Rain Garden Research at EPA’s Urban Watershed Research Facility
Emilie K. Stander, Michael Borst, Thomas P. O’Connor, and Amy A. Rowe
U.S. Environmental Protection Agency, Office of Research and Development, Water Supply and Water Resources Division, Urban Watershed Management Branch
2890 Woodbridge Ave, MS-104, Edison, NJ 08837
Emilie K. Stander l stander.emilie@epa.gov l 732-906-6898
Experimental Design of Bench-Scale Test
Few studies have quantified the ability of rain gardens to effectively manage
chemical stressors. These studies indicate that the gardens function well in
removing heavy metals and phosphorus, but the results for nitrogen have been
mixed (Davis 2007). A nitrogen species of particular interest is nitrate, which is
known to contribute to algal blooms and hypoxia in receiving waters. Current rain
garden design, which typically includes rapidly draining sandy soils with low
levels of organic matter, is not conducive to nitrate removal through
denitrification. Compost is often mixed in with the sandy media to increase
adsorption of heavy metals and phosphorus and increase organic matter content,
but without the proper conditions for nitrate removal, compost may be a nitrate
source in rain garden effluent. Alternative carbon sources should be considered.
Experimental Design of Pilot-Scale Study
Eight pilot-scale rain garden mesocosms will be tested in 2.1-m and 2.5-m diameter,
conical-bottom HDPE tanks at EPA’s Urban Watershed Research Facility (Figures 1
and 2).
40
36
34
30
Control
1 Layer
2 Layers
Figure 6. This graph shows
effluent volumes for each
treatment during the low flow
test.
Treatment
Effluent flow rates were significantly larger in the control treatment compared to
both newspaper treatments during the low flow test (F2,16 = 9.0, p < 0.01) (Figure
7a). However, during the high flow test, all three treatments displayed significant
ponding (Figure 5), and there were no significant differences among treatments
(Figure 7b). These results suggest that something common to all three treatments
was a factor in slowing drainage rates, possibly related to the migration of small
clay particles in the media (Figure 8).
freeboard
5 cm
38
32
60 cm
media
6 cm
900
360
shredded
newspaper
34.5 cm
800
340
Flow R ate (m L/m in)
2.5 cm
EPA’s rain garden research in Edison, NJ, studies enhancements of rain garden
design to promote increased nitrate removal through denitrification and plant
uptake while maintaining adequate drainage and high levels of heavy metal and
phosphorus removal.
42
Flow R ate (m L/m in)
Background
Effluent volumes were significantly larger in the control treatment compared to
both one-layer and two-layer newspaper treatments during the low flow test
(F2,15 = 11.3, p < 0.01), suggesting that the shredded newspaper retained the
added stormwater (Figure 6). There were no significant differences among
Effluent Volumes
treatments during the high flow test.
A bench-scale test (Figures 3-5) of the pilot-scale rain garden mesocosm study
(Figures 1 and 2) was conducted in 90 x 60 x 60 cm polyethylene tanks to test for
possible impacts of shredded newspaper layers on drainage. The tanks were filled
with a locally-available sand-clay media used for baseball infields in New Jersey.
The media is 81% sand, 9% silt, and 10% clay. Stormwater collected from an
adjacent community college parking lot was mixed and applied to the media
surface at low and high flow rates using a spray bar. Slotted pipes, wrapped in
geotextile to prevent clogging, at the tank bottoms allowed for effluent drainage
to collection containers. Water level sensors in the collection containers measured
effluent depth, used as a surrogate for effluent flow rate, at six-minute intervals.
Newspaper was chosen as a carbon amendment based on preliminary work by
Kim et al. (2003) demonstrating high nitrate removal in bioretention columns
amended with newspaper. Tanks were constructed with zero (as a control), one,
or two layers of shredded, unprinted newspaper to determine the effect of the
newspaper on effluent volumes and flow rates.
Volume (L)
Rain gardens are vegetated depressions designed to capture and infiltrate
stormwater runoff from impervious surfaces such as roofs, parking lots, and roads.
The potential benefits compared to traditional curb and gutter drainage systems
include peak flow attenuation in receiving waters, increased groundwater
recharge, stream baseflow maintenance, and filtration and transformation of
environmental stressors, such as heavy metals and nutrients, commonly found in
runoff. These structures are quickly gaining popularity as green infrastructure
components of low impact development planning.
Results
320
300
280
media
260
shredded
newspaper
2.5 cm
9.5 cm
media
3.75-cm diameter perforated drainage
pipe wrapped in geotextile
Figure 3. This schematic shows the bench-scale design for a tank containing two newspaper
layers. In the one-layer treatment the bottom layer of shredded newspaper was eliminated.
700
600
500
400
300
240
200
C o n tro l
O ne Layer
C o n tro l
T w o L a y e rs
O ne Layer
T w o L a y e rs
Figure 7. Effluent flow rates are shown by treatment for the low flow (a) and high flow tests (b).
Coefficient of Uniformity
(D60/D10)
Introduction
90
80
70
Figure 8. The coefficient of
uniformity at 3-cm soil depth
increments increased in the
bottom half of the media,
suggesting that smaller clay
particles may have migrated
from the top half to the bottom
half of the media.
60
50
40
30
20
10
0
0-3
3-6
6-9
9-12
12-15
15-18
Soil Depth (cm)
Future Directions
Figure 1. Elevated rain gardens
facilitate collection and analysis of
water that drains from the gardens.
Figure 2. Experimentally-added stormwater
drains through a gravel layer and out a
bottom drain to a collection tank.
Stormwater Additions in Pilot-Scale Mesocosms
Stage 1: a series of twelve 3-hour stormwater additions will be carried out during
two months to test hydraulic properties (infiltration/exfiltration rates, volumes,
and timing; soil water content using TDR; standing water depth)
Stage 2: chemical analysis of effluent (heavy metals and nutrients)
Stage 3: quantification of denitrification rates and plant uptake
The experimental design consists of four treatments, listed below, assigned to eight
mesocosms using a partial factorial design:
• high / low volume of organic matter (layers of shredded newspaper)
• presence / absence of a saturated zone at depth
• high / low hydraulic loading (different sized tanks)
• turf grass / native, herbaceous vegetation
Notice
U.S. Environmental Protection Agency
Office of Research and Development
The U.S. Environmental Protection Agency, through its Office
of Research and Development, funded and managed, or
partially funded and collaborated in, the research described
herein. It has been subjected to the Agency’s peer and
administrative review and has been approved for external
publication. Any opinions expressed are those of the author
(s) and do not necessarily reflect the views of the Agency,
therefore, no official endorsement should be inferred. Any
mention of trade names or commercial products does not
constitute endorsement or recommendation for use.
References
Figure 4. This photograph
illustrates the experimental
setup for the bench-scale study.
Figure 5. All three treatments demonstrated significant
surface ponding of the added stormwater during the high
flow test. This was an unexpected result.
Davis, A. P. 2007. Field performance of bioretention: water quality. Environmental
Engineering Science 24(8): 1048-1064.
Kim, H., E. A. Seagren, and A. P. Davis. 2003. Engineered bioretention for removal of nitrate
from stormwater runoff. Water Environmental Research 75(4): 355-367.