Flow
Rain Gardens
Introduction
Rain gardens are vegetated surface
depressions, often located at low points
in landscapes, designed to receive
stormwater runoff from roads, roofs, and
parking lots. The gardens' sandy soils
allow stormwater to infiltrate quickly
to the native soils below and eventually
contribute to groundwater recharge.
Pollutants and nutrients in stormwater
runoff are removed by rain garden
vegetation and soils through biological
and physical processes such as plant
uptake and sorption to soil particles. In
comparison with stormwater release to
receiving waters through conventional
storm drain systems, infiltrating
stormwater through rain gardens reduces
peak flows and stressor loadings. This
reduction improves the physical and
biological integrity of receiving streams by
reducing stream bank erosion and negative
effects on aquatic communities.
Current Issues
While local governments and individual
homeowners are building many of the rain
gardens in use, relatively few studies have
documented the rain gardens' pollutant
and nutrient removal capability. Typical
rain garden designs currently consist of
gravel beneath a layer of sandy soil. A
mulch layer on the soil supports plantings
of trees, shrubs, and herbaceous plants.
Although this design is successful for the
removal of heavy metals and phosphorus,
it does not provide the right conditions for
nitrate removal.
A possible explanation for this pattern is
that the currently-accepted rain garden
design inhibits denitrification, a microbial
process that converts nitrate to nitrogen
gas, which is released to the atmosphere,
removing it from the system. Microbes
that carry out denitrification typically
require wet conditions and a readily-
available source of carbon to drive the
reaction. Sandy soils drain quickly and
have low levels of available carbon,
creating poor conditions for denitrification.
Objectives
Comprehensive asset management is The
National Risk Management Research
Laboratory (NRMRL) rain garden
evaluation is part of a larger collection of
long-term research that evaluates a variety
of stormwater management practices.
The U.S. EPA recognizes the potential
of rain gardens as a green infrastructure
management tool to lessen the influence of
development on aquatic resources.
The Green Infrastructure Research
Program's long-term rain garden research
objectives are to address the issues of
pollutant and nutrient removal capabilities
of rain gardens by studying the following:
• The performance of rain gardens in
removing pollutants.
• Whether currently-accepted design
standards can be adjusted to improve
nitrate removal capabilities.
Current Research
Eight rain garden mesocosms have
been constructed in conical-bottom
polyethylene tanks, and are filled with
different experimental types and levels
of substrate. The mesocosms are also
vegetated; half of the tanks are planted
with turf grass and the other half with
native, herbaceous plants that are drought,
flood, and salt tolerant. NRMRL's
current research investigates soil carbon
amendments and saturation at depth in
these rain gardens to promote conditions
for denitrification, and thus, increase
nitrate removal.
Controlled-condition research enables
NRMRL to better manage research
projects and collect high-quality
information. Collecting data and
performing experiments at field sites away
from the laboratory limits research due
to uncertainties in weather forecasts, site
access, utility locations, vandalism, and
other logistical issues that collectively
add greatly to the costs and timelines of
research projects.
Using on-site, experimental rain gardens
enables NRMRL to collect high-quality
data necessary for evaluating engineered
structures. On-site experiments are
inherently safer than attempting to monitor
U.S. Environmental Protection Agency
Office of Research and Development, National Risk Management Research Laboratory
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public rain gardens adjacent to roadways
during rainstorms, avoiding unnecessary
risks to people and equipment.
NRMRL operates a 20-acre Urban
Watershed Research Facility that includes
stormwater mesocosms, laboratories,
greenhouses, fabrication space, a pipeline
testing facility, swale and pervious parking
lot performance testing, and storage for
equipment and supplies. This unique
facility is part of the larger 200-acre
Edison Environmental Center operated
by the U.S. EPA Region 2. This land area
allows NRMRL to undertake this research
on a scale that cannot be executed at any
other U.S. EPA facility. The eight rain
garden mesocosms, along with the supply
and collection tanks, occupy '/4-acre of this
land.
Impacts
A more complete understanding of how
rain gardens function will enable the U.S.
EPA to provide national guidelines on rain
garden design, construction, maintenance,
and monitoring which local organizations
can use to reduce pollutant and nutrient
loads to receiving waters. Reducing
pollutants and nutrients in runoff will help
our waters meet their locally-designated
uses. Rain gardens and other management
tools will help watershed managers assure
that receiving waters meet the "fishable
and swimmable" goals that Congress
outlined in the Clean Water Act, and will
better assure the continuing supply of
high-quality, potable water.
CONTACT
Michael Borst
Chemical Engineer
U.S. Environmental Protection Agency
Office of Research and Development
National Risk Management Research Laboratory
732-321-6631
borst.mike(g),epa. gov
RESOURCES
Urban Watershed Management Research
http://www.epa.gov/ednnrmrl
U.S. EPA(2008). The Urban Watershed
Research Facility, Edison, New Jersey (PDF)
EPA/600/F-08/005
Photo of raingarden courtesy of Mt Airy Rain Catchers
Recycled/Recyclable
Printed with vegetable-based ink on
paper that contains a minimum of
50% post-consumer fiber content
processed chlorine free
U.S. Environmental Protection Agency
Office of Research and Development, National Risk Management Research Laboratory
EPA/600/F-08/013
September 2008
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