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MEMORANDUM Page 2 of 3 00■ Holly Commerce Center E&E Tracy, CA �0 S I E G F R I E D Attachment 1. The site is designed with a detention and water quality basin that serves a dual purpose. The basin is designed to attenuate large storm events so that Sugar Cut is not impacted while also capturing and treating the 85th percentile storm event with a total annual capture of least 80%. For the sake of the following analysis that is intended to replace the nitrate loading calculation found at the bottom of Page 8 of the Chesney Report Dated May 6, 2019. Revised Equation per Attachment 1 nr = Wnw(1-d)+Rnb / (W+R) (See Attachment 1 for definitions of terms) For this project: W = 960 gpd x 5 dpw x 52wpy = 249,600 gals/yr = 33,367 cu-ft/yr x (1/43,560 ac-ft/cu-ft) = 0.7660 ac-ft/yr (revised from original report to match ADF in Siegfried design) n,N = 146 mg N/L (per original report) d = 30% (per original report) nb = Assumed 0.1 ppm, NO3-N (per original report) R = R, + R2 = Average annual rainfall in ac-ft/yr (see below for analysis) Total Site Area including the basin area = 45.57 acres (42.00 acres for the site plus 3.57 acres for proportional share of the basin (80.8% of 4.43 acres) Annual Rainfall = 9.5in (per San Joaquin County Standard Drawing D-3) Total Runoff = 9.5in/yr x 45.57 acres x (1/12) = 36.08 ac-ft/yr R, = Portion of annual rainfall that is absorbed into leach field areas, landscaping less evaporation from paved surfaces. For the sake of this analysis it is assumed that the site is 7% pervious and that 3% of the runoff is retained in the pavement area and evaporated. The paved areas are approximately 33.1 % of the site. R, = (35.57 ac-ft/yr x 7%) - (35.57 ac-ft/yr x 3% x 33.1 %) = 2.4899 — 0.3532 = 2.1367 ac -ft R2 = Portion of the annual rainfall that is captured by the basin and infiltrated, including any evapotranspiration. Although Attachment 1 suggests a month by month analysis this analysis has simplified the approach due to the fact that the County mandated storm water quality requirements dictate that the basin capture and treat the 85th percentile event and capture and treat at least 80% annually. Our storm water modeling estimates that in large storm events approximately 20% of the storm water will be pumped to Sugar Cut. Per the figure below from Appendix D of the California Storm Water BMP Handbook for New Development and Redevelopment for an impervious coefficient of 93% and a required capture rate of 80% the basin annually will capture approximately 70% of the total runoff and infiltrate it with a typical drawdown time of 48 hours. The basin will not be planted and will not have standing water during months when the ET rate is high, however for the sake of this analysis it is assumed that 5% of the water is lost to ET. R2 = 45.57 ac-ft/yr x (70% - 5%) = 29.6205 ac -ft R = R, + R2 = 2.1367 ac-ft/yr + 29.6205 ac-ft/yr = 31.7572 ac-ft/yr Stockton San Jose Sacramento Modesto 3428 Brookside Rd. 111 N. Market St., #300 900 Howe Ave., #200 100 Sycamore Ave, #100 Stockton, CA 95219 San Jose, CA 95113 Sacramento, CA 95825 Modesto, CA 95354 t: 209.943.2021 t: 408.754.2021 t: 916.520.2777 t: 209.762.3580 F:\17projects\17017 Holly Sugar Industrial\Civil\Septic Sizing - Pre1im\17017 Holly Commerce Center E&E Nitrate Loading Memo Ver 1.docx