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Aquifer Storage Properties

Aquifer Storage Properties. CVEG 5243 Ground Water Hydrology T. Soerens. Q. well. Pump tests are used to determine aquifer parameters (hydraulic conductivity and storage properties). h 0. h 2. h 1. Aquifer Medium. drawdown curve drawdown cone cone of depression. h 0 - h = drawdown.

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Aquifer Storage Properties

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  1. Aquifer Storage Properties CVEG 5243 Ground Water Hydrology T. Soerens

  2. Q well Pump tests are used to determine aquifer parameters (hydraulic conductivity and storage properties) h0 h2 h1 Aquifer Medium drawdown curve drawdown cone cone of depression h0 - h = drawdown Darcy’s Law (K) describes the shape of the drawdown curve at equilibrium h0 We need more information to describe the time-dependent (“transient”) behavior

  3. Specific Yield, Sy for unconfined aquifers Volume of water released under gravity drainage from a volume of initially saturated material, divided by the total volume of the material. Aquifer Storage Properties (p.58ff) 1 m3 • Specific Retention, Sr • Volume of water retained ... • Released + retained = total • Sy + Sr = n 0.10 m3 • range of Sy = 0.01 clay to 0.30 gravel

  4. Specific storage, Ss Volume of water released per unit volume from storage under a unit decline in hydraulic head. i.e., if head drops 1m while 0.01 m3 is released 1 m3 0.01 m3

  5. Storativity, (storage coefficient) for confined aquifers S = volume of water released or taken into storage per unit of aquifer storage area per unit change in head. well 1 m2 upper confining layer b = 3m lower confining layer 1 m3 3 boxes, each 1 m3 1 m3 1 m3 • S = Ss.b (unitless) • Ss = volume of water released per 1 m3 of aquifer volume per 1m change in head • example: Ss = 0.01 m-1(0.01 m3 released per 1m3 box) • Aquifer volume per 1 m2 of aquifer area = aquifer thickness = b • example: = 3m x 1m2 / 1m2 = 3m = b Ss= 0.01 m-1 • range of S = 0.005 to 0.00005 (10-3 to 10-5)

  6. Transmissivity, T T = K.b Pump test assume confined calculate T, S if S ~ 10-3 or lower  confined if S > 10-2  unconfined Water released from unconfined: drainage S = Sy confined: water expansion - minimal S ~ 10-6 m-1 compression of aquifer

  7. Assumptions uniform hydraulic conductivity aquifer not stratified saturated thickness constant unconfined - before pumping confined - constant well is 100% efficient fully penetrating no slope of water table or potentiometric surface laminar flow infinite extent Well terms static water level (SWL) pumping water level (PWL) drawdown dif between initial and PWL s = h - h0 residual drawdown (during recovery) after pumping stops, water level comes up. well yield discharge from a well (gpm) specific capacity well yield per unit drawdown varies with time radius of influence horizontal distance from center of well to limit of cone of depression (no drawdown) gets larger with time larger in confined Equilibrium Well Equations Confined Unconfined

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