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HomeMy WebLinkAboutNC0003573_Stormwater Calculations_20201210DocuSign Envelope ID: COB F24FB-5071-4B1C-AE82-E891E3AC3233 Geosyntec' consultants Geosyntec Consultants of NC, P.C. NC License No.: C-3500 and C-295 Memorandum Date: 10 December 2020 To: The Chemours Company, FC LLC From: Geosyntec Consultants of NC, P.C. Subject: Fayetteville Works Monomers/IXM Stormwater Runoff Volume Calculations for Design Storm 2501 Blue Ridge Road, Suite 430 Raleigh, NC 27607 919.870.0576 — DocuSigpa ft O.0578 www.geosyntec.com `�oR�N1CAIRo �oFESS/p,�'1'�'', SEAL 038141 VFHHGIN DE�N�c4 J� ```` _ nDocuSign/e�b-y: N�ti�� l� ,. 15D6CDF15426487... This memorandum was prepared by Geosyntec Consultants of NC, P.C. (Geosyntec) for The Chemours Company, FC, LLC (Chemours) to outline the calculations for stormwater runoff volume draining to the Monomers/IXM stormwater treatment system for the design storm. The Addendum to Consent Order Paragraph 12 (CO Addendum) specifies (in Paragraph 4) that "By June 30, 2021, Chemours shall complete installation of, and commence operation of, a system that captures and treats stormwater from the Monomers/IXM area at the Facility. " The CO Addendum also specifies that "the Monomers/IXM stormwater capture and treatment system consistently captures stormwater from the MonomersA" area in rain events up to one (1) inch within a 24- hour period and removes PFAS compounds (as measured by concentrations of indicator parameters GenX,, PMPA, and PFMOAA) at a minimum removal efficiency of 99%. " The CO Addendum requirement to capture the 1-inch, 24-hour rain event, or design storm, is consistent with North Carolina statewide guidance. The North Carolina Department of Environmental Quality (NCDEQ) Stormwater Design Manual' (Manual) (Part B — Stormwater Calculations) specifies one inch as the design storm depth, for all inland counties. The Monomers/IXM stormwater treatment system ("Treatment System") will receive stormwater runoff from the Monomers/IXM drainage area via the Cooling Water Channel (CWC) and separate stormwater-only ditch, through an off-line configuration. Stormwater and Non -Contact Cooling Water (NCCW) will be separated to create a stormwater-only channel (NCCW will be in a separate pipe), with a sump and diversion structure located in the southwest corner of the Monomers area. This is illustrated in Figure 1. ' North Carolina Department of Environmental Quality. North Carolina Department of Environmental Quality Stormwater Design Manual — Stormwater Calculations. 2017. hLtps:Hdeq.nc.gov/sw-bmp-manual engineers I scientists I innovators DocuSign Envelope ID: COBF24FB-5071-4B1C-AE82-E891E3AC3233 Stormwater Treatment System Runoff Volume Calculations 10 December 2020 Page 2 Drainage area to Treatment System Figure 1. Proposed Treatment System Drainage Area Diverted stormwater will be pumped via an offline2 pump station into a storage tank that will function as flow equalization and initial pretreatment (via sedimentation) for the Treatment System. Following treatment, treated stormwater will be piped back into the Site Conveyance Network, comingling with NCCW and flowing to Outfall 002. This concept is illustrated in Figure 2. When the capacity of the sump and diversion structure is exceeded during storm events with large total rainfall depths and/or intensities, stormwater flows will overflow the diversion, comingling with NCCW, and flow to Outfall 002. 2 An offline pump station pumps stormwater flows that do not exceed the capacity of the Treatment System. DocuSign Envelope ID: COBF24FB-5071-4B1C-AE82-E891E3AC3233 Stormwater Treatment System Runoff Volume Calculations 10 December 2020 Page 3 NCCW Untreated stormwater (potentially when rainfall is in excess aft" design storm) Treated stormwater Diversion -Storage -Treatment Flow Diagram storage/ pretreatment treatment train Existing channel (to OF 002) - 4- �14 1- Proposed pipe for NCCW *Not to scale (within existing channel) pump station Stormwater runoff greater than the design storm diversion manhole stormwater flows are diverted to the treatment system; high flows (above design storm) continue through the channel (using a weir or different elevation outlet pipes Existing concrete -lined channel (proposed for stormwater only) Figure 2. Treatment System Design Schematic This memorandum outlines the calculations for determining the stormwater runoff volume to the Treatment System, which is used to size the storage tank, from the design storm cited in the CO Addendum. It should be noted that the storage tank functions together with the Treatment System design flowrate, diversion structure(s), and pump station(s) to provide adequate capture and treatment of stormwater runoff from the drainage area. However, this memorandum only outlines the calculation of the runoff volume from the design storm, to size the storage tank. It is also important to note that the Treatment System design flowrate (not discussed herein) will be provided to treatment system vendors, in addition to PFAS concentration reduction requirements specified in the CO Addendum, and the vendor will design the Treatment System. The Simple Method for Runoff Volume, as outlined in Part B of the Manual, was used to determine the runoff volume from the 1-inch rain event (as specified in the CO Addendum) from the drainage area. This method first determines the runoff coefficient using the impervious fraction of the drainage area, as shown in Equation 1. The impervious fraction was determined using aerial imagery. The drainage area to the Treatment System was divided into the following classifications: building/rooftop, impervious (i.e., pavement), gravel/river rock, and open/undeveloped. Building/rooftop and impervious (i.e., pavement) areas were assumed to have an imperviousness of 100%, gravel/river rock areas were assumed to be 80% impervious, and open/undeveloped areas were assumed to be 11% impervious. These assumed values of imperviousness, based on land cover, were based on guidance from the DocuSign Envelope ID: COBF24FB-5071-4B1C-AE82-E891E3AC3233 Stormwater Treatment System Runoff Volume Calculations 10 December 2020 Page 4 Manual (Part B, Stormwater Calculations, Table 1)3 and best professional judgement. The approximate area of each land cover is shown in Table 1, and the resulting impervious fraction of the drainage area to the Treatment System was 0.83, or 83% impervious. Table 1. Land Cover of Drainage Area to Treatment System Land Cover Area (acre) Building/rooftop 2.1 Pavement 2.8 Gravel/river rock 8.2 Open/undeveloped 0.8 Total Area 13.9 Equation 1: Runoff Coefficient Rv=0.05+0.9x'A where, R, is the runoff coefficient (unitless); and IA is the impervious fraction (unitless). Rv = 0.05 + 0.9 x 0.83 = 0.80 The design volume was then determined using Equation 2 with a design storm depth of one inch. The drainage area to the Treatment System was determined to be approximately 13.9 acres, as illustrated in Figure 1. A portion of the drainage area has secondary containment that captures rainwater for disposal off -site (i.e., it does not drain to the site conveyance network and then to Outfall 002). Rainwater in process sumps will continue to be disposed of off -site. Rainwater captured in non -process sumps is planned to be sent to the Treatment System. To be conservative, the full 13.9 acre drainage area is assumed for use in subsequent calculations. 3 Open/undeveloped areas were assumed to be similar to "Lawns, heavy soils, flat (<2%)", in Table 1 in Part B of the Manual, which references a rational runoff coefficient of 0.15 (and imperviousness of 0.11). DocuSign Envelope ID: COBF24FB-5071-4B1C-AE82-E891E3AC3233 Stormwater Treatment System Runoff Volume Calculations 10 December 2020 Page 5 Equation 2: Design Volume DV=3630xRDxRVxA where, DV is the design volume (cubic feet); RD is the design storm depth (inches); R,, is the runoff coefficient (unitless); and A is the drainage area (acres). cu ft DV = 3630 in x ac x 1 in x 0.80 x 13.9 ac DV = 40,214 cu ft = 300,800 gal The calculations outlined above result in a stormwater runoff volume from the drainage area to the Treatment System, for the designated design storm, of approximately 300,800 gallons. The capacity of the storage tank was sized for the runoff volume from the 1-inch rain event (as specified in the CO Addendum) from the drainage area to the Treatment System, which was determined based on the Simple Method for Runoff Volume (Part B of the Manual), as outlined above. The calculated design volume was used to determine the volume capacity of the storage tank, resulting in a storage tank designed to have a volume capacity of approximately 300,800 gallons. Part C-6 of the Manual includes information on sand filters, which requires both a stormwater volume and flow -based sizing process, similar to the Treatment System. It is important to note that the Treatment System will not operate as a sand filter. NCDEQ design guidance for sand filters is simply used to aid in developing conceptual design parameters relevant to the storage tank and Treatment System design flowrate. Minimum design criteria (MDC) #3 (sediment/sand chamber sizing) notes that, "The volume of water that can be stored in the sediment chamber and the sand chamber above the sand surface combined shall be 0.75 times the treatment volume." It further notes to calculate the design volume, the design volume should be multiplied by 0.75 ("a `discount' that is allowed because stormwater infiltrates so rapidly through the sand media that the stormwater is treated throughout the storm event. By the end of the storm, the runoff from the beginning of the storm has already been treated and has exited the sand filter"). DocuSign Envelope ID: COBF24FB-5071-4B1C-AE82-E891E3AC3233 Stormwater Treatment System Runoff Volume Calculations 10 December 2020 Page 6 Although this discount factor could potentially be applied to the design volume of the storage tank, based on guidance from the Manual, the full design volume (as calculated with Equation 2) is recommended as a conservative approach. The capacity of the storage tank is sized to store the entire runoff volume from the 1-inch design storm, even though stormwater will also be treated by the Treatment System throughout the storm event, allowing for additional capacity in the storage tank.