The Effectiveness of Rain Gardens at Reducing Total Phosphorus and Suspended Solids in Storm Water Run-off

Margaret EckerDepartment of Biology, Saint Michael’s CollegeColchester, Vermont

Methods:

Total Phosphorous (TP) and Total Suspended Solid (TSS) samples were taken at five different rain gardens during multiple storm events in the summer and fall of 2010. Four of the rain gardens are located in St. Albans, Vermont with three of them on Bishop St. and one in front of the Northwestern Medical Center. The fifth rain garden is located on the University of Vermont campus in Burlington, VT. Due to the distance between these locations not all gardens were able to be sampled on the same day as it was not always raining enough to produce an outflow in both locations. However, inflow and outflow data for a specified rain garden was always taken during the same rain event.

Two TP samples were taken at the estimated highest inflow and outflow points of each garden, as well as one TSS sample taken at each of these points. The UVM rain garden did not have a point of outflow due to its layout. The garden was designed to prevent the storm water from leaving the garden, held in by a mound. Therefore, the outflow sample was taken from the retained water closest to this mound. Two of the gardens located in St. Albans had outflow pipes, the medical center garden as well as the Bishop St. down garden, and in these instances outflow samples were taken from the water flowing into these pipes. In addition to the TP and TSS samples taken, an infiltration rate test was also conducted on all of the St. Albans rain gardens. In order to test the infiltration rate of the gardens, a gallon of water was poured over a designated area and then the amount of time for an additional gallon of water to infiltrate the soil in that same area was measured.

The TP and TSS samples taken from the rain gardens were analyzed at the EPSCoR lab at the University of Vermont. Both TP and TSS were analyzed using a method borrowed from Standard Methods for the Examination of Water & Wastewater (APHA 2005) AND Wetzel &Likens 2000 (5).

Results:Introduction:

The issue of high levels of phosphorus in Lake Champlain is one that has received a lot of attention in recent years in the Burlington area. While many of the point sources of phosphorus have been identified and measures have been taken to stop these pollutant sources, it is the non-point sources that prove to be difficult to remedy. The impervious surfaces of roads, parking lots, and rooftops are the main cause of this non-point pollution, as storm water is unable to penetrate the soil and deposit the phosphorus (1). Therefore, the storm water either runs into streams, eroding the ground as it moves, or runs down streets into drains bringing all the phosphorous from the road with it. Both of these scenarios ultimately lead to phosphorus entering the waterways and eventually making its way to Lake Champlain.

Steps have been taken in an attempt to decrease the flow of storm water down steep roadways. The building of rain gardens in the Burlington area began in 2006 in South Burlington (2). The idea is that the storm water will run into these gardens where it will be held and will seep into the soil at a 30% greater rate than that of a lawn (1). This is thought to serve two purposes; the flow of storm water down the street will be decreased and the amount of phosphorous in the outflow of these gardens will be reduced due to the infiltration of the phosphorous into the garden soil (3).

Discussion:

- with the exception of the Bishop St. down rain garden, all gardens displayed an increase in total phosphorus from inflow to outflow

-this trend was not the same for TSS levels, which varied among the rain gardens.

-the UVM rain garden showed increase in both TP and TSS, however it must be noted that there was no outflow and these samples were taken from the retained water. Therefore, it is not surprising that the water contained higher levels of TP and TSS as the water had stirred up soil in the garden as it moved and had not yet infiltrated (Figure 1)

-both the Bishop St. Up and the Brown Ave rain gardens were similar in layout along the Bishop St. curb. However, the Brown Ave garden was longer and contained a grass divide. The TP data for these gardens showed similar trends. For TSS, the Brown Ave garden decreased from inflow to outflow on average (Figure 3) while the Bishop St. Up garden showed slightly increased TSS in the outflow on average (Figure 2). This is possibly due to the Bishop St. Up garden’s relatively small size and location near the top of Bishop St. where the flow of the run-off is possibly too fast during storms of this magnitude for the small garden. The run-off may be disturbing the soil in the garden and too fast for infiltration to occur (although this garden does have a high infiltration rate).

-the Bishop St. Down rain garden seems to be the most effective at reducing the TP and TSS levels (Figure 4). This could be due to a variety of factors; the length of the garden, the rock divisions in the garden with a pipe through the first division to slow the water down and direct it into the next section of the garden, and also the outflow pipe which directs the remaining water directly into the ground to further infiltrate the soil.

-the Medical Center rain garden showed increased TP and TSS from inflow to outflow, however this rain garden also had an outflow pipe that directed any remaining run-off into the ground for further infiltration.

While the data does not necessarily support the proposed hypothesis, that is the rain gardens are not as effective at reducing TP and TSS as one would expect, there are other factors in the experimental design that possibly contribute to these results. The majority of rain events that these gardens are exposed to (80%) do not have as high a precipitation volume as the storm events in this study (large enough to produce an outflow that would yield a sample) (4). Therefore, while the two gardens with outflows over the curb and back into the street run-off had increased levels of TP returning into the drainage system, these events are less frequent than the typical storm these rain gardens are exposed to. Furthermore, while the medical center rain garden showed an increase in TP, it contained an outflow pipe that this outflow water was taken from. This pipe led into the ground, where the TP and TSS would be infiltrated.

Bishop St. Up Rain Garden:

Brown Ave Rain Garden:

UVM Rain Garden:

Bishop St. Down Rain Garden:

Medical Center Rain Garden:

Acknowledgements:

Thank you to Vermont EPSCoR for funding this research. Thank you also to Meredith Simard for her guidance both in the lab and in the field, Maeve McBride for her continuous assistance throughout this project and Miranda Lescaze for all of her encouragement and support. I would also like to thank Harold Manrique Hernandez and Calines Guerrero Diaz for aiding me in the analysis of my samples and for making my lab experience so enjoyable. A special thank you to Jeff Rouleau for collecting all of the St. Albans samples.

Works Cited:

1. Andreoletti, Jessica. The Vermont Rain Garden Manual "Gardening to Absorb the Storm"

Winooski: Winooski Natural Resources Conservation District. Print.

2. "Rain Gardens." Winooski Natural Resources Conservation District. Winooski Conservation

District, 6 Nov. 2009. Web. 11 Apr. 2011.

<http://www.vacd.org/~winooski/winooski_raingarden.shtml>.

3. "Reducing Stormwater Inpacts in Heavily Developed Areas; Winooski Rain Garden

Project." University of Vermont Extension (Sept. 2007). Print.

4. "Reducing Stormwater Impacts in Heavily Developed Areas through Road-side Rain

Gardens; St. Albans City." University of Vermont Extension (Oct. 2007). Print.

5. Reference Manual for Undergraduate Interns. Vermont EPSCoR Streams Project. 2010-

2011.

outflow

inflow

*no TSS data was collected for this storm event

**during this storm event only one inflow and one outflow phosphorus sample was taken

*no TSS data was collected for this storm event

**during this storm event the phosphorus inflow average of the two samples was 0.00 mg/L

*no TSS data was collected for this storm event

*

*no TSS data was collected for this storm event

The infiltration rate for the soil in this garden was calculated to be 116.6 cm/hr

The infiltration rate for the soil in this garden was calculated to be 580 cm/hr

The infiltration rate for the soil in this garden was calculated to be 225 cm/hr

The infiltration rate for the soil in this garden was calculated to be 72 cm/hr

The infiltration rate was calculated to be

56.7 cm/hr. This infiltration rate of

the soil was calculated prior to

the garden’s instillation.

*

0

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8/2/2010 9/30/2010 10/5/2010 Average

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phosphorus inflow

phosphorus outflow

TSS inflow

TSS outflow

Figure 2. Average total phosphorus concentration and total suspended solids for the Bishop St. Up rain garden for each storm event and an average of all storms

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8/2/2010 8/22/2010 Average

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phosphorus outflow

TSS inflow

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Figure 1. Average total phosphorus concentration and total suspended solids for the UVM rain garden for each storm event and an average of all storms

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8/2/2010 9/28/2010 10/6/2010 Average

Ph

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phosphorus inflow

phosphorus outflow

TSS inflow

TSS outflow

*,**

Figure 3. Average total phosphorus concentration and total suspended solids for the Brown Ave garden for each storm event and an average

for all storms

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8/2/2010 9/28/2010 Average

Ph

osp

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d T

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phosphorus inflow

phosphorus outflow

TSS inflow

TSS outflow

Figure 4. Average total phoshporus concentration and total suspended solids for the Bishop St. Down rain garden for each storm event and an average of all storms

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8/2/2010 9/28/2010 10/6/2010 Average

Ph

osp

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rus c

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(u

g/L

) an

d T

SS

(mg

/L)

phosphorus inflow

phosphorus outflow

TSS inflow

TSS outflow

Figure 5. Average total phosphorus concentration and total suspended solids for the Medical Center rain garden for each storm event and an average of all storms

*,**

*

*

The City of St. Albans, VT contains Stevens Brook, a stream that was deemed to be severely impaired. The storm water catch basins at the bottom of Bishop Street in St. Albans were found in the 2003 Stevens and Rugg Brook Watershed Report to be an issue and in 2007 the Bishop St. rain garden project was funded (4). The project was a collaboration among the UVM Extension Lake Champlain Sea Grant NEMO program, the City of St. Albans and the Northwest Regional Planning Commission (NRPC) with help on the gardens’ designs and instillation from the Vermont Youth Conservation Corps.

The purpose of this study was to test five rain gardens, in order to determine the difference between inflow and outflow levels of total phosphorus and total suspended solids. It was hypothesized that if the rain gardens were functioning properly, the outflow storm water levels would be lower than those of the inflow.

The building of the Bishop St. Down rain garden (intersection of Bishop St. and Lincoln St.) (4)