where pi = plotting position for the ith ranked data value, from smallest to largest, i = the rank of the data value, and n = the total number of data values (Helsel and Hirsch 2002). The plotting position associated with the median of a dataset is pi = 0.5. Plotting positions represent the non- exceedance probabilities for given data values.
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Design of a Bioretention-Gravel Wetland Hybrid
An innovative bioretention system was designed and constructed based on the concepts of using the functional mechanisms of a gravel wetland, and replicating them in the footprint of a
standard bioretention system (Roseen et al 2011). From the surface, the system does not look any different than a standard bioretention system. However, in the subsurface it incorporates an anaerobic internal storage reservoir (ISR) in the same sense that is accomplished in a gravel wetland. Of primary significance in this design is the ISR, the long circuitous flow path, and the volume contained in the ISR. A traditional bioretention system has approximately 2 feet of vertical filter path length as it moves through the bioretention soil media prior to exiting by underdrain or exfiltration. A gravel wetland typically has at least a 30’ horizontal flowpath through the anaerobic ISR. A gravel wetland typically contains about 25% of the WQV in the ISR. This innovative system replicates this to a lesser degree. The ratio of the ISR/WQV is believed to be a crucial element of the design in that it is based on the phenomena that nitrate is heavily first flush weighted, and should wash off in the beginning of a storm event, or a small fraction of the WQV. The nitrate first flush was observed for storms of multiple sizes by Roseen et al (2006).
Two side-by-side systems were installed in a commercial parking lot on Pettee Brook Lane in Durham, NH, in summer 2011. Partners in this installation were the Town of Durham NH, US EPA Region 1, and the UNHSC.
These two systems contain the same BSM: 50% sand, 10% compost, 20% wood chips, 10% loam, and 10% WTR2 (identical to BSM.10 from column phase 2 and BSM3 in column phase 3). These two systems are known as Bio 5 with Cell 1 and Cell 2. Bio 5, and other systems, including a subsurface gravel wetland system (GW) and two standard bioretention systems (Bio- 3 and Bio-4), were monitored for nitrogen and phosphorus removal.
The design of the parallel systems in Durham is nearly identical, except for the difference in the sizing of the internal storage reservoir. Cell 1 was designed to capture a drainage area of 13,400 ft2 and hold 20% of the water quality volume in the ISR. Cell 2 was designed to capture a drainage area of 17,200 ft2 and 10% of the WQV in the ISR (Figure 3). The layout and long section detail of the cells are shown in Figure 11 and Figure 12. A walkway divides the cells, which are each about 32 feet in length and 6 feet wide. UNHSC specifies a minimum 30 foot horizontal flow path in the designs for subsurface gravel wetlands to allow space and time for the denitrification process to occur in the subsurface storage reservoir. The Bio-5 cells were
designed with a minimum horizontal flow path of 22 feet in the internal storage reservoir (Figure 12). Depth of BSM in these systems is 2 feet. A 6 inch pea gravel (3/8” diameter) layer lies below the BSM to prevent migration of the BSM into the crushed stone (3/4” diameter) layer. The crushed stone layer varies between 3.32 and 3.75 feet deep in Cell 1 and only 2.08 and 2.50 feet deep in Cell 2. This layer provides the internal storage reservoir; since the outlet from the system is at the top of this layer, this layer is permanently saturated. The greater depth of the stone layer in Cell 1 provides more storage space than in Cell 2 (see also Figure 13 for the cross section).
A geomembrane of high-density polyethylene (HDPE) was placed in the stone layer of each cell at a 1% slope to increase the travel distance of the water to be treated. Water is forced to travel
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horizontally around the membrane and through the stone layer to the outlet. This provides a minimum travel distance of 22.17 feet.
The cells were both designed for a maximum ponding depth of 4 inches, with an overflow grate sitting 4 inches above the BSM surface. Similar to a gravel wetland, the system outlets are orifice controlled with a design release rate of 24 hours. They are vegetated with native plants, and the surface is dotted with round river stones.
Figure 11. Layout of Durham Bio-5 Bioretention Cells.
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Figure 13. Cross-Section of Durham Bio-5 Bioretention Cells.
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RESULTS AND DISCUSSION
Soil Test Results
Materials characterization was important in this study because of the wide variation observed in materials used in bioretention soil mixes, particularly within compost and WTR (see Table 4, Appendix A for full soil reports). Of particular interest to this project are the contents of available phosphorus, aluminum and iron, measured by the Mehlich 3 extraction method. Mehlich 3 and oxalate extraction are both methods to estimate the amount of reactive element present in the soil matrix. At the PSU Agricultural lab, Mehlich 3 extraction was the method available, though oxalate extraction is a commonly preferred method in other studies due to greater accuracy (Dayton and Basta 2005; Dayton et al. 2003; Elliott et al. 2002; O’Neill and Davis 2012a; Sakadevan and Bavor 1998).
The P saturation index is defined as the ratio of reactive P to reactive Al and Fe (typically using oxalate extractable values, but estimated here using the Mehlich 3 method). A low P saturation index indicates a low available P content relative to the available Al and Fe content in a material, which is desirable for this study. Table 4 presents these values. O’Neill and Davis (2012a) propose the oxalate ratio (OR) as a measure of P leachability. OR is essentially the inverse of the P saturation index:
=( + )