Minnesota Local Road Research Board Local Operational Research Assistance Program (OPERA)

Transcription

1 Minnesota Local Road Research Board Local Operational Research Assistance Program (OPERA) September 30, 2011 Storm Water Pollutant Removal in Rain Gardens OPERA Project Number: Project Sponsor: City of Grand Rapids, Minnesota Report Authors: Tom Pagel, P.E. (1), Glen D. Hodgson, P.E (2), Scott Sikkink (3), Thomas Hammann (4) (1) Grand Rapids City Engineer and Project Leader (phone ) ; (2) Engineering Instructor, Itasca Community College (phone ); (3) Engineering Student, Itasca Community College; (4) Engineering Graduate, Itasca Community College

2 Research Problem Urban transportation systems typically include storm water facilities. Increased environmental awareness and increased regulatory scrutiny have led local roadway agencies to consider various means to mitigate the impacts of storm water discharges into receiving lakes, streams, and rivers. The City of Grand Rapids, MN is one such local agency. Figure 1 shows the location of Grand Rapids within the State of Minnesota. The City has developed and implemented a Storm Water Pollution Prevention Plan (SWPPP) as part of its application to obtain coverage under the National Pollutant Discharge Elimination System Permit No. MNR which applies to Minnesota MS4 s (Municipal Separate Storm Sewer Systems) outside of urbanized areas. The City of Grand Rapids falls under this NPDES permit. The ultimate goal of the City s SWPPP is to control or reduce the discharge of storm water generated pollutants to receiving waters. [1] This goal is of particular importance to the City because all storm waters in Grand Rapids eventually reach the Mississippi River which has been identified as a Restricted Discharge water. [2] Rain gardens have been used as a Best Management Practice in SWPPP s since the 1990 s or before. Simply put, a rain garden is a shallow depression filled with selected trees, shrubs, flowering plants and grasses designed to allow rainwater run-off to absorb into the soil. [3] (Emphasis added.) The potential benefits of allowing run-off to infiltrate into the soil have been the subject of many studies. For example, Elliott and Meyer studied the benefits on water quality characteristics of three large-sized rain gardens in the Twin Cities area. [4] Those three rain gardens were 4360 ft 2 (405 m 2 ), 4360 ft 2 (405 m 2 ), and ft 2 (4047 m 2 ) in area. Unfortunately, in many urban settings there is simply not enough space to construct a rain garden that is large enough to fully absorb storm runoff into the soil. Many types of municipal construction, operation, and maintenance projects cannot accommodate rain gardens as large as those in the Elliott/Meyers study. The operational and research question then becomes: Can modified rain gardens be constructed that provide at least some of the benefit of a conventional (larger) rain garden? Problem Solution The City of Grand Rapids began to address that question by constructing modified rain gardens as part of the 2008 street reconstruction project on 1 st Avenue NW. The idea was to construct the rain gardens between the curb and the sidewalk. On the 1 st Avenue reconstruction project the distance between the curb and the sidewalk is approximately 15 feet. (The 1 st Avenue right-of-way is unusually wide, so this distance is greater than would typically be available.) The length of the 1 st Avenue rain gardens is approximately 25 feet. Therefore the typical area of a 1 st Avenue rain garden is on the order of 375 square feet. The available space for these rain gardens is therefore at least an order of magnitude smaller than the rain gardens studied by Elliott and Meyer. The City recognized that these smaller rain gardens could not be used as

3 Figure 1 Location Map for City of Grand Rapids, MN

4 infiltration basins alone. Rather, the rain gardens would need to be connected to, and be allowed to discharge to, the storm sewer system. Therefore the rain garden design includes an infiltration pipe and an overflow pipe. In periods of high rainfall, both pipes permit discharges to the storm sewer. Appendix B contains a series of photographs that depict the design and construction of the rain gardens. Two of the rain gardens constructed as part of that project were used to collect data for this study. Figure 2 provides a map of Grand Rapids showing the location of those two rain garden sites. Research Procedure During the summer of 2011 rainfall events in Grand Rapids were monitored. When an event of sufficient magnitude occurred, field data collectors were dispatched to the two rain gardens on 1 st Avenue NW. Samples of storm water runoff that was entering each rain garden were collected. Thus, for every rainfall event there were two before rain garden samples. The discharge pipe from the rain gardens to the storm sewer was then monitored. When discharge reached a sufficient rate, samples of storm water leaving each rain garden were collected. Thus, for every rainfall event there were two after rain garden samples. Sample temperatures were recorded on site. All samples were then immediately refrigerated and kept at a temperature of 2 to 4 degrees Celsius during temporary storage. Samples were transferred to a certified testing laboratory. Water quality parameters that were laboratorytested included nitrogen (nitrate + nitrite), total phosphorous, total suspended solids, and turbidity. See Appendix A for a detailed description of the sampling protocol that was used. [The remainder of this page was left blank intentionally]

5 Figure 2 Rain Garden Locations for Study

6 Results (Data) Water samples were collected from a total of six rainfall events between June 7, 2011, and July 27, These events are listed in Table 1. Rainfall amounts were measured in a rain gauge at the study site. Table 1 Rainfall Events Dates of Events Total Rainfall (Inches) 6/7/ /15/ /21/ /22/ /27/ /27/ During the process of sample collection, storm water temperatures were obtained both before the water entered the rain garden and as the water was discharged to the storm sewer. Temperatures are listed in Table 2. Table 2-Temperature Data Date Rain Gardens Before Temperature After Temperature Temperature Change 6/7/ & /15/2011 1& /21/ & /22/ & /27/ & /27/ & Note: Temperatures are in degrees Farhenheit Note: Samples from the two rain gardens were combined for temperature readings Mean Change Std Deviation 1.05 Degrees of Freedom 5 Std Error of Mean 0.47 t-ratio for paired t-test: sided p-value 0.02

7 Data for the four parameters that were tested in the laboratory are listed in Tables 3 through 6 as follows: Table 3 Data on Nitrogen, Nitrate + Nitrite (in mg/l as N) Table 4 Data on total Phosphorous (in mg/l as P) Table 5 Data on Total Suspended Solids (in mg/l) Table 6 Data on Turbidity (in Nephelometric Turbidity Units) Table 3-Nitrogen, Nitrate + Nitrite Date Rain Garden Before Nitrogen After Nitrogen Nitrogen Change 6/7/ /7/ /15/ /15/ /21/ /21/ /22/ /22/ /27/ /27/ /27/ /27/ Note: Nitrogen concentrations are in mg/l as N (EPA Method 353.2) Mean Change Std Deviation 0.34 Degrees of freedom 11 Std Error of Mean 0.10 t-ratio for paired t-test sided p-value 0.12 [The remainder of this page was left blank intentionally]

8 Table 4-Total Phosphorous Date Rain Garden Before Phosphorous After Phosphorous Phosphorous Change 6/7/ /7/ /15/ /15/ /21/ /21/ /22/ /22/ /27/ /27/ /27/ /27/ Note: Phosphorous concentrations are in mg/l as P (EPA Method 365.1) Mean Change Std Deviation 0.23 Degrees of freedom 11 Std Error of Mean 0.07 t-ratio for paired test sided p-value 0.29 [The remainder of this page was left blank intentionally]

9 Table 5-Total Suspended Solids (TSS) Date Rain Garden Before TSS After TSS TSS Change 6/7/ N/A N/A N/A 6/7/ N/A N/A N/A 6/15/ /15/ /21/ /21/ /22/ /22/ /27/ /27/ /27/ /27/ Note: TSS concentrations are in mg/l (USGS Method I ) Note: Sample volumes were insufficient on 6/7/11 to complete TSS testing Mean Change Std Deviation Degrees of freedom 9 Std Error of Mean t-ratio for paired t-test sided p-value 0.01 [The remainder of this page was left blank intentionally]

10 Table 6-Turbidity Date Rain Garden Before Turbidity After Turbidity Turbidity Change 6/7/ N/A N/A N/A 6/7/ N/A N/A N/A 6/15/ /15/ /21/ /21/ /22/ /22/ /27/ /27/ /27/ /27/ Note: Turbidities are in NTU (EPA Method 180.1) Note: Sample volumes were insufficient on 6/7/11 to complete Turbidity testing Mean Change Std Deviation Degrees of freedom 9 Std Error of Mean 7.00 t-ratio for paired t-test sided p-value 0.21 [The remainder of this page was left blank intentionally]

11 Results (Statistical Analysis) The summary results of the statistical analyses of the various data groups are: Parameter Mean Change (before to after garden) p-value Temperature degrees Fahrenheit 0.02 Nitrogen mg/l 0.12 Phosphorous mg/l 0.29 TSS mg/l 0.01 Turbidity NTU 0.21 Based on the high p-values, the results for Phosphorous and Turbidity indicate that no conclusions can be drawn regarding the removal (or addition) of those parameters as the storm water passes through the rain gardens. The results Nitrogen are suggestive that the rain gardens may be having some beneficial effect, but no strong conclusions should be drawn with the p-value of The results for temperature strongly indicate that the rain gardens are reducing the temperature of the storm water runoff. However, the magnitude of the change appears rather small at an average of degrees Fahrenheit. The results for TSS very strongly indicate that the rain gardens are effective in reducing total suspended solids as the storm water moves through the rain gardens. In addition, the magnitude of the change in TSS is impressive. For the ten sampling events the average removal of TSS through the rain gardens was 69%. Implementation Based on the TSS results alone it appears that modified rain gardens can indeed provide some of the benefit of a conventional (larger) rain garden. (See the section on Research Problem above.) It appears that continued implementation of the City s modified rain garden design is warranted. Status The initial research project is complete and shows that modified rain gardens have potential as a Best Management Practice for controlling and reducing storm water pollutant discharges to receiving waters.

12 Additional research is needed to further quantify the potential benefits of (modified) rain gardens. Future research efforts should be directed towards: Acquiring more data on temperature (possibly including air and pavement temperatures) and nitrogen so that stronger conclusions can be drawn regarding these parameters Acquiring data that could relate rainfall amounts to pollutant removal levels A longer-term study to determine if the rain garden benefits remain as the gardens age and (potentially) become clogged with silt and debris Considering water quality parameters that were not addressed in this initial study Relating rain garden size to rain garden performance Consideration of rain garden design modifications Project Duration Approval of this OPERA research project was transmitted to the City of Grand Rapids by the OPERA Program Coordinator on November 2, Sampling of rain garden inflow and outflow during rain events began in June of 2011 and continued through July of No rainfall event in either August or September, 2011 produced runoff sufficient for a sampling event. The project report was completed and submitted to the OPERA Program Coordinator on September 30, The total project duration (including months during which no research activities occurred) spanned approximately 11 months. Project End Date The research portion of the project ended on September 30, Upon the request of the OPERA program, presentation of research results may occur in the future at one or more professional conferences. Project Costs Total project cost through final report preparation was approximately $14,000. OPERA funds used for the project totaled $8,000. The remaining project funding was provided by the City of Grand Rapids and Itasca Community College (through a grant from the Blandin Foundation.)

13 REFERENCES 1. City of Grand Rapids, Minnesota. February, NPDES Phase II Notice of Intent and Storm Water Pollution Prevention Program. 2. Mn. Rule University of Minnesota Extension Elliott, Sarah and Meyer, Mary H. (2011) Water Quality Characteristics of Three Rain Gardens Located Within the Twin Cities Metropolitan Area, Minnesota, Cities and the Environment (CATE): Vol4: Iss. 1, Article 4.

14 APPENDICES

15 City of Grand Rapids/Itasca Community College Rain Garden Study Summer, 2011 APPENDIX A SAMPLING PROTOCOL Rainfall Event Sampling Notes 1. When a rainfall event occurs, samplers will try to obtain samples before rain gardens as soon as possible after water begins to run in the gutters. 2. Samplers will try to obtain samples after rain gardens as soon as possible after discharges from the rain gardens to the catch basins begin. 3. All samples are to be kept cool. Samples should be refrigerated within a few minutes after acquisition. 4. After all 4 samples from a rainfall event are gathered, samples (in the Pace coolers) should be delivered to Wenger Hall and placed in the refrigerator in the Commons. 5. Samplers will communicate with Glen whenever they obtain samples. Rainfall Event Sampling Protocol 1. Samplers will wear safety vests and safety glasses at all times during sampling events. Rubber gloves will be worn when any sample bottle is open. 2. Samplers will set up traffic cones to warn on-coming traffic that they are working in the street. If samplers use a vehicle, it should be parked between the catch basin and on-coming traffic. 3. Each sample set consists of three samples: temperature, turbidity and TSS, and nitrogen and phosphorus. a. Obtain the temperature sample first. Use one of the collection cups. Fill the cup and insert thermometer. When the temperature of the water has stabilized, record the temperature in the field book. b. Dump out the temperature water and rinse the cup with distilled water. c. Obtain a second sample and fill the N/P sample bottle. d. Obtain a third sample and fill the turbidity/tss sample bottle. e. Place all samples back in the PACE cooler. f. Fill out the PACE sample ID form. Field Book Data For each set of (4) samples record the following information: 1. Sampler name. 2. Date and time of collection. 3. Temperature of the samples. 4. Amount of rainfall. 5. Approximate time between before rain garden sample collection and after rain garden sample collection. 6. Other unusual or important information.

16 APPENDIX B CONSTRUCTION PHOTOS PHOTO 1 Initial excavation of rain garden prior to sidewalk and curb placement PHOTO 2 Connection of infiltration pipe to catch basin

17 APPENDIX B (CONTINUED) CONSTRUCTION PHOTOS Photo 3 Overflow pipe with aggregate bedding and filter fabric connected to storm sewer Photo 4 Rain Garden backfilled with top of overflow pipe visible

18 Photo 5 First plantings installed APPENDIX B (CONTINUED) CONSTRUCTION PHOTOS Photo 6 Completed rain garden

19 APPENDIX C RAIN GARDEN PHOTOS AFTER TWO FULL GROWING SEASONS