Evaluating Bioretention Hydrology and Nutrient Removal at Three Field Sites in North Carolina

Evaluating Bioretention Hydrology and Nutrient Removal at Three Field Sites in North Carolina W. F. Hunt 1 ; A. R. Jarrett 2 ; J. T. Smith 3 ; and L. J. Sharkey 4 Abstract: Three bioretention field sites in North Carolina were examined for pollutant removal abilities and hydrologic performance. The cells varied by fill media type or drainage configuration. The field studies confirmed high annual total nitrogen mass removal rates at two conventionally drained bioretention cells 40% reduction each. Nitrate-nitrogen mass removal rates varied between 75 and 13%, and calculated annual mass removal of zinc, copper, and lead from one Greensboro cell were 98, 99, and 81%, respectively. All high mass removal rates were due to a substantial decrease in outflow volume. The ratio of volume of water leaving the bioretention cell versus that which entered the cell varied from 0.07 summer to 0.54 winter. There was a significant p 0.05 change in the ratio of outflow volume to inflow volume when comparing warm seasons to winter. Cells using a fill soil media with a lower phosphorus index P-index, Chapel Hill cell C1 and Greensboro cell G1, had much higher phosphorus removal than Greensboro cell G2, which used a high P-index fill media. Fill media selection is critical for total phosphorus removal, as fill media with a low P-index and relatively high CEC appear to remove phosphorus much more readily. DOI: 10.1061/ ASCE 0733-9437 2006 132:6 600 CE Database subject headings: Stormwater management; Best management practice; Water quality; Nutrient loads; North Carolina; Hydrology; Abatement and removal. Introduction 1 Assistant Professor and Extension Specialist, Dept. of Biological and Agricultural Engineering, North Carolina State Univ., Raleigh, NC 27695-7625. E-mail: bill_hunt@ncsu.edu 2 Professor, Dept. of Agricultural and Biological Engineering, Pennsylvania State Univ., University Park, PA 16802. E-mail: arj@psu.edu 3 Project Engineer, McKim and Creed, Raleigh, NC 27607. 4 Engineer, CH2M Hill, Raleigh, NC 27606. Note. Discussion open until May 1, 2007. Separate discussions must be submitted for individual papers. To extend the closing date by one month, a written request must be filed with the ASCE Managing Editor. The manuscript for this paper was submitted for review and possible publication on July 20, 2005; approved on October 27, 2005. This paper is part of the Journal of Irrigation and Drainage Engineering, Vol. 132, No. 6, December 1, 2006. ASCE, ISSN 0733-9437/2006/6-600 608/ $25.00. Stormwater runoff is a primary factor in the degradation of many streams and other water bodies USEPA 2000. The adverse impacts of this and other sources of pollution include closures of shellfish waters, fish kills, and a reduction of aesthetics, which consequently reduce fishing and recreational values of downstream waters. Stormwater runoff contains nitrogen, phosphorus, and heavy metals Pitt et al. 1995; Wu et al. 1998. In coastal areas of the eastern United States, nutrients are a particular concern. Excess nitrogen and phosphorus loading in coastal North Carolina NC is blamed for fish kills in NC estuaries Gray 2000. Similar impacts were noted in the Chesapeake Bay Chesapeake 2001, where government officials from Chesapeake Bay watershed states have declared a nitrogen reduction goal of 50% by 2010. State and federal regulations force mitigation of pollutants in stormwater by requiring new developments to install structural stormwater treatments called best management practices BMPs. Examples of stormwater BMPs include stormwater wetlands, sand filters, wet ponds, and more recently, bioretention areas Coffman et al. 1993b. Some states, such as North Carolina, require that all stormwater BMPs within sensitive watersheds reduce nitrogen loads NCDENR 1999. Having accurate mass removal efficiencies for BMPs, such as bioretention, is essential in practice selection. Stormwater practices, particularly those installed in areas frequented by pedestrian traffic, not only must reduce pollution, but must also be aesthetically pleasing. Communities in many states require up to 15% of large commercial developments to be set aside for medians and other open space. Bioretention may be able to meet both water quality and landscape objectives. An illustration of construction of a bioretention cell is found in Fig. 1. A bioretention area is initially an excavated basin, at the bottom of which underdrains are laid and covered with a gravel envelope. Between 0.7 and 1.2 m of fill soil media is placed over the gravel envelope, and then plants and mulch are added to the surface. Typically, runoff is captured and infiltrates the bioretention media. Captured water leaves the cell by underdrain outflow, exfiltration, or evapotranspiration; whereas high flows, occurring when the bioretention cell is full, nominally when a minimum of 25 mm of precipitation has fallen, are bypassed, either around or through the bioretention area. Viewed from the surface, the bioretention cell can be very attractive if designed and constructed properly. Pollution removal occurs at the surface and in the deeper soil media layers, making the bioretention area essentially a vegetated sand filter that is planted with shrubs, trees, or grass. Compared to other ultra-urban BMPs, bioretention is cost effective Wossink and Hunt 2003, making bioretention a principal stormwater practice used in low-impact development LID Hager 2003. However, current bioretention designs employed in the field do not substantially reduce the levels concentration and mass of nitrate-nitrogen NO 3 -N Davis et al. 2001, 2003; Hsieh 600 / JOURNAL OF IRRIGATION AND DRAINAGE ENGINEERING ASCE / NOVEMBER/DECEMBER 2006

Fig. 1. Typical steps in construction of bioretention cell: a excavation; b placement of underdrains and gravel envelope; c filling basin with soil media; and d planting vegetation at surface of bioretention bed and Davis 2005, especially compared to the BMP performance standards required by some states. For example, North Carolina requires all new stormwater practices to have nitrate-nitrogen removal rates approaching 30%. A recent University of Maryland laboratory bioretention study reported that nitrate-nitrogen removal rates ranged between 64 and 9% Hsieh and Davis 2005. The study did show, however, that bioretention areas can substantially reduce concentrations of phosphorus and several metals in runoff. For nitrate-nitrogen to be removed as nitrogen gas, an anaerobic environment with a carbon source is required. Bioretention designs currently used do not incorporate an anaerobic internal zone of stored water NCDENR 1997. Two recent studies Hunt 2003; Kim et al. 2003 demonstrated in a laboratory environment how the addition of an internal saturation layer increased nitrate-nitrogen conversion to nitrogen gas. Hunt 2003 examined nitrogen losses for a relatively low hydraulic conductivity fill media conductivity of 0.0035 to 0.011 mm/s or 0.5 to 1.5 in./h on 1.2 m 4 ft deep soil columns. Several experiments were run with variable saturated zone depths. Total nitrogen TN and NO 3 -N removal ranged from 60 to 90%. Kim et al. s 2003 research investigated ways to create an anaerobic zone in a bioretention cell and measured how the addition of an anaerobic zone increased denitrification rates. They did find substantial NO 3 -N and TN removal from column studies. Removal rates approached 100% under varying loading nutrient and synthetic event frequency and fill media amendments including straw, leaf compost, and shredded newspaper. Newspaper was identified in this study as the best carbon energy source of materials tested. Despite extensive laboratory studies conducted at the University of Maryland Davis et al. 2001, 2003; Kim et al. 2003; Hsieh and Davis 2005, very few bioretention field sites have been extensively tested. The goals of the research presented herein were to 1 corroborate laboratory findings with field studies to show that bioretention areas reduce stormwater pollutants, 2 measure water quality from field bioretention areas and calculate annual mass removal efficiencies for TN and total phosphorus TP, 3 examine the fill media s role in pollutant removal, particularly that of phosphorus, 4 examine the reduction of outflow and seasonal variations associated with outflow reduction, and 5 determine whether the drainage configuration impacted nitrogen removal. Site Descriptions and Methods Bioretention areas were installed at two locations in North Carolina: Chapel Hill and Greensboro Fig. 2. A description of both locations follows. Greensboro Battleground Crossing Shopping Center Bioretention cells at Battleground Crossing Shopping Center in Greensboro were constructed in 2000 and 2001. The Guilford Fig. 2. Map showing study site locations in North Carolina: G1 and G2 in Greensboro and C1 in Chapel Hill. Raleigh state capital and home of NCSU is shown for reference. JOURNAL OF IRRIGATION AND DRAINAGE ENGINEERING ASCE / NOVEMBER/DECEMBER 2006 / 601

Fig. 3. Creating internal water storage IWS zone in bioretention device by using upturned elbow County North Carolina soil survey indicated that the soil on site was a Madison clay loam, with a very low permeability 0.0014 to 0.0042 mm/s or 0.2 to 0.6 in./h below the A soil horizon Stephens 1977 and a perched water table near the surface 0.15 to 0.45 m or 6 to 18 in.. Average annual precipitation for Greensboro is 1,096 mm 43.1 in. NCSCO 2005. The bioretention cells included underdrains due to the in situ soil s low conductivity. Both cells had nominally 1.2 m 4 ft deep soil media layers and were planted with river birch Betula nigra, common rush Juncus effuses, yellow flag iris Iris pseudacorus, and sweetbay Magnolia virginiana at an approximate density of one plant per 4m 2 40 ft 2. One bioretention cell, G2, was constructed with conventional underdrains to prevent saturated layer formation. When the monitoring of the cells began in summer 2002, an internal water storage IWS layer was incorporated into a bioretention cell G1 to test the hypothesis that some NO 3 would denitrify into N 2 gas in a saturated, reduced environment. An IWS layer was created by a slight change in drainage configuration; in lieu of installing underdrains that simply flow by gravity to the outlet box, an elbow was installed so that an IWS zone was produced Fig. 3. An upturn in the drainage pipe forced the bottom 0.45 to 0.6 m 1.5 to 2 ft of the bioretention cell to remain saturated. The watersheds of the bioretention areas were 0.20 ha 0.50 acres each and the bioretention surface area to watershed size ratios were nearly identical 5%. This ratio lies within the standard range of 5 to 7% for typically designed bioretention cells NCDENR 1997. If completely saturated and filled to the brink of overflow, each cell could hold approximately 30 to 35 mm of runoff from the contributing watershed. A locally available organic sandy soil was backfilled over the underdrains. The soil s hydraulic conductivity was tested using the auger hole method Amoozegar and Wilson 1999 2 years after construction and was found to range from 0.021 to 0.11 mm/s 3 to15in./h, which was higher than available design guidelines Coffman et al. 1993a; NCDENR 1997. Other than hydraulic conductivity, little design guidance is provided for the fill soils NCDENR 1997. The phosphorus index P-index of the fill soil, calculated using the Mehlich-3 soil test methodology, ranged from 86 to 100 in G2, which is considered high. The P-index in cell G1 was 20 to 26, which is considered low to medium Hardy et al. 2003. Chapel Hill, University Mall The University Mall in Chapel Hill, North Carolina N.C., was constructed in the early 1970s. The Orange County N.C. soil Fig. 4. Bioretention cell C1 in Chapel Hill 8 months after construction survey shows the in situ soil to be relatively tight and to contain clay, clay loam, and silty clay White Store-urban complex Dunn 1977. The soil had a saturated hydraulic conductivity less than 0.42 mm/s 0.06 in./h below the A soil horizon. The soil typically had a perched water table 0.15 to 0.46 m 6 to18in. below the soil surface. Average annual rainfall in Chapel Hill is 1,209 mm 47.6 in. NCSCO 2005. Two separate bioretention areas were constructed at the mall during the summer and fall of 2001. As with the Greensboro site, the bioretention cells included conventional non-iws underdrains. The larger of the two cells C1 was monitored. The drainage area of bioretention cell C1 was 0.06 ha 0.15 acres. The entire drainage area consisted of asphalt pavement and was frequently trafficked. The bioretention surface area to drainage area ratio was high 14.9% due to poor grading of asphalt. If completely saturated and filled to the brink of overflow, cell C1 could approximately hold 95 to 100 mm of runoff from the contributing watershed. The Chapel Hill cell was monitored for 11 months to 1 estimate TN and total phosphorus TP pollutant load reduction, and 2 to compare TP removal to that of Greensboro cell G2, where a high P-index fill media was used. A sandy media mined from a local quarry was used to backfill the bioretention cells. The P-index of the fill media ranged from 4 to 12, which is considered low Hardy et al. 2003. The saturated hydraulic conductivity of the sandy media cores was measured 18 months after the cell was constructed and ranged from 0.009 to 0.021 mm/s 1.3 to 3.1 in./h. This permeability was within the target design range. Vegetation in the bioretention cell was spaced approximately 1 plant per 2.3 m 2 25 ft 2. Species included Southern wax myrtle Myrica cerifera, Virginia sweetspire Itea virginica, winterberry Ilex verticillata, and inkberry Ilex glabra. Fig. 4 shows bioretention cell C1 8 months after it was constructed. Field Data Collection and Analysis At each site the following sampling scheme was utilized. Two tipping-bucket rain gauges GlobalWater were installed within 10 m 30 ft of the Greensboro bioretention cells in an area clear of interference; at Chapel Hill only one rain gauge was installed and located inside the cell. Sigma 900Max automated samplers collected samples at the invert of each underdrain as it emptied 602 / JOURNAL OF IRRIGATION AND DRAINAGE ENGINEERING ASCE / NOVEMBER/DECEMBER 2006

into the drainage outlet, which also served as rectangular concrete overflow inlets. Outflow was measured by a Sigma pressure transducer as it flowed over a 30 v-notch weir and was recorded by the Sigma 900Max. Flow-composited outflow samples were collected for each 150 to 300 L 20 to 40 gal, depending on watershed size, which passed over the weir. The flow rate from the underdrains was automatically recorded by the Sigma 900Max. Inflow runoff samples were collected at each cell by a Sigma 900Max, which was activated by rainfall at all three cells. The pressure transducers were accurate to within 7 mm 0.25 in. but were sensitive to direct sunlight, dry conditions, and temperatures less than 0 C 30 F American Sigma 2003. Inflow data were collected at the entrance of all three cells. Flow into the cell was calculated based on measured rainfall amount and initial abstraction, which was estimated by site survey. In Greensboro, there was no single entry point for runoff into the bioretention cells, so it was assumed that the water entering the cells at the given sampling location would be characteristic of that across the entire drainage area. In Chapel Hill, all the runoff was channeled through a concrete flume outfitted with a 120 v-notch weir. The samples were composited on a flow basis, which captured runoff up to 50 mm 2.0 in. of rainfall. Sample volume was dependent on runoff volume. The sampler bottles each stored up to 11.4 L 3 gal of runoff, and samples were removed from the field sites within 24 h of cessation of the precipitation event and taken to one of two EPA certified laboratories. Chapel Hill samples were delivered to Tri-test, Inc. in Raleigh, and the Greensboro samples were sent to Simalabs International, in Burlington, North Carolina. Table 1. Twelve- and 10-Month Inflow and Outflow Mass Loading of Various Pollutants at G2 Cell in Greensboro and C1 in Chapel Hill Pollutant Mass in kg Mass out kg Percent reduction or increase a Greensboro Cell G2 12-month mass and efficiency a TN 2.53 1.51 40 NO 3 -N 1.19 0.30 75 TKN 1.22 1.28 4.9 NH 3 -N 0.59 0.60 0.99 TP 0.13 0.44 240 Ortho-P 0.048 0.052 9.3 Zn 0.460 0.009 98 Cu 3.70 0.02 99 Pb 0.016 0.003 81 Fe 0.13 16.8 13,000 b TSS 18 48 170 b Chapel Hill Cell C1 10-month mass and efficiency c TN 0.61 0.36 40 NO 3 -N 0.093 0.081 13 TKN 0.52 0.28 45 NH 3 -N 0.33 0.045 86 TP 0.139 0.048 65 Ortho-P 0.059 0.016 69 Note: Neither cell included an IWS zone. a Period from June 2002 to May 2003. b Dramatic increase possibly a result of iron leaching from surrounding high Fe in situ soil. c Period from July 2002 to April 2003. Estimating Annual Pollutant Mass Inflow and Outflow One of the goals of the study was to estimate annual pollutant mass inflow and outflow. At G2, flow was measured continuously from June 1, 2002, through May 15, 2003, with the exception of two periods: July 21 to August 17, 2002, and January 9 to February 6, 2003. Precipitation amounts were collected either on site or at the nearby U.S. Geological Survey USGS gauging station, Horsepen Creek USGS Gauging Station number 020939399200, 0.40 km 0.25 m from the site. Rain gauges at the site malfunctioned for nearly 2 months during icy winter periods. Water quality data from 11 storm events were collected during the 1-year period from June 2002 through May 2003, with samples collected from three storm events per season except for the summer two. At C1 flow was measured continuously from June 15, 2002, through April 21, 2003, with the exception of one period: January 17 to February 1, 2003. Precipitation amounts were collected either on site or, during portions of the winter, at the nearby Chapel Hill airport, approximately 2 m from the site. Rain gauges at the site malfunctioned for nearly 2.5 months during icy winter periods. Water quality data from 10 storm events were collected during the 11-month period from June 2002 through April 2003, with samples collected from three storm events in summer and spring and two in fall and winter. The inflow and outflow masses were either measured or calculated for individual storms and then summed so that an annual reduction or increase in pollutant load was estimated. Inflow and outflow volumes were collected for nearly every storm; pollutant concentrations were collected for three storms per season, on average. Seasonal pollutant inflow and outflow concentrations were averaged, and this average was multiplied by an individual storm event s volume to calculate both inflow and outflow loads. When samples had been collected for a specific event, these concentrations, and not a seasonal average concentration, were multiplied by the storm s volume. The calculated individual event pollutant loads were summed to establish an annual pollutant load. Results Estimating Annual Pollutant Mass Inflow and Outflow Sufficient flow data were collected and water quality samples gathered for two bioretention cells G2 and C1 to estimate longterm pollutant mass entering the bioretention cell by runoff and that leaving the cell as outflow to the storm drain network. Greensboro Cell G2 Water quality samples were collected for 11 events, with event sizes ranging from 8.1 to 95.5 mm 0.32 to 3.76 in.. The median storm size monitored was 33.5 mm 1.32 in.. The median precipitation event of 100 to 110 events annually in central North Carolina ranges between 10 and 13 mm 0.4 to 0.5 in., meaning that the majority of events exceeded the average storm size. The study was conducted during a year with well-above-average rainfall. During this 12-month period, a total of 1,440 mm 56.54 in. of precipitation fell 340 mm or 13.5 in. above normal National Weather Service 2005. As seen in Table 1, cell G2 conventional drainage configuration had high annual mass removal rates for most metals except iron, TN, and NO 3 -N. The metal removal results were similar to those found by Davis et al. 2001, 2003, JOURNAL OF IRRIGATION AND DRAINAGE ENGINEERING ASCE / NOVEMBER/DECEMBER 2006 / 603

except for iron. Additionally, the mass reduction of TN was high, even though this design did not include an IWS configuration. During an auger investigation 20 months after construction, pocket saturated zones were discovered in cell G2. While much of the bottom layer of soil was unsaturated, portions in the middle of the soil layer were observed to be saturated and potentially anaerobic. These wet zones were isolated from both the surface and the bottom of the cell and were perhaps created by thin clay lenses inside the fill media. Despite its design, G2 appeared to develop a small internal saturation zone, which would potentially provide nitrogen transformation. This finding is supported by a study conducted in Connecticut from November 2002 to December 2003 in which redox potential was measured in a pair of small bioretention areas, and it was found that conditions existed to favor denitrification Dietz and Clausen 2005. The bioretention cell released a substantially higher mass of total phosphorus, total suspended solids, and iron to the storm sewer network than that which entered the cell, a finding again similar to that of Dietz and Clausen 2005. The high TP outflow was directly attributable to 1 leaching of existing soil adsorbed phosphorus discussed in the next section and 2 increased TSS outflow Table 1. The cell released TSS and any phosphorus potentially bound to it. Iron effluent masses were evident from the first samples, which were always rusty in color. It was suspected that iron from the fill media was leaching from the bioretention cell as it became saturated, similar to hypothesized phosphorus leaching. While collected effluent concentrations for all nutrients and iron were higher than influent concentrations, effluent concentrations never reached toxic levels. Chapel Hill Cell C1 Water quality samples were collected for 10 events with event sizes ranging from 17.3 to 58.4 mm 0.68 to 2.30 in.. The median storm size monitored was 39.9 mm 1.57 in., with the majority of events exceeding the average annual storm size 10 to 13 mm or 0.4 to 0.5 in.. The monitoring period was much wetter than average, particularly in October and in the spring. During this 11-month period, a total of 1,270 mm 49.95 in. of precipitation fell 237 mm or 9.30 in. above normal National Weather Service 2005. Nutrient concentrations TKN, NH 3 -N, NO 3 -N, TP, and ortho-p were collected, and flow volumes were measured to estimate the reduction of pollutants. A summary of these data is shown in Table 1. While TN removal rates were the same between C1 and G2 40% each, TKN and NO 3 -N were considerably different. TKN removal rates in C1 were high 45%, while G2 slightly added TKN 5% to the storm drainage network. Conversely, NO 3 -N removal levels were very high in G2 75% ; whereas, they were minimal in C1 13%. This is partly explained by the fact that the fill media in C1 was a more uniform sand and was absent much organic matter, unlike the media in G2, in which organic matter was evident. Perhaps there was enough organic matter to provide for microbial action to convert NO 3 -N to N 2 in Greensboro, but not enough organic matter was present to do likewise in Chapel Hill. Mass removal rates of TP of the Chapel Hill bioretention cell were generally higher than those of cell G2 and are attributed to the P-index of the soil. Table 2 contains portions of a North Carolina Department of Agriculture NCDA soil test report for the soils at G1, G2, and C1, including the phosphorus index P-index, which is an indicator of a soil s ability to adsorb phosphorus. A low-to-medium P-index below 50 indicates a soil that has low levels of phosphorus adsorption and that could capture Table 2. Soil Test Report for C1, G1, and G2 Sites Soil tested ph CEC meq BS% P-Index a C1 5.4 6.0 1.9 2.4 63 79 4 12 G1 6.1 6.8 6.0 7.3 90 100 20 26 G2 6.2 6.6 5.6 7.0 89 94 86 100 Note: Three soil samples were collected and tested from every site in spring 2003. a Calculated using Mehlich-3 soil test methodology Hardy et al. 2003. more phosphorus. A high 51 to 100 or very high over 100 P-index indicates soils that are saturated with phosphorus Hardy et al. 2003. The results from this test reinforced the TP removal efficiencies calculated at each site. The Chapel Hill fill media P-index ranged from 4 to 12, indicating that the media was not near phosphorus saturation, and its TP removal rate was consequently 65%. Cell G2 had high P-index numbers 86 to 100, showing that the soil had reached a TP adsorption limit and consequently added TP to the storm drain network. Note that the P-index is not to be confused with the statistical p-value. Neither soil had a high cation exchange capacity CEC, expected for the types of media used at the sites. Bioretention cell C1 s CECs were particularly low 1.9 and 2.4 meq, which pointed toward the fact that the C1 media was sandier than that of G2. The report noted high base saturation percentages BS% for both soils, which indicated that there had not been a buildup of metals in either of the cells. Comparing TP Removal in Cells G1 and G2 Variation in P-Index TP effluent comparisons were made between cells G1 and G2, because student t-tests showed inflow TP concentrations were not dissimilar. Due to the variation in soil media type by P-index and because most phosphorus removal would occur toward the surface of the bioretention cell, independent of the IWS zone, a comparison of the fill media P-index can be made between cells G1 and G2. Effluent phosphorus concentrations were compared using twofactor ANOVA without replication Table 3. These data did not necessarily indicate total mass removal because flow was not measured during the entire study period at the outfall from G1. Fig. 5 highlights outflow TP concentrations for both the cell with high P-index soil media G2 and media P-index soil media G1. Outflow samples were collected from both cells for 21 storm events an event was defined as a period of continuous outflow between September 2002 and December 2003. Of these, TP concentrations from G1 were lower in 20 samples than those from G2. TP concentrations were significantly lower p 0.05 in the cell with lower P-index fill media cell G1. From the annual pollutant loading analysis done for cell G2 Table 1, it was shown that substantial amounts of TP, ortho-p, and Fe were added to the storm sewer network. It was possible that the pollutant loadings from cell G1 would have been lower than those from cell G2, due to lower outflow concentrations from G1. As seen in Table 3, the mean TP concentration from G1 was one-fifth that of G2. Applying G1 s TP concentration to runoff volumes from G2 decreases the annual TP mass leaving cell G1 estimated to be 0.09 kg less than the mass of TP that entered cell G2 0.13 kg. 604 / JOURNAL OF IRRIGATION AND DRAINAGE ENGINEERING ASCE / NOVEMBER/DECEMBER 2006

Table 3. Outflow Concentrations Flow Weighted and Statistical Comparisons from Cell with IWS Layer G1 and Conventionally Drained Cell G2 Outflow concentration Analyte Cell Mean a mg/l Standard deviation mg/l Significant? p 0.05 b TKN G1 4.10 2.0 No G2 4.90 3.5 NO 3 G1 0.28 0.43 No G2 0.3 0.42 TN G1 4.38 2.07 No G2 5.23 3.42 TP G1 0.56 0.39 Yes G2 3.00 3.4 p=0.003 Ortho-P G1 0.52 0.37 Yes G2 2.20 2.9 p=0.01 Fe G1 21 10 Yes G2 40 23 p=0.046 Note: Data collected from September 2002 through December 2003. a n=22 for analytes except for Fe n=9. b Two-factor ANOVA without replication. Seasonal Effect on Outflow Reduction Annual bioretention hydrology was examined from the most complete outflow record from this study, that of conventionally drained cell G2. Similar to research conducted by Emerson and Traver 2004 on a biofiltration median, this bioretention cell substantially reduced the volume of water entering the storm drain network. Bioretention s impact on outflow varied seasonally. Ratios comparing the volume of water leaving the cell to the storm sewer network outflow to the volume of water entering the cell as inflow runoff were calculated for 48 storms that produced runoff from June 2002 to May 2003. Much higher outflow:runoff ratios were found in winter December 21, 2002, through March 20, 2003 than at other times of the year Table 4. Runoff from one large event from August 29 to 31, 2002, produced approximately 30 mm 1.2 in. of bypass overflow. This bypass water was not included in the outflow:runoff ratio. Statistical analyses were conducted to compare outflow to runoff ratios OF:RO in Table 4 seasonally. The first two events Fig. 5. TP outflow concentrations exiting a bioretention cell G1 with a medium P-index media and one with a high P-index media G2 in Greensboro, N.C. Table 4. Comparison of Inflow Runoff and Outflow Volume from Events from Greensboro Cell G2 Day of month RF a mm RO b L OF c L OF:RO Season a June 2002 1 2 11.9 13,900 1,110 0.08 Spring 6 7 39.9 70,500 3,030 0.04 Spring 26 27 28.4 47,300 1,930 0.04 Summer b July 2002 2 9.7 9,260 1,020 0.11 Summer 3 6.6 3,090 200 0.06 Summer 4 18.3 26,800 2,750 0.10 Summer c August 2002 16 6.6 3,090 0 0 Summer 17 18 13.2 16,500 140 0.01 Summer 27 28 8.6 7,200 28 0 Summer 29 31 84.8 161,600 35,800 0.22 Summer d September 2002 1 2 Included with above Summer 15 17 29.5 49,400 6,290 0.13 Summer 18 10.7 11,300 510 0.05 Summer 26 11.2 12,300 4,370 0.35 Fall e October 2002 11 12 60.5 112,200 18,900 0.17 Fall 14 17 35.1 60,700 7,340 0.12 Fall 28 31 38.4 67,400 9,440 0.14 Fall f November 2002 5 7 29.0 48,400 5,530 0.11 Fall 11 8.9 7,720 425 0.06 Fall 12 13 29.2 48,900 5,330 0.11 Fall 16 19 43.7 78,200 9,520 0.12 Fall g January 2003 1 2 20.1 30,400 9,020 0.30 Winter 3 5 10.2 10,300 8,360 0.81 Winter h February 2003 7 14 33.5 57,600 19,000 0.33 Winter 18 37 95.5 183,200 85,500 0.47 Winter i March 2003 13 15 9.7 9,260 6,780 0.73 Winter 16 23 41.1 73,100 43,400 0.59 Winter 30 31 32.5 55,600 4,680 0.08 Spring j April 2003 7 14 64.5 120,400 18,900 0.16 Spring 18 19 16.8 23,700 1,760 0.07 Spring 26 27 16.3 22,600 2,040 0.09 Spring k May 2003 1 10.2 10,300 850 0.08 Spring 5 7 14.7 19,600 11,900 0.61 Spring 17 20 19.3 28,800 2,240 0.08 Spring Note: Only runoff data from events of at least 6 mm a runoff-producing storm were included in the table. The annual OF:RO was 0.22. 78% of water either exfiltrated the cell or left via evapotranspiration. a RF rainfall onto watershed. b RO amount of water to run off watershed. c OF outflow from cell to storm drainage system. JOURNAL OF IRRIGATION AND DRAINAGE ENGINEERING ASCE / NOVEMBER/DECEMBER 2006 / 605

Table 5. Comparison of Outflow Volume Reduction among Seasons from Greensboro Cell G2 OF:RO ANOVA p-value Season Mean Standard deviation Spring Summer Fall Winter Spring 0.14 0.18 0.25 0.85 0.002 Summer 0.07 0.07 0.14 0.001 Fall 0.13 0.10 0.002 Winter 0.54 0.21 Note: As measured by outflow:runoff ratio, any p-value less than 0.05 significance is italicized. listed in June 2002 were grouped with the dates from March 24 to May 20, 2003, to comprise the spring season. All storm events are listed in Table 4. A single-factor ANOVA test was used to compare the seasonal OF:RO ratio i.e., summer to fall, summer to spring, summer to winter, etc.. A significance level of =0.05 was chosen for the comparison. Comparisons, including the seasonal ratio mean, standard deviation, and p-value from the ANOVA tests, are given in Table 5. The three warm seasons months between the spring equinox through the winter solstice in North Carolina spring, summer, and fall, when individually compared to the cold season winter, had significantly lower p 0.05 outflow to runoff ratios. There were no significant differences in runoff reduction among the three warm seasons. The difference in seasonal outflow to runoff ratios can be partially attributed to a lower evapotranspiration ET rate in the winter compared to that of the warm seasons. Low ET rates impact both the water inside the bioretention cell and the high water table adjacent to the cell. Water that was normally lost from the cell to ET in the warm seasons would remain in the bioretention cell in the winter. In the winter the surrounding water table was higher, in great part due to low ET. The higher surrounding water table, in turn, limited the amount of exfiltration from the cell. Comparing Cells G1 and G2 Inclusion of IWS Layer Cell G1, with an IWS layer, was compared to cell G2, without an IWS layer. Comparisons of outflow concentrations from the two bioretention cells were possible for the following pollutants: TN, TKN, and NO 3 -N, because the P-index of the soil media would not impact nitrogen concentrations. Two-factor ANOVA without replication was used for analysis Table 3. Like TP results, these data did not necessarily indicate total mass removal because flow was not measured for both cells. Monitoring results indicated that, while average TN concentrations were lower in cell G1, the IWS design had no significant impact on TN outflow concentrations Table 3, the purpose for which it had been designed. Moreover, NH + 4 -N levels were higher in G1 than G2. A discussion of the nitrogen and phosphorus transformation processes that may have occurred follows in the next section. Explanation of Internal Bioretention Nitrogen and Phosphorus Processes While the total loads leaving Greensboro cell G2 were typically lower than the calculated loads entering the bioretention cell, there was an increase in concentration for every nutrient analyte studied, except for NO 3 -N. The cell with the IWS zone and media P-index soil, G1, also had its outflow concentrations exceed those of the inflow. Table 6 shows inflow and outflow concentrations Table 6. Inflow and Outflow Concentrations Flow Weighted for Greensboro cells G1 and G2 Concentration Analyte Inflow/ outflow Mean mg/l Standard deviation mg/l a Cell G1 IWS configuration a Significant? p 0.05 TKN Inflow 1.0 0.75 Yes p=0.0001 Outflow 4.1 2.0 NH 4 Inflow 0.24 0.20 Yes p=0.0001 Outflow 2.82 1.77 NO 3 Inflow 0.34 0.17 No Outflow 0.28 0.43 TN Inflow 1.35 0.70 Yes p=0.0001 Outflow 4.38 2.07 TP Inflow 0.11 0.13 Yes Outflow 0.56 0.39 p=0.00003 Ortho-P Inflow 0.05 0.09 Yes Outflow 0.52 0.37 p 0.00001 b Cell G2 conventional configuration b TKN Inflow 0.76 0.47 Yes p=0.0007 Outflow 4.90 3.50 NH 4 Inflow 0.22 0.18 Yes p=0.0015 Outflow 1.54 1.26 NO 3 Inflow 0.50 0.32 No Outflow 0.30 0.42 TN Inflow 1.27 0.55 No Outflow 5.23 3.42 TP Inflow 0.10 0.083 Yes p=0.013 Outflow 3.0 3.4 Ortho-P Inflow 0.056 0.063 Yes p=0.020 Outflow 2.20 2.90 Note: All significant increases in concentration from inflow to outflow are noted. a n=17. b n=15. for NO 3 -N, NH 4 + -N, TKN, TP, and ortho-p for cells G1 and G2. Average inflow concentrations for TN 1.27 1.35 mg/l were similar to national averages, while the TP influent concentrations 0.10 0.11 mg/l were slightly below the national average Barth 2000. There was a significant p 0.05 increase in the concentrations for both NH 4 + -N and TKN for both cells. Mean concentrations of the former increased 12-fold in cell G1 and 7-fold in cell G2. Likewise, mean concentrations of TKN increased 4 times in cell G1 and nearly 6.5 times in cell G2, and similarly, there was a significant p 0.05 increase in TP concentrations from influent to effluent for both G1 and G2. Mean concentrations increased by factors of 5 and 30 for G1 and G2, respectively. If an anaerobic zone developed throughout lower portions of the fill media, regardless of drainage configuration, there would have been very little opportunity for ammonia-nitrogen to convert to nitrate-nitrogen. However, ammonification organic N conversion to ammonia could have taken place in an anaerobic environment. The rate of ammonification is slower, though, under anaerobic conditions than under aerobic conditions. If the deeper media portions of the bioretention system were principally anaerobic and the only microbiological activities that could take place were ammonification and fixation, then the lack of nitrate- 606 / JOURNAL OF IRRIGATION AND DRAINAGE ENGINEERING ASCE / NOVEMBER/DECEMBER 2006

nitrogen 15% of the total amount of N leaving the IWS cell, G1 and the abundance of NH 4 -N 68% of TN leaving G1 were understandable. Because the media contained a carbon source, the media may have helped produce TKN. The bioretention environment may not have been conducive to the conversion of NO 3 -N to N 2 en masse because NO 3 -N was not being created from NH 4 -N. The source of the organic nitrogen and NH 4 -N was probably organic matter in the soil media. Some of the organic nitrogen could have subsequently converted to ammonia and some have remained in an organic state. This would explain a significant increase p 0.05 in the concentration of organic N effluent. Total phosphorus migration from the bioretention cell was similar to that of organic N. As litter decomposed, phosphorus could have either leached from the organic matter to the water column or become detrital accretion Kadlec and Knight 1996. Some phosphorus that became soluble and entered the water column was unable to sorb to the soil of the Greensboro cells because of the medium to high P-index. Because neither cell provided an environment to reduce soluble phosphorus, both cells G1 and G2 had higher TP concentrations in effluent than influent to the cells. Because concentrations of nutrients in the bioretention cells outflows were much higher than those of inflows, and, as noted in an earlier section, winter outflows were significantly higher than those of the warm seasons, lower mass removal efficiencies were calculated during the winter. For example, despite an annual removal of 40% TN, during the winter of 2002 to 2003, cell G2 added more nitrogen to the storm sewer system than that which entered the bioretention cell. Conclusions and Design Recommendations The following conclusions were drawn from the research presented herein: 1. Annual TN mass removal estimated for two conventionally drained bioretention cells with 1.2 m 4 ft of fill media was 40%. NO 3 -N loads were reduced by varying degrees by the two conventionally designed cells. Greensboro cell G2 removed 75%, while Chapel Hill cell C1 removed 13%. This may have been due to the possible formation of an anaerobic zone in G2 and not in C1. 2. Bioretention mass removal rates for TP at the field sites ranged from a 65% removal to a 240% increase, probably due to the type of media used in the bioretention cell. The P-index of the media in cell G2 was high 86 to 100, indicating that the media was saturated with phosphorus. In cell C1, the P-index was low 4 to12, indicative of a media that could accept more phosphorus. The lower P-index, with more available CEC sites, likely enhances the adsorption of phosphorus, thereby lowering the TP concentrations in the outflow. Soil media of this composition are recommended for use in phosphorus-sensitive watersheds. 3. Bioretention cells reduced the mass of many metals in the outflow to a very high degree. During the year-long period beginning in June 2002 and ending in May 2003, mass metal removal rates from cell G2 for Zn and Cu were more than 98%, and Pb removal rates exceeded 80%. These results echo those found by Davis et al. 2003. Most lead concentrations entering the cell were less than the MDL, and lead outflow concentrations were also lower than the MDL on all but one occasion. The cell reduced Pb loadings by 80% because outflow was nearly 80% lower than inflow. Iron mass entering the storm sewer network increased dramatically, by 13,000%, more than the inflow mass entering the cell, perhaps due to the bioretention cell s location adjacent to ironrich in situ soil or the likelihood of the fill soil becoming saturated and having its Fe content leach into the storm sewer system. 4. Outflow nutrient concentrations typically exceeded those of the inflow. This held true for both TP and TN. Were it not for a significant reduction in outflow runoff volume, the bioretention cells monitored would have added nutrient mass to the storm sewer. Significant p 0.05 increases in concentration occurred for ortho-p, TP, NH 4 + -N, and TKN at two cells, G1 IWS configuration and G2 conventional configuration. TP and TN concentrations increased 5- to 30-fold and 3- to 4-fold, respectively, from inflow to outflow. 5. Unlined bioretention cells can reduce total outflow to the storm drainage network, even in clayey soils. During the one year studied, outflow volumes were less than 50% of runoff volumes entering the bioretention cell, indicating the importance of ET and exfiltration. This reduction in outflow is essential when computing load removal. 6. Seasons and their related weather had a significant impact p 0.05 on outflow volume from bioretention cell G2. During the warm seasons spring, summer, and fall there was a significantly lower ratio of outflow to inflow than during the winter but not a significant difference in the outflow to runoff ratio among the warm seasons. Because mass removal rates are based upon inflow and outflow, the mass removal rates were much higher in the warm seasons than in the winter. 7. The impact of drainage configuration on TN removal was not determinable. Outflow from the IWS design at cell G1 had slightly lower concentrations of TN, but these results were not statistically significant. It was observed that cell G2 had saturated pockets, despite its design, which may explain somewhat similar TN effluent concentrations. Implications of the research on design include new design standards for media selection and outlet configuration. Each would be pollutant dependent. A bioretention area designed to remove phosphorus should certainly be required to use low P-index soils. If a bioretention area is designed to remove nitrogen, then other factors such as fill media soil organic content and hydraulic conductivity need to be considered. State-assigned TN and TP removal efficiencies for bioretention cells may need to be updated. Additional field and laboratory research is needed to continue refining bioretention design standards. Acknowledgments The writers would like to thank the research sponsor: the North Carolina Water Quality Workgroup of the UNC Water Resources Research Institute. The City of Greensboro Scott Bryant and Ron Small and the Town of Chapel Hill Fred Royall provided invaluable support. Thanks also go to the following members of the NCSU Biological and Agricultural Engineering Department: L. T. Woodlief, Amy Moran, Mike Shaffer, and Dave Bidelspach. References American Sigma. 2003. User s manual for Sigma 900 and Sigma 900Max, Mission Viejo, Calif. JOURNAL OF IRRIGATION AND DRAINAGE ENGINEERING ASCE / NOVEMBER/DECEMBER 2006 / 607

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