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i r. APPENDIX B Extraction Well Design 'rhe ex"aclion well was designed for!lows between land S g:lllr m per minute. The selection ofs-cen slot size and tiler pack size were based on the methods discussed in Groundwater and Wells,second edition,Cha ler 12(1986, j Driscoll.Fletcher,Johnson Division,St.Paul Minnesota} Known: 12 inch diameter borehole.6 inch diameter casing,twenty feet of screen in saturated zone (longer screen would allow larger drawdown but may cause contamination orsoil in deeper zones during non-pumping periods). s ' fjased on lillmologic description and grain size analysis the soil in the saturated zone is silty-sand and has a hydraulic I PA conductivity in the range of 10 to 100 gallons per day per foot squared(gpd/It2)(Grututdwaterand tells, pg. 75) a 13 'Ina 1,natuEw flow to the cx�mcdofl well: surface area of aquifer exposed at"borehole is pi times the diameter times the length: r E 3.14 x 1 It.x 20 ft.=62.8 square feet. natural 110w to well equals the surface area times the hydraulic conductivity: 62.8 sq.li.x 100 gpd/ft.2=6.280 gpd or 0.01 cubic feet per second Deterntinatinn of Minimum Olen area of Screen: i i Area of casing available for flow assuming a worst case drawdown of 15 feet equals: Area=I.D.x pi x length Area=.5 ft.x 3.14 x 5=7.8 square feet 3 - Required open arra equals the natural flow divided by the preferred minimum entrance velocity. The preferred velocity i is O.I feet per second. Area=O.Olcfs/0.1 fs=0.1 square foot. i Assuming half of the open area will become blocked the required open area is 0.2 square feet. In terms of per cent l �E this is 2.6% (.2 sq.ft/7.8sq.ft.). Determine Filter pack Size- Froin igFroin the grain size curve(Figure B-1 and Table B-1)the uniformity coefficient (UC)is calculatgrain ed: UC= diameter at 6U per cent passing (00p)divided by grain size diameter at 10 percent passing all at UC=.22mm/0.006mm=36 'fb determine the filter pack size a grain size curve was constructed parallel to the formation curve but bounded by two other curves;a)a curve with a UC of 2.5 drawn through the D30p times 6 and;b)a curve with a UC of 2.5 drawn through the D30p times 9. These curves are shown on Figure B-1. I ~, Based on information supplied by the Lonestar htdustries, Lonestar number lc sand provides an acceptable grain size distribuition for the filter pack. t L Determine Screen Sing; - i The formation has a UC greater than 2.5 so select slot size allowing up to 20%of filter pack to pass. A screen slot ! size of 0.02 inches is the largest slot size acceptable. i Check on flow capacity_oFscreeo:-12.squ.tre inches Perfootr---------------- — --- [ - ------ - ---- ------- --— open area times 0.5(addcipated-open area loss over---- ------ time)limes 5 feet(worst case available length of screen) times 0.31 (converts to gpm)equals 9 gpm, 9 gpm is -- more than expected so slot size will not limit well production.